NationStates Jolt Archive

Don't know if this has been gone over here, but here's the question:

Imagine a giant treadmill the length and width of an airplane runway which moves backward at the same speed as the aircraft's takeoff velocity.

Would the plane be able to take off from this treadmill?

Yes. The plane is not being moved forward by it's wheels.

No.

Yes. Gartref is right. This whole thing has been hashed out a million times. Draw the free body diagram if you don't believe it.

I'll say it again, no. Lets say the fixed wing aircraft takes off at a velocity of 70 kts. If the treadmill, which the aircraft is taking off from, is moving in the opposite direction at a speed of 70 kts as well then there is no airflow over the lifting surfaces of the aircraft, thus no flight is achieved. Bernoulli's Principal or the theory behind lamanir flow wings has no effect in this example.

I'll say it again, no. Lets say the fixed wing aircraft takes off at a velocity of 70 kts. If the treadmill, which the aircraft is taking off from, is moving in the opposite direction at a speed of 70 kts as well then there is no airflow over the lifting surfaces of the aircraft, thus no flight is achieved. Bernoulli's Principal or the theory behind lamanir flow wings has no effect in this example.

The treadmill will not affect the speed of the airplane.

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.

Don't know if this has been gone over here, but here's the question:

Imagine a giant treadmill the length and width of an airplane runway which moves backward at the same speed as the aircraft's takeoff velocity.

Would the plane be able to take off from this treadmill?

What kind of aircraft, what are the power plants, and how many?

It works mainly because of the jet engines thrusting. Now, if it were say, a glider, then no, you would get no lift.

The treadmill will not affect the speed of the airplane.

Right. But if the treamill is acting in the opposite direction of the aircraft always at the same speed (take a look at the OP question) then there should be no airflow over the lifting surfaces. Groundspeed and airspeed are comepletely different.

EDIT: Sorry but I'm going to the bar to get drunk, I'll Wilgrove take over.

No.

You need air to go over the wings of a plane to take off. If a plane is on a treadmill, then it will move because the prop or jet will create thrust. But the plane cannot take off unless air flows over the wings. If you pointed a giant high speed fan at the airplane, then it could take off. But you have to remember that the airplane is not moving forward relative to the earth or atmosphere, so as soon as the plane leaves the treadmill and the wind of the fan, it will drop like a stone.

Right. But if the treamill is acting in the opposite direction of the aircraft always at the same speed (take a look at the OP question) then there should be no airflow over the lifting surfaces. Groundspeed and airspeed are comepletely different.

Yea but what if it's a prop aircraft, wouldn't the wind from the prop give some winds for lift?

What kind of aircraft, what are the power plants, and how many?

It's an airplane so that implies it has power plants which are sufficient for it to take off under ordinary circumstances. Why does anything else matter?

It works mainly because of the jet engines thrusting. Now, if it were say, a glider, then no, you would get no lift.

If an airplane moves along a runway, it can get air to travel over its wing to create lift. If an airplane is on a treadmill and is not moving relative to the air around it, there will be no lift.

Yea but what if it's a prop aircraft, wouldn't the wind from the prop give some winds for lift?

Prop provides thrust, not lift (of course if I remember theres other stuff like couples involved, but I don't).

oh for...
Someone get an RC plane and put it on a treadmill and post the vid already!

Right. But if the treamill is acting in the opposite direction of the aircraft always at the same speed (take a look at the OP question) then there should be no airflow over the lifting surfaces. Groundspeed and airspeed are comepletely different.

EDIT: Sorry but I'm going to the bar to get drunk, I'll Wilgrove take over.What propels the aircraft foreward? The tiny wheels which do not appear to be attached to some sort of engine or the giant fucking jet turbine?

If it is the giant fucking jet turbine(tm), then the treadmill would have zero effect on the plane. It would merely increase the rolling friction of the jet, which tends to be insignificant in comparison to the thrust produced by the giant fucking jet turbine.

All the treadmill is going to do is spin the planes wheels faster. The plane is not going to be moved backward by the treadmill because it gets it's power by pushing against the air - not the ground. The treadmill is not moving the air, just the ground - so it has almost no effect on the airspeed of the plane.

Planes tend to need to move to get lift, if you're holding it stationary it's probably not going anywhere.

EDIT: ^^^ I forgot the wheels are free. Yeah it would take off.

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.

Diamond isn't metal.

Diamond isn't metal.
:rolleyes:

What propels the aircraft foreward? The tiny wheels which do not appear to be attached to some sort of engine or the giant fucking jet turbine?

The device that happens to provide thrust, be it a prop or turbine. Regardless, the a/c (aircraft) will still start off on the ground. In this case the ground, a treadmill, constantly moves in the opposite direction of the aircraft. Thus takeoff speed (relative to the air, the only thing that actually has an effect on a wings ability to provide lift) is nil. This is however a closed system right? No headwind and such to affect the a/c.

Yes. The plane is not being moved forward by it's wheels.
No, but the wheels aid it in moving forward which brings air over the wings for lift. A treadmill leaves it sitting in place = no air flow = no lift.

All the treadmill is going to do is spin the planes wheels faster. The plane is not going to be moved backward by the treadmill because it gets it's power by pushing against the air - not the ground. The treadmill is not moving the air, just the ground - so it has almost no effect on the airspeed of the plane.

Shh don't make it too easy.

http://img.photobucket.com/albums/v719/Lacadaemon/stupid.jpg

See, it's not that difficult.

Its interesting that alot of people seem to think yes. Apparently, many of us do not understand flight or physics.

Here (http://youtube.com/watch?v=-EopVDgSPAk)

and here (http://youtube.com/watch?v=kHUnAU0MyHM)

go and run your own experimentations.

No, but the wheels aid it in moving forward which brings air over the wings for lift. A treadmill leaves it sitting in place = no air flow = no lift.

Is that so?

Tell us more ! :D :D :D :D :D :D :D

No, but the wheels aid it in moving forward which brings air over the wings for lift. A treadmill leaves it sitting in place = no air flow = no lift.

I suppose that's why seaplanes never actually manage to take off.

If the plane dosn't have any air moving around the wings, then it can't generate ANY lift, so its just going to stay on the treadmill going nowhere since its engines are counteracting the speed of the treadmill on the wheels.

Its interesting that alot of people seem to think yes. Apparently, many of us do not understand flight or physics.


Apparently! :D :D :D :D :D :D


Digging yourself a deeper hole there amigo.

The device that happens to provide thrust, be it a prop or turbine. Regardless, the a/c (aircraft) will still start off on the ground. In this case the ground, a treadmill, constantly moves in the opposite direction of the aircraft. Thus takeoff speed (relative to the air, the only thing that actually has an effect on a wings ability to provide lift) is nil. This is however a closed system right? No headwind and such to affect the a/c.But it is going to be moving forward in relation to the air. The treadmill is only spinning free spinning wheels. The effect of being on a treadmill is going to increase the rolling friction, but it will still be several order of magnitude less than the trust (10^6 Newtons less than the trust). Relative to the air, the a/c is going to be moving 99.9999 percent of its usual take off speed, relative to the air. It will be moving 199.9999 percent its take off speed relative to the treadmill, but as you said the a/c velocity relative to the treadmill has no effect.

Furthermore, a live experiment agrees with me. http://www.youtube.com/watch?v=-EopVDgSPAk The treadmill does not move the air that the plane pushes against to get its trust. The plane moves forward relative to any fixed point besides the treadmill (including the air), the plane takes off.

I deeply and profoundly regret and apologize for neglecting to tick the 'make results public' button when I put the poll up.

It would have been a very accurate list of nsg's worst and dimmest :)

I'm predicting a lot of deleted posts in this thread. :p

I would have to say that if the treadmill was keeping up with how much the engine of an aircraft is pushing it forward, IE if the aircraft is putting out about 40 knots of airspeed and the treadmill for all intent and purposes is keeping up by adding 40 knots in the opposite direction, efficectly keeping the aircraft in the same spot, then no air is generated beneath the wing, and thus no lift.

However, one does have to wonder if ground effect play into the situation.

Ground effect in aircraft, most notably lifting effects in fixed wing aircraft, that have been utilized to create Wing-In-Ground effect vehicles. Similar effects also occur on helicopters and other aircraft, for a variety of reasons. Lifting ground effect is also being researched as a possible way to make trains run on a cushion of air - see Ground effect train.

I'm predicting a lot of deleted posts in this thread. :p

Yeah, I got the first.

EDIT: But I realized my mistake quickly, at least.

Here (http://youtube.com/watch?v=-EopVDgSPAk)

and here (http://youtube.com/watch?v=kHUnAU0MyHM)

go and run your own experimentations.

Meh the problem I have with those two "experiments" (if they can be called that), is that we don't know how fast the aircraft or the treadmill is going. I mean on the one with the kid, for all we know, the kid could be making the airplane go faster than the treadmill.

We need MythBusters to bust this! Someone get Jamie and Adam on the phone!

I deeply and profoundly regret and apologize for neglecting to tick the 'make results public' button when I put the poll up.

It would have been a very accurate list of nsg's worst and dimmest :)


Brilliant.

I would have to say that if the treadmill was keeping up with how much the engine of an aircraft is pushing it forward, IE if the aircraft is putting out about 40 knots of airspeed and the treadmill for all intent and purposes is keeping up by adding 40 knots in the opposite direction, efficectly keeping the aircraft in the same spot, then no air is generated beneath the wing, and thus no lift.

However, one does have to wonder if ground effect play into the situation.You are also assuming that if the treadmill moves at 40 knots, then the a/c is going to move 40 knots slower. Is that the case?

If you have a toy car available, place it on a piece of paper and quickly move the piece of paper out from under it(in the direction that the car rolls). Does the car move as fast as the paper? Why? How does this relate to the airplane question?

Quoted for future reference, to wit, the Hall of Shame

I would have to say that if the treadmill was keeping up with how much the engine of an aircraft is pushing it forward, IE if the aircraft is putting out about 40 knots of airspeed and the treadmill for all intent and purposes is keeping up by adding 40 knots in the opposite direction, efficectly keeping the aircraft in the same spot, then no air is generated beneath the wing, and thus no lift.

However, one does have to wonder if ground effect play into the situation.

Remind me never to fly Wilgrove Airlines!

I'll say it again, no. Lets say the fixed wing aircraft takes off at a velocity of 70 kts. If the treadmill, which the aircraft is taking off from, is moving in the opposite direction at a speed of 70 kts as well then there is no airflow over the lifting surfaces of the aircraft, thus no flight is achieved. Bernoulli's Principal or the theory behind lamanir flow wings has no effect in this example.

Proof that a little knowledge can be a dangerous thing





Recanted!

Saved!

If the plane dosn't have any air moving around the wings, then it can't generate ANY lift, so its just going to stay on the treadmill going nowhere since its engines are counteracting the speed of the treadmill on the wheels.

Is this answer surprising?


No, but the wheels aid it in moving forward which brings air over the wings for lift. A treadmill leaves it sitting in place = no air flow = no lift.

For posterity...

Yea but what if it's a prop aircraft, wouldn't the wind from the prop give some winds for lift?

This coming from an airplane pilot! Christ!

No. Equal forces and acceleration in opposite direction will prevent movement. Without movement, a plane cannot take flight on its own.

Another member in the legion of demerit.

Uh.... seriously?
What level of physics have you attained? Inertia states that an object in motion will stay in motion or an object at rest will stay at rest unless acted upon by an outside force. A toddler pulling an airplane would be an outside force.


No, it's because the wheels push backwards against the ground in order to move the object they are attached to (the plane) forwards. It's the same principle as with a car.

The Legion of Demerit may have found its new commander!

I'm predicting a lot of deleted posts in this thread. :p

I'm predicting I was only the most obvious of the smattering of trolls in this thread.

Meh the problem I have with those two "experiments" (if they can be called that), is that we don't know how fast the aircraft or the treadmill is going. I mean on the one with the kid, for all we know, the kid could be making the airplane go faster than the treadmill.

We need MythBusters to bust this! Someone get Jamie and Adam on the phone!


No we don't.

