KF3 wing convert to KF4?

[NotSoAmish]

I would like to know if it would be feasable to add cap strip assemblies to the bottom of a KF3 wing, removing the undercamber and acheiving a higher max airspeed. Is the top side airfoil section on the KF3 close to a KF4?

[Av8r3400]

The ribs have a completely different profile on the top and bottom curves, and no the bottom of a IV rib is not flat.

Someone posted a photo comparing them a while back. Also the slight differences between the Avid and 1-3 Kitfox ribs...

[NotSoAmish]

I found the avid airfoil HH compared to the undercambered one here. It does look like it would end up less thick compared to the avid high speed rib.
http://www.teamkitfox.com/Forums/sho...hlight=airfoil

[DBVZ]

The Avid speed rib is the one with the flat bottom. Thicker, more interior volume, allows for the 18 gallon/side wing tanks. The "speed" part is due mostly to the shorter wing span.

[jtpitkin06]

Trying to convert a Kitfox 3 into a four is akin to making a Cessna 120 into a Cessna 152. It's a big leap for little gain and may not be feasible.

I understand your quest for speed. Sometimes, if what you want is a faster airplane, it's best to sell what you have and get a faster airplane. Or, you can enjoy flying a Kitfox 3 for what it is while puttering around low and slow.

Just a few of the known differences between Kitfox Model 3 and 4-1200:

Wing airfoil, thickness, camber, leading edge extrusion.
Flaperon chord increase, airfoil change
Flaperon control system
Flaperon attach method
Lift Struts diameter and wall thickness
Lift strut carry through tubing
Rudder area height and chord
Rudder attach method
Vertical stab 10 inches taller
Vertical stab bracing
Optional electric trim
Landing gear bungee attach bracing
Thicker windshield
Windshield profile
Turtledeck
Bumped doors for elbow room

As you can see, the changes are not minor. Perhaps Denny should have named the models the 333 and the 444 to avoid assumptions that they are almost identical. The changes between 3 and 4s are as great as the leap from Model 4 to current Model 7.

Low and slow is not a bad thing. I fly with a buddy in his Luscombe. If we get 90 mph on a cold day we laugh at the blinding speed and just smile that we get more flying time on the way to lunch than the pilots in faster airplanes.

So, lets say you modify your Model 3 wing with the Model 4 airfoil and you get a whopping 10% increase in cruise speed. Your trip to get a hamburger used to take 60 minutes and now you do it in 54. Who notices? How about a big cross country flight? Ok, a 1000 mile 10 hour flight to Oshkosh now takes you just 9 hours. It still takes two days!!!

Now, if you just want to own the worlds fastest Model 3 Kitfox.... that's another story. I can appreciate that.

Enjoy the ride.
John Pitkin

[NotSoAmish]

Good Lord John. You have it figured out each way I turn. All excellant point to keep in mind. You even thought about the Oshkosh flyin factor. I give up. Guess I am always trying to change things. My brother and I grew up with a father who had his own drawings of the what became the Wankel engine, before the Wankel went to patent. We have always looked to improve things like he did. I guess it is more important to have the extra HP of the Viking engine for critial climb situations as opossed to just going faster. Thanks.

[Peteohms]

John Pitkin has good points. There are a few things you can do to speed up a III. Murle Williams told me you can add a sharper leading edge to the wing and speed up some. You can add fairings to various places too. But, if you go too fast exceeding the design vne I suppose you could cause problems elsewhere. My Kitfox III cruises about 85mph. I can get 90 out of it but I prefer not to push the engine. I've flown from Austin, Tx to Oshkosh, Wi twice and survived a flight to Sun N Fun and back in March/April. I enjoy the long trips. My advice is dance with the girl who brought you.

[egp8111]

Notsoamish,

Like you I was looking as ways to make my KFIII faster during a rebuild. I installed the plastic leading edge from KF and a thicker windshield. With the 912 mine will "easily" blow through red line. I've yet to get the throttle fully open before I'm off the ground. A 912(80hp) is all the motor you want on a KFIII, anything more is overkill IMHO. The airplane will let you know what speed it wants to cruise. I cruise at 80-85 at 4200rpm. I can easily make it go faster but it just does feel happy. I don't know what the effect of the new leading edge on the earlier airfoil as I never flew without it. My impression is that it lowers the overall drag of the wing but does not have much effect on it's overall speed range. All I can say is mine does not exibit any negitive effects. Murle Williams is a good source of info about the modification.
I would advise against getting too creative with alot of unproven mods or engines. Stick with what has proven to work, you'll be flying a lot sooner and safer. Get it flying and enjoy it for what it is, you'll soon forget about wanting to go fast !

EG

[dholly]

Back on topic... as you can see, one cannot simply scarf on new lower capstrips to convert the KF3 under cambered rib to an equivalent of the newer KF4 Riblett airfoil. Even if you could replicate the rib portion of the airfoil, you would still need to replace the asymmetrical KF3 flaperon with the newer symmetrical airfoil flaperons, plus add the PVC leading edge extrusion which forms an integral portion of the airfoil.

What you may consider instead, assuming you haven't already assembled your KF3 wings, is to simply build them with the new KF4 ribs. Of course, if you do this, you will also need the new flaperons as well as the KF3 > KF4 flaperon mixer control modification kit (old p/n 19065.000). The kit is comprised of two brackets that get bonded into the fuselage to mount the new mixer assembly, the new mixer assembly itself, new flaperon push-pull tubes, new control column, flap handle, bellcranks and control push-pull tubes. Basically you are replacing your flaperon control system.

This was a factory option offered towards the end of the model 3 run and there are a number of KF3s sold with the new KF4 wing option. IMHO, the new wing has proved to be the equal of the earlier under cambered airfoil wing for S.T.O.L. performance, and far superior for cruise performance. You no longer hit that aerodynamic wall at ~90mph so common with the under cambered Avid/KF wings, and we know many KF 4s cruise well above those speeds. I should think the new KF4 airfoil would work just as good or better on a lighter framed KF3 with similar engines.

So if you're not trying to turn your KF3 into a KF4 but, rather, are simply looking for notable performance improvements, IMHO this is -very- worthy of consideration. If the net cost boiled down to new ribs and flaperons, plus the flaperon control system parts (parts you might even be able to source used ie., KF4-6 flaperons $500 on Barnstormers right now hint, hint!), less the sale proceeds from your original unused parts, it could be a relatively inexpensive out-of-pocket mod with no appreciable increase in build time. My only caveat is I don't know if Skystar retained or allowed for an increase of Vne when selling the option to fit the new wing, flaperons and control system to the model 3. I heard they did but I haven't found anything definitive in writing.

With the benefits of the new wing so well-documented, that bang-for-the-buck makes a lot of sense to me. Personally, if I had an unstarted KF3 kit or wings I would do this in a heartbeat. I can't think of any other single modification that would be as cost effective and result in such a notable performance gain for a model 3. Your only other alternative really is to spend many more hours fairing every little thing on your airplane for (maybe) a couple mph gain, but you will still hit that KF3 wing aerodynamic wall well short of KF4 cruise speeds.

Or, fly it 'built by the book' as most do, still a ton 'o fun!

[3benny3]

Why could one not just use the KF IV ribs when building a new wing? Is there a picture comparing a KF IV rib to a KF III? What difference in the flaperon brackets are there? And if you used KF IV flaperons why would you have to use the newer mixer? Sorry, just don't understand completely.