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Flight and gravity

swellman said:
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?
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None of these answers is correct.

The correct answer is: Yes, because without gravity, it would just be coasting, not flying.
:D
 
Boy, there's a lot of misinformation in this thread. Of course an airplane would be able to fly a stable course with no gravity. It would just be flying at a somewhat nose-down attitude, making for a zero angle of attack on the wings. Actually, it wouldn't need to be quite this nose-down, because the wings would need to provide just a little lift to compensate for the down force of the fuselage's pointing down. Any plane which has enough elevator control movement to suddenly go into a zero-g dive could handle this.
 
CurtC said:
... It would just be flying at a somewhat nose-down attitude, making for a zero angle of attack on the wings...
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Manyconventional wings are assymetric, and produce lift with a zero angle of attack. I think it would be difficult to get a conventional wing to have a zero angle of attack in zero g.

Walt
 
It would seem that some combination of low/zero/negative angle of attack on the wings with a properly sized tail control surface in opposing force/moment could achieve stable flight in zero g, assuming the power plant is configured appropriately.

Now the hard part - how do you steer it (in a stable fashion)? Is a coordinated turn easier or harder than in a gravity environment?

I cannot see an obvious answer, but then this stupid problem is making me lose sleep...
 
davefoc said:
I think Walter Wayne is absolutely right that a plane could easily be designed to fly in zero gravity [...]
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Err - zero gravity. Nothing falls. A brick can fly in zero gravity.
 
Fly, in that you can control its direction of motion by altering its control surfaces and speed. And that you can make it head any direction you choose, including down.
 
rwald,

It's not a big deal, but presumably you realize that there is no naturally preferred direction in this gravity-free environment? Hence the word 'down' is meaningless.
 
Maybe "down with respect to the plane" would be more accurate? The plane does have its own coordinate system, so that's the one I was referring to.

Also, this is the third time in so many days I've had my posts corrected on scientific or mathematical grounds. Maybe I should read them over before submitting them...
 
WalterWayne wrote:
Many conventional wings are assymetric, and produce lift with a zero angle of attack. I think it would be difficult to get a conventional wing to have a zero angle of attack in zero g.
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Almost all wings are asymmetric! There are a few wings, such as on stunt planes, with a symmetric airfoil. But I think they all need a positive angle of attack to produce lift. I guess it depends on precisely how angle of attack is defined, but if the wing doesn't throw air downwards as it passes, it can produce no lift.

For the example standard plane in zero g, you just need to tilt it over until it no longer throws air downwards.

A plane in zero g would also require much less power to maintain cruise speed, since its energy isn't being spent throwing all that air down. And it wouldn't be too hard to coordinate turns - to make a level turn, just turn your wings 90 degrees and pull up on the elevator.

This really isn't that hard.
 
The point is (and I think that most would agree with this), that airplanes designed for zero-g would be structurally different from most one-g planes (with symmetric wings, elevators which push down as often as up, etc.), and would also be piloted differently.
 
One thing that stands out is that this is a very poor question to include in homework. The better the student understands flight, the more confused they'll get trying to answer it. It might be useful as a moderated class discussion, but the idea that there is a scientifically 'right' answer is quite disturbing.

My response would be that the question is badly formed - the notion of flying involves the notion of overcoming the tendency to fall under gravity. You can argue that an aeroplane could move along the same path in an atmosphere with or without gravity, but it wouldn't be flying since a brick with an engine could move along that path too. Bricks can't fly, even with engines. In fact, because of lift the direction of thrust would have to be significantly different for an aeroplane to achieve a given speed in the same direction as under gravity.
 
Beausoleil said:
One thing that stands out is that this is a very poor question to include in homework. The better the student understands flight, the more confused they'll get trying to answer it. It might be useful as a moderated class discussion, but the idea that there is a scientifically 'right' answer is quite disturbing.
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Agreed. This question really just drives a 3rd grade student into fishing for the yes/no answer. While I posted the problem because it made an intriguing thought problem, it is a pi$$ poor grade school science question.

Might as well ask something like "if humans could breath underwater, could we walk around on the sea bed?" Well if we ignore the generally accepted principals of evolution, assume some sort of ballasting is present and all agree not to swim - sure we could.
 
swellman said:


Agreed. This question really just drives a 3rd grade student into fishing for the yes/no answer. While I posted the problem because it made an intriguing thought problem, it is a pi$$ poor grade school science question.

Might as well ask something like "if humans could breath underwater, could we walk around on the sea bed?" Well if we ignore the generally accepted principals of evolution, assume some sort of ballasting is present and all agree not to swim - sure we could.
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So, was this question out of a textbook, or did the teacher make it up? If it was from a book, which one?
 
garys_2k said:

So, was this question out of a textbook, or did the teacher make it up? If it was from a book, which one?
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I checked back with my friend whose daughter had this assignment. In his words:

"Good timing.......just got back from the teacher conference.

It was just on a worksheet that had questions on the section they were doing, part of which was gravity.

She (the teacher) also thought it wasn't the greatest question. I gave her the printout of the chat from today and yesterday [i.e. this forum - swellman], she was pretty impressed at all the furvor over the subject. She agreed that it wasn't the greatest question and will cross it off next time."

My misunderstanding - it wasn't a textbook, but a handout worksheet (not authored by the teacher).

Now for my next challenge: convince a friend that she really doesn't need to sleep on a sheet of magnets "for her back"...
 
swellman said:

Now for my next challenge: convince a friend that she really doesn't need to sleep on a sheet of magnets "for her back"...
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Bring her a new bed. Tell her that this new bed has vastly upgraded magnets. They are made of a new material that holds a much larger magnetic field.

After she sleeps on a couple weeks and reports how well it works, then tell her there isn't a magnet in it at all.

Then, she might be ready to listen to why there is no reason to believe that magnets have any effect on bio tissue and that the only people who says it does have a bunch of magnets they want to sell.

Or, she might just be ready to kick your ass.
 
Wings generate lift by being flat on the botom and curve on the top. This generates an area of pressure on top of the wing that is lower than air pressure on the bottom. The higher air pressure below the wing pushes it up.
 
CurtC said:
How does being curved on top create lower pressure?
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Bernoulli's principle. The pressure that a stream of fluid applies against a surface at right angles to it decreases as the speed of the fluid increases.

Putting a convex curve onto the top surface of the wing forces the air travelling over the top to move a greater distance (in the same amount of time) as air moving over the flatter bottom of the wing. That "higher speed on top" creates less pressure there compared to the bottom, so the higher pressure on the bottom forces the entire wing UP.

Pretty cool, eh?
 
But of course that is not the only way wings generate lift. After all, planes can fly upside down, and some, notably the Wright's machine, have flat wings.
 
roger said:
But of course that is not the only way wings generate lift. After all, planes can fly upside down, and some, notably the Wright's machine, have flat wings.
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These situations usually pitch the wings to cause the same effect, faster moving air over the upper surface.
 
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