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Old Nov 13, 2010, 02:47 PM
jackerbes is offline
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USA, ME, Ellsworth
Joined May 2008
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Originally Posted by philipa_240sx View Post
Hi Jack,

Glad to see you had some success with the KFM3 wing.

I didn't have much luck with my KFM3 wing, but I notice some differences that may have accounted for it:

1) The BB33 KFM3 wing has ailerons placed much closer to the 75% step.
Steps too close to the aileron is often cited as the cause of poor aileron performance but I'm still not sure that it is the only factor in the poor performance. It might be a combination of things that are at work here. I seem to notice the following are present when wings are cited as "poor performers":

1 - There is dihedral

2 - There is an open hinge line close to the step.

I built one poor performing KFm3 with ailerons and realize now that it also had an open aileron hinge line. When I built the second I had moved on to using full length tape hinges that effectively sealed that open hinge line. And that second wing worked much better. In that type of hinge (image attached) the hinge line is on the top surface of the wing and the inverted "V" hinge opens to the bottom.

I sort of wonder now if having a smooth surface from the step and to and over the aileron hinge is not a factor. Like it promotes smoother air flow or something?

2) I had approx 3 deg dihedral.

Basically what would happen is the plane would tend to pitch up when banking. Possibly stalling the wing and ailerons. The end result was mushy response and I would have to bring the nose down, then try again. The CG and incidence were spot on so I could never explain the phenomenon.
That 3 degrees works out to about .84" or almost 7/8" of an inch per tip and that is not a lot of dihedral but I wonder if it isn't still a factor too. I don't recall my not so good KFm3 as being that way or that bad.

Did you notice if the aileron performance seemed OK when coordinated rudder/aileron were used and if you were flying more in a "real airplane with passengers in the back drinking coffee" style? Or was is that the disappointment was not more that it would not fly well on ailerons only (i.e., when flown more in the "yank and bank" style)?

I notice my BC12D Taylorcraft with a near flat KFM2 aileron wing is much more well behaved. Nice turns are a snap with this wing and only a touch of rudder is needed for a nice co-ordinated turn.
And the BC12D has similar dihedral to what the KFm3 did? And the same kind of hinges the KFm3 did?

Thanks for the insight. Perhaps I will try another KFM3 winged BB in the future!
I encourage you to try it. If we keep messing with this we may figure something out here. If you or I took the trouble re-do the hinges on the lower performing KFm3 wings with sealing tape hinges it might tell us something. Like maybe the smoother air flow makes better ailerons or something.

Then there is the fact that our expectations can be a factor and differ too. I tend to just take whatever I get unless I find or feel a need to pursue getting something that was not there.

For example, I consider it normal to use coordinated controls on a high winged plane with ailerons. It is just the nature of the beast. The business of messing with differential ailerons never came up with me because I don't try to get "yank and bank" performance out of a "don't spill the coffee" plane.

So I was quite pleased when the 48" flat SS wing turn out to have great aileron response without turning into something that was hard to keep in smooth and level flight. I credit at least some of that to the KF wing but I also had a similar experience with the flat wing on the SPADET LC-40 I built.

The SPADET has a flat-bottomed Clarke-Y kind of wing and it was very pleasant to fly on ailerons only or with rudder and ailerons. The SPADET had a high wing too and the rolls were not very axia. But I think axial rolls and high wings exclude each other at least to some extent.

My wing was made with the KF step lines straight across the wing from 50% and 75% of the tip chord. So that puts most of the aileron a little further away from the step and that might be a factor.

But if you build a flat KFm3 like this one and don't get enough roll rate out of it I would be *very* surprised. I was somewhat taken aback with what I found especially since I had reduced the width and length of the ailerons a little.

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