Expert: Paul Soderman - 9/29/2008

Question
Dear Paul,

I have a question regarding how an aircraft might behave after a real engine failure in a cessna 152/172. The training that we receive as pilots for PPL include a simulated engine failure where the prop is still windmilling. we are told to instantly trim the aircraft for best glide speed as per POH. My question is suppose if you have a real engine failure with lets say the prop completely stopped, will the nose become comparatively more heavier than a windmiling prop and therefore have a comparatively steeper descent angle ?  would there be drastic difference in the behaviour of the aircraft compared to the simulated engine failure in terms of controlling the aircraft.

Answer
Anil
I don't have enough flight experience to answer your question from first-hand knowledge.  However, there are a few things we can figure out.  We know that with engine failure we want an optimum glide angle and optimum glide speed.  This translates to best L/D (see http://www.auf.asn.au/emergencies/aircraft.html#vbg).  So the question becomes how to minimize drag - does a stopped propeller have more or less drag than a windmilling propeller.  Conventional wisdom is that the windmilling propeller has more drag.  But Sprunger found that windmilling propellers can have more or less draq than a stopped propeller depending on pitch and blade length (http://www.goshen.edu/physics/PropellerDrag/thesis.htm).  At some point the drag can be equal.  So you need to estimate the drag effects of your specific propeller before you will know how the glide behavior will change if the propeller stops.  The first reference suggests that it is not recommended to stop a windmilling propeller because conditions will change and you have enough to deal with anyway.  Good luck.
Paul