Kitfox Airfoil Mods

[mclayton]

Does anyone know of a mod to the airfoil for Models I, II, III that is supposed to improve the performance? The Kitfox I am rebuilding has a plastic extrusion epoxied to the leading edge of the wing which causes the leading edge to have a shorter radius, and the front of the wing to be a little bit lower than the original airfoil.

The only thing I have been able to find so far is an article in Sport Aviation which refers to a mod made to a Kitfox III by Murle Williams, based on data supplied to him by Harry Riblett. The mod was said to consist of a piece of 1" PVC sawn in half lengthwise, epoxied to the leading edge, and then faired with something called "Feather Fill". Since I am rebuilding my wings, I wondered whether the mod is worth retaining.

[SkySteve]

What engine does your model 1 have? Also, what consistent cruise did it produce with the leading edge mod?

[HighWing]

I am curious if the M Williams mod is what you have on your airplane. The factory has been selling a leading edge PVC extrusion since the Model IV came out in the very early 90's. Could it be what you have? or is yours definitely the Pipe faired in with the Feather Fill? I would think that if Harry Riblett was an advisor on that mod, it would be good to keep. Incidentally, Feather Fill, is a propriatory "bondo"like filler that is thickened with microballoons - dust like hollow glass spheres, commonly referred to as micro in the composite community. It is very light and extremely easy to sand. Tons are used in building a composite airplane - just kidding about the tons, but it is used everywhere to fill, faire, and prep for finish.

Lowell

[mclayton]

Originally, my Model II had a Rotax 532. It will now have an HKS 700E. As far as the mod, I really don't know if it is the one the factory has sold or not. When I mentioned it to John McBean, his only comment was that it would change the airfoil, which is certainly true.

It is definitely not the 1"PVC pipe the Murle Williams was reported to have used for his prize winning Kitfox III. However, it looks to me as though it would have pretty much the same result, as the protrusion from the original leading edge is 1/2 inch. I thought perhaps it was a third party mod kit for the aircraft.

[dholly]

Hey Mike-

Late reply but I have heard two stories regarding the leading edge extrusion. First is that Kitfox decided to add the extrusion after the model 4 rib re-design to a.) sharpen or reduce the leading edge area in an attempt to increase speed, and b.) help move the center of gravity forward slightly for the heavier engines. Second is that the extrusion was actually an integral part of the Riblett designed laminar flow airfoil from the git go. Then again, maybe the truth lies with a combination of all of the above, as I believe the template to ensure the correct positioning of the extrusion to the spar was included in all model 4 build manuals.

In any event, it is my understanding the extrusion was never designed to be added to the under cambered airfoil used on the KF I-III, although many builders did once it became available. Only read a couple of posts extolling a slightly lower stall or landing speed after install, but no appreciable increase in cruise speed. Several Avid owners who added the extrusion to their under cambered STOL and HH airfoils also realized little more than added build time, expense and weight for the effort. In fact, there was another, similar leading edge extension designed for the Avid STOL wing to add ~1-1/2" ahead of the spar using aluminum sheet that never took hold, presumably because of negligible gains. Seems the original design of both under cambered airfoils is hard to improve upon.

FWIW, the original builder of my KF III did not use the extrusion; I opted not to add any extension or extrusion when building my Avid Mk-IV HH wings; I did decide to use the KF leading edge extrusion when building my custom Avid+ wings based on Avid's flat-bottom speed rib (a very, very similar profile to the Riblett laminar flow airfoil). In a situation where the extrusion is already attached to your KF-II spars, assuming it is positioned correctly (best to follow the top of the rib profile in the template as much as possible??), I would just recover over vs. removing it. IMO, it does have some aesthetic appeal but, perhaps most importantly, to my knowledge no dangerous flight characteristics were ever reported when fitted to the under camber wings.

-Doug

[rogerh12]

Doug;

When you said this:
"First is that Kitfox decided to add the extrusion after the model 4 rib re-design"

When was this model 4 rib mod made? What did it entail?

Thanks

Roger

[napierm]

What motor mount are you using for your HKS?

[dholly]

When was this model 4 rib mod made? What did it entail?

Roger,

I've attached an old newsletter that details model changes including the new airfoil and leading edge extrusion (note: pgs. 2 and 3 are out of order).

Also attached are a couple of pics that will help you visualize the difference between the KF model I-III under cambered vs. KF model IV and up flat bottom rib profiles. Pic 1 shows the nearly identical airfoils of the KF I-III and Avid STOL/Heavy Hauler (blue tape) ribs. Pic 2 shows the Avid HH rib against an Avid Speed rib (again, very close to Riblett's airfoil for the KF IV and up).

Hope that helps.

[rogerh12]

Ok, I see it. Wow that is a real difference, I guess I must have had the undercambered on my model 2 (which I bought as flying), but know I have the streght on my model 4 as i just varnished the ribs !!!!

Thanks !!!!

Roger

[mclayton]

Thanks for all the helpful replies. I also had a good conversation with John McBean. He thought that it might improve the stall characteristics as well as the top speed, although only slightly. Thanks to Doug for all the info he supplied on the history of the airfoil mods.

In any event, I have decided to keep the mod and not worry about it. It will likely have a minimal beneficial effect, but no harmful effects, based on all the info I received.