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View Full Version : How come airplanes can fly inverted?

natedog74
3rd Nov 2003, 09:48
If the airfoil is upside-down and so the camber is opposite of what it is in non-inverted flight, doesn’t Bernoulli’s law mandate that the difference in pressures would exert DOWNWARD force? In like manner, Newton’s law would have the air deflected UPWARD. Resultant ‘lift’ force would therefore act in addition to, not opposite the force of gravity. Or would it?

FatFlyer
3rd Nov 2003, 10:39
IF you are inverted, the nose attitude has to be much higher ie stick pushed further forward to get enough angle of attack to get the lift than you would for normal flight or have symetrical wings

Flight Detent
3rd Nov 2003, 19:42
Yeah Mate!,
The upside down aircraft will always have a much greater angle-of-attack to offset the effect of the wing.
If you have ever seen a couple of aircraft flying the 'mirror' formation, that is, one inverted and the other right way up, together, you can easily notice the difference in their attitudes.
Cheers

Fat Dog
3rd Nov 2003, 20:44
Unless you happen to be flying something like a CAP232 with symmetrical wings...

The African Dude
3rd Nov 2003, 20:47
Well, a door will generate lift through circulation if you fly it at the right angle of attack! But that's a silly example.

The camber on the wing allows for the aerofoil to be more efficient at producing lift at normal (non-inverted) angles of attack.

As the AofA increases the lift generated also does, as you know... when flying inverted there is less lift generated due to the shape of the wing - you're correct.. but increase the angle of attack and you can still generate enough to support the aircraft. Bernoulli's still works - the camber isn't significant enough to stop that. Maybe if you were flying inverted at flaps forty you might have a bit of a problem (those who can actually fly correct me?:O)

That's why you can fly inverted, but only at greater angles of attack!

(I think that's right, anyway.)

Muchos regardos

Andy

VivaTheBeaver
3rd Nov 2003, 20:55
Check out NASA's foilsim program at:

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

You can create an airfoil without camber or with negative camber then adjust the angle of attack to see how much lift would be generated.

vtb

Miserlou
4th Nov 2003, 03:54
Viva,

To which Beaver do you refer? The type in general or a specific example?

Noise Unit
4th Nov 2003, 17:58
3 points.

1) The leading edge of subsonic aerofoils is rounded. As the angle of attack is changed, the stagnation point moves around this leading edge changing the relative distances to the trailing edge and hence the velocity vector of the flow leaving the trailing edge. Therefore, when inverted you have to move the stagnation point further round the leading edge to overcome th camber and generate lift (it would be easier to explain with a diagram).

2) While some aircraft use a symmemtrical aerofoil, the wing is usually set at an angle relative to the body to ensure that the wing produces lift with a reasonable body attititude. Hence an aircraft with a symmetrical aerofoil will have to have a greater body angle when inverted to get the same angle of atack on the wing.

3) The other point to recognise is that you need to have a fuel feed system that will continue to function while inverted. If not the inverted flight tends to be rather short!!

Specnut727
5th Nov 2003, 12:51
I like Fat Dog's comment on the Cap 232, and I've noticed this on Yaks, Sukhois, Extras and Edges.

At an airshow some time ago I saw a 'champion' pilot fly a Sukhoi the full length of the display area with the wings vertical, and lots of angle of attack. Not very efficient, but obviously enough grunt to counter all that drag.

ft
7th Nov 2003, 18:50
First off, Bernoulli is less responsible for lift generation than Newton. As a previous poster stated, a barn door with an angle of attack will fly with enough thrust/airspeed. The distance from stagnation point to trailing edge does not really matter. What matters is how much the wing deflects the air down.

The mounting angle of the wing is called angle of incidence and will indeed typically mean that aerobatics aircraft with symmetrical airfoils will still have to hold a steeper pitch attitude when inverted. In fact, great care is taken in designing the visual appearance of aerobatics aircraft to make sure they appear to be aligned with the direction of flight for the typical manoeuvres, as this is a key factor in scoring.

Flying with the wings vertical is called knife edge flying. The vertical component of thrust and the lift generated by the fuselage keeps the aircraft in the air. Takes a bit of rudder authority... but I’ve often seen them cheating, having a “mere” 80-85 degrees of bank. ;)

Cheers,
Fred

Oktas8
8th Nov 2003, 13:45
This debate rears its ugly head quite often doesn't it?

IMHO Newton's laws offer an excellent description of why a wing generates lift, but it has weaknesses. Bernoulli's principle offers an excellent description of why a wing generates lift, but it has weaknesses. That summarises much of the argument between the proponents of the theories.

Aircraft Flight: A Description of the Physical Principles of Aircraft Flight by Barnard & Philpott contains a simple yet accurate description of why aircraft fly. I've quite enjoyed reading it.

Along the way - a wing of infinite span (or one bounded by wind tunnel walls) is more efficient at generating lift than a real world wing. But it has less downwash... So there's more to it than talking about deflected air as one would imagine.

This is a great forum isn't it? But now I've taken us off topic...
O8