# Do airplanes need gravity to fly?

Okay, so I understand Bernoulli's principle about generating lift and what not, but I am curious if any of that lift generation would matter if there was no gravity to oppose it. Suppose one was in a giant outer space airport with the pressure and air that of earth, and it began its takeoff roll, would the plane be controllable in the three dimensions? Would minimal engine power be required to fly it? What else can you tell me and derive from this hypothetical situation?

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• Kim
Lv 5
8 years ago

It is an interesting question. The airplane could work and be stable. With a cambered wing, you'd have to operate the aircraft with a negative angle of attack (push the stick forward to get a nose-down attitude) to get zero lift. Of course, "down" in this case means with respect to the aircraft, not with respect to gravity! It would be better to use a symmetric wing section. Then it could fly straight with zero angle of attack. If you pull the stick back, then the aircraft would go to a positive angle of attack and would fly a loop. If you pushed the stick forward, then the nose would go down and you'd fly an outside loop.

The dihedral effect would not do you any good because there would be no gravity to make you start to sideslip if you rolled a little, so you'd have to control the ailerons to keep the roll angle straight.

You could not just roll to turn, but instead you would have to roll to a new angle and then pull up to fly a partial loop until you were pointed the way you wanted. Then you'd have to straighten out.

If you stepped on the rudder, it would cause the airplane to yaw. While it was yawing, one wing would be moving ahead and going slightly faster than the other wing. On a normal airplane with lift in gravity, this causes more lift on that wing, which causes a roll. But in this zero gravity situation, the wing would normally be at zero lift, so going faster would not cause a roll. But the sideslip angle would cause a sideforce on the fuselage and that would make you curve in a horizontal path. So you could turn just using the rudder. I guess that means you didn't have to roll to turn the way I said before.

Anyway, yes, the airplane would be controllable in all three dimensions (roll,pitch, yaw).

It would still need power to fly because there would still be drag from the air. But there would be much less induced drag (only when maneuvering). When flying straight, you'd only have profile drag. So it would require less power to fly.

To land, you could fly along a horizontal surface and then create some "downward" force using the wings at a small negative angle of attack so you would have a normal force to allow any kind of friction braking from the wheels. When you finally came to a stop, you'd be in free fall, so nothing would really hold you down. Maybe the propeller could be swivelled to push downward against the surface so you could get out with your magnetic boots and tie the airplane down to the tiedowns.

Very fun question.

• 8 years ago

For an aircraft to fly staight and level at a constant speed, GRAVITY = WEIGHT, and DRAG = THRUST.

If there is matter present, i.e. air and ground, then lift will still need to equal weight, as it will HAVE weight, even if just a little. Note, to have an atmosphere LIKe Earth, then gravity must be the SAME as on earth, or there would not be the pressure as on earth!

It would further require engine power similar to on earth, as drag exists, but maybe some saving due to lack of large wing size. Because the air would be thinner than on earth, the aircraft would need to travel faster, so drag will be similar to on earth in the end.

The 2 pairs of forces must be in equilibrium, in flight as described above.

Of course, as the air is so thin at your hypothetical airport in space, the cabin would need to be pressurised for occupants.

• 8 years ago

Interesting question! Well, when you learn about aviation you see the diagram about how the airplane stabillizes in flight by balancing thrust and drag, lift and gravity. Without gravity, airplanes would at least look very different. They'd need to generate negative as well as positive lift, and control them to climb or dive (you couldn't glide! You couldn't stall! How could you land?) Also balance is very important in an airplane, I mean balance of weight, left and right as well as fore and aft. That wouldn't be important in a weightless situation.

But you know a planet with no gravity would have no atmosphere anyway, right? All the atmosphere would just drift away! So no air pressure, so no lift either way.

• 8 years ago

The airspeed of the plane, and the angle of attack of the wing, are both adjusted to have the lift force MATCH the weight of the plane. And either the pilot, or the pilot assisting systems, will adjust these parameters accordingly.

If the aircraft as it is presently built, were operating in zero gravity, it would be incredibly confused when trying to fly. A skilled pilot might be able to compensate for it. Any pilot assisting software, probably would crash. I wouldn't trust any important passengers or cargo to it, though. Not until it is designed to be able to handle the loss of gravity.

Stunt pilots can indeed cruise a plane upside-down. It it is a common aerobatic move. It usually requires wings that are specifically built for it. You couldn't do this with a commercial aircraft, as its wings just aren't designed to handle that kind of stress.

In order to fly in zero gravity, the aircraft needs to be able to somehow nullify the lift from its wings. It could do this, by activating openings in its wings, so that oncoming air just passes through, instead of getting deflected downward. Or, it could have a set of counter-wings, that produce negative lift, both sets being independently adjustable in their angle of attack.

Likely, in order to be able to control its motion in the three dimensions, it would need at least three jet engines on its tail, and use them the way that rockets use their rocket nozzles.

Rockets use their rocket nozzles to steer, by thrusting one of them less than the others, angling the spacecraft, and then making a full thrust while maneuvering. An aircraft would do the same thing. Except with jet engines instead of rocket nozzles.

• 8 years ago

it wouldn't need gravity to fly because there is nothing keeping it from staying on the ground. and it wouldn't require nearly as much fuel. and the three dimensions part is excluded because it's self explanatory