RC Aircraft Proving Grounds - Aerodynamic Center Stabilizer & Longitudinal Stability

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Aerodynamic Center
Stabilizer & Longitudinal Stability

Longitudinal stability refers to stability around the lateral axis. It is also called pitch stability.

Longitudinal stability depends on the location of the center of gravity (c of g.) This is the most important thing to realize as a pilot.

If the aircraft is loaded within the approved c of g envelope it will have positive static longitudinal stability. That is critical, because an aircraft with negative longitudinal stability would be impossible to fly for more than a few moments. It would require tremendous concentration to avoid over controlling such an aircraft.

Just as with directional stability, discussed previously, longitudinal stability depends on the weather veining. The only complication is that for most aircraft there are two wings producing lift affecting the longitudinal stability, whereas the directional stability depended only upon one wing, the fin.

Additionally, longitudinal stability is compromised by the fact that the main wing must be close to the c of g since net lift must act opposite to weight. Conversely directional stability was easier to achieve because the fin was placed well behind the c of g and was not required to produce any force under normal flight conditions.

From the above you can easily guess that the stabilizer must be a major contributor to longitudinal stability. This is in fact the case.

Most aircraft would be completely unstable without the horizontal stabilizer. The stabilizer provides the same function in longitudinal stability as the fin does in directional stability. When the angle of attack changes it tends to pitch the aircraft back to its original angle of attack. The main wing, on the other hand, may be stable, or unstable, depending on the exact location of the c of g. We will consider the main wing's contribution to stability on the next page.

The movie below shows how the stabilizer keeps the aircraft flying at the original angle of attack. In this movie the aircraft remains at zero degrees angle of attack.

Trim Angle of Attack

The tail is used to trim the aircraft to maintain a certain angle of attack. Once a given elevator deflection is established the aircraft will stay at that angle of attack regardless of changes in wind direction or airspeed.

The next movie shows how the tail keeps the aircraft flying at a given angle of attack. In this example the wing is at approximately 10 degrees angle of attack. You can cause a wind gust which will momentarily change the angle of attack. However, because the aircraft is stable it will quickly return to the trimmed angle of attack. You should be able to surmise that this will happen even though the airspeed might change.