NationStates Jolt Archive

I read a forum that is dedicated to general aviation. One of the threads that has held my interest has to do with how well pilots understand the aerodynamics that make their airplanes fly.

The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is stated on the forum: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

Now, my assumptions are these:
1. The conveyor belt mechanism doesn't move the atmosphere.
2. The conveyor belt will move in the direction opposite to any movement of the airplane.

3. The aircraft is not a vtol, nor is it a helicopter. Think J-3 Cub
( Good try, guys)


The general consensus among pilots is that the airplane will fly. I don't think so, but I'm going to see if there is any interest in this topic before I post my answer.

The wheels of the plane are not used to propel the plane, it moves air with it's engines. The wheels just allow it to roll. So the plane will move forward, just as well as if there weren't a conveyor, the wheels just turn twice as fast.
Or the plane take off vertically. VTOL

In another version the conveyor moves at the speed the wheels turn at, I just noticed it's nto the case here

No. Assuming it is set up so that the plane stays still (I assume that's what you're getting at) then there is no strong air flow over the wings required for lift.

Interesting..

But the plane will take off, as the engines work by moving air, and not ground. Air moves independantly of the ground (thankfully!). It should look just like a regular takeoff, apart from the speed at which the wheels are turning (twice the KIAS).

No, it won't take off.

Airoplanes work by creating a faster airflow over the top of the wings than under the bottom. That's what creates the lift. This is done by having a longer top edge, which means the air has to move faster, and is thus less dense than the slow moving air on the underside of the wing. The engines are only used to move the engine through the air.

If the plane is rendered stationary due to the conveyor belt, and there is no wind, then there would be no airflow over the wings, meaning, as somebody pointed out earlier, that no lift would be created.

Nice idea though. A similar problem is with a train:

A train is on some scales. There are birds on top of the train. if they take off, the weight of the train becomes less. Fairly obviously. Now, if the birds are inside the train, and they take off, does the weight of the train change?

If the plane is rendered stationary due to the conveyor beltBut it isn't.
If the plane isn't moving, the conveyor belt isn't moving either.
If the plane does move, the conveyor belt moves in the opposite direction, and the wheels mturn at twice the usual speed. But the plane still moves.

And again, there's the possibility of vertical take off.

My first thought was 'No'. That was later strengthened to 'Hell, no'. If we look at what is required for flight, we know that Thrust = Drag and Lift = Weight. I think we can agree that this setup the plane can accelerate along the ground, so Thrust will equal Drag. Part one done.

Now, how about Lift = Weight? Aside from vectored thrust, and this isn't one of those sorts of airplanes, nor is it a helicopter, I can only think of one way to get lift. That is to get differential pressures on the wing. Can you have a pressure differential when there is no airflow over the wing. No. The most basic theory is that Lift = q*C_l. where q is dependent on the square of the freestream velocity, V**2.

So when we get away from the airflow directly behind the propellor, is there any flow over the wing? Not if the conveyor is working properly and the forward speed of the aircraft is matched. If V=0, then L=0 and there is no lift.

Here's the conclusion of an article that tries to convince us otherwise
http://www.avweb.com/news/columns/191034-1.html

A few moments later, the roaring Continental, spinning that wooden Sensenich prop, has accelerated the J-3 and Manfred to 25 mph indicated airspeed. He and the airplane are cruising past the cheering spectators at 25 mph, while the conveyor has accelerated to 25 mph eastbound, yet it still has no way of stopping the airplane's movement through the air. The wheels are spinning at 50 mph, so the noise level is a little high, but otherwise, the J-3 is making a normal, calm-wind takeoff.

As the indicated airspeed passes 45 mph, groundspeed -- you know, relative to where all those spectators are standing beside the conveyor belt -- is also 45 mph. (At least that's what it says on Manfred's GPS. Being brought back to life seemed to create an insatiable desire for electronic stuff.) The conveyor is also at 45 mph, and the wheels are whizzing around at 90 -- the groundspeed plus the speed of the conveyor in the opposite direction.

