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Old 8th Jan 2011, 12:15
  #17 (permalink)  
CliveL
 
Join Date: Dec 2010
Location: Europe
Age: 84
Posts: 286
As D120A already pointed out, there is a moment Cm0 in the aerodynamic center. This in turn implies that it is not exactly the point where the Lift is excerted, otherwise you wouldn't have a moment at this point. Normally the aerodynamic center is somewhat in front of the Center of Lift (i.e. Center of Pressure). The pitching moment of Airfoils usually increases with increasing AoA.
I don't know what you mean by "in" the aerodynamic centre. Cmo is by definition the pitching moment at ZERO lift and the concept of centre of pressure in conjunction with zero lift is meaningless. As I said, Cmo is produced by a couple, which has no centre of pressure.
You can't talk of airfoil pitching moment without also specifying the point about which the moments are being measured. Usually this is taken as 25% chord for the precise reason that as incidence (AoA) changes the pitching moments about 25% chord don't vary much because the lift acts near there.

Most airfoils have positive camber (mid-thickness line above the line joining LE and TE) and this produces a negative Cmo, so if you measure pitching moments about 25% chord when the airfoil is lifting and that lift acts at the aerodynamic centre (also at 25% chord) then the pitching moment will remain equal to Cmo and the effective centre of pressure will be located at Cmo divided by lift coefficient aft of the measuring point. This means, as has been said, that the CP is behind the aerodynamic centre but that it moves towards the aerodynamic centre as lift increases.

How the pitching moment varies with increasing AoA depends entirely on the datum you assume for pitching moment. If you choose somewhere between the leading edge and 25% chord then the pitching moment will become more negative as AoA is increased. If you choose a point aft of 25% chord the moment will become more positive (nose up).
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