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3 Answers

Constant-speed prop windmilling

Asked by: 1307 views Aerodynamics, Aircraft Systems, General Aviation

In the event of an engine failure, the best glide speed is stated as being 85 KIAS (flaps up and at 3,000 lbs.) What do you do with the windmilling propeller? High or Low RPM setting? I read someplace that if you go to the LOW RPM setting (increase pitch) that it reduces drag a little and will extend your glide a tiny bit. The HIGH RPM setting flattens out the prop blades and increases drag. What are your thoughts? Thanks.

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3 Answers

  1. Jim F. on May 10, 2016

    I’m going to assume, seeing that you referenced ‘the prop’ on a 3,000 pound airplane, that you’re talking about a typical single-engine recip…

    So I’m gonna answer by asking a couple questions of my own:
    If you lost your engine (in order to cause a windmill), do you have oil pressure?
    If you don’t have any oil pressure, how can the prop be actuated out of high speed/low pitch?

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  2. Jeff Nunan on May 10, 2016

    Jim F.
    Thank you for responding to my question.
    In your opinion, would oil still be circulating as a result of the prop windmilling?

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  3. John D Collins on May 11, 2016

    As Jim points out, you may not be able to control the pitch of a constant speed prop if you have zero oil pressure, but on the other hand if you have zero oil pressure, it is often the case that the engine will freeze or the prop will rotate at a speed well below normal. The bad news is you lost the engine, the good news is that you will often glide much further, but it depends on the aircraft, engine, and prop.

    In my Bonanza, it is not a small effect, it is huge. If the engine failure does not involve loss of oil pressure, the prop position is dramatic. Anywhere much above the full rear stop, and the prop will turn at 1800+ RPM. At the full rear stop, it slows to 1100 RPM. The descent rate with the prop at the rear stop is around 800 to 1000 FPM. Anywhere much forward of the rear stop, the descent rate is at least 1400 FPM. Glide range is dramatically improved if the prop can be controlled.

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