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we are analyzing an aircraft's lateral-directional control characteristics with self-made code. We have observed that the aileron's influence on the body axis yaw moment is quite sensitive to the angle of attack.

My question is if this makes any sense or if there is something wrong.

For Alpha = 0°

Alpha = 0°

For Alpha = 5°

Alpha = 0°

Edit: our model analyses the effects of the individual effects we simulated in CFD, relying on the superposition principle. For every component (including the ailerons) the following operations are made:

\begin{equation} \label{eq:decompforcas} \begin{bmatrix} F_x \\ F_y \\ F_z \end{bmatrix} = \begin{bmatrix} \cos{\alpha}\cos{\beta} & -\cos{\alpha}\sin{\beta} & -\sin{\alpha}\\ \sin{\beta} & \cos{\beta} & 0\\ \sin{\alpha}\cos{\beta} & -\sin{\alpha}\sin{\beta} & \cos{\alpha} \end{bmatrix} \begin{bmatrix} -qSC_D \\ qSC_Y \\ -qSC_L \end{bmatrix} \end{equation} \begin{equation} \label{eq:momentodocomponente}\begin{bmatrix} 0 & l_z & -l_y\\ l_z & 0 & -l_x\\ -l_y & l_x & 0 \end{bmatrix} \begin{bmatrix} F_x \\ F_y \\ F_z \end{bmatrix} + \begin{bmatrix} -qScC_l \\ qScC_m \\ -qScC_n \end{bmatrix} = \begin{bmatrix} M_x \\ M_y \\ M_z \end{bmatrix} \end{equation}

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  • $\begingroup$ You use the same reference length for all moments, which is uncommon. Normally, c$_m$ is multiplied with l$\mu$ and both c$_l$ and c$_n$ are multiplied with wingspan b or ½b. The adverse yaw over AoA looks wrong but without the code and the CFD results I cannot tell you why. $\endgroup$ Jul 31 at 17:56
  • $\begingroup$ You're right about the reference length. I missed that when writing the equations, but it is done right practice, thanks! $\endgroup$
    – rubemnobre
    Jul 31 at 18:28
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That coukd very well be, with the up-going aileron in the “shade” of the wing’s upper surface. The adverse yaw effect from aileron hooking described in this answer mentions that the effect depends on angle-of-attack or camber of the wing.

At zero AoA and zero camber there would be no yawing moment due to aileron deflection.

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  • $\begingroup$ Actually, our model is based on the superposition of the effects we simulated in CFD. As such, this doesn't include the aerodynamic effect you mention. My guess is it is related to the aerodynamic-body axes transformations. $\endgroup$
    – rubemnobre
    Jul 31 at 16:21
  • $\begingroup$ @rubemnobre Normally adverse yaw increases with angle of attack, since induced drag grows with AoA as well. The pyplot figures look wrong and a factor of more than 2 cannot be explained by an error in the transformation. $\endgroup$ Jul 31 at 17:53
  • $\begingroup$ Good observation on how the transformation cannot explain a factor of more than 2. $\endgroup$
    – rubemnobre
    Jul 31 at 18:31
  • $\begingroup$ Could it be that we only analyzed the aileron coefficients for a single angle of attack and assumed that it would vary the same in different angles? $\endgroup$
    – rubemnobre
    Jul 31 at 18:32
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    $\begingroup$ Linear extrapolation of aerodynamic functions would not work. $\endgroup$
    – Koyovis
    Aug 3 at 5:20

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