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what airfoil to use to prevent tip stall - 7/4/2003 7:45:13 AM   
tbyrd


 

Posts: 17
Joined: 10/8/2002
From: Monroe, LA
Status: offline 		I am cutting foam wings for SSC class combat planes ( and oh, by the way you have to try this SSC class combat, it's faaaantastic) and I am currently trying different designs of tapered, constant thickness wings using flat bottom airfoils. I am trying to create a template for my root airfoil that would allow it to stall first before the tip. I would like to avoid using any washout at the tips just to keep the cutting a little simpler. Would a sharper leading edge at the root give me an "earlier" stall ( stall at a lower AoA) than the larger radius LE tip airfoil? Remember, this will be a tapered wing but of constant thickness.

So far, the wings I have cut do fly great and really maintain their energy in a tight turn, but I want to see if I can further improve these wings.

Any recommendations would be appreciated.
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what airfoil to use to prevent tip stall - 7/4/2003 9:34:38 AM   
KJohn


 

Posts: 264
Joined: 2/19/2003
From: Medina, OH, USA
Status: offline 		It is my understanding that if the root area has a fairly sharp leading edge and it transitions the outboard leading edges which have a very blunt radius it will be similar to introducing washout. I believe the 'Aquila' sailplane used this principal. The wings were built with no washout. However the wingtips were swept 'up' on that design. So the combination of blunt leading edges near the tips and the upswept wing tips may both be needed to achieve the flight characteristics desired.

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what airfoil to use to prevent tip stall - 7/4/2003 11:12:18 AM   
BMatthews



Posts: 9004
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Other things you can try is to thicken the outer airfoil by about an extra 2 or 3% compared to the root. The thicker airfoil is more resistant to stalling thanks to the extra thickness and probably the extra camber if you're using a flat bottomed airfoil. If you use something like Profili to thicken it try increasing the camber by an extra .5 to 1% and move the max camber position forward to about the 40% point. This is all very crude in this day of high zoot airfoil design using pressure distributions and all but these rules of thumb were suggested in many model magazines back in the early days. All else being equal thicker sections and sections with more camber stall later. Of course the extra camber will make inverted stalls occur easier so you may want to just stay with a thicker airfoil. But watch how you thicken it so the camber stays constant. Moving the max thickness of the airfoil forward a bit may also help resist the stall.

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what airfoil to use to prevent tip stall - 7/5/2003 6:41:59 AM   
tbyrd


 

Posts: 17
Joined: 10/8/2002
From: Monroe, LA
Status: offline 		thanks for the responses!

My current wing has a 1" constant thickness and a 2" taper in the chord. The tip airfoil is basically a shortened version of the root but still retaining the same thickness, so the tip is actually thicker in proportion to it's chord than the root airfoil. From what you folks have recommended, I plan to move the tip's max thickness forward a bit, sharpen the root's leading edge, and maybe even move the roots max thickness more to the rear.

On a side note, I notice that the Funtana, the new 3D ARF from Horizon, features a tapered chord but with a constant thickness. The add says it has great low speed stability.

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what airfoil to use to prevent tip stall - 7/5/2003 7:02:01 AM   
Tall Paul



Posts: 4692
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		I wonder how you will detect the cure to a problem you say you don't have.

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what airfoil to use to prevent tip stall - 7/5/2003 11:34:57 AM   
BMatthews



Posts: 9004
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		[QUOTE]Originally posted by Tall Paul
I wonder how you will detect the cure to a problem you say you don't have. [/QUOTE]

You're right Paul. I missed that bit.

Tbyrd, you'll have to wring out the first versions that much harder to find a weakness to fix.

At any rate it's fun to experiment.

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what airfoil to use to prevent tip stall - 7/5/2003 2:53:42 PM   
KJohn


 

Posts: 264
Joined: 2/19/2003
From: Medina, OH, USA
Status: offline 		I see what the others are stating. You say that the wings you are cutting fly great but you want to try to improve them. Are you noticing anything specific you would like to focus on fixing? For example, are the current wings handling well in the various flying characteristics except they are a too slow on the straight legs?

