Physics of inverted flight
 

This is probably a stupid question, but here goes:

As we all know, many aircraft are capable of flying inverted and performing all sorts of aerobatic stunts.

As I understand it, wings keep the aircraft aloft due to the lower pressure maintained over the top of the wing as it is curved.

How is it, then, that planes flying inverted are capable of maintaining a positive rate of climb? Surely the overwheling force on the wings is downwards?

And what about stalls? Surely the airflow over the inverted wing is going to get messed up very quickly?

I know there is a logical explanation to this, but I can't figure it out with my limited knowlege... :)


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There's nothing like an airport for bringing you down to earth.

Most non-aerobatic aircraft have, for various aerodynamic reasons, cambered wings. That means that if one imagines a centre line through the cross section of the wing travelling from the leading to trailing edge that line is curved. A wing this shape can produce lift upside down (but is not very good at it), because much of the lift is derived from the angle of attack at which the wing is presented to the oncoming airflow, so that if the horizontal stabiliser is used to make sure that in inverted flight that there is a positive angle of attack, lift will be produced.
Aerobatic aircraft tend to have non-cambered wings i.e. the cross section looks identical which every way up it is. As long as an appropriate angle of attack is maintained this sort of wing can produce as much lift inverted as it can the right way up.
If lift were solely a result of the cross sectional shape of the wing, then no cambered wing aircraft could fly upside down. But it isn't. It has more to do with a a wingish shaped construction maintaining a positive angle of attack.

Thanks for your informative answer!

keendog wrote: "Most non-aerobatic aircraft have, for various aerodynamic reasons, cambered wings."

Just to amplify that a little, the asymmetry of a wing is by no means essential in producing lift.

A symmetric aerofoil produces plenty of lift as you give it an angle of attack. Typically you get a lift coefficient (just think of it a a measure of the lift) of about 0.1 per degree of AOA. So the lift coefficient is +0.5 at 5 degrees up, zero at zero degrees and -0.5 (pushing down) at 5 degrees down.

If you camber the aerofoil, you offset the angle at which you get zero lift. So you might get +0.5 at 3 degrees up, +0.2 at 0 degrees AOA, zero at 2 degrees down and -0.5 (pushing down) at 7 degrees down.

Why bother to camber the aerofoil at all? Why not just give all aeroplanes symmetric aerofoils and get them to fly at slightly higher AOAs? Well the minimum drag tends to occur close to zero degrees AOA: so you can get a better lift/drag ratio by cambering the aerofoil so its AOA is zero (or close) at the typical lift coefficient you want it to be flying at most of the time.

Here is another question. They say a symmetrical wing produces no lift at zero AoA. Just wondering what actually happens inflight if I was to lower my nose to try and achieve this?

Initially, you would start to descend. But at least two other factors spring to mind (there are probably others):

1. The descent would alter the angle of attack as the air started to flow slightly up relative to the wing, and the wing would then start to create lift again.

2. As you descend, your speed would increase. As that happened, the wing would create more lift. Lift is proportional to the square of IAS (might be TAS - I could never remember).

So I reckon that to maintain the zero lift condition, you would have to continually lower the nose. Presumably there is some limiting condition to this process, or else you would just end up in a vertical dive.

Isn't aerodynamics fun! :)

[This message has been edited by low flyer (edited 19 September 2000).]

NASA have already designed an aircraft to do this, a modified 707 used for zero g training of space crew, popularly known as the 'Vomit Comet'.

By accelerating to Vne doing a 3g pull up to near vertical, the flight controller takes over and pushes the stick to give zero lift until its nearly in a Vne dive. It then repeats the process (hence the name).

With zero lift you experience 0g and hence follow a parabolic trajectory with the vertical speed falling at 10 m/s^2(about 20 knots/s) this is sustained for 20-30seconds.

Hi folks,

Just to throw a bit more in about a symmetrical wing producing no lift at zero AoA.

I do glider aerobatics, and have flown many times in an aircraft called the Fox. It is a highly aerobatic glider, with a symmetrical wing section (similar performance both erect and inverted).

I know that this glider is very easily stalled in a nose-up and nose-down attitude. It is a weird feeling when you drop the nose sharply, and then kick in a bit of rudder.......you flip straight into an inverted spin!

Barney