RC Aircraft Proving Grounds - Aerodynamic Center Directional Stability

BACK to AC Homepage.

This web site is designed for 800 x 600 screens with 16 bit resolutions.

It's best viewed with

Click Here To Start.

Aerodynamic Center
Directional Stability

Static directional stability is a measure of the aircraft's resistance to slipping. The greater the static directional stability the quicker the aircraft will turn into a relative wind which is not aligned with the longitudinal axis.

The main contributor to the static directional stability is the fin. Both the size and arm of the fin determine the directional stability of the aircraft. The further the vertical fin is behind the center of gravity the more static directional stability the aircraft will have. (This is often called the weather veining effect, because it works the same way as a weather vein.)

As mentioned previously all rotational motions of the aircraft occur around the center of gravity. Directional stability refers ro motions around the normal axis.

The Size and Arm of the Fin Determines Directional Stability

Watch the diagram to the left. It will change if you click on the links within this paragraph.

If a gust of wind strikes the aircraft from the right it will be in a slip.

The fin now has an angle of attack.

Which causes the aircraft to yaw until the slip is eliminated

You can also see this effect in the movie on the previous page.

Dynamic Directional Stability

When the aircraft slips the fin will turn the aircraft back into the relative wind. In most aircraft this tendency is nearly dead beat. This means that the aircraft does not overshoot and yaw back and forth much, it just turns quickly into the relative wind and stops there. However, if the aircraft has a lot of angular moment it will overshoot and slip in the opposite direction. The fin will then yaw it back and so on. The nose may wind up yawing back and forth several times before it settles down to non-slipping flight.

The amount of dynamic directional stability depends upon the static directional stability, as stated above and, upon the moment of inertia of the aircraft (around the yaw axis.) An aircraft with most of the weight concentrated in the fuselage will have a low moment of inertia. But, an aircraft will a large amount of fuel in the wings and several engines mounted out on the wings will have a much higher moment of inertia and therefore will tend to have less dynamic directional stability.

In the above movie the aircraft has tip tanks. This give the aircraft much more moment of inertia than the aircraft in the movie on the previous page.

Pilots should expect conventional twins to display less dynamic directional stability than single engine aircraft due to the greater moment of inertia of the twin.