Page 122 - Aeronautical Engineer Data Book

98 Aeronautical Engineer’s Data Book
U 2
2
Drag (D) per unit width = C l
D
2
Moment (M) about LE or
U 2
2
1/4 chord = C l
M
2
per unit width.
C , C and C are the lift, drag and moment
D
M
L
coefficients, respectively. Figure 6.3 shows
typical values plotted against the angle of
attack, or incidence, ( ). The value of C is
D
is often used for the
small so a value of 10 C D
rises towards stall point
characteristic curve. C L
and then falls off dramatically, as the wing
rises gradually,
enters the stalled condition. C D
increasing dramatically after the stall point.
Other general relationships are:
• As a rule of thumb, a Reynolds number of
6
Re 10 is considered a general flight
condition.
increases steadily for
• Maximum C L
7
5
Reynolds numbers between 10 and 10 .
decreases rapidly up to Reynolds
• C D
6
numbers of about 10 , beyond which the
rate of change reduces.
• Thickness and camber both affect the
that can be achieved. As a
maximum C L
increases with thickness
general rule, C L
and then reduces again as the airfoil
generally
becomes even thicker. C L
increases as camber increases. The
achievable increases fairly
minimum C D
steadily with section thickness.
6.3 Pressure distributions
The pressure distribution across an airfoil
section varies with the angle of attack ( ).
Figure 6.4 shows the effect as increases, and
the notation used. The pressure coefficient C p
reduces towards the trailing edge.