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Folding props are increasingly used on self-launch gliders in order to reduce drag. Here is an example of 1m dia. props from LZ Design's FES system:

LZ Design FES folding props

For self-launch gliders, due to minimally-sized power plants the props are optimized for the safety-critical climb phase, leaving cruise performance as an afterthought.

At full-scale power levels, i.e. 10-30kW, how are design considerations different for folding props vs. standard fixed-root props?

In specific, could an existing folding propeller be retrofit by simply having a root which pivots, or would the already-structurally optimized root of a folding propeller make this unworkable?

[*] Examples at What are the advantages/disadvantages of a forward-folding propeller?

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    $\begingroup$ asking if something is "reasonable" is opinion based. You'd better ask how much complexity is would add (objective, easily related with costs), how much weight it would add (objective and easily related with performances) and/or how much it would improve efficiency (objective, easily related to performance). BTW, I fail to see why you want to optimize such propeller for cruise as gliders don't use it for cruise. $\endgroup$
    – Manu H
    Apr 21, 2020 at 3:49
  • $\begingroup$ Usually, the propeller is retracted inside the fuselage so there is no need to fold the blades. $\endgroup$
    – Manu H
    Apr 21, 2020 at 3:53
  • $\begingroup$ @ManuH, the picture you linked to is the pylon style of motorized glider. Another style uses the blades pictured above, which are 1m in dia. and manufactured by LZ Designs as part of their FES system. I also disagree that gliders don't cruise. There is a world of sustainers which serve only for cruise, being too weak to effectively climb. Typical cruise is for returning to the departure airport when there is no longer usable lift, however some pilots cruise in order to do a "soarfari". $\endgroup$
    – Kenn Sebesta
    Apr 21, 2020 at 4:21
  • $\begingroup$ I only saw that in RC models. In manned gliders, I already saw feather as the center of the propeller is far from the side. One exception I know is the Stemme S-10VT for which blades and only blades retracts inside the fuselage but don't fold $\endgroup$
    – Manu H
    Apr 21, 2020 at 6:11
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    $\begingroup$ the cruise part to return home is marginal and not a normal case. The properler's main use is to climb, thus there is little incentive to optimize it for other phases as it can do the job even if not optimize for and this job is a marginal and not usual job. $\endgroup$
    – Manu H
    Apr 21, 2020 at 6:19

2 Answers 2

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Yes, although contest rules to date haven't yet motivated doing both at once. In F1D international competition, propellers are commonly variable pitch (Indoor Model Airplanes: The Best of Indoor News and Views, ed. Tim Goldstein, pp. 138-144) to better exploit the gradually diminishing power during a flight, and sometimes variable diameter (ibid., pp. 119-126) to reduce drag like a folding propeller would. Although not a contest winner, R/N Models' "Maverick" kit included a folding propeller purely for drag reduction.

No: if you need variable pitch for efficiency from a variable-power engine, and folding for drag reduction, it's simpler to reduce drag by feathering (changing pitch so far that the propeller won't windmill while unpowered) instead of by folding.

No, for a special case: For propellers of the size you show, 8 to 16 inches diameter, 300 to 2500 watts electric power, a typical flight profile alternates full power climb with zero power glide (hopefully finding thermals to stay aloft). Partial throttle only wastes energy. Proving that mathematically is hard, but it has been verified with numerical simulation. "Cruising" is best done with a burst of climb (what the propeller is best at) followed by a long glide (what the wing is best at). At any rate, for a full on / full off power source, a single pitch is optimal.

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    $\begingroup$ Thanks for your thoughts and links. FYI, the pictured prop is 1m in diameter. The application is full-scale flight with 15-25kW of power, and so I wouldn't be surprised to learn that there's somewhat marginal utility in comparing to small scale model airplanes. For instance, in 13.5m FES gliders the saw-tooth/dolphin approach to covering ground has proven to be less efficient then level cruise. $\endgroup$
    – Kenn Sebesta
    Apr 21, 2020 at 4:10
  • $\begingroup$ BTW, a feathered prop is very draggy, to the point of destroying the plane's ability to climb in anything other than strong thermals. Folded props do not suffer this pathology and have a minimal impact on drag of 5-10% overall. $\endgroup$
    – Kenn Sebesta
    Apr 21, 2020 at 13:09
  • $\begingroup$ Thanks for indicating the scale. Sure, the rules are different there. Particularly about feathered props: that big, the nonzero pitch at each point along the blade results in essentially a speed brake, even though the pitches cancel out so the prop doesn't spin. $\endgroup$
    – Camille Goudeseune
    Apr 21, 2020 at 15:18
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It is aerodynamically feasible, but it is not very practical.

Variable pitch is only worth the weight and complexity if the plane has a wide speed range, able to fly either very slowly or very fast, and needs to remain efficient throughout. If the speed range is not great then the aerodynamic benefits will be minimal and it is not worth doing.

Combining it with a folding prop poses no aerodynamic issues other than making the mechanism sufficiently compact to avoid unacceptable drag. But it does pose engineering ones; can a suitably compact mechanism be made strong, stiff, light and reliable enough for a realistic cost?

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  • $\begingroup$ i.stack.imgur.com/VhG4q.jpg suggests that the efficient performance range for a pure climb prop is very narrow. It looks like even a 10 degree shift in pitch angle could increase range by 50%, which is substantial on a small battery pack. Do you think that this graph generally holds for all props, or is it only props designed with variable-pitch in mind which can have such a broad operation envelope? $\endgroup$
    – Kenn Sebesta
    Apr 21, 2020 at 17:47
  • $\begingroup$ The whole point of variable pitch is to broaden the operational envelope. I doubt that any graph holds for more than the prop it was measured on, especially when the design parameters and optimisations of fixed and varying pitch are so different. $\endgroup$
    – Guy Inchbald
    Apr 21, 2020 at 20:15

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