Unless this magic conveyor belt is also accelerating the atmosphere, I can't see how any indicated airspeed is created. The conclusion ends up with some nonsense about how this would be different if the wheels provided the thrust, instead of the propellor.

But it isn't.
If the plane isn't moving, the conveyor belt isn't moving either.
If the plane does move, the conveyor belt moves in the opposite direction, and the wheels mturn at twice the usual speed. But the plane still moves.

And again, there's the possibility of vertical take off.

If the plane is rendered stationary I thought as the conveyor belt moves the plane in the opposite direction and exactly equal to the movement of the plane, rendering it stationary. As the air is also stationary in relation to the plane (although not the conveyor belt), there is no lift, so no take off.

Interesting..

But the plane will take off, as the engines work by moving air, and not ground. Air moves independantly of the ground (thankfully!). It should look just like a regular takeoff, apart from the speed at which the wheels are turning (twice the KIAS).
You're the one I'm looking for.

Where does the IAS come from? All the prop is doing is providing the thrust that is required to fly. Where is the lift coming from?

I'll agree that the prop provides enough velocity to lift the tail, but only because the tail is directly in line with the prop. Where does the freestream velocity on the wings come from?

Of course not.... The wheels will turn though. And air sucked through the engines. But there will be no lift, cause the wind is blowing with a few hundred kph too slow

A train is on some scales. There are birds on top of the train. if they take off, the weight of the train becomes less. Fairly obviously. Now, if the birds are inside the train, and they take off, does the weight of the train change?
Only if they could magically suspend themselves without producing any downward force. Like being helium balloons. The act of flying around in the train car just replaces their sitting weight with the force required to stay aloft.

You're the one I'm looking for.

Where does the IAS come from? All the prop is doing is providing the thrust that is required to fly. Where is the lift coming from?

I'll agree that the prop provides enough velocity to lift the tail, but only because the tail is directly in line with the prop. Where does the freestream velocity on the wings come from?

First of all, a plane that can take off with 25 KIAS is a pretty high-lift wing. With a wing like that, an engine pushing air over the wing can probably get you to take off.

For aircraft like the C-17 that have blown flaps on the main wing, i.e., high wing lift generated by engine airflow, yes it could take off.

I don't believe for a second, however, that this applies to most aircraft.

There's also a ground effect that can give you some lift, but you still have to be moving in relation to the ground.

Only if they could magically suspend themselves without producing any downward force. Like being helium balloons. The act of flying around in the train car just replaces their sitting weight with the force required to stay aloft.

Yay!

Sends lots of people into confused huddles that one. I love these things, got any more?

Yay!

Sends lots of people into confused huddles that one. I love these things, got any more?
Nah, I was really looking for someone to defend the 'Yes it can fly' side. I wrote a letter to the columnist and never heard back from him.

Then, there's always the "If you jump right before an elevator crashes, will you escape injury".

If the plane is rendered stationary I thought as the conveyor belt moves the plane in the opposite direction and exactly equal to the movement of the plane, rendering it stationary. As the air is also stationary in relation to the plane (although not the conveyor belt), there is no lift, so no take off.How could the conveyor keep the plane stationary with respect to the ground? It's moving with respect to the air, and all the conveyor does is move the wheels a bit more.
Admittedly the wheels aren't perfectly frictionless, but consider the case where they are. Then no matter the speed of the conveyor, there is no effect on the plane, at all. So any trust of the plane will move it with respect to the air.

First of all, a plane that can take off with 25 KIAS is a pretty high-lift wing. With a wing like that, an engine pushing air over the wing can probably get you to take off.

For aircraft like the C-17 that have blown flaps on the main wing, i.e., high wing lift generated by engine airflow, yes it could take off.

I don't believe for a second, however, that this applies to most aircraft.

There's also a ground effect that can give you some lift, but you still have to be moving in relation to the ground.

