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5 Answers

Constant Speed Prop.

Asked by: 6944 views Aircraft Systems

1.  Single engine piston constant speed props. do not feather. Any reason why?   2. When oil pressure is lost some single engine constant speed props. default to high pitch and some default to low pitch. Why don't they all default to one or the other (low or high pitch prop. ) ?  With a loss of oil pressure  engine failure is usually not far behind. A single engine airplane with a constant speed prop. (as with any aircraft), you want to reduce drag to enhance the glide ratio/range. My understanding is placing the prop. in the high pitch position in this situation (engine failure) reduces drag yet in normal operations high pitch increases resistance (drag) of which reduces RPM.   Thank you for the feedback.    

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5 Answers

  1. Wes Beard on Sep 28, 2013


    All of the single engine constant speed airplanes that I have seen fail to the low pitch / high RPM setting while all multi-engine airplane will feather (high pitch / low RPM) if oil pressure is lost to the constant speed propeller.

    If the oil system loses all fluid, the constant speed prop will hit its mechanical stops and the engine will eventually seize as you stated in the question.

    Most single engine manufacturers are more concerned with a problem occurring in the constant speed prop where the oil pressure can no longer move the solenoid against the spring. The oil system is fine so the manufacturers wants to default the propeller to a position that produces maximum power. If the default for the prop was to go to feather, a problem with the propeller governor will cause an engine failure. That is not good.

    On multi-engine airplanes manufacturers are more concerned with the increased drag the propeller produces if the prop is not feathered. The increased drag may be enough to cause the airplane with the one remaining engine to slowly descend to the ground. If the oil system fails, the engine is going to quit and the prop should go to feather… if it was designed the other way it would be a very bad thing. So in this case, the manufacturers decided that if oil is lost to the propeller governor or engine to feather the prop and as a result… fail the engine via pilot shutdown.

    The most important thing to remember is that the manufacturers have determined what the safest option is and designed the propeller governor to fail in the safest condition.

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  2. Dan Chitty on Sep 28, 2013

    Thank you Wes. You make several good points.

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  3. Sam Dawson on Sep 28, 2013

    An exception to the rule are aerobatic airplanes with constant speed props. They will often fail to a course pitch. Next time you see a Super Decathlon on a ramp look at the prop.

    The reason for this is the possibility of a prop failing during a maneuver that would result in an instant and catastrophic overspeed, such as a vertical downline.

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  4. Dan Chitty on Sep 28, 2013

    Thank you Sam.

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  5. Reno Psaila on May 28, 2014

    If the prop is electric, one still has control over pitch angle, unless battery power is also lost.
    In this case should ‘cruise’ pitch be selected to increase L/D ratio?

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