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Old Aug 17, 2010, 12:09 AM
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Palo Alto, California, United States
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Serrated prop leading edges

Check out this thread in the high performance electric forum:

http://www.rcgroups.com/forums/showthread.php?t=1293011

It seems that some competitors in the F5D event are using serrated LE props on competition models. They also may be using much larger props, so a 1:1 comparison of the serrations may not be possible.

I wouldn't have thought serrated leading edges would work well in this application, but maybe there is something useful going on here?

Steve
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Old Aug 17, 2010, 01:59 AM
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Colorado
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Aren't tubercles just leading edge stall delaying devices? That's the impression I got from the articles I've read about them. I wouldn't be surprised if two or three LE steps or a few vortilons would do just as well.

--Norm
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Old Aug 17, 2010, 05:22 PM
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I thought that propellers at these Reynolds Numbers weren't operating with much separated flow and therefore didn't need turbulators. It seems the serrated LE would increase CLmax but possibly hurt profile drag in the attached flow regime. That's why I'm surprised that this modification helps performance.


Steve
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Old Aug 19, 2010, 09:07 AM
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Well the low speed blade stall of pylon racers has been mentioned in the thread you linked to in the OP so I won't rehash that. Something more obscure that I have wondered about is the affect of centrifugal force on the attachment line. Wouldn't that cause spanwise flow just like sweep? It sure looks that way to me. Assuming that I'm right about that then leading edge features could be expected to have the same effect on propellers as swept wings. Along those lines I am reminded that last November NASA acquired a Gulfstream GIII for the "Environmentally Responsible Aviation" program. It will be used for researching compliant trailing edge controls and bumps on the leading edge that are supposed to disrupt, or inhibit the formation of, the Tollmien-Schlichting wave. Apparently the spacing of these bumps depends on both the Reynolds number and the Mach number.

Here are some references (I haven't read any of them. A lot of this stuff is beyond me, it just happened to pass through my inbox):

Quote:
Carpenter, A., Saric, W. & Reed, H. 2008. Laminar flow control on a swept wing with distributed roughness. AIAA-2008-7335, AIAA Applied Aerodynamics Conference, Honolulu,
2008.

Carpenter, A., Saric, W. & Reed, H. 2008. Roughness receptivity in swept-wing boundary layers-Experiments. Presented at the 61st Annual Meeting of the APS Division of Fluid Dynamics.

Carpenter, A., Saric, W. & Reed, H. 2009. In-flight receptivity experiments on a 30-degree swept-wing using micron-sized discrete roughness elements. AIAA-2009-0590, AIAA Aerospace Sciences Meeting, Orlando, 2009.

Chang, C.L. 2004. LASTRAC.3d: Transition prediction in 3D boundary layers. AIAA-2004-2542.

Rhodes, R., Carpenter, A., Reed, H. & Saric, W. 2008. CFD analysis of flight-test configuration for LFC on swept wings. AIAA-2008-7336, AIAA Applied Aerodynamics Conference, Honolulu, 2008.


http://flight.tamu.edu/instrumentation/images/pdre3.jpg

http://flight.tamu.edu/instrumentati...s/IRlaptop.jpg
-- Norm
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Old Aug 19, 2010, 07:12 PM
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Quote:
Originally Posted by mlbco View Post
....That's why I'm surprised that this modification helps performance. Steve
Hi

me too, but let's first see some real, certified results.
Any one can claim a lot of things, without me implying that in this case the claim may be without justification.

W.
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Old Aug 20, 2010, 01:36 AM
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Some things to think about:

1) Serrations of this scale can cause lift variation across the blade span which increases induced losses of the lifting blade.

2) Can a highly serrated leading edge maintain the leading edge suction necessary for low section drag?

3) The blade chord seems to be substantially reduced with the serrations. Could the reduction in chord be an overriding benefit compared to any profile drag coefficient increase?

4) What boundary layer effect do the serrations provide that could not be better accomplished with vortex generators or a boundary layer trip?

5) Are the serrations providing a benefit that is (or is not) highly dependent on Reynolds Number?

Steve
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