A J-3 has a pretty low stall speed. It might be 25 KIAS. But, for the 'yes' hypothesis to be correct by direct engine action, the conveyor belt wouldn't even matter. The plane could be tethered to the ground, i.e. never untied the tail tiedown, and when the engine is at full power, the airplane would fly.

I remember talking to a guy that flew B-17s during WWII. One of the scramble takeoff procedures was to fire off all four engines at once. Of course, one of the crew had to hold the stick way back, so the plane would stay on the tail wheel. He told me about a pilot that forgot to hold the nose up. The forces generated by those engines on the tail surfaces rotated the tail up in the air and tipped the airplane up on its nose.

if it is a VTO(L) sure it can fly.

How could the conveyor keep the plane stationary with respect to the ground? It's moving with respect to the air, and all the conveyor does is move the wheels a bit more.
Admittedly the wheels aren't perfectly frictionless, but consider the case where they are. Then no matter the speed of the conveyor, there is no effect on the plane, at all. So any trust of the plane will move it with respect to the air.
What I tried to state was that the conveyor moved in an equal and opposite direction to the aircraft. If the airplane accelerated to 25 knots ground speed to the east, the conveyor would be moving at -25 kts east, or 25 kts to the west. The plane should remain stationary to a fixed object off of the conveyor, e.g. a control tower on the airport.

Does that explanation change your answer at all?

What I tried to state was that the conveyor moved in an equal and opposite direction to the aircraft. If the airplane accelerated to 25 knots ground speed to the east, the conveyor would be moving at -25 kts east, or 25 kts to the west. The plane should remain stationary to a fixed object off of the conveyor, e.g. a control tower on the airport.

Does that explanation change your answer at all?No, because the movement of the plane isn't with repect to the conveyor. So even thoguht he conveyor moves in the opposite direction, it doesn't move the plane back. It only makes the wheel turn twice as fast than if it where to stand still.

How could the conveyor keep the plane stationary with respect to the ground? It's moving with respect to the air, and all the conveyor does is move the wheels a bit more.
Admittedly the wheels aren't perfectly frictionless, but consider the case where they are. Then no matter the speed of the conveyor, there is no effect on the plane, at all. So any trust of the plane will move it with respect to the air.

Ohhhh, you're right! If the wheels are frictionless, then the forward thrust created by the engines (or propellor) would move the plane, and the wheels would rotate twice as fast (as you said). What is confusing I think is that the conveyor belt does not provide any drag on the plane. The body of the plane moves through the air, thus uplift.

I stand corrected.

NB: If you could jump at the speed of a freefalling lift then you would, technically, survive. However, we have neither the strength to jump that quickly, or the ability to execute a jump in the timeframe that would be available. Plus I suspect that a crashing elevator would result in the ceiling of the elevator hittign the floor of the elevator, so even if you had rendered yourself stationary, you'd probably be crushed by it!

Now, my assumptions are these:
1. The conveyor belt mechanism doesn't move the atmosphere.
2. The conveyor belt will move in the direction opposite to any movement of the airplane.

If item 1 is correct, and the atmosphere is stationary, then the only problem I see is mechanical failure of the undercarriage (or undue drag by same) preventing the engine(s) (whether jet, propeller, even rocket) from letting the plane achieve the designed take-off IAS. (The retardant force of on-ground wheel-only friction at twice the IAS speed (Fn(2x), where x is the required take-off speed) plus all the rest of the drag coefficient (Fm(x, which is essentially the aerodynamic drag) is greater than the maximum thrust (Ft(max)).

In the simplest model, Fn(2x) may be considered negligable, and the gear (wheels/skids/whatever) assumed to have no delitarious effect on speed, nor speed have a delitarious effect on the gear, in which case the plane acts as normal, though the rushing conveyor might disorientate the pilot if no other external references exist.

