RC Aircraft Proving Grounds - Aerodynamic Center Main Wing Stability

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Aerodynamic Center
Main Wing Stability

Most aircraft have a main wing and a separate stabilizer. On the previous page we learned that the stabilizer is a major contributor to the longitudinal stability of the aircraft (hence the name.) On this page we will consider the contribution of the main wing.

If the main wing was well behind the c of g, as the stabilizer is, it would be just as stabilizing. Unfortunately, the wing must be much closer to to the c of g, since we need to generate a lift force which will oppose weight. Remember that weight acts at the c of g by definition.

The movie below shows how the wing is stable if it is behind the c of g.

Main Wing Stability

Earlier we learned that if the wing is behind the c of g it is stable. Now we can be more precise and say that if the ac of the wing is behind the c of g the wing is stable. Use the movie below to clarify the concept.

If the ac is ahead of the c of g the wing will be unstable. This concept is explained in the movie below.

Stability Summary

When the aerodynamic center is behind the c of g the wing is longitudinally stable. When the aerodynamic center is ahead of the c of g the wing is unstable. If the ac is exactly at the c of g the wing will have neutral longitudinal static stability.

Pitching Moment Vs. Angle of Attack

To facilitate a graphical analysis of the stability on an aircraft we will use Coefficient of Pitching Moment vs. Angle of Attack graphs.

Pitching moment is calculated using an equation which is very similar to the lift or drag equations we have used earlier. However, keep in mind that the earlier equations generated units of force. Moment on the other hand has units of force times distance. Therefore, it should not surprise you that the equation for pitching moment is:

M = C_M x S x ½pV² x c (c = arm, C_M = Coefficient of pitching moment.) By definition positive values of M are nose up moments, and negative values are nose down moments.

In the graph to the left we see that as C_L increase C_Mdecreases. In other words as angle of attack increases a nose down moment develops. This is a stable situation.

As long as the C_M vs. C_L graph has a negative slope the wing is stable. The steeper the slope the more stable the wing.