I just looked into SSC and understand it now but I do not know the shortfalls of present designs for that class. It would seem to me that you would need a tactical advantage of some kind. Generally these are speed and maneuverability. So maybe you are looking for a faster roll and tighter circles (with less stall than the next fellow). The engine appears to be 'limited' in rpm and bore size so tactics would be of interest and that would place demands on the wing for stable yet tight turns and fast roll rates. Guess you also need some luck

< Message edited by Keith Johnson -- Jul 5 2003 10:17AM >

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what airfoil to use to prevent tip stall - 7/6/2003 1:58:27 AM   
tbyrd


 

Posts: 17
Joined: 10/8/2002
From: Monroe, LA
Status: offline 		easy folks,


I'm sorry I didn't directly state my problem

facts:

my combat wings do seem to fly well during most flight regimes

my combat wings DO tip stall when they do stall

having flown a fair amount of combat ( SSC mainly), I know that you are often pushing the limits of the flight envelope of your plane

suprises, such as tip stalls or "snaps" are not good when you have other things to worry about, oh let's say..... someone trying to cut your ribbon!!!

my combat wings do tip stall when they do stall

a more gentle, straight forward stall would be more desirable

my combat wings do tip stall when they do stall

I have moved the CG forward and yes it tames everthing down including being competitive.

Oh, and did I mention that my wings tend to tip stall?

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what airfoil to use to prevent tip stall - 7/6/2003 2:27:04 AM   
tbyrd


 

Posts: 17
Joined: 10/8/2002
From: Monroe, LA
Status: offline 		OK

here's MORE info for you guys to chew on

I do know that the tapered airfoils seem to be the way to go in that they mainain their energy through turns better than the constant chord wings I have seen

although the ssc minimum wing area is 400 sq in, the more competitive designs have at least 500 sq in

I have been able to acheive an almost stall proof wing in the past, but it was a constant chord, very low aspect ratio ( 12" chord, 36" span ) and it would turn on a dime, but after that tight turn it bled off so much airspeed (much like delta wings) that you were a sitting duck for a few seconds until you got your speed(enegy) back up.

I am not sure that a really high aspect ratio, constant chord wing with the span needed to get me to 500+ sq in wing area could be built strong enough for continuous combat. That's where the tapered wing gives you the strength at the root but the narrow tip allows for a more efficient turn.

Many parrallels can be made between lessons learned in SSC and what we know about what influenced fighter plane design throughout history. Maintaining your speed is very important to survival. Because you can build these planes so quickly and on the cheap, one can afford to do alot of experimenting.

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what airfoil to use to prevent tip stall - 7/6/2003 9:02:30 AM   
BMatthews



Posts: 9004
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		What sort of planform are you using now? And is this a flying wing or conventional design?

Just wondering if you're tapering the wing too much. For models I've usually seen that you don't want the tips to be less than 70% of the root if you want to avoid undue tip stalling.

The other thought is that you may be better off using proper airfoils rather than using that contstant thickness core with the airfoil foreshortened at the tip. It may be too blunt and the air can't follow around the curves?

Could you trace around the templates and scan them in or do something similar so we can see what you're working with?

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what airfoil to use to prevent tip stall - 7/8/2003 4:15:20 AM   
banktoturn



Posts: 713
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline 		tbyrd,

Try a less cambered wing, even a symmetric airfoil, as opposed to a flat bottom. Not only is your thickness increasing toward the tip, your camber is as well. To control tipstall, use a little less taper.

Really, there is no difference, in terms of simplicity, between using different airfoils at the root and tip and using washout. Since you seem to value simplicity, a constant airfoil with less taper sounds like your kind of solution ( mine too ).

banktoturn

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