Wings

[SmBaugh]

Gentlemen,

I was just wondering about something and invite opinions from your experience and expertise. I'm a pure amateur when it comes to aerodynamics, but for some (quirky) reason I find that what I can understand of it to be fascinating!

The wings in WWII US fighters and other aircraft often used aerodynamic twist" (instead washout) by reducing the thickness of the same airfoil at the tips. So, for instance, the Grumman F6F-5 Hellcat used an NACA 23015.6MOD airfoil at the root (15% max thickness) and the NACA 23009 at the tip (9% max thickness).

Would this work on models as well to move the stall point on the wing inboard? (I.e., reduce nasty tip stall characteristics.) (I was wondering if this also had to do with reducing weight at the tips also.)

Here's some background to my question and some ideas to ponder. Take a look at this fascinating old study sometime:

http://naca.larc.nasa.gov/reports/1940/naca-report-703/

It has a paragraph admitting that wings of different span which are otherwise essentially the same can have quite different stall characteristics. This means that models with our low Reynolds numbers might not act like like full scale aircraft which we model.

NACA Report 703 also suggests that using an airfoil with higher *camber* at the tips may help move the stall inboard. Seems like something we could try on a model. The benefit of this over using washout is reduced drag perhaps?? I don't know. Maybe it's not worth the effort. . . .

I have heard in connection with model wings that thicker airfoils give greater lift. But I wonder why this is so because *theoretically* (i.e., infinite span), thicker airfoils don't necessarily give more lift. For instance, here is sample data from Abbott and Doenhoff, _Theory of Wing Sections_ pp. 498 and 505:

NACA 23012 (i.e., 12% thickness)
Clmax = 1.8 @ 18 deg. alpha

NACA 23021 (i.e., 21% thickness--"fatter")
Clmax = 1.5 @ 15 deg. alpha

[Clmax = maximum lift coefficient; "alpha" = angle of attack]

Note in this data that if you use NACA 23012 on the tip and NACA 23021 on the root in a rectangular wing (taper ratio = 1) with all other variables being equal (e.g., no sweep, no washout), you will theoretically have an effective washout of 3 degrees (not counting the differences in the Cl curve). It is this theory I was wondering about. It may not prove true for wings we can make for our models though.

Sorry this is so rambling. They are just questions I was wondering about.

Thanks in advance,

Steve

[Richard Miller]

Steve,

Those are a lot of good questions. Answer one yourself, or a couple: What sort of model are you contemplating? General layout and size n' like that?

That's one fine site, the NASA file.

-Richard

[SmBaugh]

Hi Richard,

Well, actually I was just thinking in abstract. But I can see how your questions are important.

Let me say in general that I'm mostly interested in older high wing light planes and biplanes from the 1920s to -50s (the Army O- planes, Stinson Gull Wing, Beech Staggerwing [!], maybe the Sopwith Triplane). So I'm not really interested in these questions in connection with a Hellcat or other high speed, high performance, or aerobatic wings at present. With the kind of planes that interest me, the stall should be very gentle (i.e., off the tip).

I'm contemplating doing my first scratch built from the FF rubber plans by Earl Stahl for a 32 1/2" Taylorcraft 0-57 Grasshopper. I'm guessing that might adapt into a fair electric R/C plane. Here's a link for the plans:

http://www.theplanpage.com/esp.htm

Increasing the camber of the wing tip airfoil seems like an interesting thought. Could one build a wing on something like the O-57 with a highly cambered airfoil across the span. After flight tests of this wing, then it seems like I could then cover the bottom of the wing with Reynolds wrap or something to about 3/4 out of the semi-span making it a flat airfoil, but leave the tips with high camber for second flight tests. This might save having to build two wings for a simple test.

Just more thoughts!

Regards,

Steve

[Richard Miller]

Steve,

Airfoils like the Eiffel 400 were popular in the late '30s, big thick things with loads of underchamber, and I took that in with my mother's milk. The last one I built, in the '50s, just fell out of the air, but I didn't have the good sense to stomp on it.

Meanwhile, enter Carl Goldberg, who I had the very great pleasure of knowing. Carl built lots of wings for test purposes and lots of others for models like the Zipper and Sailplane and 1/2A Blazer and Gentle Lady. These convinced me that you're probably not going to do a lot better than a simple flat-bottomed section of maybe 9% thickness, high point 35-8%.

Tips - Among the myriad hlg I built, also in the '50s, were some with a reverse ellipse, but they didn't seem to fly any better than the usual elliptical planform. What I'd failed to do, and what made a lot of difference, I later learned, was carving a sufficient amount of wash out in the tips.

It seems to me axiomatic to want to reduce lift presssures near the tip. The loss there is proportional to the square of the [effective] chord dimension and of the lift coefficient. Too small invites stalling. This business of airfoil and wing efficiency just goes on and on. It's one I'm working on all the time.

And those models you mention! Invite me for the flying sessions please. They're among my favorites.

-Richard

[SmBaugh]

"And those models you mention! Invite me for the flying sessions please. They're among my favorites."

Hehehe. I'll be glad to assuming I can someday get to build something that flies!

Thanks for your help and ideas.

Steve