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Folding or Feathering Propellers

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Topaz

Super Moderator
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Joined
Jul 29, 2005
Messages
15,993
Location
Orange County, California
Okay, I'm not switching designs (yet), but if I did, the new beastie would be a motorglider. With a bit more accent on 'glider' than 'motor', but still with enough cruise chops to make 350nm<R<500nm trips with two on board.

One of the 'make it or break it' items on a checklist for such an airplane is getting the prop out of the relative wind while soaring. Or at least get its drag cut down to a bare minimum. My choices (as I see them at the moment) are, in order of personal preference:
  1. Feathering prop: Doesn't need to be constant-speed or even infinitely-variable pitch. I just need to be able to feather the blades when the engine is stopped - a two-position prop, changeable in-flight. This would be my preference if I can find one for the right power/price.
    • Pros: Simplest installation. Bolt it on, hook up the controls, and go.
    • Cons: Can't find one for sale in the right price/power range.
  2. Folding prop: Can be fixed-pitch, with blades that fold. Could be that they fold aft (a pusher installation), fold forward (tractor installation - see the Caret motorglider), or fold radially (Stemme S-10).
    • Pros: One of the lowest-drag alternatives, and potentially the lightest of the three for a given drag reduction.
    • Cons: Can't find one of these for sale at any price/power range.
  3. Retractable prop: Until the recent advent of electric motors, the standard setup for self-launching sailplanes (as opposed to motorgliders) has been to bury the motor in the fuselage and drive a mast-mounted prop through a belt drive. The mast folds back into a bay in the fuselage which is then covered by doors. (See the attached photo.)
    • Pros: The lowest-drag alternative, best for soaring. Uses a standard, low-cost propeller.
    • Cons: With the exception of the radial-folding option (Stemme), most of these aren't designed for continuous cruising operation, and all the usual difficulties with PSRU design apply, multiplied by the requirement to fold it back into the fuselage.
Jet engines and jettisonable propellers are non-starters for reasons of cost and practicality, respectively, but you've got a good imagination! Let's also say that I consider the option of simply throttling down to zero-thrust for soaring to be a last-ditch, last-resort sort of option. When I'm soaring, I don't want the motor turning over at all. It's rather the point of the exercise, after all.

Have I missed any options? What else could be done, on a budget and with a first-time builder's skillset?

Assuming I chose a feathering or folding prop, where can such items be had? A couple of (admittedly brief) web searches turned up two results: a) feathering props (for really big engines) that are more expensive than a small house and, b) nothing at all. Have I missed the super-secret low-power feathering/folding prop manufacturer, or is such a beast as rare as a Nessie sighting? BBerson, how much is a replacement prop for your G-109? Their web site is pretty much unusable these days. *waits in trepidation for sticker shock*

If I instead went with a self-launcher-style retractable prop, what can be done to make such an installation reliable for cruising-flight use?

I think I could engineer a Stemme-style radially-folding propeller, but I'd rather buy off-the-shelf any day, and I'd still have to build a PSRU-like offset-drive to allow the pass-through-the-hub shaft that retracts the nose-cone. More fussy than I want to deal with, even though it works fine for Stemme.

What to do?
 

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