It is obvious, the props will propel the plane at its usual speed relative to everything but the belt on the treadmill. The wheels will spin faster, but the plane will reach the same speed and will get lift.

So perhaps if the treadmill could reach a speed substantially faster than the typical take off speed of the plane and generate a huge amount of friction on the wheels, it could keep the plane from taking off. By the OP's scenario, it will still take off.

Meh the problem I have with those two "experiments" (if they can be called that), is that we don't know how fast the aircraft or the treadmill is going. I mean on the one with the kid, for all we know, the kid could be making the airplane go faster than the treadmill.

We need MythBusters to bust this! Someone get Jamie and Adam on the phone!

Yes, we need a treadmill like they have for cars - the treadmill doesn't go, it just seats the wheels. The wheels spin the treadmill at the speed they are turning.

Quoted for future reference, to wit, the Hall of Shame

Don't be a dick.

Meh the problem I have with those two "experiments" (if they can be called that), is that we don't know how fast the aircraft or the treadmill is going. I mean on the one with the kid, for all we know, the kid could be making the airplane go faster than the treadmill.

We need MythBusters to bust this! Someone get Jamie and Adam on the phone!

hence my "Do your own experiment."

By doing it yourself, you can easily calcuate the speed of the RC plane. and put the treadmill to an equal or even greater speed.

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.
Yes, diamond is the hardest substance known to man. (I assume you meant mineral when you said metal.) However, it is also very brittle, and because of that breaks very easily. I know this because I did a little research into it when I thought "Well, we have a ton of diamonds, so why aren't we making diamond bullets, or diamond armor, or diamond knives?"

It's a common misconception, but the truth is that it may be the hardest, but that doesn't mean that it's the sturdiest.

On the subject of airplanes taking off...

Dunno.

I'd guess no, though, since it would need air to take off.

Yes, diamond is the hardest substance known to man. (I assume you meant mineral when you said metal.) However, it is also very brittle, and because of that breaks very easily. I know this because I did a little research into it when I thought "Well, we have a ton of diamonds, so why aren't we making diamond bullets, or diamond armor, or diamond knives?"

It's a common misconception, but the truth is that it may be the hardest, but that doesn't mean that it's the sturdiest.


That's 2.

Remind me never to fly Wilgrove Airlines!

You'd probably push the aircraft out of it's Center of Gravity envelope anyways.

This coming from an airplane pilot! Christ!

Sorry, in my trainings and several hours of flight, we didn't cover the whole Conveyor belt vs. aircraft debate. We messed with silly stuff like Dead Reckoning, VOR navigation, flying in IFR weather, and MVFR weather. So sorry if the Conveyor Belt vs. Aircraft debate is kinda low on the list of things that peak my interest about flying and the aviation world.

I'll visit my instructors on Monday and tell them to include this into their lesson plans for future student pilots, even though it has nothing to do with the safe operation of an aircraft.

If you put an airplane in front of a fan, and control it's propellers in a way that the traction is always almost equal to the drag generated by the wind, then it will fly.

A treadmill, however, makes no difference.

That's 2.
What?

Quoted for future reference, to wit, the Hall of Shame

Armchair play-acting engineer

Its true, I am not an engineer. I am a science major though and I have taken some high level physics classes. But you didn't know that did you?

No. It wouldn't gain any lift.

To clarify: when i saw the word "treadmill", i translated that into Dutch as "tredmolen", which is something like a hamster wheel, except human-sized. I couldn't see an aircraft take off from that, even if it were runway sized. Seeing the video, i realized my mistake...

Car vs. Plane

A car is sitting on a suitable conveyor belt, the conveyer belt l is designed to automatically meet the speed that the wheels of the car is going.

As the car engine starts the energy from the engine is transfered onto the wheels. As the wheels start turning, the conveyerbelt matches.

There are 2 speeds that are important. Car relative to conveyor belt and Car relative to ground.

As the wheels spin, driven by the axles the coveyerbelt counters, lets say the car gets up to upto a certain speed and 80km/hr (50 miles/hr) is shown on the spedomitor.

The conveyor belt (most likely thanks to newtons laws of action and reaction) is perfectly also going 80km/hr (50 miles) in the opposite direction.

Using the Vectors, the car is going 80km/hr (100 miles/hr) relative to the conveyor belt

but 0 kms/hr (0 miles/hr) compared to the ground as the movement of the of the wheels compared to the conveyor belt is exactly the same as the conveyor belt relative to the ground.

Hence no overall movement.

A plane is put on the same conveyorbelt, the planes wheel are not driven by any direct force and only respond to external stimulus. The engines of the plane start creating thrust, as thrust pushes the whole plane forward the wheels spin, the conveyor belt which counters the movement makes the wheel spin 2 twice as fast as any force placed on the wheel only affects the wheel. Meanshile the plane is moving forward and air starts flowing under the wings creating lift.

The Conveyor Belt's speed exponentially grows until it or the wheel can no longer handle the stress and one of them breaks or the plane moves off the conveyor belt. This achieving lift.

However if the plane is put in a long enough wind tunnel and the wind tunnel is programmed to create a wind that counters the movement of the airplane. The plane will still move forward but will provide no lift. As overall wind movement is 0.

its true, the plane would take off because the ground doesn't matter. ohh


i am ashamed. :(

Its true, I am not an engineer. I am a science major though and I have taken some high level physics classes. But you didn't know that did you?

Email the head of the physics department and post his reply and then your apology.

I dare ya :D

Its true, I am not an engineer. I am a science major though and I have taken some high level physics classes. But you didn't know that did you?

I suggest you change major right now.

No. Equal forces and acceleration in opposite direction will prevent movement. Without movement, a plane cannot take flight on its own.

I suggest you change major right now.

I did, I had to think about it for more than a couple of minutes though.

No. Equal forces and acceleration in opposite direction will prevent movement. Without movement, a plane cannot take flight on its own.

Did you not look at my fine diagram?

its true, the plane would take off because the ground doesn't matter. ohh


i am ashamed. :(

Hallelujah, rebuke sin an you are saved, we have another brother, born-again :)

Sorry, in my trainings and several hours of flight, we didn't cover the whole Conveyor belt vs. aircraft debate. We messed with silly stuff like Dead Reckoning, VOR navigation, flying in IFR weather, and MVFR weather. So sorry if the Conveyor Belt vs. Aircraft debate is kinda low on the list of things that peak my interest about flying and the aviation world.

I'll visit my instructors on Monday and tell them to include this into their lesson plans for future student pilots, even though it has nothing to do with the safe operation of an aircraft.

You never covered lift, weight, thrust and drag?

No. Equal forces and acceleration in opposite direction will prevent movement. Without movement, a plane cannot take flight on its own.

Oh my friend, I was once like you. But soon I hope you will see the truth.

One of us, we accept him.

Theoretically, yes.

Physically, no! unless you have a treadmill as big as a runway.

Sorry, in my trainings and several hours of flight, we didn't cover the whole Conveyor belt vs. aircraft debate. We messed with silly stuff like Dead Reckoning, VOR navigation, flying in IFR weather, and MVFR weather. So sorry if the Conveyor Belt vs. Aircraft debate is kinda low on the list of things that peak my interest about flying and the aviation world.

I'll visit my instructors on Monday and tell them to include this into their lesson plans for future student pilots, even though it has nothing to do with the safe operation of an aircraft.You never covered lift, weight, thrust and drag?

... I'm more worried that they didn't cover LANDINGS!!! :eek: :p

You never covered lift, weight, thrust and drag?

Yes we did. Thrust is equal to drag, and lift is equal to weight.

Yes we did. Thrust is equal to drag, and lift is equal to weight.

So no, in other words. To start moving you would need thrust to be greater than drag, wouldn't you? F=ma and all that crap.

An actual free body diagram for a Boeing 747 at rest. The numbers and formulas can be found on Wikipedia (you may have to do some derivation), but the result is the plane will be accelerating forward.

http://img481.imageshack.us/img481/5328/img001pt3.jpg

The image is sideways. My scanner driver does not support rotating the image, and I do not have the Gimp/Photoshop/etc installed.

This question has been asked and answered thoroughly. In case you still have doubts then you just need to ask the smartest man in the world for the answer.http://www.straightdope.com/columns/060203.html and his followup http://www.straightdope.com/columns/060303.html

Final solution:


http://i45.photobucket.com/albums/f63/azmjs/Solution.jpg

Depends on the plane and situation.

In case the airflow from the engines passes through the wings and creates enough lift then the plane does lift off, otherwise you just have a highly unstable wind tunnel. However, if the experiment is run in a tunnel one could speculate that the airspeed in the tunnel will be homogenous enough for a lift off (well, there's the roof..).

Of course, if the plane's engines can generate lift simply by running the plane can also rise from a treadmill (eg. VTOL planes, exotic planes with engines as wings, etc..)

In normal case though, the engines themselves *do not* generate enough lift for the aircraft to take off. It's the friggin' airflow generated by the speed of the wings and other lifting surfaces (incl. engines) relative to the air surrounding them that does it - Relative speed which in this scenario is nearly nil.

edit:
Basically the situation is equal to an airplane trying to take off with wheels locked into a place.

edit 2:
Scratch that...The situation isn't anywhere like that.

The plane is able to take off because it can move relative to ground even while rolling on the treadmill almost regardless of how fast the treadmill is rolling because the thrust it exerts is independent of the wheels and thus the treadmill.

edit 3:
Or is it? A plane has to taxi and has to roll over runway before it can lift off. What if it can't go forward because of the rolling treadmill?

ARRRGH!

~ brain meltdown ~

Lift is created by air moving across the wings. Whether airplanes take off or don't take off is not a factor of their forward momentum, but the lift the wings generate. In other words, a treadmill will neither help nor hinder the taking off of an airplane. Only lift, weight, drag and thrust affect them.

Yes, of course it will. The wheels will spin faster, the plane will not be otherwise affected.

Yes, of course it will. The wheels will spin faster, the plane will not be otherwise affected.
A-ha!

Moving to an idealized world where wheels and treadmills are of inifinite durability, completely smooth and can roll infinitely fast.

What happens is that the mass of the treadmill and wheels grows too high for the plane to generate enough lift for a lift off.


In normal world though the wheels will just spin faster making the liftoff somewhat harder but not impossible because the treadmill can't force the plane to stay in place.

The plane's wheels will spin at double frequency but the plane will take off just the same.
Take off and flight are achieved by the velocity of the plane and the geometry of its wings. Velocity is achieved by the engines' thrust that move the plane through the surrounding air. The relative movement of a runway or treadmill would have no influence on that at all.
A plane is not a car. A car's movement depends on the car's interaction with the street (a treadmill there would indeed be effective) while a plane's movement depends on it's interaction with the air. Big difference.

The plane's wheels will spin at double frequency but the plane will take off just the same.

That's not true: The wheels can spin at arbitrary speed depending on the velocity of the treadmill under them.

If the treadmill and the wheels are properly configured the airplane can liftoff even without any wheelspin.

That's not true: The wheels can spin at arbitrary speed depending on the velocity of the treadmill under them.
If the treadmill and the wheels are properly configured the airplane can liftoff even without any wheelspin.Read the OP. :rolleyes:
... at the same speed as the aircraft's takeoff velocity. ...

Read the OP. :rolleyes:

That's limiting the perspective compared to the name of the topic! wail! :D

A plane is not a car. A car's movement depends on the car's interaction with the street (a treadmill there would indeed be effective) while a plane's movement depends on it's interaction with the air. Big difference.

I think everyone here understands that, yes and no voters alike, but the brain has a way of picking the first theory that comes to mind and sticking to it (treadmill moving backwards = plane standing still = no lift).

We're not dumb. We just don't think before we vote, or sometimes talk. ;)

I'll say it again, no. Lets say the fixed wing aircraft takes off at a velocity of 70 kts. If the treadmill, which the aircraft is taking off from, is moving in the opposite direction at a speed of 70 kts as well then there is no airflow over the lifting surfaces of the aircraft, thus no flight is achieved. Bernoulli's Principal or the theory behind lamanir flow wings has no effect in this example.

precicely there will be no thrust so there will be no lift

Yes, it is possible if the Force of the treadmill that pulls the plane backward is smaller than the Friction Force of the wheels.

