The key to understanding the effect of center of gravity (CG) location on airplane pitch stability depends on thoroughly believing that an airplane in flight rotates around its CG. This is an oft repeated datum, but one that is disbelieved almost as often as it is heard, due to confusion about the wings, lift, the tail and their interrelationship with the CG.
A BASIC MISCONCEPTION
If one pictures a simple side view of an airplane in flight, it is accurate to visualize lift as a string that holds the plane aloft, attached about 30% back from the leading edge of the wing. The attach point of the string is standardly called the “center of lift.” Visualized in this manner, it is a common leap to conclude that the airplane pivots around this point.
Since we agree an airplane actually pivots around the CG, this conclusion would be accurate only when the CG is located precisely in line with the center of lift. When the CG is anywhere else, ahead of or behind the center of lift, AIRPLANES DO NOT PIVOT AROUND THE WING.
THE TRUTH ABOUT PITCH STABILITY
Last month, I defined static stability as the initial tendency of a system to resist disturbance, and dynamic stability as the tendency of a system over time to return to its original state after a disturbance.
The most popular explanation of airplane pitch stability incorporates a second major misconception, suggesting that dynamic stability (return to starting flight attitude) is the most important consequence of CG location. As stated last month, this is incorrect: THE KEY IMPORTANCE OF CG LOCATION IS ITS EFFECT ON STATIC, NOT DYNAMIC STABILITY. An understanding starts with the concept of a weathervane.
Weathervanes are systems designed to turn, or face, into the relative wind. Begin by imagining the simplest type of weathervane, an arrow. Seen from the side, the arrow has a point at one end and feathers at the other. If you mount the arrow on a vertical shaft and allow it to pivot around the mounting point, you may have created a weathervane. Whether or not the arrow actually rotates into the relative wind depends on the location of the pivot. If the pivot is placed somewhat forward, such that the arrow structure has the most air resistance on the feather side, the arrow will weathervane. If the pivot is moved back toward the feathers, there will be a point when the air resistance on both sides is balanced, with the result that the arrow will not rotate at all when blown on from the side. We can call this point the “aerodynamic center” of the arrow.
By extension, if the pivot is moved even further aft, the arrow will pivot “backwards” and face away from the wind.
This simple visualization is key to understanding static pitch stability. Picture the plane as an arrow. Because we are examining pitch stability, visualize the plane mounted on a horizontal pole, the lateral axis, that passes through the CG. As the nose is raised and lowered, the plane pivots on this axis.
Like the arrow, the airplane has an aerodynamic center: a location where the total air resistance of fuselage, tail, and wing is perfectly proportioned front and rear. As with the arrow, when the pivot point (the CG) is placed in front of this location, the plane will rotate (pitch) up or down into the relative wind.
If the CG is placed at the exact aerodynamic center, the plane will not weathervane. In practical terms, this would mean that when you raise the nose and increase angle of attack, the nose will not tend to drop back down on its own. When you lower the nose and reduce angle of attack, it will not want to push back up. This is called “statically neutral” and would be a genuine pain to fly.
THE PAYOFF
Common sense and the regulations require that airplane designs must be statically stable, and you now possess the key to understanding the importance of CG location: the aft limit must never be as far back as the aerodynamic center of the plane. As long as this rule is followed, your plane will have a tendency to self-correct from pitch inputs, leading inevitably to a life of milk and honey and lottery jackpot wins.
