Flight and gravity

[swellman]

Question from a third grader's science homework - does an airplane need gravity to fly? There seem at least three possible answers:

1. An atmosphere would not form or stay contained if gravity was absent. (the wisea$$ answer)

2. No, because lift is a function of only velocity and air density. (this is the book answer)

3. Yes, because a standard aircraft balances lift, drag, thrust, velocity and pitching moments to maintain stable flight. Remove the gravity vector and the aircraft becomes unstable without radical redesign/trim.

Any predictions on the stability margin of an aircraft in condition number 3? Or is this answer out to lunch?

 

[Houngan]

Sure, and I have a real-world example. Make a paper airplane, then cut elevators into the backs of the wings. Bend them up, and give it a toss. It should almost immediately flip nose-up, then buffet and stall. Of course, without gravity, the plane would never stall, but it would still have WAY too much lift.

 

[Oso]

What happens when you throw a glider in the International Space Station? The only difference would be that you need less velocity to generate lift. #2 sounds pretty good to me.

 

[Andonyx]

Sure, and I have a real-world example. Make a paper airplane, then cut elevators into the backs of the wings. Bend them up, and give it a toss. It should almost immediately flip nose-up, then buffet and stall. Of course, without gravity, the plane would never stall, but it would still have WAY too much lift.

Wouldn't it continually fly in loops until it ran out of energy to fight drag?

 

[no one in particular]

Number two is a ridiculous, incomplete answer. Without gravity (or an enclosure) there is no air density, atmosphere dissipates to something like a few molecules every few feet. Certainly there is nothing to provide lift. Number one is the only answer. The only exceptions to this are the rare microgravity-in-atmosphere situations (the “Vomit Comet”, a vessel in orbit, or, say, an elevator with the cable cut). Unless you are flying something inside of something else that is in freefall, it is a pointless question. If the book expands on the answer then it is fine, if it does not then it is misleading.

 

[Oso]

Number two is a ridiculous, incomplete answer. Without gravity (or an enclosure) there is no air density, atmosphere dissipates to something like a few molecules every few feet. Certainly there is nothing to provide lift. Number one is the only answer. The only exceptions to this are the rare microgravity-in-atmosphere situations (the “Vomit Comet”, a vessel in orbit, or, say, an elevator with the cable cut). Unless you are flying something inside of something else that is in freefall, it is a pointless question. If the book expands on the answer then it is fine, if it does not then it is misleading.

It appears you were correct swellman, #1 is the wisea$$ answer.
You win the million.

 

[rwald]

Without gravity, airplanes would need some sort of feature to send them back down to the Earth. I mean, they could use normal aerodynamic lift to move upwards (assuming that the atmosphere didn't dissipate), but they couldn't get back down. That's the real problem, as I see it.

 

[Ladewig]

2. No, because lift is a function of only velocity and air density. (this is the book answer)

If there is no gravity, why would an airplane need lift?

Heck, if there were no gravity but enough dense air, a running outboard motor could propel itself forward (with a little rebalancing).

 

[Oso]

OK, let's start over. An airplane typically has wings, flaps, ailerons, elevators, a vertical stabilizer and a rudder. They all would perform, and would be needed for, the exact same functions with or without gravity. As I mentioned above think about what would happen in the ISS.

 

[rwald]

I don't think so. Without gravity to counter lift, the airplane would just fly in a large vertical circle (as someone else pointed out). Also, it would be impossible for the airplane to ever descend, short of aiming downwards and then aiming straight. Flying without gravity would be very different than with.

 

[Oso]

I don't think so. Without gravity to counter lift, the airplane would just fly in a large vertical circle (as someone else pointed out). Also, it would be impossible for the airplane to ever descend, short of aiming downwards and then aiming straight. Flying without gravity would be very different than with.

Here's a link. Go down to the model and play with the controls, and think about what they do.
http://travel.howstuffworks.com/airplane6.htm

A plane here on earth would also fly in vertical circles (at least until it broke up). Stunt pilots do it all the time.

Yeah, it would probably be harder to land, also harder to stop, but the question wasn't about landing.

 

[Walter Wayne]

I think there is a lot more to figuring this out.

For instance, when the plane is in a stable position torque from gravity, thrust, lift (wings and tail) and drag equal out.

Now without gravity, the wings and tail will impart a large upward component and change the angle of attack and drag vector significantly. Assuming the downward (wrt plane) component of drag is primarily due to the surface area of the wings and tail, they may actual be distributed similarly to the lift. However, given the angle of attack would the wings, elevators and ailerons work as advertised?

Also, in terms of the vertical circle, many modern aerobatic and fighter planes are more than capable of pulling -1 g, and supersonic planes have different wing profiles than we are normally taught.

Walt

P.S. Given that one can build a wing using a flat board and angling it with respect to air velocity, I believe we can make a balanced plane for any gravity. However, my mind is curious about a conventional wing in 0 g with air density as normal.

 

[davefoc]

I think Walter Wayne is absolutely right that a plane could easily be designed to fly in zero gravity (assuming the little problem about the atmosphere leaking into space can be overcome).

I'm not sure that a plane designed for an environment with gravity could fly level, climb and descend in a zero g environment.