Edit: But the difference between the two forces should be very great!

After thinking about this long and hard I have decided that this r being trick question.

I found that the question to implied that the treadmill is able to prevent the airplane from moving forwards. This is not the case.

As soon as the engine provides enough thrust to get the wheels rolling then the treadmill ceases to have any additional effect on the acceleration of the plane.

Unless of course the treadmill was moving so fast that it caused the wheels to spin so fast that the insides of the wheels are able to grip the plane, and therefore cause it to roll forwards onto its front.

Well, by actually reading the OP:
Don't know if this has been gone over here, but here's the question:

Imagine a giant treadmill the length and width of an airplane runway which moves backward at the same speed as the aircraft's takeoff velocity.

Would the plane be able to take off from this treadmill?

The question is actually - after long consideration and several false assumptions - reduced to:

Can the wheels of a commerical airliner spin at double the take off speed without braking up?

With a Boeing 747 they'd have to spin at around 580 (http://www.aerospaceweb.org/question/performance/q0088.shtml) km/h.

For comparison space shuttles touchdown at a speed of ~350 km/h (wiki).

edit:
Alternatively, at a maximum takeoff weight, do the rolling wheels induce enough drag to prevent a take off?

Well, by actually reading the OP:


The question is actually - after long consideration and several false assumptions - reduced to:

Can the wheels of a commerical airliner spin at double the take off speed without braking up?

With a Boeing 747 they'd have to spin at around 580 (http://www.aerospaceweb.org/question/performance/q0088.shtml) km/h.

For comparison space shuttles touchdown at a speed of ~350 km/h (wiki).

edit:
Alternatively, at a maximum takeoff weight, do the rolling wheels induce enough drag to prevent a take off?

Yes they can spin twice the take off speed its a requirement for all airliners from the FAA CAA and NATS.

The answer to the question, of course, is NO.

For the same reason that if you were running flat out on the treadmill and then jumped into the air, you would not break the world record for the long jump. In fact, you would land in exactly the same spot.

DO NOT TRY THIS AT HOME!

When you land, the belt of the treadmill will still be going flat out and you will either be catapulted backwards off the end of the belt or you will fall forward flat on your face and then be shot off the back of the treadmill.

If you don't believe me, go ahead and try it! But don't do it home. Do it at the gym so everyone can have a good laugh :p

After thinking about this long and hard I have decided that this r being trick question.

I found that the question to implied that the treadmill is able to prevent the airplane from moving forwards. This is not the case.
I'm so used to indulging bad analogies and examples that I voted "no" and was going to reply "No an airplane can't take off if you keep it still relative to the air but a treadmill isn't enough to stop an airplane from moving. You need to come up with a better example of a situation where an airplane is held in place."

After reading the debate all I can say is wow, it's amazing some people actually think jet engines apply force to the ground/treadmill like car engines rather then to the air.:eek:

PS. This thread is brilliant, very good question.

Well, by actually reading the OP:


The question is actually - after long consideration and several false assumptions - reduced to:

Can the wheels of a commerical airliner spin at double the take off speed without braking up?

With a Boeing 747 they'd have to spin at around 580 (http://www.aerospaceweb.org/question/performance/q0088.shtml) km/h.

For comparison space shuttles touchdown at a speed of ~350 km/h (wiki).

edit:
Alternatively, at a maximum takeoff weight, do the rolling wheels induce enough drag to prevent a take off?

Well, to be fair, the question that you quoted, which I wrote, doesn't mention a commercial airliner anywhere.

All the tangential, trivial speculation aside, at least you're on the team of truth ;)

Of course, if the airplane were to increase it's ground speed to twice it's takeoff velocity, then yes, it would takeoff!

The answer to the question, of course, is NO.

For the same reason that if you were running flat out on the treadmill and then jumped into the air, you would not break the world record for the long jump. In fact, you would land in exactly the same spot.

DO NOT TRY THIS AT HOME!

When you land, the belt of the treadmill will still be going flat out and you will either be catapulted backwards off the end of the belt or you will fall forward flat on your face and then be shot off the back of the treadmill.

If you don't believe me, go ahead and try it! But don't do it home. Do it at the gym so everyone can have a good laugh :p

Somebody's laughing, believe you me.

Thanks for the advice though, ace ;)

Oh, wow, I didn't realise how few people here are able to take abstracted mental images and apply realistic physics to them. I'm highly surprised that more people voted no than yes, considering the general level of intelligence on this forum.

I bet those who think that the plane would not be able to take off also disbelieve that 0.999... = exactly 1.

I would have to say no - because cost over-runs on building a ginormous treadmill would prevent you from purchasing jet-fuel.

Oh, wow, I didn't realise how few people here are able to take abstracted mental images and apply realistic physics to them. I'm highly surprised that more people voted no than yes, considering the general level of intelligence on this forum.

I bet those who think that the plane would not be able to take off also disbelieve that 0.999... = exactly 1.

If the airplane were to increase it's engine speed to allow it to remain in one position on the runway/treadmill, how coud it possibly takeoff? It would, in effect, be stationary. just as you would be if you were running on it.

If, on the other hand the engine speed was increased to double it's stationary position it would travel twice as fast as the treadmill and therefor reach it's takeoff velocity...........

.999... does not = exactly 1

:fluffle:

If you think an airplane on a gigantic treadmill will take off, welcome to the Idiot Club. You join the ranks of Gomer Pyle, Gilligan and other lovable TV personas renowned for their, well... intellectual shortcomings.

Speed through the air and speed on the ground are not the same thing. An aircraft takes off by virtue of its speed through the air, not the speed at which the wheels on the landing gear are turning. This is why they build wind tunnels where the air is moving to test scale models of aircraft, not go down to the local Sears and pick up a treadmill (which, at scale, could reproduce conditions on a model of adequate size for research purposes), which would be much, much cheaper than a wind tunnel.

Also, this is why airplanes take off and land IN TO The wind instead of with it. It's not that it's going to screw up the airplane to take off in the same direction as the wind is blowing, so long as wind speeds are reasonable, but it's more fuel efficient because it takes less speed relative to the ground for the aircraft to maintain lift. 20mph wind is 20mph (relative to the ground) slower that an airplane has to go to take off or maintain lift whilst landing. Now, if you had 200mph winds blowing at an aircraft head on, but the wheels were still on the ground, it would take off. But I wouldn't suggest attempting to fly an airplane in 200mph winds.

In short, the plane's wheels may be turning, but its wings are essentially remaining still in the AIR, thus there is no airflow, thus there is no lift. The jet engines provide horizontal thrust that pushes the plane and causes air to flow over the wings, causing lift. There are engines that are capable of providing lift without airflow, but we call these magical devices rockets and they're usually very, very large and powerful.

Then again, as ridiculous as the idea of a treadmill of that size is, it's remotely possible that it could cause enough turbulence in the air to cause it to move at the right speed for the aircraft to take off, but it would stall and crash as soon as it escaped the treadmill's relatively minute influence.

If you think an airplane on a gigantic treadmill will take off, welcome to the Idiot Club. You join the ranks of Gomer Pyle, Gilligan and other lovable TV personas renowned for their, well... intellectual shortcomings.

Thank you for inviting me to the idiot club! :p This means a lot coming from it's president! :)

Thank you for inviting me to the idiot club! :p This means a lot coming from it's president! :)

As your new Commander in Chief, I command you to bite me.

If you think an airplane on a gigantic treadmill will take off, welcome to the Idiot Club. You join the ranks of Gomer Pyle, Gilligan and other lovable TV personas renowned for their, well... intellectual shortcomings.

Speed through the air and speed on the ground are not the same thing. An aircraft takes off by virtue of its speed through the air, not the speed at which the wheels on the landing gear are turning. This is why they build wind tunnels where the air is moving to test scale models of aircraft, not go down to the local Sears and pick up a treadmill (which, at scale, could reproduce conditions on a model of adequate size for research purposes), which would be much, much cheaper than a wind tunnel.

Also, this is why airplanes take off and land IN TO The wind instead of with it. It's not that it's going to screw up the airplane to take off in the same direction as the wind is blowing, so long as wind speeds are reasonable, but it's more fuel efficient because it takes less speed relative to the ground for the aircraft to maintain lift. 20mph wind is 20mph (relative to the ground) slower that an airplane has to go to take off or maintain lift whilst landing. Now, if you had 200mph winds blowing at an aircraft head on, but the wheels were still on the ground, it would take off. But I wouldn't suggest attempting to fly an airplane in 200mph winds.

In short, the plane's wheels may be turning, but its wings are essentially remaining still in the AIR, thus there is no airflow, thus there is no lift. The jet engines provide horizontal thrust that pushes the plane and causes air to flow over the wings, causing lift. There are engines that are capable of providing lift without airflow, but we call these magical devices rockets and they're usually very, very large and powerful.

Then again, as ridiculous as the idea of a treadmill of that size is, it's remotely possible that it could cause enough turbulence in the air to cause it to move at the right speed for the aircraft to take off, but it would stall and crash as soon as it escaped the treadmill's relatively minute influence.



The idiot club eh?

Christ this thread is comedy gold.

Pray tell, sage, what is the most fundamental, basic, first-principle function of the wheels on an airplane during takeoff?

As your new Commander in Chief, I command you to bite me.

I can't bite you now. I'm on the treadmill.

If the airplane were to increase it's engine speed to allow it to remain in one position on the runway/treadmill, how coud it possibly takeoff? It would, in effect, be stationary. just as you would be if you were running on it.

If, on the other hand the engine speed was increased to double it's stationary position it would travel twice as fast as the treadmill and therefor reach it's takeoff velocity...........

.999... does not = exactly 1

:fluffle:

I sincerely doubt that the force of friction applied by the planes wheels on their ball bearings, and by their ball bearings on their axles is sufficient that it would require anything remotely near 1/2 power of its engines to cause it to be stationary on a treadmill.

But keeeeep digging :)

Well, to be fair, the question that you quoted, which I wrote, doesn't mention a commercial airliner anywhere.

All the tangential, trivial speculation aside, at least you're on the team of truth ;)

Well, the implied premise threw me off at first; that a treadmill can be roughly equal to wheel brakes at keeping the plane stationary relative to ground while engines are running.

I'm still not entirely convinced that any plane can take off from a running treadmill airstrip under every circumstance eg near maximum take-off weight.

The principle is sound though: Rolling wheels can't exert enough force - under the premise of only "double speed" - to stop the plane from going forward.

I sincerely doubt that the force of friction applied by the planes wheels on their ball bearings, and by their ball bearings on their axles is sufficient that it would require anything remotely near 1/2 power of its engines to cause it to be stationary on a treadmill.

But keeeeep digging :)

Hahahahahahahaha,. Who said anything about ball bearings and 1/2 speed?

What I said was that if the airplane were to increase it's ground speed to twice the speed of the treadmill, it would takeoff.

If you can't understand that, I suggest you try to break the world long-jump record from your treadmill!

:headbang:

Speed through the air and speed on the ground are not the same thing. An aircraft takes off by virtue of its speed through the air, not the speed at which the wheels on the landing gear are turning.


You're almost there, Ice.

Your assumption that the question stated that the airplane is travelling at the same speed as the treadmill is just that...An assumption on your part.

The aircraft would need to increase it's groundspeed (ie to double that of the treadmill) to take off. As the treadmill is travelling at the same speed as the airplanes' takeoff speed, it would be neccessary for the airplane to reach twice it's normal take off speed. Having acheived this, the airplane would takeoff.

There is nothing to suggest it would not!

I'm so used to indulging bad analogies and examples that I voted "no" and was going to reply "No an airplane can't take off if you keep it still relative to the air but a treadmill isn't enough to stop an airplane from moving. You need to come up with a better example of a situation where an airplane is held in place."

After reading the debate all I can say is wow, it's amazing some people actually think jet engines apply force to the ground/treadmill like car engines rather then to the air.:eek:

PS. This thread is brilliant, very good question.I was just beginning to wonder. Is there anyway in which a treadmill can be used to prevent a plane from taking off.

Consider an infinitely long treadmill. The treadmill is wired up so it will move the airplane forward in such a manner that it will prevent the wheels of the airplane from turning. Is that enough to stop the airplane from achieving take off?

