Game Development Reference
In-Depth Information
Figure 10-26 shows moment coefficient data for the NACA 2412 airfoil.
1
The moment coef-
ficient shown in the figure is relative to the quarter chord point. It is fairly constant for angle of
attack values ranging from -10 to 10.
Figure 10-26.
Moment coefficient data for the NACA 2412 airfoil
While the moment about the aerodynamic center of an airfoil is relatively independent of
the angle of attack, this is not the case if the center of rotation of the moment is evaluated
somewhere else on the airfoil. The moment taken about the center of gravity, for example, will
change if the angle of attack is changed.
Trim
Figure 10-26 showed the moment that can be generated in an airfoil, but the same considerations
can be applied to an airplane as a whole. Consider the tailless airplane shown in Figure 10-27.
The lift from the wing acts through the center of pressure of the wing. If the center of gravity is
aft of the center of pressure, a positive (counterclockwise) moment is created about the center
of gravity that will pitch the nose of the plane upwards. Obviously, if this moment isn't coun-
terbalanced, the plane will flip over backwards.
To counteract the moment generated by wing is the reason planes have a horizontal stabi-
lizer as part of the tail. The horizontal stabilizer is a lifting surface; in effect it is a small wing.
When a tail with a horizontal stabilizer is added to the airplane, as shown in Figure 10-28, the
lift generated by the horizontal stabilizer creates a negative (clockwise) moment that pushes
the nose of the plane down and counteracts the moment generated by the wing.
