NationStates Jolt Archive

I've got an aircraft (A one-off, actually... a converted prototype.)
that has a average weight of 46,000lbs.

It's got a pair of 26,000lb-thrust (30,000lbs w/ afterburning) engines; so the total thrust is greater than the weight of the aircraft.

Now, my question is: can I reasonably claim that this thing can hover by simply pointing its nose straight up?

No, too unstable would flip over, need stabilizers of sorts for a true 90 degree angel.

Well, it has RCS and vectoring.
The original aircraft was a maneuverability experiment that's infamous for having killed pilots with excessive Gs...

You might be able to keep it vertical if you had momentum and continually compensated with the wing stabilizers speed and thrust doesn't call into account being vertical.

BUMP so someone can confirm his opinion. Is he right?

BUMP again.

He's generally right. A thrust-to-weight ratio of greater than one does mean that it could theoretically hover. An F-15 can accelerate going straight up, and could theoretically hover. However, stability is a definite issue if the thrust reaction vector does not go through the exact center of gravity of the aircraft.

Momentum means motion, and so that's not an option if hovering. With computer-controlled thrust vectoring (assuming complex three-axial so you can do yaw, pitch, and roll, not too far off given a two-engine craft) it is again possible, but tricky.

In the end, though... it's quite possible it wouldn't be designed with that fine a control because it's simply unneccessary. A jet fighter doing that is just a target and it could be tricky setting up and getting out of a hovering position. Still, that's your call.

--Hearty Aerospace Pirate Cap'n Scolo

Well, the way I see it;

The plane's nose goes up. The plane's forward motion slows as greater surface area is presented.
Ventral RCS jets fire, slowing the aircraft to a stop, and the aircraft begins to hover.

Computer fires RCS jets and adjusts thrust to maintain balance.
To go up or down, thrust is adjusted.
To go sideways, RCS jets are fired.

To land, the aircraft drops to 50-60 feet, cuts engine, and fires RCS jets on top of nose and bottom of tail to rotate 90 degrees. Ventral RCS (and VTOL vents) are fired to slow the drop. (This is a emergency maneuver; difficult to perform without turning the aircraft into a smoking wreck)

To stop hovering, thrust is maxed and the aircraft begins to move upward. When sufficient speed is reached, aircraft uses elevators to flatten out and resume normal flight.

Bump-whump-a-thump-bump-whump-bang-bump-thump-boom.

The plane's nose goes up. The plane's forward motion slows as greater surface area is presented.
Ventral RCS jets fire, slowing the aircraft to a stop, and the aircraft begins to hover.
Only problem with that is it doesn't take into account the plane's tendancy to a) stall or b) flip over backwards given those circumstances. As you increase angle of attack, sure, you get more drag and you slow down, but you also divert more air downwards and thus get more lift. As long as circulation around the wing goes the right way, you get lift pulling you back... in a stall, circulation goes backwards and thus the wing actually produces negative lift. Not good.

Computer fires RCS jets and adjusts thrust to maintain balance.
To go up or down, thrust is adjusted.
To go sideways, RCS jets are fired.
Sideways laterally (plane-relative) I can see. Going up or down, you've a lot of problems from the wing giving you 'lift' that you don't want. Sideways vertically (plane-relative) is just nasty because, again, the planform of the wing acts as a big giant drag-producing wall. Inefficient in the extreme, but doable.

The problem with RCS jets is that they usually have the moment to turn the craft but not really the force to move it so much. Sure, the Harrier can hover and move sideways and what not, but those maneuvers are reliant on the thrust-vectoring of the main exhausts. It could be done, given a fast enough computer and enough fuel, but using reaction control thrusters for motive force would be slow going.

To land, the aircraft drops to 50-60 feet, cuts engine, and fires RCS jets on top of nose and bottom of tail to rotate 90 degrees. Ventral RCS (and VTOL vents) are fired to slow the drop. (This is a emergency maneuver; difficult to perform without turning the aircraft into a smoking wreck)
This just makes me flinch. It also suggests the aircraft doesn't have a wing... at which point you're describing the Delta Clipper (that conical VTOL rocket-thing that they were testing as a Space Shuttle replacement) and it should just land on its tail anyway.

How about, to land, it just levels out at a decent altitude and does a normal approach? The problem with your plan is that you take a plane (vertical) with no horizontal motion and then make it a plane (horizontal) with no horizontal motion. That screams a flat spin, because nothing is moving over your control surfaces like it should. Best plan of attack is not just level out but dive... which means you'd need a lot more than a few tens of meters.

To stop hovering, thrust is maxed and the aircraft begins to move upward. When sufficient speed is reached, aircraft uses elevators to flatten out and resume normal flight.
The thrust isn't a problem; this is essentially the F-35 VTOL system. I fear the problem is again your lifting surface wanting to flip the plane backwards (which will happen long before the elevators become useful). This can be addressed with RCS but that would be extremely fuel inefficient... then again, VTOLs are by nature fuel inefficient because they take an airplane with lift-producing wings and forces it to act like a brute-force rocket stuck onto a brick.

As an aerospace engineering student, the idea is fascinating but kinda cringe-forming. Again, it's all possible, but not necessarily feasible especially when fuel requirements are taken in. Just as a curious question... why do you want it to tail-stand anyway?

Well, you see, I've landed it vertically in a RP already, and then later I thought "Is that actually possible?".
Why? Two reasons.

1. The landing gear was stuck.
2. The forward VTOL vents were inoperative (broken hydraulic line).

So I ended up pancaking the thing near the runway...
Killed the underwing mounts, the landing gear is now hopelessly mangled, and I think I may have fractured a turbine blade.

But a bit of patching to the hydraulic line for the forward VTOL, and it flies! :P

(Yes, it's insanely durable... been through more than fifteen emergency landings, some of them avoiding the term "crash" simply because the thing still flew afterwards.)