Exploring Inverted Flight: Understanding Aerobatic Plane Wings

[Gonzolo]

Hi, is the wing on an aerobatic plane symetrically shaped? Is that why it can fly inverted? Is the angle of attack of the wing the same whether it's flying inverted or right-side-up?

 

[chroot]

The oft-cited Bernoulli effect, due to the asymmetry of the wing, is actually not a major factor in the generation of lift. You are correct that the angle of attack is largely what matters. Airplanes generate lift mainly by pushing air down.

The angle of attack may or may not be the same when the plane is inverted -- the exact angle of attack needed to maintain altitude depends on a host of conditions, including thrust, air density, wing design, and so on.

- Warren

 

[LURCH]

Yes, aerobatic aircraft as well as military fighter planes have symmetrical wing cross sections.

 

[Gonzolo]

Thanks guys.

What about a helicopter, could it fly with a symmetric blade? I assume the pitch-angle of attack could be adjusted. I've heard that small-scale models can do impressive manouvers, but can they maintain inverted flight?

 

[enigma]

Practically any plane with practically any airfoil can maintain inverted flight for a while. In the simulators, one of the test-pilots had the space shuttle do a single barrel roll (much to the shagrin of the engineers). When you're upside down, you simply need to have a negative enough angle of attack. If you were right side up, it would be below the zero-lift angle of attack. This is much easier for symmetrical airfoils (which have a zero degree zero-lift angle of attack), which is why stunt and combat aircraft typically have them. The only thing preventing other aircraft from maintaining inverted flight is structural concerns... not aerodynamic concerns.

The "Bernoulli Effect" which is so often taught to beginners in physics doesn't exist. That situation presupposes that the flows must meet up at the trailing edge of the wing. They don't. The flow on the top side of the wing actually gets there faster in most cases. Bernoulli's equation states that faster air means lower pressure air, and it applies in any incompressible flow... symmetric, unsymmetric, underwater... wherever. You can't just turn it off when the angle of attack is non-zero and say "now we're considering Newton's Third Law only" any more than you can stop considering Newton's second when you start doing analytical dynamics. All of Newton's Laws (which includes Bernoulli... it's derived from N2) apply all the time.

 

[SplinterIon]

This is a little off topic...

It's amazing - but I've had some pretty heated debates with people (one who even was an assistant curator of an aircraft museum) who refuse to hear any explanation for lift other than 'Bernoulli's Effect'. And what's worse is that this stuff is written everywhere - don't believe me - go to your nearest aircraft museum and check out the boards that explain lift.

It's way too ingrained in the current school system in North America -

 

[enigma]

Well, the real reason - the only reason - for lift (or any other force for that matter) is a pressure distribution. N3 is a side effect (air goes down, plane goes up). N3 doesn't explain how the lift happens though. To explain it or quantify it, you need:

[tex]\frac{d}{dt}mV=F[/tex]
[tex]\frac{d}{dt}mV=\int{\int{p * dA}} [/tex]

And to explain the pressure differences, you need Bernoulli or Thermodynamics.