In this example the treadmill will still be making up a considerable part of the Force that is contributing the airplane's acceleration. So once the plane leaves the ground it loses that force and begins to decelerate and coming into contact with the ground again.

I still don't think this would prevent the airplane from taking off, but I think it would force the airplane to travel at a much faster speed before it was able to take off successfully.

You're almost there, Ice.

Your assumption that the question stated that the airplane is travelling at the same speed as the treadmill is just that...An assumption on your part.

The aircraft would need to increase it's groundspeed (ie to double that of the treadmill) to take off. As the treadmill is travelling at the same speed as the airplanes' takeoff speed, it would be neccessary for the airplane to reach twice it's normal take off speed. Having acheived this, the airplane would takeoff.

There is nothing to suggest it would not!

Gazza - you're not getting it either. The plane can take off at normal speed. It doesn't need double speed. Why? Because the treadmill is not slowing down the plane. The treadmill is only making the plane's wheels spin faster.

Imagine that you are on a treadmill wearing rollerskates. Imagine also that you are holding a rope attached to the wall. The force you need to pull yourself with the rope along the treadmill towards the wall will be almost exactly the same regardless of how fast the treadmill is moving. The only thing that will change is how fast your rollerskate wheels are turning.

shit, i voted the wrong option. this question alway tricks me.

I don't know if this has been mentioned before but...

Thrust increases to maximum available power. Treadmill matches. After a while, tire friction increases to the point where temperature exceeds tire integrity. Tires burst, plunging the plane into its fuselage, where it tears itself to pieces and explodes.

Lawsuits ensue.

Gazza - you're not getting it either. The plane can take off at normal speed. It doesn't need double speed. Why? Because the treadmill is not slowing down the plane. The treadmill is only making the plane's wheels spin faster.

Imagine that you are on a treadmill wearing rollerskates. Imagine also that you are holding a rope attached to the wall. The force you need to pull yourself with the rope along the treadmill towards the wall will be almost exactly the same regardless of how fast the treadmill is moving. The only thing that will change is how fast your rollerskate wheels are turning.

That's just it. The airplane does takeoff at normal speed. Aerodynamics says the plane will only takeoff at it's normal takeoff speed, no less. Because the treadmill is travelling at the normal takeoff speed in the opposite direction, for the airplane to be able to become airborne, it will need to be travelling at twice the speed of the treadmill!

It's not rocket science... But it's got you lot in a twist.

:cool:

I don't know if this has been mentioned before but...

Thrust increases to maximum available power. Treadmill matches. After a while, tire friction increases to the point where temperature exceeds tire integrity. Tires burst, plunging the plane into its fuselage, where it tears itself to pieces and explodes.

Lawsuits ensue.

Haha...I like it! :)

You're almost there, Ice.

Your assumption that the question stated that the airplane is travelling at the same speed as the treadmill is just that...An assumption on your part.

The aircraft would need to increase it's groundspeed (ie to double that of the treadmill) to take off. As the treadmill is travelling at the same speed as the airplanes' takeoff speed, it would be neccessary for the airplane to reach twice it's normal take off speed. Having acheived this, the airplane would takeoff.

There is nothing to suggest it would not!

The wheels of the plane would turn twice as fast as ordinarily, but the speed of the wings and fuselage of the plane relative to a fixed point on the ground would be no more than if it weren't on a treadmill.

In short, the plane's wheels may be turning, but its wings are essentially remaining still in the AIR, thus there is no airflow, thus there is no lift. The jet engines provide horizontal thrust that pushes the plane and causes air to flow over the wings, causing lift.The turning of the plane's wheels is a passive movement only caused by the moving of the plane. The wheels are not used for propulsion.
How can you state something like the bolded part and yet claim that the plane can't take off?? That's completely contradictory.

...for the airplane to be able to become airborne, it will need to be travelling at twice the speed of the treadmill!

Why?

Why can't the airplane take off with it's normal thrust power? How does the treadmill impede the plane's airspeed?

How does the treadmill impede the plane's airspeed?It doesn't.

It doesn't.

I know. I'm trying to get Gazza to realize it.

Why?

Why can't the airplane take off with it's normal thrust power? How does the treadmill impede the plane's airspeed?

Because it needs to compensate for the fact that if the engine was turned off (or at idle) the airplane would be going backwards at takeoff speed!

Why can't the airplane take off with it's normal thrust power? How does the treadmill impede the plane's airspeed?

The whirling treadmill adds resistance through the increased tire rotation.

Because it needs to compensate for the fact that if the engine was turned off the airplane would be going backwards at takeoff speed!

Only if you measure take off speed relative to the treadmill instead of the ground or air surrounding the plane.

Because it needs to compensate for the fact that if the engine was turned off (or at idle) the airplane would be going backwards at takeoff speed!
Who is turning off what engine? ??

The whirling treadmill adds resistance through the increased tire rotation.That's negligible.

Only if you measure take off speed relative to the treadmill instead of the ground or air surrounding the plane.Speed relative to the treadmill is irrelevant. The plane's speed through the air is all that matters for flight.

Because it needs to compensate for the fact that if the engine was turned off (or at idle) the airplane would be going backwards at takeoff speed!

Assuming the engine isn't turned off.... Again, how does the treadmill impede the airplane's airspeed?

To understand the reasons why the airplane would need to be travelling at twice the speed of the treadmill, you need to understand the way an airplane becomes airborne.

Before taking off a pilot will set his/her flaps and elevator to a position which will cause the airplane to become lighter than air at a certain speed whilst travelling along the ground. (takeoff speed) If that speed, whatever it is, is not reached, the airplane will not take off.

If, in the treadmill question, it were possible to increase the throttle gradually until the airplane appeared to be stationary on the treadmill, it would not take off.
Because the treadmill is moving (as stated) at the airplane's normal takeoff speed, the airplane, if not on a treadmill would take off.

The question now becomes, at what speed would the airplane need to be travelling at to become airborne.....Answer...Twice the speed of the treadmill.

I can't explain it any simpler than that!! :headbang:

To understand the reasons why the airplane would need to be travelling at twice the speed of the treadmill, you need to understand the way an airplane becomes airborne.

Before taking off a pilot will set his/her flaps and elevator to a position which will cause the airplane to become lighter than air at a certain speed whilst travelling along the ground. (takeoff speed) If that speed, whatever it is, is not reached, the airplane will not take off.

If, in the treadmill question, it were possible to increase the throttle gradually until the airplane appeared to be stationary on the treadmill, it would not take off.
Because the treadmill is moving (as stated) at the airplane's normal takeoff speed, the airplane, if not on a treadmill would take off.

The question now becomes, at what speed would the airplane need to be travelling at to become airborne.....Answer...Twice the speed of the treadmill.

I can't explain it any simpler than that!! :headbang:

You are not understanding the difference between the airplane's speed relative to the treadmill and the airplane's speed relative to the air. Perhaps you need to read the entire thread. It's been explained various ways.

Assuming the engine isn't turned off.... Again, how does the treadmill impede the airplane's airspeed?

.|..

You are not understanding the difference between the airplane's speed relative to the treadmill and the airplane's speed relative to the air. Perhaps you need to read the entire thread. It's been explained various ways.

I'll read the entire thread if you'll read this (http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html)!

I chose a site which explains in simple terms just for you! :fluffle:

I'll read the entire thread if you'll read this (http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html)!

I chose a site which explains in simple terms just for you! :fluffle:You should better read up on aerodynamics, so you won't confuse it with treadmilldynamics...

I'll read the entire thread if you'll read this (http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html)!

I chose a site which explains in simple terms just for you! :fluffle:

The more condescending you get, the more you're going to feel stupid when it finally clicks in your mind.

But since I am dealing with an expert, I won't attempt to dumb down my answer anymore...

Draw a free body diagram. There is an upward normal force from the road, a downward gravitational force and an upward lift force. As long as the plane remains on the ground, these balance because the vertical component of acceleration is zero so the vertical component of the net force is zero.

There is a thrust force from the exhaust pushing the engines that points horizontally forward, and there is friction backward. The wheel is not sliding on the conveyor belt because the belt moves along with it so there is only rolling friction and it is quite small, certainly smaller than the thrust or the plane wouldn't get off the ground even without a conveyor belt. Adding the vectors, the net force is forward, so the acceleration is forward.

A jet could I suppose but I wouldn't think an airplane could.. It would basically just be standing still rolling on the treadmill and get no lift. Unless it is going forward faster than the treadmill is going backwards .. but then the treadmill wouldn't be a factor. I dunno.

Don't know if this has been gone over here, but here's the question:

Imagine a giant treadmill the length and width of an airplane runway which moves backward at the same speed as the aircraft's takeoff velocity.

Would the plane be able to take off from this treadmill?

No. You would need a wind tunnel to enable takeoff from a fixed position. The plane achieves lift by increasing speed to increase air pressure on the wings. It's a function of how fast air is moving over the wings, not how fast the plane itself is moving.

The more condescending you get, the more you're going to feel stupid when it finally clicks in your mind.

I didn't mean to be condescending! I was getting frustrated.

Sorry everyone. I overlooked one very important point.

When everyone else around you thinks your wrong, maybe they're right!

:(

I didn't mean to be condescending! I was getting frustrated.

Sorry everyone. I overlooked one very important point.

When everyone else around you thinks your wrong, maybe they're right!

:(Welcome to enlightenment :D

Quoted for future reference, to wit, the Hall of Shame



Remind me never to fly Wilgrove Airlines!



This coming from an airplane pilot! Christ!







You don't have to know how the inner workings of an automobile works or why it works in order to be a good driver do you? Same goes for planes.


And I still don't understand the treadmill thing. So no matter how fast that treadmill is going .. the craft will still take off at the same speed it would if it was just taking off from a stationary platform? Interesting. Would it lift off of the treadmill? And within the same time/distance as it would from the stationary platform/ground?

Lot's of science-challenged people here...

Probably been said already, but the answer is in the free-body diagram. The thrust overcomes any drag and pulls the aircraft forward. When it reaches a speed above stall, then the plane will fly.

Quoted for future reference, to wit, the Hall of Shame



Remind me never to fly Wilgrove Airlines!


This coming from an airplane pilot! Christ!


Remember that Wilgrove also thought that his engine would fail if the battery went dead...
http://forums.jolt.co.uk/showpost.php?p=13065771&postcount=3

Actually, the prop wash can lift the tail in a tail-dragger configuration, but that's about it.


Still, the answer is yes, the airplane will take off. The wheels are a red herring...All they do is reduce friction between the ground and the airplane. They can't impede the takeoff, unless a bearing seizes or a tire blows. Neither of which is likely with a light aircraft.

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.

Fool. Dragonforce is the hardest metal known to man.

Yes the thrust is never being applied to the ground and the lift is generated by speed relative to the air not the speed relative to the ground

A jet could I suppose but I wouldn't think an airplane could.. It would basically just be standing still rolling on the treadmill and get no lift. Unless it is going forward faster than the treadmill is going backwards .. but then the treadmill wouldn't be a factor. I dunno.

What does the lift have to do with ground movement at all?

Lot's of science-challenged people here...

Probably been said already, but the answer is in the free-body diagram. The thrust overcomes any drag and pulls the aircraft forward. When it reaches a speed above stall, then the plane will fly.

No kidding ... I am kind of depressed actually I thought we cleared this up last time it came up

Ok here is the simplest analogy I can think of

Think of a toy plane and your treadmill at home, if you hold the toy plane on the treadmill the wheels spin really fast yes? lets say you make the treadmill go faster or slower still no movement in the plane right?

Now try moving that plane forward ... it works just fine right? no matter what the speed of the treadmill.

Its the same for the plane as the thrust is being applied using the air not the wheels the only thing effected is the speed of the wheels spinning, assuming you have wheels that can take it you are fine for takeoff

So no matter how fast that treadmill is going .. the craft will still take off at the same speed it would if it was just taking off from a stationary platform?

Yes.

But the rolling conveyor belt induces some forces that the plane must overcome.

and no...