I believe the issue is whether at some speed and elevator setting the plane can develop more lift out of the tail assembly than the wings. My guess is yes, because it just seems that at some speed with the elevators pointed down the rear of the plane will have more lift than the front of the plane and the plane will pitch forward.

 

[swellman]

P.S. Given that one can build a wing using a flat board and angling it with respect to air velocity, I believe we can make a balanced plane for any gravity. However, my mind is curious about a conventional wing in 0 g with air density as normal.

I agree. This is the most intriguing part of the problem. If one imagines a huge volume of air with no gravity, why build an entire airplane? All that is really needed is a propulusion system. But actually designing an aircraft that can operate in such an environment and then controlling it calls for a fresh approach.

How about a circular wing built around the fuselage containing the engine? Or if you want to be economical, a square with four lifting/control surfaces?

 

[MRC_Hans]

Mmm

Premise 1: Despite the lack of gravity, there is an air-pressure (so we must be inside something).

Premise 2: We are talking about a conventional plane.

Premise 3: By "flying" in this environment we mean move around in the air in a controllable way (since, in zero gravity, anything can fly).

Basically, the answer is yes, a conventional plane might be steered around in such an environment. However, ther would be a number of difficulties (some of which have already been mentioned):

1) The lift generated by the wings would send it off course, and would have to be countered by steering in the opposite directions. Some types of planed might not be capable of generating enough "down" elevator.

2) In order to be steerable, for its control surfaces to work properly, the plane would have to travel at near normal flying speed, even if this is not needed for lift. This would give problems when you wanted to stop.

3) Related to the #2, landing, or docking, would be impossible without special equipment. While the plane could be steered into contact with a landing strip and braked with spoilers or the like, once the speed fell below that where the control surfaces worked, it would float away out of control.

4) Since the directional stability of a normal plane is an interaction between lift and gravity, most planes would be quite unstable and would have to be steered constantly.

5) Last, but not least, a lot of practical problems would exist, in the area of jubrication, fuel feed, carburettors, etc. etc. Even aerobatic planes are built to fly upside up most of the time.

Hans

 

[Houngan]

OK, let's start over. An airplane typically has wings, flaps, ailerons, elevators, a vertical stabilizer and a rudder. They all would perform, and would be needed for, the exact same functions with or without gravity. As I mentioned above think about what would happen in the ISS.

True enough, except that the wings generate the majority of lift inherently, just by their shape. So all the control surfaces would be inadequate to counter that lift. If you wanted to make a plane to fly in a theoretical zero-G-with-atmosphere environment, you'd need to make the wings symmetric on the cross-section so that they didn't generate lift one way or the other. Stunt planes are made this way, because their engines are so grossly overpowered, they can fly by control surfaces alone. (Think horizontal helicopter.)

H.

 

[MRC_Hans]

True enough, except that the wings generate the majority of lift inherently, just by their shape. So all the control surfaces would be inadequate to counter that lift.

No, that is not correct. The wings are (usually) shaped to give lift, but on most planes, the elevator will be able to change the angle of attack (AOA) sufficiently to remove the lift of the wings. Thats what you do when you dive.

Some plane types, especially types optimized to be easy to fly may not have sufficient elevator effect to do this, but any plane with (even moderate) aerobatic capabilities, and fighter planes can do it. In general, I would expect that any plane that is capable of flying inverted will be able to "fly" in zero gravity.

Hans

 

[Walter Wayne]

2) In order to be steerable, for its control surfaces to work properly, the plane would have to travel at near normal flying speed, even if this is not needed for lift. This would give problems when you wanted to stop.

I'm trying to think if the control surfaces do work properly. Whenever wings are discusses their limits are described in terms of an angle where the bottom of the wing is plowing through the air.

In our thought experiment, the top of the wing is plowing through the air. What is the stall angle in this direction and how well to the control surfaces work?

Thats what I meant original about a wierd angle of attack.

Walt

 

[Walter Wayne]

True enough, except that the wings generate the majority of lift inherently, just by their shape. So all the control surfaces would be inadequate to counter that lift. If you wanted to make a plane to fly in a theoretical zero-G-with-atmosphere environment, you'd need to make the wings symmetric on the cross-section so that they didn't generate lift one way or the other. Stunt planes are made this way, because their engines are so grossly overpowered, they can fly by control surfaces alone. (Think horizontal helicopter.)

H.

I believe these plans derive much of the lift from the angle of attack of the wings. Not just the control surfaces.

Walt

 

[Agammamon]

An airplane won't fly in a vertical circle in 0-g. What would happen is the wings would provide a constant upward force (like normal) with no compensating downward force (gravity); this means the plane would continue to rise until it hit something (you could certainly fly it in a vertical circle simply by pitching the nose up to increase the wings angle of attack). What you could do to counter this is when you reach your desired altitude, alter the shape of the wing to eliminate lift thereby allowing straight and level flight. To descend you merely have to alter the wing's shape again to that it is basically upside down and provides "lift" in a downward direction. All of this assumes that you have designated up and down somehow, since in microgravity these directions are pretty much arbitrary.

Of course in 0-g you don't need wings to stay aloft anyway. A small reaction motor would be sufficient to lift you.