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I've read many times, including here on this very site that the commonly known explanation of flight is wrong, and that airplanes can fly because the shape of their wings deflects air down. This makes sense, but as far as I can tell it doesn't explain upside down flight or symmetric wings.

The images I've seen show an inclined wing, which forces the air to go downwards. But how can planes fly upside down then?

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2 Answers

Colin's answer is right. Let me see if I can clarify a little bit.

First, forget that old Bernoulli explanation. It's not wrong, but it confuses everybody.

If you create a simple symmetrical teardrop-shaped airfoil, and place it in a wind stream, then the air will flow past it, and it you turn it at an angle to the wind, it will deflect the wind stream, and it will feel a lateral force. The vertical stabilizer of any airplane (with or without a rudder) works that way.

Now make an airplane's wings the same way - symmetrical airfoil. Then the plane can create positive lift by tilting the nose up, and negative by tilting it down.

Some aerobatic aircraft are designed exactly that way, as in this example. They can fly inverted just as easily as normally.

Most airplanes are not designed for inverted flight, so they use an airfoil designed for efficient upright flight, but they can still develop negative lift if called upon to do so. (Example: The Cessna 172, a typical small plane, is stressed for 4g positive, and 2g negative.) ASIDE: That means if the airplane, as loaded, weighs 1000kg, it is designed to withstand upward force of 4000kg and downward force of 2000kg. The latter can happen if the pilot stumbles into a downdraft at even a normal cruising speed.

Fun Fact: Inverted flight is interesting for other reasons as well.

  • the ground is above you, and you have to press "nose down" to maintain altitude or climb.
  • to turn left, you have to bank right, and vice-versa.
  • to turn to a heading, you have to realize that the compass is backwards.

So it takes practice!

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@LarianLeQuella: I'd be interested to see if you have anything to contribute to this. Is there a way to "think about" piloting inverted that keeps you out of trouble? –  Mike Dunlavey Oct 20 '11 at 22:29
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Had an F-15 Instructor pilot explain to me what keeps airplanes in the air. That is money and gasoline. Neither money nor gasoline care which way up is. And that is why some planes can fly inverted and others can't. Money (design) and gasoline (engine) –  Jerry Oct 21 '11 at 1:53
    
@Jerry: Gasoline does care in same cases. Or rather, the fuel pumps do. –  Zan Lynx Oct 21 '11 at 2:43
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@Jerry: Aerobatic planes need special fuel tanks. Rather than having an outlet at the bottom, they have a hose with a weight on the end, so no matter which way is "down" that's where they drain from. At least that's one way it's done. They also need a special vent arrangement. Then there's the issue of oil sump and carburetor (if they're not fuel-injected). –  Mike Dunlavey Oct 21 '11 at 3:25
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Upside-down or right side up, flight works the same way. As you stated, the wing deflects air downward. When inverted, the pilot simply controls the the pitch of the aircraft to keep the nose up, thus giving the wings sufficient angle of attack to deflect air downwards.

Most airplanes are designed with some positive angle of attack "built-in," meaning that there is some angle between the wings and the fuselage so that the wings have a small positive angle of attack while the fuselage is level. This is why the floor isn't tilted tailwards when you're in an airliner in level flight. So when upside down the nose has to be held a bit higher than usual, and the other flight systems (including the pilot!) must be designed to handle it, but there is nothing really special about upside-down flight.

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So the idea is that upside down flight is not exactly upside down, because if it was then the wings would be pointing the wrong way? –  Javier Badia Oct 20 '11 at 19:50
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@Javier - yes it's not perfectly level upside down. Although some aerobatic planes with almost symetrical wings can get very close. –  Martin Beckett Oct 20 '11 at 19:57
    
I'm not sure what you mean by "not exactly upside down." The aircraft can roll completely over if it is designed to do so safely. –  Colin K Oct 20 '11 at 19:58
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Oh because its got its nose raised? I wouldn't really say that this means it's not upside down. The nose is often raised during regular flight. It just depends how fast you're flying, and what else is going on. You can fly a Cessna 150 at something like 45 knots on full flaps; you'll have the throttle turned way up and the nose will be pointed like 20 degrees above the horizon, but its still flight:) –  Colin K Oct 20 '11 at 20:02
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The whole point here is that there is absolutely no difference between regular and inverted flight. In both cases the wings must be angled to deflect air downwards. The angle that the fuselage takes when the wings are angled properly is simply a function of the way the plane is built. –  Colin K Oct 20 '11 at 20:42
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protected by Qmechanic May 8 '13 at 21:44

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