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Old Jan 22, 2012, 10:57 PM
Build straight - Fly twisty
Whiskers's Avatar
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No, it's a foam (EPO) 1.8M RTF with a convectional wing section with a mild undercamber.
Usual remarks by guys who see it fly are how well it 'floats.'
Being an old free-flight guy from back in the early mists of time I know that, relative to a good sailplane, it's actually falling out of the sky.
But that's OK. It just means I have to be more skilled at finding good lift, and to fly with precision so I don't lose in one clumsy turn the precious height I've managed to gain in the previous 5 turns.
Let me get this straight here and now.
I think flying a powered glider under considerable power for the majority of the flight is totally missing the point. Radian owners, I've noticed, seem to enjoy this. I don't give them a hard time about it. If that's fun for them, that's good.
Here, in Scratchbuilt Foamies, having fun seems to be our main aim.
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Old Jan 22, 2012, 11:17 PM
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In that case, you could flatten out the undercamber by covering the bottom. It will improve penetration, and will only cost a small amount of float.

I was at a seminar with Dr. Robert Liebeck from McDonnell-Douglas back in college (he was a pioneer of the techniques for computer-optimizing airfoils). When developing airfoils for max lift, his computer typically came up with sections that had extremely low thickness (so thin that there was not enough room for an adequate wing spar) and a lot of undercamber. He found that he could add quite a lot to the underside of the airfoil without hurting the lifting capabilities very much.

In your case, the flow is probably separating on the underside as soon as you try to fly much faster than thermalling speed. Slightly convex on the bottom would be better, but just covering across the undercamber to make it flat under there will help significantly, without costing much low-speed performance.
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Old Jan 23, 2012, 03:11 AM
Build straight - Fly twisty
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Huh? When did I say anything about lack of penetration? That's fine. I never have any problem gliding back to base. In fact that's a game I play for my own amusement: When I decide to terminate the flight I'll go to high altitude and require a strictly no power glide on to the ground at my feet, or as near as possible. (10 demerit points for every step to where it finishes )
Good fun.
The crime of adding a smidgen power during a glide was committed just to give this draggy thick-winged bare foam RTF glider a few minutes ( probably 5 min in a 45 min flight) pretending to be a 'real' sailplane.
And you know what? It made me think, "I must build something that really glides like that.

The airfoil: I just looked, and I'm pretty sure it's the dear old RAF-32. That's a link back to the long lost days of my youth.
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Last edited by Whiskers; Jan 23, 2012 at 03:26 AM. Reason: RAF-32 identification.
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Old Feb 09, 2012, 06:20 PM
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Hello, I was wondering if there has been any windtunnel tests of the KF airfoil since the NASA test? If not, is it because NASA did not have favorable results on the KF airfoil?
Thanks, teflyer
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Old Feb 09, 2012, 06:42 PM
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yes.
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Old Feb 09, 2012, 06:49 PM
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sorry, but to which question was that an answer to? The second question?
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Old Feb 09, 2012, 07:27 PM
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yes.
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Old Feb 09, 2012, 09:07 PM
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yup, it just doesn't work!

Craziest thing, several hundred guys all over the world are having this mass hysteria condition where they all think it's a good airfoil and even some seem to think that their flights are longer, glides better, and performance improved over other airfoils. Strangest case I've ever heard of......
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Old Feb 09, 2012, 09:38 PM
Jack
USA, ME, Ellsworth
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Quote:
Originally Posted by teflyer View Post
Hello, I was wondering if there has been any windtunnel tests of the KF airfoil since the NASA test? If not, is it because NASA did not have favorable results on the KF airfoil?
Thanks, teflyer
The NASA test were not oriented towards wings with the same shape as we are building, as seen in post #2 on this thread:

** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** - http://www.rcgroups.com/forums/showthread.php?t=1117276

The tests, done a lone time ago, were more oriented towards testing for military or commercial use in full sized aircraft.

We, in RC model building, are building different airfoils than were tested originally, are using more inexpensive materials, building with much different wing loadings, and with much higher power to weight ratios than are used for full sized aircraft. This has just turned out to be a wonderful venue for the wing.

Advocates of the more standard symmetrical and semi-symmetrical airfoils used in many other RC planes often regard the KF wings as we are using them as unsuitable and even unworthy of any further consideration. And some even go out of their way to criticize them for their higher drag and similar reasons.

In flying venues where adding a little more power is cheap and easy to do, the drag of the KF airfoil is not a negative factor. And in some respects, it might even be that the drag contributes something to the qualities of the way we are building and using the KF airfoil.


The bottom line is that if you want to build quickly with more readily available materials, the Kline-Fogelman airfoil is an excellent choice and has excellent flying abilities.

It is less suited to some types of planes than others. For trainers and simple sports flying it really shines. It is a very stable airfoil for slower flying and has very gentle stall characteristics. It is good for building first or entry level planes for learning aerobatics and 3D flying. The low building costs and easy repairs or replacement of parts or even entire planes make it especially attractive to those that have to consider the costs of the hobby.

KF airfoils are fine to just OK for entry level gliders for getting started with. If you want to fly gliders competitively, scratch built foam gliders with KF wings will not attain the low drag and penetration of the better gliders with thinner wings and high performance airfoils. But it is perfectly suitable for gliders flown for sports or self amusement. You can build down to the same wing loadings as much more expensive and competitive gliders, you can thermal, but you are unlikely to be able to attain the same kind of sink rates or fly competitively against

You really have to build one to appreciate it...

Jack
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Old Feb 09, 2012, 10:13 PM
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Thanks guys!
So would the KF airfoil be suited for smaller items such as UAVs which range from speeds of 70mph to 200mph? Would the KF airfoil be simpler to contruct with metal due to the straight lines of the KF airfoil? I am trying to see if I would be able to do an experiment with the KF.
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Old Feb 10, 2012, 08:12 AM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
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Flat slabs of foam and KFm airfoils (as commonly used here) made for each other. In fact the sections used reflect the sizes of the most usual sheet foam.
Constructing a metal KFm wing would be more difficult than a metal conventional wing. There would also be a slight weight advantage for the conventional wing.
Curving sheet metal gives it 'free' rigidity. Flat metal panels need support or they will do anything but remain flat.
Of course a KFm airfoil can be made with mostly curved surfaces, and that's part of the problem. If you asked if a NACA 1408 would work, I'd say yes. If you asked about a Gottingen 625 I'd say forget it. There is no simple answer to KFm wing questions because there are so many shapes in use.
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Old Feb 10, 2012, 10:35 AM
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New York
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Quote:
Originally Posted by springer View Post
yup, it just doesn't work!

Craziest thing, several hundred guys all over the world are having this mass hysteria condition where they all think it's a good airfoil and even some seem to think that their flights are longer, glides better, and performance improved over other airfoils. Strangest case I've ever heard of......
yup. "craziest thing" i think youve summed up the situation quite well.

as we now know from actual testing by rcgroups members et al, flights are not longer, glides are not better, and performance is not improved. yet massive following. and certainly my statement regarding nasa is true. strange indeed.

maybe not so strange though when you consider roughly 3/4 of the population actually believes in angels and devils.
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Old Feb 10, 2012, 10:46 AM
treefinder
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Actually not much stranger than spending $1200+ on a toy that you have to wait in line to take turns flying for 5 minutes.
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Old Feb 10, 2012, 10:56 AM
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i agree with that. its cheapo foamies for me. and i will say the kf makes them even more fun.
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Old Feb 10, 2012, 10:57 AM
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yup! tru dat!
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