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Thread: KF3 wing convert to KF4?

  1. #21
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    Default Re: KF3 wing convert to KF4?

    From others, I have "heard" the 912 80 HP engine has an installed weight of 165 lbs, not counting the engine mount or firewall accessories (battery, gassolator and fuel pump), but does include fluids and the propeller (Can anyone verify this ????)

    As far as the VW, I should point out the VW direct drive can actually be built anywhere between 55-85 HP, and they are all basically the same weight (about 165 lbs without intake, exhaust ect…). The difference in HP ratings are because the displacement range is from 1600-2273 cc, and the compression ratio can be set to run only Avgas, or reduced down to allow 87 Octane Car gas (and anywhere in between). Once you get over 1910 cc’s, you do have to use a Stroker crank and the cost of the long block about doubles (a 1910cc engine will still give you about 65 HP max on Avgas) Also, aluminum cylinders can be used to reduce the engine weight by 10 lbs and a “Fly-wheel” drive version is available too, it weighs about 7 lbs less.

    The Honda engine looks really sweet though, I would love to give one a try, but I bet the 178 lbs weight quoted does not include the intake, exhaust and other things that would likely give a total installed weight (with prop) of about 200 lbs, which would be a little less than what a Subaru Ea-81 engine or certainly less than a Corvair engine install would be. Parts would be cheap though, I bet.

    As far as my plans for my Kitfox-4, the particular VW engine I am planning on building will be the 2273 cc VW engine, set to run on 91 octane car gas (which we have here locally, made without alcohol) and will produce about 75 HP max or 68 continuous at high speed cruise. With a full electrical system and dual ignition, oil cooler and Ellison injector carb, the total installed weight is estimated to be about 185 lbs, but I might spend the extra $1000 to get the aluminum cylinders, which would drop the weight down to about 175 lbs (not counting the battery, gassolator and fuel pump on the firewall). A light weight (4.4 lbs) ATP ULTRASTART RED battery on the firewall will also help to keep the total engine install weight down.



  2. #22

    Join Date
    Oct 2011
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    Canada
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    Default Re: KF3 wing convert to KF4?

    Quote Originally Posted by rogerh12 View Post
    From others, I have "heard" the 912 80 HP engine has an installed weight of 165 lbs, not counting the engine mount or firewall accessories (battery, gassolator and fuel pump), but does include fluids and the propeller (Can anyone verify this ????)

    As far as the VW, I should point out the VW direct drive can actually be built anywhere between 55-85 HP, and they are all basically the same weight (about 165 lbs without intake, exhaust ect

    Roger -- the problem with VW is higher RPM needed when using direct drive. With PRSU you might be bedter perfomance but more weight.....

    Jabiru - lower RPM than VW -

    Rotax hard to beat over all. Price sucks I know but some guys will go out and spend 60k plus to build a Kitfox
    same plane sells built for less than half to cost to build from kit.
    This helps make the Rotax engine pricing look better.

    Some enjoy building but the reason why home builders started was it was supposed to be cheape toi build than buy a Certified plane........
    Cessnas are now cheapoer used than ever.

    Dave

  3. #23

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    Default Re: KF3 wing convert to KF4?

    158lb sure makes the 165 lb alo more achievable. No need to wight prop as every engine will need one.

  4. #24

    Default Re: KF3 wing convert to KF4?

    Nice engaging conversation here. Thanks everyone. I will ask Jan and Viking what his total weights are and what does his 178 lb. number include.

  5. #25
    Senior Member dholly's Avatar
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    Default Re: KF3 wing convert to KF4?

    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!
    -Aeropro CZ Aerotrek A240 Tri-Gear SLSA 912uls
    -Airdale Avid+ on CZAW Amphibs 'FatAvid Floater' (building)
    -Kitfox 4-1200 TD 912ul (sold)
    -Kitfox Model III TD 582 (R.I.P.)
    -Avid Flyer Mk-IV TD (sold)

  6. #26

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    Oct 2011
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    Default Re: KF3 wing convert to KF4?

    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.

  7. #27
    Super Moderator Av8r3400's Avatar
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    Default Re: KF3 wing convert to KF4?

    Benny, yes that is a possibility. BUT, there is a chain reaction of additional changes that will be needed.

    • Model IV wings mount the flapperons differently, so IV+ Flapperons and hangars will be needed.
    • Model IV flapperons have different control needs so the IV mixer is needed.
    • Model IV mixer is different so there will, at least, need to be modifications to the flap control and torque tube pushrods/linkages
    • ...and so on and so on

    This is do-able. Definitely. You will need to be up on your fabrication skills, though. There is a non-posting member here, a good friend of mine, doing something very similar to this. He is making an Avid-Kitfox IV hybrid.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  8. #28

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    Oct 2011
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    NC
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    Default Re: KF3 wing convert to KF4?

    Anyone have any pictures of the difference in the flapperon hangers, or exactly what kind difference it is?

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