De Havilland 2-speed prop - how does it work technically?

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Zacke
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De Havilland 2-speed prop - how does it work technically?

Post by Zacke »

Hey guys,

is the pitch of the DH 2S prop changed by oil pressure (like most constant speed props) or is it done through a other mechanical force or linkage? As far as I remember the pitch only can be changed while the engine's running.

Thanks in advance,
Zacke

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Killratio
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Re: De Havilland 2-speed prop - how does it work technically?

Post by Killratio »

It is indeed.

Originally I thought it was a mechanical connection not hydraulic but have since found someone with an original bracket airscrew manual.

The counterweights will drag the blades back to coarse pitch by centrifugal force. Pulling the the Bowden back causes the oil pressure to build (a two way valve lets oil into the propellor) forcing the blades to fine pitch. Pushing the Bowden in allows the oil pressure to fall (two way valve reverses allowing the oil to drain from the hub), letting the counterweights act. It stands to reason therefore that without the engine running, there is no oil pressure to force the blades to fine and no centriugal force to take them to coarse.
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Tomas Linnet
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Re: De Havilland 2-speed prop - how does it work technically?

Post by Tomas Linnet »

Thanks Darryl

I was counting on you to answer this :D
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Re: De Havilland 2-speed prop - how does it work technically?

Post by Zacke »

Thanks again Daryll!

Now it's also more clear to me why to start and stop the engine with coarse pitch.

So if you lose youre engine/oil pressure the prop will go into coarse pitch?

Regards,
Zacke

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Re: De Havilland 2-speed prop - how does it work technically?

Post by Killratio »

I'm not sure to be honest. With engine windmilling, there may be enough oil pressure
to keep the fine setting. It would perhaps depend on the reason for the failure. Loss
of oil preeure would cause the blades to go coarse. I'm pretty sure the oil pump is
engine driven not electric. The pitch is controlled using engine oil. So it "depends"?
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Re: De Havilland 2-speed prop - how does it work technically?

Post by Killratio »

Tomas Linnet wrote: 20 Aug 2021, 04:12 Thanks Darryl

I was counting on you to answer this :D
:wink:

Glad to be of service Tomas .
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Re: De Havilland 2-speed prop - how does it work technically?

Post by Tutmeister »

Prop going coarse with engine failure prob not a bad thing anyway!
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Re: De Havilland 2-speed prop - how does it work technically?

Post by Killratio »

Agreed, certainly not a bad thing!

Whilst not having the benefits of a full "feather" it would extend the glide distance by a bit.

BBMF Spitfire operating notes call for pitch to coarse in all engine failure situations. Early Pilot's notes "Forced
Landings"sections are concerned only with the choice of undercarriage position really! The Mk IX Notes have an
emergency section and that says that "the glide may be lengthened considerably by moving the propellor speed
control fully back" (coarse).
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