If there's significant drag, then the 'best horizontal veolocity' decreases below the IAS. The plane will never stand still, though, even under high drag conditions. You can solve for "what if the plane happens to be standing still at attempted speed x". The conveyor's systems will spot that the plane is not moving and not move the conveyor, thus if the plane is able to move at all (i.e. assuming Fn(2x) is not as great or higher than Ft(max), which could not occur on any practicle non-catapulted plane) the convey it will move at a speed greater than zero against the conveyor.

(VTOL also cannot happen, unless you unassume the 'air unaffected by conveyor' theory, and tie conveyor-speed and the plane's attempted-air speed together, instead, and the air exactly as the conveyor, in which case the plane will fly as normal, but with a ground-speed adjusted to zero. That's jiggery-pokery is way byond the origional postulation, though.)


[Post-edit: In the time taken to write and edit the above, it looks like the conclusion has been bashed out by two others. Apologies for the subsequent repetition]

No, because the movement of the plane isn't with repect to the conveyor. So even thoguht he conveyor moves in the opposite direction, it doesn't move the plane back. It only makes the wheel turn twice as fast than if it where to stand still.
Okay, I think the light is almost ready to come on. But humor me with one more answer to one more question. And asking may do more to help me than your answer, but since I have to compose the question anyway, I may as well post it.

The airplane accelerates from 0 to some positive ground speed. While it is accelerating in one direction, the conveyor is matching the ground speed in exactly the opposite direction. Let's look at the aircraft from the control tower, now. Isn't the airplane fixed at the same point in my window as it was when it was at 0 ground speed? So, shouldn't I be able to walk out and touch one of the wings, so long as I don't step onto the conveyor?

I think this is the important part. The plane depends on movement over the ground to build up the KIAS needed for lift. The thrust provided by the propeller only overcomes friction and drag to provide the forward motion. If the ground keeps moving, the airplane is essentially stationary in the atmospheric coordinate system.

What am I missing?

Where I got stuck kind of, was that the wheels are free to turn in respect of both the plane and the conveyor belt. If the wheels could only turn at the speed the plane was supposed to be travelling, then it wouldn't work, as the conveyorbelt would indeed negate it, but as the wheels can travel as fast as they like, and they are not powered (as they are in a car) they just provide a distraction. The forces are working on the body of the plane.

If the wheels were frictionless, if the plane was stationary, and the conveyor belt moved, the wheels would turn and the plane would remain stationary. That is sort of the situation, but with a force (the thrust) acting on the body of the plane to propell it forward.

The airplane accelerates from 0 to some positive ground speed. While it is accelerating in one direction, the conveyor is matching the ground speed in exactly the opposite direction. Let's look at the aircraft from the control tower, now. Isn't the airplane fixed at the same point in my window as it was when it was at 0 ground speed?No, consider a table with tablecloth and glasses on it, now pull very quickly at the cloth, if you do it right, you pull it out from under the glasses and they hardly move.

The thing is, you have a force exerted by the plane, and a force exerted by the conveyor (through the drag on the wheels) on the plane. But the former is much greater than the latter, so the plane has a net force pushing it forward at all times.
And also, the conveyor moving backwards a meter doesn't necessarily bring back the plane a meter (just like pulling the table cloth hardly moves the glasses if you pull fast). With frictionless wheels there wouldn't even be any effect.

I think this is the important part. The plane depends on movement over the ground to build up the KIAS needed for lift. The thrust provided by the propeller only overcomes friction and drag to provide the forward motion. If the ground keeps moving, the airplane is essentially stationary in the atmospheric coordinate system.

What am I missing?You're missing how fast the conveyor has to move to keep the plane stationary. It has to cancel out thrust not movement. And because the conveyor acts on the wheel, and not directly on the plane, you're just pulling the conveyor along under it, instead of pulling the plane back.

Okay, I think the light is almost ready to come on. But humor me with one more answer to one more question. And asking may do more to help me than your answer, but since I have to compose the question anyway, I may as well post it.