If the belt spins too fast the wheels of the plane will break. Also, in an idealized situation if the speed is relativistic then there are other things to watch out for, like increased gravitational field :p

Interesting. Would it lift off of the treadmill? And within the same time/distance as it would from the stationary platform/ground?

No.

How big a difference....well, that depends on the resistance the rolling wheel induces as it spins faster. The estimates run somewhere between 'some what' and 'marginal' ;)

Yes.

But the rolling conveyor belt induces some forces that the plane must overcome.

and no...

If the belt spins too fast the wheels of the plane will break. Also, in an idealized situation if the speed is relativistic then there are other things to watch out for, like increased gravitational field :p



No.

How big a difference....well, that depends on the resistance the rolling wheel induces as it spins faster. The estimates run somewhere between 'some what' and 'marginal' ;)

Yeah most people propose an idealized situation ... otherwise we would make them start explaining how the hell the treadmill the size of a jet works lol

First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

Here lies the problem: the vast majority of you uber-smug pedants who mock with disdain those who aren't seeing this experiment your way are explaining the problem poorly. Not all of you, but most of you. The above explanation is the clearest one I've read in the first few pages. Always remember that in trying to educate, you should never belittle.

Here lies the problem: the vast majority of you uber-smug pedants who mock with disdain those who aren't seeing this experiment your way are explaining the problem poorly. Not all of you, but most of you. The above explanation is the clearest one I've read in the first few pages. Always remember that in trying to educate, you should never belittle.

That is a pretty good explanation ... I tried to do it with my holding a toy plane analogy ...

That is a pretty good explanation ... I tried to do it with my holding a toy plane analogy ...

Yes, that one worked for me, too. Especially if the toy plane is actually capable of flight. You can actually feel the plane pulling against your hand while you hold it motionless.

Yes, this has been done before - and yes, it can take off ;)

People who do not believe that are kindly requested to construct their own experiment involving a belt/threadmill and something propelled by for instance a fan or rubber bands at the sides of the belt. And to retake basic physics ;)

Remember that Wilgrove also thought that his engine would fail if the battery went dead...
http://forums.jolt.co.uk/showpost.php?p=13065771&postcount=3

Actually, the prop wash can lift the tail in a tail-dragger configuration, but that's about it.


Still, the answer is yes, the airplane will take off. The wheels are a red herring...All they do is reduce friction between the ground and the airplane. They can't impede the takeoff, unless a bearing seizes or a tire blows. Neither of which is likely with a light aircraft.


Yup, the most basic, fundamental, first-principle function of the wheels is to reduce (ideally to negate) friction between the plane and the ground.

Nothing beyond understanding this is required to answer the question correctly and understand why the correct answer is correct.

Right. But if the treamill is acting in the opposite direction of the aircraft always at the same speed (take a look at the OP question) then there should be no airflow over the lifting surfaces. Groundspeed and airspeed are comepletely different.

EDIT: Sorry but I'm going to the bar to get drunk, I'll Wilgrove take over.

Actually, the OP says the treadmill is moving backwards at the same speed as the aircrafts takeoff velocity...

So the correct answer would be yes, it is possible for the aircraft to take off... The aircraft would just have to be moving at twice its takeoff velocity...

If the craft is moving at 140kts, then it still will reach the neccessary 70kts relative to air-speed velocity to takeoff.

In light of recent facts, I must still conclude, even understanding that "the plane can just go twice the speed of the runway," that it is not possible. Let's take, for instance, a 747...

Additional Argument Numero Uno:

A typical 747 at typical weights in typical weather on typical runways requires 8000-10000 feet of runway to get to its takeoff speed of about 150mph. Pardon the inferior non-metric units of measure. As the plane is moving fastest on the ground during takeoff (the 747 can be going as slow as 125mph for a landing, I believe), the wheels are probably designed with a bit of wiggle-room to sustain around 150mph for an un-extended period of time.

Shortly put, you're telling wheels designed around 150-200mph to sustain up to 300mph for not only the normal amount of time it takes the aircraft to "equalize" with the treadmill, but the additional time it takes (all of it over the 150mph mark, mind you) to get to 300mph and take off without overheating, stressing, breaking and/or catastrophically failing. Fat chance.


Additional Argument Numero Dos:

It takes ~8000 feet for a 747 to achieve 0-150mph on a runway. I'm not sure how long it takes for a 747 on the ground to achieve 150-300mph, which is roughly half its typical cruising speed, might take a bit longer. I decided to load up that old classic, Flight Simulator, because it's the closest I can get to trying it on a real plane and noticed that it took roughly 2/3rds as long to get from 0-150mph than it did to get from 150-300mph. Thus, you're going to need a treadmill appx. 1/3rd longer than a typical runway, and as the original question states, the treadmill is as long as a standard runway if the flight characteristics in FSX carry over in any aspect whatsoever on the most realistic settings to the real world, which I cannot say for sure that it does or does not.

For the record, here's a 747 taking off on a ~7200ft runway.
http://www.youtube.com/watch?v=feGZ4l5fk4Q

Helping thoughtstep:
Can a hovercraft take off from a moving runway ?

You mean like the kind they cross the English Channel with?

Yes, for thesame reason as turning the threadmill 90 degrees and putting it next to the Space Shuttle won't stop that from launching.

Alternatively, the plane is a Harrier.

Actually, the OP says the treadmill is moving backwards at the same speed as the aircrafts takeoff velocity...

So the correct answer would be yes, it is possible for the aircraft to take off... The aircraft would just have to be moving at twice its takeoff velocity...

If the craft is moving at 140kts, then it still will reach the neccessary 70kts relative to air-speed velocity to takeoff.

again, no, not even close.

Yes, this has been done before - and yes, it can take off ;)

People who do not believe that are kindly requested to construct their own experiment involving a belt/threadmill and something propelled by for instance a fan or rubber bands at the sides of the belt. And to retake basic physics ;)
The only problem with that particular experiment is treadmill speed is impossible to correlate with speed the plane would achieve moving forward on the ground by thrust from the engines.

from a theoretical perspective, the plane would remain absolutly stationary on the treadmill as the wheels spun freely.

Practically the plane would be dragged back slightly, due to friction on the treadmill on the wheel, and friction between the wheel and the axil it's attached to. So the treadmill would exert SOME backwards pressure, purely due to friction.

The primary questions are:

1) does the plane have enough thrust to overcome this additional friction and reach takeoff velocity. Probably, because I doubt the friction would amount to much more than the equivalent of a stiff tailwind.

2) can the wheels remain on as they spin at twice the normal velocity? Probably, but that's the main question, is will the wheel just break spinning that fast?

The only problem with that particular experiment is treadmill speed is impossible to correlate with speed the plane would achieve moving forward on the ground by thrust from the engines.

Why ? The threadmill does not effect the forward speed of the plane at all.
Thinking it would is the error most people make here ;)

Feel free to permantly make the threadmill move back at twice the maximum velocity your "plane" could achieve. As long as your simulated engines do not propel your "plane" through traction with the runway and the wheels underneath can turn freely enough there will be no problem.

from a theoretical perspective, the plane would remain absolutly stationary on the treadmill as the wheels spun freely.

No, if the wheels move completely freely the plane will take off exactly the same way as it would if the runway was not moving.

as mentioned, the problem is that people think a plane works like a car, it doesn't. A plane does not propel itself through a drive system connected to its tires like a car does. A plane is propelled by its engine turbines. The wheels merely serve to reduce friction on the runway.

No, if the wheels move completely freely the plane will take off exactly the same way as it would if the runway was not moving.

sorry, I mean, if the turbine engine on the plane was not active IE no force propelling it forward. Theoretically (IE without friction) a stationary plane without its turbines active will sit there, as the wheels spin.

Why ? The threadmill does not effect the forward speed of the plane at all.
Thinking it would is the error most people make here ;)
My or anyone else's opinion of what would happen is irrelevant. It doesn't make the experiment you proposed any less flawed.

oooh, ok, is TPH done pretending to be an expert on the law and is now pretending to be an expert on aerodynamics? Fortunatly this question has been already addressed (http://www.straightdope.com/columns/060303.html).

My or anyone else's opinion of what would happen is irrelevant. It doesn't make the experiment you proposed any less flawed.

What is wrong with making the threadmill move back at twice the theoretical maximum speed of the "plane" you use ? Yes, its speed will not vary like in the question - but since it will constantly be higher I fail to see the problem...

from a theoretical perspective, the plane would remain absolutly stationary on the treadmill as the wheels spun freely.

Practically the plane would be dragged back slightly, due to friction on the treadmill on the wheel, and friction between the wheel and the axil it's attached to. So the treadmill would exert SOME backwards pressure, purely due to friction.

The primary questions are:

1) does the plane have enough thrust to overcome this additional friction and reach takeoff velocity. Probably, because I doubt the friction would amount to much more than the equivalent of a stiff tailwind.

2) can the wheels remain on as they spin at twice the normal velocity? Probably, but that's the main question, is will the wheel just break spinning that fast?
Yall need to stick to pretending to know the law ...

Go find a 1st year physics student. Ask him to draw a freebody diagram of the forces acting on the airplane. That should be enough to convince anyone that has a lick of science training behind them that the plane will take off, unimpeded by the minuscule amount of friction that is added by the wheel/bearing/treadmill configuration.

Maybe we've never added this one little kicker into the explanation.
Sums of forces and sums of moments need to be zero for a body in equilibrium -- in this case, a stationary aircraft. I need to have a force in the Drag direction that is equal to the force in the Thrust direction before I'll believe that this airplane is stationary. Friction isn't it. Tying the airplane to the treadmill would work, but that's not part of the problem, as posed to us.

But let's ask a very important question... What is happening when the treadmill moves in opposition to the wheel motion. The obvious answer is that the treadmill moves. That's about it, as the answer to this problem is trivial.

Let's try one more thing to help the helpless...

What happens when a plane lands on this moving treadmill? I'm talking a power-on landing, because it makes a better example...

We line up on the treadmill, settle in on the glideslope and hit the numbers. Or where the numbers would be because they're moving. After touching down, we decide it was fun enough to do again, so we add power. Is the airplane stuck to the runway at this point? I don't think so. In fact I know it is not, because I believe in the sums of forces and moments and that a freebody diagram shows them NOT to be in equilibrium. No, the airplane accelerates and we rotate, lift off, and do it again. Wheeeeee!

Harrier.

-snip-

As I said (but I know, I know, this fancy "book lernin'" ain't your thing), theoretically a plane placed on a moving treadmill will remain stationary, as the wheels spin freely, rather than being dragged backwards.

In practice it would be dragged back slightly.

I didn't say the plane was ON. I said a plane would sit there and not move, as the wheels spun freely, only dragged back by a bit of friction.

Harrier.

Lol

Yes.

But the rolling conveyor belt induces some forces that the plane must overcome.


Let's quantify this friction force that must be "overcome". In the airplane world, we call it rolling resistance.

Rubber on concrete has a kinetic coefficient of rolling friction of about 0.012. That means that if the airplane needs 100 lb-f of Thrust to take off, it must generate a about 101 lb-f to overcome the rolling resistance of the runway.
[F = uma; where u is the coefficient of rolling resistance]

Show me how the moving runway can contribute any more friction than this to the forces acting on the aircraft. The fact is that it can't. Once the wheels are rolling, that's all the resistance that they will see.

1) does the plane have enough thrust to overcome this additional friction and reach takeoff velocity. Probably, because I doubt the friction would amount to much more than the equivalent of a stiff tailwind.

Yeah. If you think about it, the engine has to have enough power to overcome that friction when the plane is taking off on a normal runway too. (If the friction was greater than the thrust could overcome, the plane would never be able to move, regardless of the surface it is sitting on.


2) can the wheels remain on as they spin at twice the normal velocity? Probably, but that's the main question, is will the wheel just break spinning that fast?

I'd guess they would be designed too spin much faster than the take off speed, just because they have to deal with landing stresses, which are far greater.

If there's so little friction in the wheels of aircraft, then why don't they just stay still while the earth rotates beneath them? It would save on fuel costs.

Put me down on the "Yes" camp.

