Page 125 - Aeronautical Engineer Data Book

Basic aerodynamics 101
Using common approximations, the following
equations can be derived:
9 d
x AC
= – (C Ma )
c c dC L
where C Ma = pitching moment coefficient at
distance a back from LE
x AC = position of AC back from LE.
c = chord length.

6.5 Centre of pressure
The centre of pressure (CP) is defined as the
point in the section about which there is no
pitching moment, i.e. the aerodynamic forces
on the entire section can be represented by lift
and drag forces acting at this point. The CP
does not have to lie within the airfoil profile
and can change location, depending on the
magnitude of the lift coefficient C L . The CP is
conventionally shown at distance k CP back from
the section leading edge (see Figure 6.5). Using

Lift and drag only cut at the CP
C

Lift

M LE M AC
Drag

x AC
Aerodynamic centre
Lift

M Drag

k CP
Centre of pressure (CP)
Fig. 6.5 Aerodynamic centre and centre of pressure