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Old Sep 14, 2012, 10:32 AM
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Originally Posted by ibillwilson View Post
In my quest to understand things a little better, I found the following thread talking about the "NASA droop": http://www.rcgroups.com/forums/showthread.php?t=63140

The droop looks a lot like what I did when I added a strip to the bottom of the leading edge. Mine wasn't 40% of the wingspan, however, so I'm not sure how much of an effect it had vs curling up the trailing edge.

Regards!
Bill...

Richard Whitcomb designed the supercritical wing, which is flat on top and cambered on the bottom with a small cusp on the underside near the trailing edge. It almost looks like the beginnings of a small step. The purpose of the flat upper surface was to reduce the sonic boom.

He was also the inventor of the coke bottle fuselage.

~ Dick
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Old Sep 17, 2012, 01:20 PM
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This seems interesting...

Just found this video that was posted this month...seems that the small step improves the L/D

KFm4 Airfoil Testing (2 min 22 sec)
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Old Sep 17, 2012, 01:31 PM
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i cant view flash. is there any other info on small step improving ld?
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Old Sep 17, 2012, 02:12 PM
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i cant view flash. is there any other info on small step improving ld?
No. I've asked but no reply so far. Obviously, If I learn anything I will post it.

~ Dick
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Old Sep 17, 2012, 04:18 PM
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hopefully there will be a response. im very curious to see if theres a happy medium between friendly stall and improved efficiency. thanks again for keeping us tuned in.
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Old Sep 18, 2012, 03:47 PM
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Does anybody know if the Applied Aeronautics group at UIUC has ever tested a KF airfoil section as part of their low-speed airfoil program? They've tested a lot of airfoils specifically at the request of RC modeling groups.

http://www.ae.illinois.edu/m-selig/uiuc_lsat.html

Near the bottom of that page is the following text:

Quote:
Our tests are supported in large part by a group of dedicated model makers / volunteers focused on a common interest in model aviation. If you are interested in helping and want to make a wind tunnel model for testing at UIUC, see the wind tunnel model construction notes for more information. For flapped models, brackets are added to the ends to secure the flap for specific settings. If you have in interest in a particular airfoil that you'd like to see tested, contact us.
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Old Sep 24, 2012, 01:59 AM
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Originally Posted by ibillwilson View Post
Does anybody know if the Applied Aeronautics group at UIUC has ever tested a KF airfoil section:
The R/C part of the program ended a long time ago but if someone has a few thousand dollars burning in their pocket they could probably get UIUC to do a KF test.

The first step is for someone to define a set of co-ordinates for a KF airfoil. Pretty much everyone I've seen is different.

Rick.
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Old Oct 09, 2012, 09:07 AM
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The success of the KF airfoil at low Re numbers might have nothing to do with the theory posited by Kline and Fogelman. Wind tunnel tests done with a corrugated airfoil modelled after a generic dragon fly mid-span section show the entrainment of vortices behind the corrugations at Re = 34,000. The corrugated airfoil had higher drag at low AoA, but a higher stall AoA, and higher L/D since the airfoil attained a higher coefficient of lift before stall. The step on the KF airfoil might cause the favourable attributes frequently cited for these airfoils by the entrainment of a vortex(s) in the manner observed for this corrugated airfoil.

If this is true then there is room for a lot of work to determine the Re number range for which the the effect can be used without undo drag; best leading edge shapes; cord lengths for corrugations; relative height of the corrugations; best shapes for the bottom of the airfoil etc.
It would be wonderful if somebody with a wind tunnel at their disposal could do some systematic testing - one variable at a time!
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Old Oct 09, 2012, 09:46 AM
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Sounds like a great project for a retired engineer type!
Interesting comments, but I think the KF theory is just that: entrained vortices that increase lift at high AoA, and mitigate stall severity along with increasing drag somewhat. And the work by Viking with multiple steps seems to further correlate with the "corrugations" idea.
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Old Oct 09, 2012, 10:10 AM
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But what about the KFm1? It seems to be a different animal altogether. Does the entrained vortex theory explain the effectiveness of the bottom step for mitigating stall?
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Old Oct 09, 2012, 10:50 AM
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"..But what about the KFm1?..."

That is the one that proves that Bernoulli's principle should not be applied to flight, only to the flow of liquids. Other than that we are all still wondering what the heck is going on there...

Jack
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Old Oct 09, 2012, 01:01 PM
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Originally Posted by ibillwilson View Post
But what about the KFm1? It seems to be a different animal altogether. Does the entrained vortex theory explain the effectiveness of the bottom step for mitigating stall?
FWIW...My personal take on it is that the flat upper surface of the KFm1 operates part stalled almost all of the time. Hence no sudden onset of stall and no sudden drop in lift. Stall becomes a more gradual process. The down side is that operating partly stalled most of the time drastically increases drag.

Others may disagree, and that's perfectly fine, however I've yet to see any sensible explanation as to how a step on the bottom of an airfoil could prevent flow separation on the top of the airfoil... After all, the air flowing over the top of the wing has no 'idea' what the bottom of the wing looks like
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Old Oct 09, 2012, 09:00 PM
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I managed to find that article: http://www.public.iastate.edu/~huhui...orrugation.pdf

Definitely worth a read.
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Old Oct 09, 2012, 11:59 PM
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I think that you are right on this Jetplane. In the range of Re. #'s where separation occurs before transition to a turbulent boundary layer I suspect that there would be little if any difference in performance between a flat plate and an otherwise similar Kfm1.
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Old Oct 10, 2012, 06:28 AM
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Originally Posted by Mitchell Covell View Post
I managed to find that article: http://www.public.iastate.edu/~huhui...orrugation.pdf

Definitely worth a read.
Yep,

That's an interesting read. It does show very well the benefits of turbulation at low Re numbers, but I suspect they structured the test in a way that was very biased toward gaining favourable results for their corrugated airfoil idea. For instance:
  • Why only test against a streamline airfoil designed for high Re # and a flat plate?
  • Why not test against a streamline airfoil specifically designed for use at very low Re numbers?
  • Why not compare the corrugated airfoil to a low Re airfoil that had conventional turbulation strips added?
  • Why only one Re number tested?
I'm guessing the single Re was due to time/budget constraints but it would have cost nothing extra to compare the corrugated airfoil against something that represented the best that traditional low Re airfoil technology had to offer.


Steve
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