Derivatives 21 Part 2

Static Stability Derivatives (part 2)

Forces induced by aircraft rotation

Whenever the aircraft rotates it rotates about its center of mass. This is so by definition, as any other rotation is interpreted as a combination of movement and rotation. Movement means movement of the center of mass and rotation means rotation about the center of mass. This definition means that even burning fuel is interpreted as movement (unless the center of mass just happens to stay put). The FAR in-editor panel shows three columns with rotation derivatives — pitch up rotation (q), roll right rotation (p) and yaw right rotation (r).

The motivation for calculating aircraft rotation derivatives is that, as the aircraft rotates the movement (speed and direction) of the individual aircraft parts, such as fins, wings and fuselage parts is affected. As a result some (additional) force is applied to the aircraft. The (additional) force is seldom that large and it ceases as soon as the rotation stops. As such a disadvantageous rotation derivative is unlikely to prevent your aircraft from flying. The rotation derivatives are still interesting, in the sense that beneficial rotation derivatives are a nice luxury.

A well-known disadvantageous rotation derivative, known as 'adverse yaw', is so common that it is almost considered a flawed airplane design when not addressed.⁽¹⁾

Resistance to rotation (heaviness), Mq, Lp and Nr

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Longitudinal and lateral finesse, Zq, Xq, Yp and Yr

-- TODO --

Adverse yaw, Np

-- TODO --

Adverse roll, Lr

-- TODO --

⁽¹⁾: I wanted to cite a quote backing up my statement, but did immediately find any good ones stating that adverse yaw is an airplane design flaw; maybe my comment is a bit of an exaggeration then. I did however find a nice explanation of adverse yaw in general at kitplanes.com.