washout by profile

[HELModels]

I'm rebuilding a wing for my experimenter. The original wing had washout in the outer 1/3 wing panels by heating the covering. With the new wing, I have the opportunity to pick an airfoil which will give me some washout by virtue of shape. The main panels are built and playing with Xfoil, I took the foil for the main panel and reduced the camber by 50%. Using the oper visc, I see that the alpha that gives Cl 0f 0.00 on the reduced camber foil is higher by about 1.5 degrees. Is this enough to give me good washout?

[biber]

More interesting than the change in alpha_0 could be the difference in alpha_max. You may want to check that aswell in the polars you have already on hand. That alpha of maximum lift coefficient should be higher for the tip airfoil than for the root airfoil to get docile stalling behaviour.

biber

[HELModels]

Hmmm. Xfoil doesnt want to even get near stall for the airfoil I want on the tip. Best I can tell, the lower camber foil stalls about 2 degrees higher. At an angle of attack of 2 degrees, the main foil is at a Cl of 0.9 and the tip foil is at 0.6 and lower drag. This should work, unless somebody has a better suggestion.

[space_case]

Xfoil doesnt want to even get near stall for the airfoil I want on the tip.

Can you further explain this comment?

[HELModels]

Space, I get convergence errors, bails out, chokes. Anyway, Xfoil and the real world dont always coincide. I mean, it all boils down to how accurately the wing is built.

What I did tonight was took the built part of the wing and laid it on a flat surface and then took a piece of sheet and traced the upper portion only of that foil onto the sheet, with the bottom edge of the sheet flush with flat surface. What this gives me is a flat bottom foil for the tip.

I think this might be the equivalent of reducing the camber And increasing the thickness. It looks pretty good, better than an Addidas Rod Laver size 12.

[space_case]

Space, I get convergence errors, bails out, chokes.

Yes, visc can be a little picky.

Anyway, Xfoil and the real world dont always coincide. I mean, it all boils down to how accurately the wing is built.

You need to build a CNC cutter, and start matching up the 'truth' airfoil to your cores. That way you can really drive yourself nuts! (please don't ask me how I know!)

What I did tonight was took the built part of the wing and laid it on a flat surface and then took a piece of sheet and traced the upper portion only of that foil onto the sheet, with the bottom edge of the sheet flush with flat surface. What this gives me is a flat bottom foil for the tip.

Not sure if I followed you but it sounds like a solid TLAR approach if I ever have heard one.

I think this might be the equivalent of reducing the camber And increasing the thickness. It looks pretty good, better than an Addidas Rod Laver size 12.

Always use a Converse All Stars when available! They fly way better and have lower drag. Plus they cost less and are more tolerant to manufaturing errors.

[Sail 'n Soar]

I'm a little confused by what you mean by "washout by virtue of its shape." There are two primary purposes of adding washout, the first being reducing the tip-stall tendency by reducing the angle of attack of the outer wing portions relative to the local air flow (the second being reduced induce drag.) For a highly tapered wing without washout, the outer wing sections are working at a higher angle of attack than the inboard sections, which will cause them to stall earlier, i.e., tip stall.

If your goal is to delay tip stalls for a given taper wing without adding washout, choose an outer airfoil with increasing camber from the root, not less, making the effective taper ratio smaller and raising the CLmax at the tip relative to inboard. Better yet, stick with washout.

[HELModels]

Jerry,
What I was looking for is a margin of difference between the stall angle of attack at the root vs. tip. This wing is basically a straight taper, i.e., 1:1 taper ratio. I could use the same foil throughout and jack up the tip to get the desired aspirin effect.

For whatever reason, lack of education, I bought some crap, whatever, I thought reducing camber and increasing thickness would work nicely.

[HELModels]

No Dummy, dont increase the thickness, reduce the thickness and keep the camber. Xfoil confirms this, even if no one else will. Build and learn.

[Sail 'n Soar]

Jerry,
What I was looking for is a margin of difference between the stall angle of attack at the root vs. tip. This wing is basically a straight taper, i.e., 1:1 taper ratio. I could use the same foil throughout and jack up the tip to get the desired aspirin effect.

For whatever reason, lack of education, I bought some crap, whatever, I thought reducing camber and increasing thickness would work nicely.

For your straight taper wing reducing the camber toward the tip will reduce the induced drag (drag as a result of creating lift), but the total drag impact will be relatively small unless you are using a very high camber foil toward the root. That's the plus side. The minus side is that you will be creating a wing that will result in the outer wing sections stalling sooner than they would have with the constant foil. In general, it would be easier to build a tapered wing with a constant foil than an equivalent constant cord wing with a varying camber foil. In addition to being easier to build, the "equivalent" constant foil tapered wing will also be aerodynamically superior for "modest" taper wings. (What's equivalent or modest will have to wait for future discussion.)