‘POWER-ON’ STABILITY

As mentioned before, to achieve balance in flight does not imply stability. When power is on, the stability equations also vary, generally making the model less stable because the propeller acts as a small forewing in front of the fuselage. The more power the propeller is applying to the air, the more destabilising its effects. Fortunately, this can usually be catered for by a slight increase in the static margin, but variations of torque when power is applied suddenly or reduced are less easy to trim out. In particular, when opening the throttle in order to ‘go round again’ after an aborted landing, the ailerons and rudder of a model are operating in low speed airflows and lose some effectiveness, while the sharp increase in the torque rolling force can be quite large. This is a common cause of accidents.

It is less well known that a high thrust line is more favourable for pitch stability than a low one. Hence, although the trimming arrangements will differ for the power-on and power-off conditions, because the balance of forces is different, once trimmed the high thrust line aircraft will be slightly more stable under power than when gliding.

This effect is offset to some extent by the propeller destabilising forces, but the combination of high power and low thrust line is unfavourable for stability in both respects, whereas high thrust line and propeller effects tend to cancel each other out to some extent.