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Old Oct 11, 2012, 03:36 PM
Rockin Robbins is offline
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buyer of the farm
United States, FL, DeLand
Joined Mar 2009
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Originally Posted by C₄H₁₀ View Post
Looks like Eli/Chuck finally took the 40. Good riddance....
Well, I'm sorry I didn't take your first advice. I didn't want to get the guy banned...

Originally Posted by C₄H₁₀ View Post
Now, RR, I don't believe I ever covered the topic of the efficiency of high-performance molded props. I'm hungry and food is a long walk away so I'll keep it brief, but it comes down to stiffness as much as anything. If any prop can hit the efficiency levels of APCs or Graupners, it's doing pretty well. Those props are commonly replaced in the high-performance world with small (4.7"-5" diameter) folders made by Georgi Mirov or some such supplier for two main reasons: 1) The stiffness of the carbon blade allows for less depitching at high RPMs, allowing more performance, and 2) The folding design protects the prop on landing. It's not so much a matter of absolute aerodynamic efficiency as simply how well the prop can cope with the extremely high power levels. If someone flew a parkflyer with a GM carbon folder, it most likely wouldn't be any different than an APC or Graupner in terms of performance... But on a 220mph F5D plane, you could have the most efficient nylon prop ever made and it could still be outperformed by a crappy composite screw.

The other aspect would be the really big (17"+) folders used in hotliner applications. We can assume that they have relatively good aerodynamic performance just because of the people who make them, but I honestly don't know where they fall on the absolute scale. Those props are also commonly run with twisted yokes to affect the average pitch of the blade, which generally isn't good for efficiency but is still done for a variety of reasons. Since a majority of these props are developed for and used in high-level competition, I suspect that there's not much room for improvement.

I think there was a thread in High Performance that actually addressed the topic of relative prop efficiencies in that arena, and those guys are miles ahead of most of us when it comes to air-screwing technology.
Makes perfect sense. So these guys are in that region where money ceases to be much of a factor in wanting to go fast. Yup I can understand why they are miles ahead of us. They're doing stuff we can't even think about and maybe didn't want to.

Glad they're out there pushing the boundaries of what's possible and there will be some fallout of cool stuff coming our way from their work.

One thing that really surprised me about the Brandt, J.B. and Selig, M.S., "Propeller Performance Data at Low Reynolds Numbers," 49th AIAA Aerospace Sciences Meeting, AIAA Paper 2011-1255, Orlando, FL, January 2011. Study was the finding that the flexible GWS props operated at much lower efficiency because of blade flutter. You know, blade flutter might actually act differently than a merely flattened blade. It could actually increase load on the motor and INCREASE amp draw while it just creates drag and turbulence instead of thrust.
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