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Author Topic: Washout?  (Read 557 times)
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carpetbagger
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« on: March 17, 2014, 07:31:12 PM »

I have one glider ready for covering/finishing/rigging, and another glider plan in the works.

Glider A is 69" span R/E/T (electric) and the plans did not call for washout, a tiny detail I kind of glossed over in the excitement of the build. Do y`all glider experts think it may need washout? Or as  last minute fix, stall strips?

Glider B is slightly shorter span, less aspect ratio, and will be running ailerons = sort of barn door style as if the designer left the inner trailing edge available for flaps. B is more slope oriented, as in fast and presumably aerobatic. No washout on those plans either, although I could tweak both ailerons up a smidgen.

I ask because many decades ago when I was building and flying A1 A2 towline and gas free flight models I seem to recall building a tad of washout in the wings. 
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lincoln
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« Reply #1 on: March 18, 2014, 12:47:37 AM »

An untapered, or moderately tapered wing with no sweep will fly just fine with no washout, though a very small amount may make it a little more efficient at all but the fastest speeds.

The more taper and sweep you have, the more washout you'll need.

Of course it's more complicated than that. For instance, if the airfoil changes along the wing, that will affect the amount of washout needed. Basically, you want to look at the "zero lift angle" of the airfoil. For instance, if I recall correctly, a Clark Y has to be 2 or 3 degrees nose down before it stops lifting. A symmetrical airfoil is already t here. So if you have a wing with a Clark Y at the root and a symmetrical foil at the tip (not necessarily optimal, just an example), you already have some washout built in when the angle of attack is the same.

I suspect a long, skinny wing will be more finicky about washout than a stubby one will. I know it's more finicky about angle of attack as a whole. Spins and stalls with a 17:1 aspect ratio can be pretty exciting.
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carpetbagger
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« Reply #2 on: March 18, 2014, 07:15:29 AM »

Thanks, good info. Wings are what I would call moderately tapered, as in enough taper to look sweet, but not all that radical. Skinnier wing is 11:1 aspect, and the sloper w/ailerons is 7:1, kind of stubby and has less taper but a thinner airfoil section.

Methinks I will give them a toss when ready and see what they do. I can always tack on stall strips if they have a snap roll tendency.
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lincoln
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« Reply #3 on: March 19, 2014, 07:52:07 PM »

Here's an easy approximation for figuring this stuff out. Make a graph of the local chord of your wing on a half span. In other words, you're sketching half your wing if it was modified to have a straight trailing edge. Then draw an ellipse, just as wide, that has the same area, matching where the wing is. I'm hoping the attached sketch (done by hand with Paint) will clarify. Hypothetically, the real lift distribution is halfway in between these two at each chord. So you can draw a line to represent that, as I've done on the attached sketch. Where the local chord is less than the local lift, that's where a stall is likely to start. It's sort of like a local wing loading. I've indicated that in red. (It was a much brighter red before Microsoft got to it.) You want the wing to stall first near the root. I've drawn two examples here, one for a lot of taper, one for a straight, "Hershey bar" wing.

BTW, the above is an oversimplification that ignores wing twist. You'd want some washout everywhere the line is red. I haven't figured out how to account for that, though I suspect Schrenk did in his original paper.
http://naca.central.cranfield.ac.uk/reports/1940/naca-tm-948.pdf
Lots of other really cool stuff there, just do a search on "magic naca archive".

Lincoln

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