The airplane accelerates from 0 to some positive ground speed. While it is accelerating in one direction, the conveyor is matching the ground speed in exactly the opposite direction. Let's look at the aircraft from the control tower, now. Isn't the airplane fixed at the same point in my window as it was when it was at 0 ground speed? So, shouldn't I be able to walk out and touch one of the wings, so long as I don't step onto the conveyor?

I think this is the important part. The plane depends on movement over the ground to build up the KIAS needed for lift. The thrust provided by the propeller only overcomes friction and drag to provide the forward motion. If the ground keeps moving, the airplane is essentially stationary in the atmospheric coordinate system.

What am I missing?

Just draw the free body diagram. You'll see quite obviously that the aircraft has to accelerate.

The wheels are rotating freely, so all it means is that if the conveyor is moving at the same velocity (but in the opposite direction) to the aircraft, they'll have to turn twice as fast as they normally would with a staionary runway. As they don't provide any thrust though, this shouldn't be a problem.

Jeez, why is this question so popular. I've seen it posted on a few websites and surely the answer is: unless its a VTOL, then hell no it cant take off.

snip

No, it will not. I will answer a question with another question. Do you feel a wind the same speed as you are running on a treadmill? How about on a excersize bike? You could have the plane and treadmill running at 80kts and still not have the airflow nessisary to lift the aircraft off the treadmill runway.

Jeez, why is this question so popular. I've seen it posted on a few websites and surely the answer is: unless its a VTOL, then hell no it cant take off.

And you would be completely and utterly wrong, as explained above by several people ;)
Once the engines have overcome the friction between wheels and belt it would simply start moving forward, just like it would if the whole conveyor belt hadn't been there. To make sure you understand: if you were sitting in the control tower you would see the plane moving forward normally. It does not stand still on the belt. Even if the belt went back at ten times the speed of the airplane the airplane would still move forward normally (provided the wheels can turn freely and are not ripped off).

Which means it can take off as always.

No it wouldn't take off. As the plane goes faster, the wheels spin twice as fast as its speed. At some point the plane's wheels will melt and burn out because the company that made them never thought they would have to spin that fast. With its wheels gone the plane's exposed undercarriage will hit the conveyor belt, creating sparks that ignite the fuselage and destroy the plane in one massive explosion. Many lawsuits follow.

No it wouldn't take off. As the plane goes faster, the wheels spin twice as fast as its speed. At some point the plane's wheels will melt and burn out because the company that made them never thought they would have to spin that fast. With its wheels gone the plane's exposed undercarriage will hit the conveyor belt, creating sparks that ignite the fuselage and destroy the plane in one massive explosion. Many lawsuits follow.

Always you people with your stinking reality have to ruin the perfect world of hypothetical experiments!

Okay, I think the light is almost ready to come on. But humor me with one more answer to one more question. And asking may do more to help me than your answer, but since I have to compose the question anyway, I may as well post it.

The airplane accelerates from 0 to some positive ground speed. While it is accelerating in one direction, the conveyor is matching the ground speed in exactly the opposite direction. Let's look at the aircraft from the control tower, now. Isn't the airplane fixed at the same point in my window as it was when it was at 0 ground speed? So, shouldn't I be able to walk out and touch one of the wings, so long as I don't step onto the conveyor?

Yes.

I think this is the important part. The plane depends on movement over the ground to build up the KIAS needed for lift. The thrust provided by the propeller only overcomes friction and drag to provide the forward motion. If the ground keeps moving, the airplane is essentially stationary in the atmospheric coordinate system.


