Suppose we have an aircraft flying in steady, level flight at zero angle of attack. In this configuration, the velocity vector $\vec{V}$ is perfectly aligned with the $x$ axis. At some time $t=t_0$, the pilot applies a finite positive rudder input $\delta_{r}$ which produces a yawing moment on the aircraft. My question is what will happen to the velocity vector immediately after the rudder input gets in.

From my understanding, as soon as the the pilot inputs the rudder, the plane produces a positive side force (in the $y$ direction), which causes a negative moment about the $z$ direction. We can write this as something like $N = -l_vY_v$, where $l_v$ is the distance from the vertical tail to the CG, and $Y_v = C_{L_v}Q_vS_v$ is the side force as mentioned above. We can write this in non-dimensional form as the yaw moment coefficient, i.e. \begin{equation} C_n \equiv \frac{N}{Q_wSb} = -\eta_vV_v\frac{\partial C_L}{\partial\delta_r}\delta_r, \end{equation} where $\eta_v = Q_v/Q_w$ and $V_v = l_vS_v/(Sb)$. So, a positive rudder deflection will give a negative yaw moment, as seen in that equation, proportional to $C_{L_{\delta_r}}$. However, this doesn't tell me anything about the velocity vector. Intuitively, I would think that after a long time when the aircraft has time to stabilize itself, the velocity vector would point along some angle $\beta$ from the $x$-axis relative to where to was before. However, I am not sure what would happen immediately. Maybe there is an immediate sideslip angle, and over time it decreases (assuming the aircraft is stable). Thanks for the help in advance!

  • $\begingroup$ Immediately, the only unbalanced force is $Y_v$. $\endgroup$
    – Jan Hudec
    Jan 29, 2019 at 6:13
  • $\begingroup$ The yaw will produce an angular acceleration about the z axis, thus an angular velocity, and the yaw will produce an induced roll angular acceleration, thus an angular velocity. Compared to the moving coordinates of the plane the velocity stays in the x axis mainly, but the velocity, with respect to a fixed coordinates system on the earth, will be changing direction. $\endgroup$
    – user40476
    May 30, 2019 at 5:48


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