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  #1  
Old 05-30-2007, 03:42 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default Reflex Flap Setting

If you go to http://www.ouchrockgallery.com then click on "Events" then click on "Taylor 100 Air Race" then scroll down the thumbnails at the left side of the display until you come to the red Lancair Race #36 in flight in turn 3, you will see an interesting thing (in addition to the rudder and elevator gap seals). If you look at the left aileron you will see it is down and think OK, he is rolling out of the turn but if you look at the other aileron it appears to be down also. Then after some thought realize that the flaps up above the normal airfoil configuration. Has anyone tested this on an RV to determine the best speed position of the flaps? In my RV-6A, except for the lower flap skin extension under the fuselage to form a gap seal when the flaps are up, this would be easy to implement by adjusting the linkage.

Bob Axsom
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  #2  
Old 05-30-2007, 03:51 PM
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Bruce Reynolds Bruce Reynolds is offline
 
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Location: Salem, Oregon
Posts: 54
Default flaps up 3 degrees

The RV-10 has both the ailerons and flaps reflexed 3 degrees.

Bruce Reynolds
RV-6A
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  #3  
Old 05-30-2007, 05:06 PM
tin man tin man is offline
 
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Location: northern california
Posts: 297
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Bob
Many of us west coasters have tried this. If you have manual flaps you can figure it out for yourself by going flying and releasing pressure on the flap handle @ high speed until it is in trail with zero pressure. Note that position and that'll get you close.
Tom
RV3
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  #4  
Old 05-30-2007, 08:54 PM
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Kevin Horton Kevin Horton is offline
 
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Location: Ottawa, Canada
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Default

I played around a few months ago with XFoil and AVL trying to see what they predicted about reflexing the flaps on an RV. XFoil provides predicted 2D airfoil lift and drag, and AVL predicts 3D lift, drag, etc on a wing and tail.



XFoil, which calculates 2D airfoil performance, does show a small reduction in drag by reflexing the airfoil. For example, at a CL of 0.15 (typical RV CL at high speed), the airfoil section CD is about 0.0062 with flaps at 0 degrees, and about 0.0052 with flaps reflexed up 5 degrees. The lift for a given angle of attack is decreased too, as you might expect. This means the inboard wing will make less lift, so the outboard wing has to make more lift. You would think this would mean that the outboard wing would see a drag increase. But, the NACA 23013.5 airfoil actually has its minimum drag at a CL of about 0.45. So increasing the CL on the outboard wing might decrease the drag a bit. Of course there is always a pay back, and in this case the additional lift on the outboard wing will lead to stronger wing tip vortices, and that means more drag.

AVL does rudimentary 3D analysis (i.e. it attempts to account for the variation in CL along the span, wing tip vortices, etc). It predicts that the whole aircraft CD is:

RV-8, 193 kt, MSL, aft CG, flap 0 deg -> CD = 0.03424
RV-8, 193 kt, MSL, aft CG, flap -5 deg ->CD = 0.03651
RV-8, 193 kt, MSL, aft CG, flap + aileron -5 deg ->CD = 0.03875

Note: AVL was originally created to do analysis of RC gliders, so it assumes that the fuselage is of circular cross section. This means that the fuselage drag, and fuselage/wing interference drag is not correct, so the absolute values of the whole aircraft CD are probably not correct. The variations between conditions is probably reasonable though.

Summary - there is probably little benefit from reflexing flaps and/or ailerons on short wing RVs.
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  #5  
Old 05-30-2007, 09:17 PM
tin man tin man is offline
 
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Location: northern california
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Not necessarly. the above calculations are assuming you have a perfect airfoil. Do you? Are your flaps in perfect trail accross the flap span of the wing? Any twist? Top skin tweeked up a little bit at the rear spar ? how big of a gap betwen the top of the flap skin and the bottom of the wing skin? everything perfect?
One beautiful thing about homebuilts is you can go and try a little tweak here and there. Don't depend on someones first hand experience or someone elses
formulas and calculations. Go out and try it yourself. Don't remember if it was the above poster but someone tried to convince this board that a piper pitot static mast wouldn't work on an RV wing. He was wrong as many posters included myself had perfectly good results with said Piper mast.
Cheers
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  #6  
Old 05-30-2007, 09:31 PM
Bob Axsom Bob Axsom is offline
 
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Default Thanks Kevin

I am always looking for something and this looks promising. I will have to worry this one for a while because there are some implementation complexities on my plane at the flap and fuselage interface. I need to have everything tested before the AirVenture Cup race on July 22. Not that I expect to win anything but I would like to improve over last year once again.

One thing that was mentioned by Marc Frederick at Taylor Texas recently was that the gap between the ailerons and flaps is a significant source of drag. I have a couple of half baked ideas but nothing really useful - this is a real back burner possibility.

Bob Axsom
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  #7  
Old 05-30-2007, 09:53 PM
the_other_dougreeves the_other_dougreeves is offline
 
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Location: Dallas, TX (ADS)
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We get about +10 kt in the CT with "reflex" flaps at -6 deg. In Europeans get -12 deg flaps, but this makes the CT too fast for the US LSA rules.
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  #8  
Old 05-30-2007, 09:56 PM
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n5lp n5lp is offline
fugio ergo sum
 
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Location: Carlsbad, NM
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Default

Quote:
Originally Posted by tin man
...Don't remember if it was the above poster but someone tried to convince this board that a piper pitot static mast wouldn't work on an RV wing. He was wrong as many posters included myself had perfectly good results with said Piper mast.
...
Well I don't know if it was the above poster either, but the above poster is professional test pilot Kevin Horton, for my money the most knowledgable person on the forum for matters like flight testing and aerodynamics. Look through his posts. You will find that he relies on facts but has the background to sometimes hazard educated opinions, which will be clearly labeled as such.

I really hate to see him thrown in with garage tinkers, such as myself.
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  #9  
Old 05-31-2007, 12:11 AM
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petehowell petehowell is offline
 
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Location: MN
Posts: 1,780
Default Well Said

Quote:
Well I don't know if it was the above poster either, but the above poster is professional test pilot Kevin Horton, for my money the most knowledgable person on the forum for matters like flight testing and aerodynamics. Look through his posts. You will find that he relies on facts but has the background to sometimes hazard educated opinions, which will be clearly labeled as such.

I really hate to see him thrown in with garage tinkers, such as myself.
Well Said Larry. Tinkering is fun and fine, but Kevin keeps us grounded in the science of it all. He is a much appreciated resource here.
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Pete

Amateur Plane - RV-9A N789PH - 1300+ Hrs
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  #10  
Old 05-31-2007, 08:34 AM
Reheat Reheat is offline
 
Join Date: Jan 2005
Location: Southeast
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Default

I believe...
That the Lancair has a laminar flow airfoil with a high pitching moment. At high speed this requires a lot of trim drag. Reflexing the flaps relieves this condition somewhat. RV's on the other hand have an airfoil with a near zero pitching moment, thus no such benefit will be realized.

I've been wrong before, but...
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