But, as it seems the question is still open, let me explain my thinking:

It doesn't matter if it's jet-powered, prop-powered, rocket-powered or has some other weird air-pusher/puller mechanism. It's pushing on the air. Planes do not go forward due to pushing against the ground, and leaving the ground does not (excluding certain STOL configurations, possibly) remove any effectiveness of the engines, so a backwards-travelling ground does not increase the difficulty.

Where it would fail is if the 'rolling resistance' of the wheels, skids, skis, repulsor pods or anti-gravity beams reduces the effective air-speed at the engine's top throttle to below take-off speed. And there'd probably be other problems happening before that. Where the resistance is negligible and/or well within the limits, you just get wheels turning at twice the speed, etc...


Oh, and a plane in a wind-tunnel where the air is made to go backwards at take-off speed (assuming no curious edge-effects and turbulance with the tunnel sides causing problems), the plane will take off stationary relative to the ground and appear to hover, much as if you were moving your observation-post along on the back of a (fast) truck to keep pace with the moving aircraft as it took off from the back of another flat-bed similarly zooming along a runway carefully throttled to keep the forwards/backwards force between it and the jetting/propping/rocketing plane zero while take-off occurs...

Of course, if you allowed the engines to push as hard as they like while generally preventing a flow of air over the wings (e.g. ducting the propulsion-air onto and away from the engines, or arranging for a tail-wind as fast as the plane is travelling to make it windless across the lift surfaces even while the engines 'think' they are moving forward as normal) you could get a plane going plane-speeds but not lifting off. As if it had no wings. Which would be stupid and/or interesting, depending on circumstances, but well beyond the remit of the original question. ;)

from a theoretical perspective, the plane would remain absolutly stationary on the treadmill as the wheels spun freely.

Practically the plane would be dragged back slightly, due to friction on the treadmill on the wheel, and friction between the wheel and the axil it's attached to. So the treadmill would exert SOME backwards pressure, purely due to friction.

The primary questions are:

1) does the plane have enough thrust to overcome this additional friction and reach takeoff velocity. Probably, because I doubt the friction would amount to much more than the equivalent of a stiff tailwind.

2) can the wheels remain on as they spin at twice the normal velocity? Probably, but that's the main question, is will the wheel just break spinning that fast?

A plane is not a car.

Just tell me: how does a plane take off normally?

The answer to the question, of course, is NO.

For the same reason that if you were running flat out on the treadmill and then jumped into the air, you would not break the world record for the long jump. In fact, you would land in exactly the same spot.

DO NOT TRY THIS AT HOME!

When you land, the belt of the treadmill will still be going flat out and you will either be catapulted backwards off the end of the belt or you will fall forward flat on your face and then be shot off the back of the treadmill.

If you don't believe me, go ahead and try it! But don't do it home. Do it at the gym so everyone can have a good laugh :pThat is a bad analogy. In the case of the plane, the plane pushes on the air to move forward, but the ground is countering your movement. To equate it to walking, you push on the ground to move, but the wind is countering your movement.

Can you walk into a 2mph wind?

As I said (but I know, I know, this fancy "book lernin'" ain't your thing), theoretically a plane placed on a moving treadmill will remain stationary, as the wheels spin freely, rather than being dragged backwards.

In practice it would be dragged back slightly.

I didn't say the plane was ON. I said a plane would sit there and not move, as the wheels spun freely, only dragged back by a bit of friction.
Nah, I got the three R's down pretty well. It's the fine print I tend to skip. That's why the in-house counsel is my best pal.

My mistake and my apologies.

A plane is not a car.

I'm aware, had you bothered to read you would have noted I said that already (http://forums.jolt.co.uk/showpost.php?p=13153569&postcount=152).

Oh, wow, I didn't realise how few people here are able to take abstracted mental images and apply realistic physics to them. I'm highly surprised that more people voted no than yes, considering the general level of intelligence on this forum.

I bet those who think that the plane would not be able to take off also disbelieve that 0.999... = exactly 1.I'm surprised that the only cases of hard physics being used (ie the two free body diagrams) have yet to be disputed by the plane cannot take off camp. We produced numbers saying that the plane will still be moving relative to a fixed point (say a control tower or the air).

That is a bad analogy. In the case of the plane, the plane pushes on the air to move forward, but the ground is countering your movement. To equate it to walking, you push on the ground to move, but the wind is countering your movement.

Can you walk into a 2mph wind?

Why do people in this thread keep talking about aircraft propulsion systems getting their thrust by pushing on the air?

Propellors generate an area of lower pressure in front of them and are sucked forwards and jets rely on newtons 3rd in much the same way as rockets.

My mistake and my apologies.

No problem, I admit, in the context of the overall discussion my words could have been read to mean the plane trying to accelerate on a runway.

I merely meant a plane lying on a conveyer belt, it would simply sit there, while the wheels spin. Again, from a practical matter it would be dragged backwards due to friction, but in a theoretical system, it would simply sit there, until the engine was turned on.

But yes, we're both right, a non active plane would sit there, an active plane would move forward (ignoring friction). I admit my wording could have been clearer however, and reading back I understand how it could be misconstrued.

I'd guess they would be designed too spin much faster than the take off speed, just because they have to deal with landing stresses, which are far greater.

I can't give you a light aircraft example, but let's look at a military aircraft... We normally make an normal approach and landing in the A-6E at about 120 Indicated, KIAS, +/- depending on fuel. For a no-flap, no-slat approach, the airspeed was 180 KIAS. Now factor in the density differences at various altitudes and you'll find that 180 KIAS at sea level is much slower, in terms of true airspeed, KTAS, that 180 KIAS at 5 or 6 thousand feet (Denver). All of which needed to be accommodated without failure.

So, yes, there is a big safety factor in wheel and tire design.

The weight of the airplane will msot likely crush the treadmill enough that it doesn't work, at which point it will accelerate and take off normally.

As for diamond, (1) they've created a couple harder materials (aggregated diamond nanorods and ultrahard fullerite) and (2) although it's very hard it's also fairly brittle.

Why do people in this thread keep talking about aircraft propulsion systems getting their thrust by pushing on the air?

Propellors generate an area of lower pressure in front of them and are sucked forwards and jets rely on newtons 3rd in much the same way as rockets.
Maybe you've found the key to understanding... A rocket is placed, in a carriage , on a treadmill...

The rest is left to the student for homework.

from a theoretical perspective, the plane would remain absolutly stationary on the treadmill as the wheels spun freely.

Practically the plane would be dragged back slightly, due to friction on the treadmill on the wheel, and friction between the wheel and the axil it's attached to. So the treadmill would exert SOME backwards pressure, purely due to friction.

The primary questions are:

1) does the plane have enough thrust to overcome this additional friction and reach takeoff velocity. Probably, because I doubt the friction would amount to much more than the equivalent of a stiff tailwind.

2) can the wheels remain on as they spin at twice the normal velocity? Probably, but that's the main question, is will the wheel just break spinning that fast?
1) Easily. Check one of the free body diagrams that has been posted.

2) Knowing Boeings QA, yes (and assuming that most others have similar practices). The plane could probably lands safely at cruising speed (typically triple take off speed), once.

1) Easily. Check one of the free body diagrams that has been posted.

The wheel can't spin 100% freely, so friction will create some backwards force. Free body diagrams, not knowing the frictional coeficient in question, can't answer that.

Why do people in this thread keep talking about aircraft propulsion systems getting their thrust by pushing on the air?
For my part, simplification, although I fully appreciate how this diverges from reality as you delve deeper.

Propellors generate an area of lower pressure in front of them and are sucked forwards and jets rely on newtons 3rd in much the same way as rockets.
Of course, strictly a propeller's creation of higher pressure behind (while creating the lower pressure in front) is around half the effect forcing the 'airscrew' element forward, while a jet must have a partial (if essentially negligible) 'sucking' effect[1] that a rocket does not. ;)

(As long as the airwards cross-section is sufficiently non-impeding and leewards spilling of the air sufficiently lacking turbulence, the engines can essentially do what they want.)

Actually, reminds me of the historic rubbishing of space-rocketry by a news-paper where they said how rockets would not work in vacuum because "there isn't anything to push against"... ;)


Post-posting footnote: [1] Compared with the air-impeding effect that a stationary engine not letting the air in, in order to compress, that is. Although even that is debatable, if you delve deeply enough within how various forms of engine works. Obviously energy of forward motion, as is derived from chucking out the exhaust, is also absorbed in the work of getting the heretofore oblivious air that just happened to be lying in the 'wrong' part of the plane's flight-path to be gathered in, compressed, allowed to be (with all due haste, without even observing proper etiquette) introduced with a given quantity of fuel and then cruelly find themselves involved in an arrange marriage of oxygen and fuel, the result being discarded rearwards with all expediency... And take a look at Ramjets... During supersonic operation of the airframe, the intake of air is deliberately baffled so as to be travelling sub-sonically through the combustion chamber, which is technically equivalent to putting a kind of air-brake on, only with the knowledge that the resulting thrust more than compensates for this. Scramjets work with super-sonic throughput of air to maximise the efficiency (less air-brake effect, in part, but with the need for a 'flame-holding' device to keep combustion spreading 'forward' at up to 30 times the speed of sound... or rely on the air being denser through compression). Or I could be entirely wrong about all of this, it's ages since I researched this sort of thing, so maybe I'm misremembering... :)

The wheel can't spin 100% freely, so friction will create some backwards force. Free body diagrams, not knowing the frictional coeficient in question, can't answer that.What does the backward force out of friction amount to? Will it take 75% of engine thrust to compensate?

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.

Sorry, I just thought........oh, never mind.


Just about skipped right past that one......

The wheel can't spin 100% freely, so friction will create some backwards force. Free body diagrams, not knowing the frictional coeficient in question, can't answer that.

But you don't need to know μ. The fact that the plane could take off under normal conditions means that the engine hasto have enough thrust overcome any friction. The fact that it is now on a conveyor belt is irrelevant.

What does the backward force out of friction amount to? Will it take 75% of engine thrust to compensate?
Don't any of you read????

Rolling resistance, which is very different than kinetic or static friction is about 0.012 for rubber on concrete. That means, for every 100 pounds force of thrust that a given airplane requires to take off in a frictionless world, it will have to generate an extra 1 pound force of thrust to take off on a real runway.

It also means that for every extra pound that an aircraft weighs, that aircraft will need to generate another 1 percent of it's weight as thrust to overcome rolling resistance...

F - umg = 0; u = rolling resistance, mg = weight.

Don't any of you read????

Rolling resistance, which is very different than kinetic or static friction is about 0.012 for rubber on concrete. That means, for every 100 pounds force of thrust that a given airplane requires to take off in a frictionless world, it will have to generate an extra 1 pound force of thrust to take off on a real runway.

It also means that for every extra pound that an aircraft weighs, that aircraft will need to generate another 1 percent of it's weight as thrust to overcome rolling resistance...

F - umg = 0; u = rolling resistance, mg = weight.So rolling resistance on the normal runway is different from that on the hypothetical conveyor?

I don't really understand the fascination with this, but yes the aircraft can take off.

The force of the jet engines is what drives it forward, just as if a giant finger came out of the sky and pushed it. Try it - get a Matchbox plane, put it on an actual treadmill and push it. I think you'll find it moves.

The wheel can't spin 100% freely, so friction will create some backwards force. Free body diagrams, not knowing the frictional coeficient in question, can't answer that.

But thats minimal ... it may make a very small if any difference in a smaller plane and almost nothing to something like a jet fighter which has multiple times the thrust needed for takeoff

But you don't need to know μ. The fact that the plane could take off under normal conditions means that the engine hasto have enough thrust overcome any friction. The fact that it is now on a conveyor belt is irrelevant.

it's not 100% irrelevant. The wheels, as we established, are spinning at twice the speed it normally would, which means, over the same period of time, twice the surface area comes in contact with the ground, which does increase the friction generated.

More rotations of the wheel means more surface area exposed in the same amount of time, which means more friction, which means more energy lost to friction, which means the engines need to outpot more power than normal to compensate.

Which they most likely are able to do. But we can't say that 100% without knowing the frictional coeficients and the maximum engine output. We CAN say that in most normal conditions with most normal planes, this isn't a problem

again, no, not even close.

