Page 124 - Aeronautical Engineer Data Book

100 Aeronautical Engineer’s Data Book
Arrow length represents the magnitude of pressure coefficient C
p
P ∞ = upstream
pressure

S

α 5˚
Stagnation point (S)
moves backwards on
the airfoil
lower surface

S

(p – p )
∞
Pressure coefficient C =
p
1 V 2 α 12˚
2
Fig. 6.4 Airfoil pressure coefficient (Cp)
6.4 Aerodynamic centre
The aerodynamic centre (AC) is defined as the
point in the section about which the pitching
) is constant, i.e. does
moment coefficient (C M
). Its theoreti­
not vary with lift coefficient (C L
cal positions are indicated in Table 6.1.

Table 6.1 Position of aerodynamic centre
Condition Theoretical positon of the AC

< 10° At approx. 1/4 chord
somewhere near the chord line.
Section with high At 50% chord.
aspect ratio
Flat or curved plate: At approx. 1/4 chord.
inviscid, incompressible
flow