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List:       flightgear-flightmodel
Subject:    Re: [Flightgear-flightmodel] Windmilling propellers
From:       "Donald C. Simon" <donsimon () firstva ! com>
Date:       2003-05-21 0:07:18
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Jon,

For all engines I am familiar with the engine is mechanically connected to
the propeller, thus if the engine is off and windmilling, it is still
turning over. To windmill the engine, the propeller must develop enough
torque to overcome engine friction. The torque coefficient, which determines
how much torque it can develop, is a function of blade angle. At each
advance ratio there is also a thrust coefficient (which is usually negative
for a windmilling prop) associated with the torque coefficient.

NACA TR 641 is an excellent resource for windmilling propellers and engine
friction. If you look at the data charts you will see that each blade angle
has an advance ratio where the negative thrust is a maximum value - it is
here where a windmilling propeller acts as an airbrake. On a multi-engine
aircraft this can cause loss of directional control. At high blade angles
both thrust and torque coefficients are very small values. This is a
"feathered" prop.

Sincerely,

Don Simon
Troy, VA



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