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GATE 2020 AE – Question 22

Flight Mechanics & Space Dynamics · Static stability: stability and control derivatives, longitudinal, directional and lateral stability, hinge moments and stick forces · 1 mark · Multiple choice

For a conventional airplane, which signs apply to $C_{l\delta_r}=\partial C_l/\partial\delta_r$ and $C_{n\delta_r}=\partial C_n/\partial\delta_r$, where δr is rudder deflection?

  1. $C_{l\delta_r}>0,\ C_{n\delta_r}<0$
  2. $C_{l\delta_r}<0,\ C_{n\delta_r}>0$
  3. $C_{l\delta_r}<0,\ C_{n\delta_r}<0$
  4. $C_{l\delta_r}>0,\ C_{n\delta_r}>0$

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Correct answer: (A) $C_{l\delta_r}>0,\ C_{n\delta_r}<0$

Explanation

Using the control-deflection convention intended by the source, rudder-generated vertical-tail side force gives a negative yawing moment and, because the tail-force line acts above the centre of gravity, a positive rolling moment. Therefore A. The signs depend on the defined positive rudder deflection; the source assumes the conventional aerodynamic control convention rather than explicitly drawing it.

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