Movement of aerodynamic centre

[QJB]

Hi just having a bit of trouble understanding this concept.

So the aerodynamic centre of an aircraft is the point at which the pitching moment does not change with a change in angle of attack. Similar in nature to the centre of gravity. I can understand this for incompressible, inviscid flow but what about when the effects of friction and compressibility are considered? Wouldn't the moment change at high angles of attack due to flow seperation affecting the pressure distribution over the wing?

Cheers,

J

[HarryMann]

Yes, I imagine it was nominally a subsonic parameter for 2D flow. It is a useful concept though, rather than an actuality or exact position. And is used. Its usually between .2 mac and .3 mac

[stubby1]

In supersonic moves back to 50% mac:ok:

[photofly]

It does move around; but not by very much in normal flight for most airfoils in use. See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090937_1993090937.pdf for some graphs.

[Jane-DoH]

stubby1

In supersonic moves back to 50% mac:ok:

At least on a straight wing, on a swept wing I don't think you get a full shift to the 50% chord until the shockwave angle off the root equals the sweep angle of the wing.

[Pugilistic Animus]

It all really depends on many things the Rn,...the Mach number ...the NACA designation, trailing edge angle...no simple answer really exists to the original question....it's experimentally determined as is most of aerodynamics:)