Not quite, in the air with a strong enough headwind one can have a groundspeed of 0kts or a groundspeed in the opposite dirrection. Another way is by headding straight up, your groundspeed would be zero. What is happening is the plane on the convayor is at a takeoff speed, however there is no airflow over the wings. In the above senerios, each aircraft had a source of airflow, the first was a natural airflow in the effect of a headwind. The second the aircraft had additional speed to go vertical, this is keeping airflow moving over the wings. Unless you were to introduce airflow to get the plane to become airborne, the plane will just sit there. Sence we are talking of a hypothetical situation, lets introduce a large fan into the equasion. The aircraft is sitting on the convayor with the fan giving the aircraft the same amount of airflow it would have if it were at that airspeed as it is traveling on the convayor belt. The aircraft would lift off and fly, however once it travels outside the cone of air, the aircraft would stall and fall back into the cone or crash into the ground. It would make a form of artificial windsheer. Anyhow, hope this answers your question.

What am I missing?

I think you have it.

Just draw the free body diagram. You'll see quite obviously that the aircraft has to accelerate.

The wheels are rotating freely, so all it means is that if the conveyor is moving at the same velocity (but in the opposite direction) to the aircraft, they'll have to turn twice as fast as they normally would with a staionary runway. As they don't provide any thrust though, this shouldn't be a problem.
Damn those basics. Or rather, damn me for forgetting them.

Thanks, that clears it right up.

The airplane accelerates from 0 to some positive ground speed. While it is accelerating in one direction, the conveyor is matching the ground speed in exactly the opposite direction. Let's look at the aircraft from the control tower, now. Isn't the airplane fixed at the same point in my window as it was when it was at 0 ground speed? So, shouldn't I be able to walk out and touch one of the wings, so long as I don't step onto the conveyor?

No - the plane is moving forward normally, just as if the belt had not been there.

Damn those basics. Or rather, damn me for forgetting them.

Thanks, that clears it right up.

My first physics prof. used to insist that every problem got a free body diagram - whether they needed one or not - just on general principle.

Over the years, that habit has saved me a lot of grief.

Jeez, why is this question so popular. I've seen it posted on a few websites and surely the answer is: unless its a VTOL, then hell no it cant take off.
No, the force acting to push the aircraft forward will push the aircraft forward, no matter what. The only thing that would keep the aircraft from moving forward is if the atmosphere was also driven in the same direction as the conveyor. The thrust force is not dependent on the ground, so the aircraft will accelerate, relative to the atmosphere.

Drawing a free-body diagram of the problem is the key. For the aircraft to remain stationary, the forces would have to sum to zero, but there is no force acting against the thrust. Thus, there is no alternative but for the airplane to accelerate and eventually fly.

And you would be completely and utterly wrong, as explained above by several people ;)
Once the engines have overcome the friction between wheels and belt it would simply start moving forward, just like it would if the whole conveyor belt hadn't been there. To make sure you understand: if you were sitting in the control tower you would see the plane moving forward normally. It does not stand still on the belt. Even if the belt went back at ten times the speed of the airplane the airplane would still move forward normally (provided the wheels can turn freely and are not ripped off).

Which means it can take off as always.

Please explain how the aircraft would overcome the friction of between the wheels and convayor without some lift. A propellor driven aircraft might have a chance with airflow moving over some of the lifting and control surfaces to move off the convayor. I could not see a jet aircraft pulling this off. The only way I could see this working is if you had something to brute force the aircraft off the convayor, Eg: Jato, Thrust vectoring.

My first physics prof. used to insist that every problem got a free body diagram - whether they needed one or not - just on general principle.

Over the years, that habit has saved me a lot of grief.
Yep. I got wrapped up in the aerodynamics and forgot about the basics.

Please explain how the aircraft would overcome the friction of between the wheels and convayor without some lift. A propellor driven aircraft might have a chance with airflow moving over some of the lifting and control surfaces to move off the convayor. I could not see a jet aircraft pulling this off. The only way I could see this working is if you had something to brute force the aircraft off the convayor, Eg: Jato, Thrust vectoring.
Friction is negligible. Once the static friction is overcome, the engine will provide more than enough thrust to overcome the dynamic friction. Think of a normal taxi and takeoff. The same wheel bearing friction is there, the same tire friction is there. Maybe it adds up to 120 lb. of static friction and becomes 40 lb. of dynamic friction after the plane starts to roll. That's something a 50-60 Horsepower engine can easily overcome. There's even more drag from the aircraft, but even there, you might only be dealing with 30-40 lb. for a small aircraft.

