That's relatively easy, then.

Consider an element ds of an arm. At time t it will be (in polar relative to hub centre) at some position (r=r(t), θ=θ(t)). The tangential acceleration is ##r\ddot{\theta}+2\dot{r}\dot{\theta}##. See

http://en.wikipedia.org/wiki/Polar_c...ector_calculus
If the element has mass dm (=ρds if uniform density) then the reactive torque is ##-(r\ddot{\theta}+2\dot{r}\dot{\theta})r.dm##