Why is it, with less airflow, you'll have less control effectiveness?
When deflected, the control surfaces (ailerons, elevator, rudder) cause an aerodynamic moment about the Aerodynamic Centre. A moment has a moment arm and needs to have a length reference - the aerodynamic moments are defined with reference to wing dimensions: wing span for rolling and yawing moments, and Mean Aerodynamic Chord for pitching moments. If we have a look at the pitching moment P:

$$ P = C_{r_{\delta e}} \cdot \delta_e \cdot q \cdot S \cdot MAC$$

With:


$C_{r_{\delta e}}$ = elevator coefficient (dimensionless)
$\delta_e$ = elevator deflection
$q$ = dynamic pressure = $\frac {1}{2} \cdot \rho \cdot V^2$
$A$ = wing area
MAC = Mean Aerodynamic Chord


$C_{r_{\delta e}}$, A and MAC are constants. So: pitching moment of the aircraft is proportional to elevator deflection, and to the square of the airspeed. Fly twice as fast, and the pitching moment from a certain elevator deflection will be four times as high.