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May 22, 2011, 12:33 AM
Slope and thermal
Question

How much drag from larger folding props?


I wonder how much extra drag is caused by wider folding props blades with more pitch?...

I have been flying my 3.4 metre Supra with a 18x10 Graupner folding prop for the last year. The climb was never that good to start with and got worse as the batteries aged. The easiest fix was to use an 18x11 Aeronaut folding prop. The climb is much better now.

I notice that the 18x11 Aeronaut has a much wider blade, and combined with the extra pitch, it looks more draggy that the 18 x 10 thin blade Graupner. The 18x11 Aeronaut sits well against the fuselage. It is the root of the prop that is much bigger than the Graupner.

The 18.5x12 Aeronaut looks ridiculously large on the narrow Supra fuselage. It also pushes the amps on my power train a bit closer to the edge of what it can take than I would like.

Does anyone know how much glide time is lost from the wider blade, higher pitch props? May be as a percentage of flight time?

EricSoar
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May 24, 2011, 07:48 PM
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No prop drag experimenters out there?

Way back when I experimented with small and large props on a 2 M sailplane. As I recall, I once compared the glide with a 12 x 8 vs a 9 x 5 folding prop. The motors were different, but about the same weight. I also made sure that the CGs were the same. The glide was noticeably hindered by the larger prop. I seem to recall about a 10 -15 % reduction in glide times for a given height. Not a huge effect, but since then, I have tended to go with higher voltage and smaller prop systems.

I have also thought about rigging one of those prop-folding gizmos to keep the prop up against the fuse, but I haven’t yet done so.

FF
Jun 03, 2011, 01:45 AM
Registered User
AH Castor, 2500 mm, 980 grams

1) Mega Ac16/15/5 + 3.1:1 GB + Ae 14x10 + MGM 25A, total 160 grams
Sink rate measured by the altimeter, flown 10 flights to 100 m in evening on Tuesday 31st May, almost absolutely quiet air (before storm): I got two values, either 0.48 m/s or 0.5 m/s, allegedly depending on the way the prop had folded (hanging down blade thus may mean 4% more drag)

2) Axi 2814/10 + Ae 9.5x5 + MGM 40A, total 165 grams
Flown this morning between 5:45 and 6:15, much more scatter in the results due to more vivid air, in 4 flights the sink rate ranged between 0.42 to 0.46 m/s

With the Axi setup two clicks down on elevator trim were required when compared to the geared setup. Also the model "feels smoother" during the flight (CoG shift?)

Please take these values with care, as a pure indication, there could be other factors unconsidered. Of course more investigation will be needed before any conclusion can be made.

Jan


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