If an aircraft has a takeoff velocity of 70kts, and is on a runway moving backwards in stationary air at a speed 70kts; the aircraft will need to be moving at a speed (relative to 70kts backward movement of the runway) of 140kts (making its velocity 70kts), meeting its required takeoff velocity (of 70kts) [-70 + 140 = 70]... That is all I said... It's absolutely true... If the aircraft has a 70kt wind against its back, it will also need a speed of 140kts to maintain the necessary 70kt velocity for lift.... if it has 70kt wind against its nose, it can be stationary and still maintain lift.... "speed" and "velocity" are different things.

it's not 100% irrelevant. The wheels, as we established, are spinning at twice the speed it normally would, which means, over the same period of time, twice the surface area comes in contact with the ground, which does increase the friction generated.

More rotations of the wheel means more surface area exposed in the same amount of time, which means more friction, which means more energy lost to friction, which means the engines need to outpot more power than normal to compensate.

Which they most likely are able to do. But we can't say that 100% without knowing the frictional coeficients and the maximum engine output. We CAN say that in most normal conditions with most normal planes, this isn't a problem
Please, don't get ridiculous. The energy lost to friction is negligible if you just want to know whether the plane will fly or not. Friction does not determine the answer to the question at issue.

it's not 100% irrelevant. The wheels, as we established, are spinning at twice the speed it normally would, which means, over the same period of time, twice the surface area comes in contact with the ground, which does increase the friction generated.
And increases the strain of wheel bearings which in turn will increase the friction coefficent.

If an aircraft has a takeoff velocity of 70kts, and is on a runway moving backwards in stationary air at a speed 70kts; the aircraft will need to be moving at a speed (relative to 70kts backward movement of the runway) of 140kts (making its velocity 70kts), meeting its required takeoff velocity (of 70kts) [-70 + 140 = 70]... That is all I said... It's absolutely true... If the aircraft has a 70kt wind against its back, it will also need a speed of 140kts to maintain the necessary 70kt velocity for lift.... if it has 70kt wind against its nose, it can be stationary and still maintain lift.... "speed" and "velocity" are different things.

again, not even close, because whether the treadmill is moving at 10kts backwards, or 70kts, or 700kts, it will not move the aircraft backwards. If you put an aircraft, turned off, on a moving treadmill it will not move the plane, the plane's wheels will just spin wildly, and the plane won't move. THe plane does not need extra force to overcome the movement of the treadmill, the movement of the treadmill is irrelevant.

What WILL change is the rotations per minute of the wheels, but the rotation of the wheels is quite independant of the speed (or velocity) of the aircraft.

As it's been explained many times in this thread, the treadmill, and its speed, is irrelevant, no matter how fast it's moving (ignoring friction, as I explained earlier)

A plane doesn't work like a car.

If an aircraft has a takeoff velocity of 70kts, and is on a runway moving backwards in stationary air at a speed 70kts; the aircraft will need to be moving at a speed (relative to 70kts backward movement of the runway) of 140kts (making its velocity 70kts), meeting its required takeoff velocity (of 70kts) [-70 + 140 = 70]... That is all I said... It's absolutely true... If the aircraft has a 70kt wind against its back, it will also need a speed of 140kts to maintain the necessary 70kt velocity for lift.... if it has 70kt wind against its nose, it can be stationary and still maintain lift.... "speed" and "velocity" are different things.But the conveyor runway does not put the plane into any backwards movement, it only makes its wheels spin at a higher frequency. The 70kt are in relation to the surrounding air, not the conveyor.

Please, don't get ridiculous. The energy lost to friction is negligible if you just want to know whether the plane will fly or not. Friction does not determine the answer to the question at issue.

It's PROBABLY negligible, but your insistance that it doesn't matter at all demonstrates an inability to think of the issue.

We don't know the plane in question. If the plane engine is only capable of doing enough work to allow the plane to take off on a smooth stationary runway and no windspeed, then the engine will not be able to overcome the extra friction.

Now, for practical purposes this doesn't really matter, the extra friction probably amounts to no more than a small tailwind, but without knowing exact details of the plane in question, it's impossible to answer with absolute certainty, we'd need to know the maximum output of the engine, and the frictional coeficient to be able to answer that with certainty.

It works mainly because of the jet engines thrusting. Now, if it were say, a glider, then no, you would get no lift.

If it is a modern jet engine it most likely is a turbofan (which has very little jet thrust).

I say no.

it's not 100% irrelevant. The wheels, as we established, are spinning at twice the speed it normally would, which means, over the same period of time, twice the surface area comes in contact with the ground, which does increase the friction generated.

More rotations of the wheel means more surface area exposed in the same amount of time, which means more friction, which means more energy lost to friction, which means the engines need to outpot more power than normal to compensate.

Which they most likely are able to do. But we can't say that 100% without knowing the frictional coeficients and the maximum engine output. We CAN say that in most normal conditions with most normal planes, this isn't a problem

Rolling friction, Fr=cmg: where c is a dimensionless constant.

It's independent of velocity, so there is the same frictional force whether the aircraft is traveling at 5kts or 500kts ground speed.

It's a statics problem, so there is no need to worry about energy.

I don't really understand the fascination with this, but yes the aircraft can take off.

The force of the jet engines is what drives it forward, just as if a giant finger came out of the sky and pushed it. Try it - get a Matchbox plane, put it on an actual treadmill and push it. I think you'll find it moves.

That's a good way of thinking about it. I think.

If it is a modern jet engine it most likely is a turbofan (which has very little jet thrust).

I say no.

His terms may be wrong but the idea of the engine acting on the air rather then the ground is sound

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.

------------------

but diamond isint a metal...

no but it could probably take off from one point against large fans?

no but it could probably take off from one point against large fans?

Why not? And Just like the treadmill question it is all about air moving over the wings the speed of the ground (or treadmill) really have nothing to do with the flight of the airplane

Stable control of the airplane probably depend on the fans moving along with the plane though

I voted yes.

If the conveyor moved backwards at the same speed the wheels span, however, I believe both would end up moving at an infinite speed. Then the universe would end. Quickly.

Stable control of the airplane probably depend on the fans moving along with the plane though

At least close enough to handle it ... most planes can take off with SOME crosswind

I voted yes.

If the conveyor moved backwards at the same speed the wheels span, however, I believe both would end up moving at an infinite speed. Then the universe would end. Quickly.

I've just spent a few minutes trying to figure out the maths for this... Is it right?

I've just spent a few minutes trying to figure out the maths for this... Is it right?

No as the wheels are free spinning the airplane would accelerate to take off and they would no longer be spinning due to the treadmill or takeoff.

But if the response of the conveyor to the increase of the speed of the wheels was immediate, Wouldn't the wheels also speed up immediately? Infinite speed would be hit straight away? (This is only if the conveyor is moving in response to the wheels, not the output of the engine as in the OP)

Assuming the plane has sufficient thrust to overcome the increased rolling friction caused by the wheels being forced to spin faster, the plane will take off.

The treadmill is largely irrelevant in any ideal model because the plane is not dependant on pushing off the ground to obtain its thrust. If we assume no rolling friction, (which we will in an ideal model), the plane will remain stationary without any use of its engines as per Newton's laws of motion. The tread mill is moving, but so what? The plane is not dependant on it, and without friction, the plane's wheels will spin but the plane will remain stationary.

So long as force can overcome the increased friction in a real life model, then the plane will take off.

Let's put it this way....

Righty, So this treadmill is stationary. No-one said it isn't there for it is.

Right, now, this plane is being propelled by jets or blades, i don't really care.

Now, seeing how the plane is not strapped to the treadmill, it can move freely. HOW?!?!

Well, betwen the body and the treadmill we have another treadmill called THE WHEEL.

now, minimize friction, to the point of it not existing. Right, so the plane is moving, and thanks to its wheels moving in qual ammounts, the treadmill is not! (if the wheels weren't moving, there's your problem)The treadmill may move a bit, but take off would occur anyways.

:headbang: damn it, i forgot, you fullas like to make it complicated with maths....But remember, the jet is a free separate entity to the runway, and even if it was strapped down....then it won't take off will it?:rolleyes:

The wheel can't spin 100% freely, so friction will create some backwards force. Free body diagrams, not knowing the frictional coeficient in question, can't answer that.You can make a reasonable guess. The coefficients of friction for the bearings are rather low, most of it comes from the actual weight of the plane. Either way, it is pretty well known (in the industry at least) that even at double takeoff speed the friction of the barrings is going to be much smaller than the trust.

it's not 100% irrelevant. The wheels, as we established, are spinning at twice the speed it normally would, which means, over the same period of time, twice the surface area comes in contact with the ground, which does increase the friction generated.

More rotations of the wheel means more surface area exposed in the same amount of time, which means more friction, which means more energy lost to friction, which means the engines need to outpot more power than normal to compensate.

Which they most likely are able to do. But we can't say that 100% without knowing the frictional coeficients and the maximum engine output. We CAN say that in most normal conditions with most normal planes, this isn't a problemThe coefficient for my Boeing example would have to be about 0.05 for the plane to sit still. While we do not know for certain what the real coefficient is, it would be less than that.

AH! but you see, the plane doesn't rely on the wheels for movement!

put a car on a treadmill, there's your problem....but a plane....

Aye, the wheels may not move 100% but close enough, otherswise how would th plane taxi and take off while on the run-way? The wheels can move fast enough for it to take off.....;)

Therefore, because of the jets propelling, we'd all be fine, TAKE OFF!

check my last post about 3 posts back for the entire theory...

as long as an airfoil is pushed through the air, it don't make diddley what the ground is doing. the runway could be dancing a hula, this has no direct effect. since nothing the ground/runway/treadmill is doing has ANY effect on the airfoils movement through the air. it's not pushing against the ground, so nothing the ground does, prevents its relative movement through the air. the aircraft will accelerate, reguardless of what the ground does just fine. accellerate relative to it anyway, because it is the air, not the ground, it is pushing against.

thus whatever the ground does, other then violently shaking up and down with sufficient force to destroy the landing geer and thus MAKE it dependent on ground movement. if that were to happen and the treadmill going, it would then travel backward at the tredmill's rate.

otherwise it don't make nada por nada. just means the wheels on the landing gere will be spinning at twice their normal take off rate, or four times or whatever, but even without them, if it had something to skid on that wouldn't overheat and set the damd thing on fire, it's still pushing against the AIR, not the ground!

otherwise none of them would ever be able to take off, even without the treadmill.

(i'd be interested in those treadmill bearings though, and how the were kept cool and lubricated. which does not pretain to the origeonal question, but could be useful elsewhere.)

=^^=
.../\...

The wheel can't spin 100% freely, so friction will create some backwards force. Free body diagrams, not knowing the frictional coeficient in question, can't answer that.
The nice thing is that frictional forces are proportional to a coefficient of friction. The real nice thing is that this coefficient is constant. Once you have the bearing/wheel/runway thing set up, the coefficient of rolling resistance doesn't change. No matter how fast or how slow -- it stays constant.

The forces due to rolling resistance can change, but they depend on a change in aircraft weight. These forces would actually decrease as the aircraft accelerates and the wings start producing lift. Now some induced drag is produced, but that's a different matter.

So the actual amount of rolling resistance is immaterial to the solution, for an aircraft that could move under its own power and take off on a motionless runway.

if an airplane COULD take off from a treadmill, would it go straight up in the air?

if an airplane COULD take off from a treadmill, would it go straight up in the air?
You mean blow up?

there is only one way to prove this, outrages government spending!

we need to use billions of dollars of tax money to build a massive treadmill and plane to use on thise treadmill.

Pfft! Airplanes don't fly. Who told you all that nonsense.

if an airplane COULD take off from a treadmill, would it go straight up in the air?

No, it would just move over the threadmill as if the threadmill was standing still.

Or lift straight up if it is a harrier ;)

simple answer is hell no. wings require lift. lift is supplied by air flow. no air flow, no lift, no lift off. speed along doesn't make take off.

question answered! now lets move on to more important questions like if a tree falls in the woods and noone hears it, does it make a sound?

I don't know, but heres a more important question:
What would happen if a diamond car crashed into a diamond wall? I mean, diamond IS the hardest metal known to man.

