The Aerodynamic Centre

[bibhu]

Why does the aerodynamic moment on the aerofoil not change with the angle of attack of the aerofoil? What’s the magic behind it? Does every flying surface have an aerodynamic centre where the aerodynamic moment is independent of its angle of attack?

 

 

[InigoMontoya]

Who ever said it doesn't?

 

Thought experiment: 90 degree angle of attack.

[MigL]

Typical airfoils are designed to give an aerodynamic centre at 1/4 chord. For supersonic flight, the aerodynamic centre shifts back to 1/2 chord due to lift ( pressure difference ) being caused more by shock wave interaction than air speed difference.

There is some movement from these values at different angles of attack but, for purposes of stability ( pitch divergence ) it is designed to be as small as possible.

 

The 90 deg. example given above is a bad example because turbulence and loss of lift ( stalling ) is reached at very small angles of attack. Even angles of 30 deg. can only be reached by re-energising the airflow over the wing using vortex generators like leading edge root extensions ( LERX) such as on the F-16, F18, MiG-29 and Su-27 families, or foreplanes/canards like EF-2000 or Rafale.

Edited March 27, 2012 by MigL

[Enthalpy]

The aerodynamic centre does move with the angle of attack, even before stalling. In today's profiles it's wide behind 1/4 of the chord. Some profiles are designed to give passive pitch stability to avoid a stabilizer, but that's difficult, needs other sacrifices, and is uncommon.

 

A canard doesn't necessarily help avoid stalling, and can be quite detrimental in this aspect.

 

Root extensions aren't vortex generators - two different methods.