And you would be completely and utterly wrong, as explained above by several people ;)
Once the engines have overcome the friction between wheels and belt it would simply start moving forward, just like it would if the whole conveyor belt hadn't been there. To make sure you understand: if you were sitting in the control tower you would see the plane moving forward normally. It does not stand still on the belt. Even if the belt went back at ten times the speed of the airplane the airplane would still move forward normally (provided the wheels can turn freely and are not ripped off).

Which means it can take off as always.

So the friction of the wheels does not impede the aircraft in any way at all? Because if they do, you could just ramp up the speed of the conveyor untill the friction overcomes the thrust of the plane. The conveyor would have to be going very fast to do this but there wasnt anything about an upper limit of the conveyor speed in the first post.

EDIT: Argh, after reading the first post again i see he says the conveyor speed only changes to what speed the plane would be going if it were moving, not just changed up and up to stop it taking off. Yeah, it can take off, thought the engines would have to be going a bit faster than usual.

Technically, the opposing movement of the convayor would cancel out the thrust. Just as a person on a treadmill or a car on the E-check rollers.

The forward movement generated by the thrusters is movement relative to the air around the plane, not relative to the conveyor belt under the plane. The air around the plane does not move along with the conveyor belt. The conveyor belt - regardless of which way it moves - therefore has no impact on the speed the plane gets from the thrusters.

Technically, the opposing movement of the convayor would cancel out the thrust. Just as a person on a treadmill or a car on the E-check rollers.
No, and here is what I missed. The thrust that is acting on the aircraft is not through the wheels. If a person or an auto would try this stunt, it wouldn't work for exactly that reason. But the thrust is in the form of accelerated air -- that's the difference, there is no significant drag or friction force that is acting against it.

The forward movement generated by the thrusters is movement relative to the air around the plane, not relative to the conveyor belt under the plane. The air around the plane does not move along with the conveyor belt. The conveyor belt - regardless of which way it moves - therefore has no impact on the speed the plane gets from the thrusters.

Ok, got it now.

So the friction of the wheels does not impede the aircraft in any way at all?

Not after the forward thrust has overcome it (wheels can turn ;)). Which in this situation is no problem for the engines.

Because if they do, you could just ramp up the speed of the conveyor untill the friction overcomes the thrust of the plane.

Correct in practice - though that would rip the wheels of first. Much easier to just chain the plane down ;)

I read a forum that is dedicated to general aviation. One of the threads that has held my interest has to do with how well pilots understand the aerodynamics that make their airplanes fly.

The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is stated on the forum: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

Now, my assumptions are these:
1. The conveyor belt mechanism doesn't move the atmosphere.
2. The conveyor belt will move in the direction opposite to any movement of the airplane.

3. The aircraft is not a vtol, nor is it a helicopter. Think J-3 Cub
( Good try, guys)


The general consensus among pilots is that the airplane will fly. I don't think so, but I'm going to see if there is any interest in this topic before I post my answer.


A plane doesn't push the ground to push itself forward. It pushes or pulls the air, with a propeller or jet engine. The plane will gain speed at the normal rate, slightly decreased because of the added frictional force between the wheels and axles because the turn rate of the wheels is double what it normally would be.

A small airplane gains airspeed at what? 100 mph? A large passenger liner, maybe twice that? The wheels would have to be spinning a great deal faster than 200 or even 400mph for there to be any issue of a blowout--especially since the nominal weight of the airplane would decrease as it picked up speed.

The plane would take off, as long as the wheels didn't explode first from all that spinning.

Once airborne, all the little fairies would keep it safe until it was time to land.

I don't see why not, though it is hard to envision. If the air moving beneath the wings hits the proper velocity and the plane is moving forward...it'll take off. I don't think spatial location relative to the ground is relevant.