No, no, no. Dragonforce is the hardest metal known to man.


Also, the plane has to be moving forward relative to the air and not the ground in order to take off. A treadmill makes no difference whatsoever. Bascially, what's been rehashed again and again for fifteen pages.

This poll depresses the hell out of me. Airplanes are not cars damn it! It doesn't matter if the treadmill under it is moving at the speed of light in a ideal model, the plane will still take off. The wheels will roll, but the plane won't move backwards because of Newton's laws of motion!

simple answer is hell no. wings require lift. lift is supplied by air flow. no air flow, no lift, no lift off. speed along doesn't make take off. Where is the air flow when a plane takes off normally? Exactly, there is none. The lift is created by the movement of the plane through the air.

Where is the air flow when a plane takes off normally? Exactly, there is none. The lift is created by the movement of the plane through the air.

ok i'll go s..l..o..w..e..r.. try running down the street. you feel air against your body. that is the lift required on the wings to get the plane off the ground. now try running on a treadmill. you don't feel this air flow so no lift. any questions??

ok i'll go s..l..o..w..e..r.. try running down the street. you feel air against your body. that is the lift required on the wings to get the plane off the ground. now try running on a tread mill. you don't feel this air flow so no lift. any questions??That's only because a person's movement is created by pressing against the ground/treadmill. The plane however is gaining speed by pressing against the surrounding air.

Again: a plane is not a car.

ok i'll go s..l..o..w..e..r.. try running down the street. you feel air against your body. that is the lift required on the wings to get the plane off the ground. now try running on a treadmill. you don't feel this air flow so no lift. any questions??

yes, because a person running on a treadmill remains stationary. An airplane does not, because the wheels can spin without moving the plane.

http://www.straightdope.com/columns/060203.html

http://www.straightdope.com/columns/060303.html

Considering the wording of the OP, it is obvious that the plane takes off. The wheels will spin twice as fast, though.

That's only because a person's movement is created by pressing against the ground/treadmill. The plane however is gaining speed by pressing against the surrounding air.

Again: a plane is not a car.

are we agreeing or disagreeing with eachother?? i'm saying that a plan can't take off if its on a treadmill going in an equal, but opposite direction. please tell me we are or i have no faith in the human race. :headbang:

yes, because a person running on a treadmill remains stationary. An airplane does not, because the wheels can spin without moving the plane.

where is the lift coming from?? a wing needs lift. who ever thinks this is possible (defying physics) will be rich!! hurry and patent this and sell it to the navy.

if you think it can take off, then you believe that a plane can take off in a total vacuum (not cleaning vacuum). this is NOT possible.

Remember that Wilgrove also thought that his engine would fail if the battery went dead...
http://forums.jolt.co.uk/showpost.php?p=13065771&postcount=3

Actually, the prop wash can lift the tail in a tail-dragger configuration, but that's about it.


Still, the answer is yes, the airplane will take off. The wheels are a red herring...All they do is reduce friction between the ground and the airplane. They can't impede the takeoff, unless a bearing seizes or a tire blows. Neither of which is likely with a light aircraft.
That's the best post evar!:D

Fool. Dragonforce is the hardest metal known to man.

Lies! It's mithril!!!!

where is the lift coming from??

Bernoulli's principle, life comes from air flowing over the wings as the plane moves forward.

Really, you seem to be under the severe dillusion that the movement of the treadmill affects the movement of the plane. It doesn't, and the movement of the treadmill below the wheels of the plane is entirely irrelevant to the movement of the plane. The treadmill will not cause the plane to move backwards. You really should have grasped at least that much by now...

simple answer is hell no. wings require lift. lift is supplied by air flow. no air flow, no lift, no lift off. speed along doesn't make take off.

question answered! now lets move on to more important questions like if a tree falls in the woods and noone hears it, does it make a sound?
So you understand lift. Do you understand thrust? Isn't the airplane still producing thrust? Why, yes it is. Are the wheels producing any opposing force? Yes, but it's no more than for any ordinary situation, i.e. without the treadmill. So what will happen? The airplane will accelerate in the thrust direction and eventually become airborne...

QED..

Bernoulli's principle, life comes from air flowing over the wings as the plane moves forward.

You really should have grasped at least that much by now...

I like to think that it's the Kutta condition at the sharp trailing edge that really makes lift possible... But Bernoulli is an okay introduction.

are we agreeing or disagreeing with eachother?? i'm saying that a plan can't take off if its on a treadmill going in an equal, but opposite direction. please tell me we are or i have no faith in the human race. :headbang:

I severely hope he is not agreeing with you, as you are flat out wrong, as the speed of the treadmill does not matter because, as he pointed out, a plane does not work like a car does, and the wheels are not connected to any drive shaft, unlike a car.

The wheels on a plane, in fact, aren't connected to anything really, and will just spin twice as fast, but that's about it.

That's the best post evar!:D
I take it you are familiar with the device that is called a magneto... And it's complete independence from the battery system.

I like to think that it's the Kutta condition at the sharp trailing edge that really makes lift possible... But Bernoulli is an okay introduction.

true enough, but considering the kind of person we appear to be dealing with, best stick to the basics

Bernoulli's principle, life comes from air flowing over the wings as the plane moves forward.

Really, you seem to be under the severe dillusion that the movement of the treadmill affects the movement of the plane. It doesn't, and the movement of the treadmill below the wheels of the plane is entirely irrelevant to the movement of the plane. The treadmill will not cause the plane to move backwards. You really should have grasped at least that much by now...

u aint picking up what i'm putting down. i'm well aware where the lift comes from, but if the plane is on a treadmill that's acting equal and opposite them planes velocity, then there is no air flow. geez......:headbang: it's all about lift on the wings. again i'll go s..l..o..w.. there will be no lift (airflow) to get the plane off the ground.

u aint picking up what i'm putting down. i'm well aware where the lift comes from, but if the plane is on a treadmill that's acting equal and opposite them planes velocity, then there is no air flow. geez......:headbang:

Why isn't there any air flow?

u aint picking up what i'm putting down. i'm well aware where the lift comes from, but if the plane is on a treadmill that's acting equal and opposite them planes velocity, then there is no air flow. geez......:headbang:

Ku-ball...Show me the equal and opposite force. SHOW ME THE FORCE!!!

Use one of the conveniently posted free body diagrams to describe it...

u aint picking up what i'm putting down. i'm well aware where the lift comes from, but if the plane is on a treadmill that's acting equal and opposite them planes velocity, then there is no air flow. geez......:headbang:

yeah, that emote seems appropriate. What you don't get is that the treadmill does NOT act "equal and opposite" to the plane's velocity (although as an aside, since velocity is a vector, it can not be both simultaniously equal and opposite, since velocity is direction dependant, something opposite will not be equal, though it can have an equal value).

The wheels on a plane are not connected to a drive shaft like they are on a car. If you take a plane, with the engine OFF, and drop it on a treadmill, it won't go anywhere. The treadmill will move under the wheels, and the wheels will just...spin. The wheels aren't connected to any drive shaft, them spinning won't move the plane. The treadmill will just cause the wheels of the plane to spin, but the plane, with it's engine off, won't move.

When the plane turns its engine on and begins moving forward, again, the treadmill will do nothing. A plane is not driven by its wheels, it is driven by its turbine. The wheels merely serve to reduce friction with the ground.

are we agreeing or disagreeing with eachother?? i'm saying that a plan can't take off if its on a treadmill going in an equal, but opposite direction. please tell me we are or i have no faith in the human race. :headbang:You fail. The movement of the plane caused by its engines is independent from the treadmill's movement.

In fact there's a fairly easy test. Get a piece of paper, a pencil and a flat surface. Drag the paper across the surface, at a stead pace. As it is moving, drop the pen on it. Does the pen move with the paper, or stay in place and just spin as the paper moves under it?

You can't start with the pen on the paper the suddenly move it, as that is a matter of acceleration not speed, and we're just talking about basically dropping a plane on a moving treadmill.

u aint picking up what i'm putting down. i'm well aware where the lift comes from, but if the plane is on a treadmill that's acting equal and opposite them planes velocity, then there is no air flow. geez......:headbang: it's all about lift on the wings. again i'll go s..l..o..w.. there will be no lift (airflow) to get the plane off the ground.

The plane moves forward regardless of the threadmill/conveyor belt.
Therefor there is airflow beneath the wings.
Therefor the plane can take off.

In fact there's a fairly easy test. Get a piece of paper, a pencil and a flat surface. Drag the paper across the surface, at a stead pace. As it is moving, drop the pen on it. Does the pen move with the paper, or stay in place and just spin as the paper moves under it?

You can't start with the pen on the paper the suddenly move it, as that is a matter of acceleration not speed, and we're just talking about basically dropping a plane on a moving treadmill.

Problem: a plane does accelerate. It has a force working on it to attempt to move it forward - unlike the pen.

Now.... what you really have to do is get that treadmill going and the jet engine to counteract the force so that it's holding still (relatively of course :p) and then suddenly STOP the treadmill, so that the plane blasts forward at the proper speed and doesn't have to worry about the distance necessary to accelerate.


...now that's not to say that it would be comfortable for whoever happens to be sitting in the plane to go from essentially 0 to however fast the plane needs to take off, but that's moot. We're just talking about getting the plane to take off.

And I didn't take the time to read all the posts here, so if somebody already got this, awesome. If not, then I have to sit and wonder why nobody thought of this already...

Now.... what you really have to do is get that treadmill going and the jet engine to counteract the force so that it's holding still (relatively of course :p) and then suddenly STOP the treadmill, so that the plane blasts forward at the proper speed and doesn't have to worry about the distance necessary to accelerate.

Unfortunately a threadmill cannot stop the plane. It would be a great spacesaver on airports and aircraftcarriers if it could.

I take it you are familiar with the device that is called a magneto... And it's complete independence from the battery system.

Yeah, that got covered when my CFI taught me the preflight checklist.

Yeah, that got covered when my CFI taught me the preflight checklist.
I'm still not sure what Wilgrove was taught, or what he thinks the left/right/both part of the checklist is testing.

Problem: a plane does accelerate. It has a force working on it to attempt to move it forward - unlike the pen.

I was talking about a turned off plane, not an active one. If the pen remains stationary, so will a plane without the engine active.

In this experiment, the argument is that the plane remains stationary because its engine counterbalances the backwards movement. So WITHOUT the engine it would move backwards.

Except it doesn't.

if an airplane COULD take off from a treadmill, would it go straight up in the air?

Some early (prototype) VTOL jet fighters did that...

yeah, that emote seems appropriate. What you don't get is that the treadmill does NOT act "equal and opposite" to the plane's velocity (although as an aside, since velocity is a vector, it can not be both simultaniously equal and opposite, since velocity is direction dependant, something opposite will not be equal, though it can have an equal value).

The wheels on a plane are not connected to a drive shaft like they are on a car. If you take a plane, with the engine OFF, and drop it on a treadmill, it won't go anywhere. The treadmill will move under the wheels, and the wheels will just...spin. The wheels aren't connected to any drive shaft, them spinning won't move the plane. The treadmill will just cause the wheels of the plane to spin, but the plane, with it's engine off, won't move.

When the plane turns its engine on and begins moving forward, again, the treadmill will do nothing. A plane is not driven by its wheels, it is driven by its turbine. The wheels merely serve to reduce friction with the ground.

Ah, that makes sense.

Whoa, what are you saying there? If we throw out the treadmill (about time, anyway) and substitute wind, then you can have wind at equal and opposite velocity to the aircraft. Net result is zero groundspeed, right?


Damn, there's that fine print stuff again...equal, opposite, vector

Wouldn't the wind have to be constantly accelerating to match the engine's thrust?

Don't know if this has been gone over here, but here's the question:

Imagine a giant treadmill the length and width of an airplane runway which moves backward at the same speed as the aircraft's takeoff velocity.

Would the plane be able to take off from this treadmill?

ok after all the hate mail i recieved i took a closer look at the question :) i was assuming that there was more of a hard connection with the treadmill, but there's not, so yes it will take off. that's my final answer. i would act independantly. the plane could acheive lift.