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/showthread.php?t=2676&highlight=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

[NotSoAmish]

The more pointed leading edge PVC parts are in our parts inventory. We also have the re-enforced flaperon link points and mass dampening weights for the flaperons. We have also increased our vertical fin leading edge area for about 15% increase and are buying the aileron differential mod kit. Still need to find out if our windshield stock material has been upgraded to the thicker one. I guess we are really after the best possible flying pleasure instead of speed.
If we go 4200 rpm with the Viking we should be consuming somewhat less than 3.5 GPH.

[Av8r3400]

Keep in mind the 1-3 were designed for the 75# 582 engine. A 135# 912 is pushing the weight limitations of this design.

I fear that Eggenfellner's 200# Honda auto-conversion will not add much performance and could possibly cause more problems than it solves on this model plane.

[Dave F]

582 readsy to roll is 110 lbs

912 is about 165 lbs

Honda -- not sure but 180 to 200 lbs ready t o go.
Impressive static test showing about 40 to 60 pounds more thrust from the honda so let's say 10% more
So take in account another 40 lbs or so that iwll cost you about 200 fpm climb rate loss so the 10% more Thrust might not make up what is lost in adding wieght.

SO many just continue to build tanks --adding weight that is just plain dumb.

My opinion but I do like the honda engines.

[Geowitz]

Not to spitefully answer questions you weren't asking, but I agree with the previous 2 posts. You may have a bigger concern with the weight and balance. On models 3's and earlier you need to pay extra close attention to not being nose heavy.

[NotSoAmish]

Vikingaircraft specifications are that it is 13 lbs. heavier than the 912ULS and the same weight as the Jabiru 3300. Are there no 912ULS in any KF3? The Viking is 178 lbs.

[DesertFox4]

No model 3's with a Rotax 912ULS (100 hp) that I know of. Several with a Rotax 912UL (80 hp) engine. I flew one for 3 years with the 80 hp Rotax on it and it was a great performer. Not sure I'd want any more horsepower on that airframe though or more weight on the front. The 80 hp. Rotax with the right prop. could easily accelerate past VNE in level flight using 80 to 85% power.

[rogerh12]

This year at Oshkosh I saw a "JUST" aircraft with a Honda Viking motor mounted. I was surprised at how compact it was. It looked good too, and the owner liked it quite a bit too. Here is a pic

http://barnstormers.com/tmp_images/65/b6/.watermarked_c57269eecc691148655d6f09ad9f5e21.jpg (http://www.barnstormers.com/ad_manager/listing_images.php?id=618766&ZOOM=ca20710804653636abd65dad16ccddc6)
I figure it weighs in around 185 lbs, but that's just a guess, might be closer to 200 lbs.
email me if you want the whole set in high deffinition.
Roger

[Av8r3400]

I spent quite a bit of time talking to these guys, too. Remember a Just Escapade is not a Model III Kitfox. It's a much larger and heavier aircraft that can handle the weight and thrust of these motors.

[desertfox1]

A Just Escapade is not a model 3, more like a model 4 with a model 3
airfoil and a longer fuselage. A 1232 airframe that became 1320.
I try to build the lightest airplanes i can each time, but I have flown
"heavy" planes that are very nice to fly, just giving up some performance.
Looking forward to see more real world experience with the Viking and
it will take experimenters to provide that for us. My planes are model 5
and 7 SS but I sometimes fly a friends heavy model 4 with 1 degree
forward swept wings to offset the heavy engine. What a blast, not as
fast but 1500 feet per minute and solid as can be. My point is that if
someone builds an airplane that is not what we fly, he is not dumb, he
is an experimenter, and we can give what guidance he asks for.

Just my opinion

Phil

[NotSoAmish]

Herea are the engine weight comparisons from vikingaircraftengines.com?
Viking 178 lbs.
Rotax 912 ULS 165 lbs.
Jabiru 3300 178 lbs.

and from Great Plains documents

VW 2180 164.5 lbs.
VW 2276 165 lbs.
VW 2180 red drive 180 lbs.
VW 2276 red drive 180.5 lbs.

Can someone explain what it is about the KF3 frame that concerns them with trying this 178 lb. Viking engine? is it torque? So no one has a VW with a reduction drive in a KF3? And what about all the KF 3s with lotus floats?

[rogerh12]

I can't comment on the other engines, but the redrive VW is very heavy (total install weight with prop) and does not get anywhere enough cooling at Kifox airspeeds to be usable ( others have found).
The direct drive VW is installed in a few Kitfox's and has a total installed weight (with wood prop) of about 185 lbs, which is on the high side, but workable (75-85 HP). A large oil cooler must be used to cool this engine in the Kitfox, but air cooling of the fins is not an issue with proper baffling. A required 60-62" diameter prop limits cruise speeds as not much pitch can be used (though 100 mph+ cruise speeds have been reported in at least one install & and a decient rate of climb).
Also, the Subaru EA-81 is used on some versions of the Avid, and despite the extra weight they still fly good because of the extra power to offset the extra weight of the engine, and also water cooling that works even at kitfox speeds. Power off the redrive at the prop is about 85 HP (though this is all info off the grape vine, I have not seen anything in print)

[Av8r3400]

I would suggest taking these numbers with a large grain of salt.

I have a 912 UL (80 hp) on my engine stand, with oil and Kitfox IV mount still attached, no coolant or prop. Per my scale it weighs 158 pounds.

Are they comparing a fully assembled weight on the Rotax/Jab versus a striped down Honda with no PSRU, oil, coolant, electrical system, etc, etc?

Remember these are sales brochure numbers.


Like the debates about VW motors, I (personally) wouldn't say don't do it. I honestly hope it works out for you and exceeds your expectations on performance. We sure could use more engine alternatives. I'm just trying to offer a devil's advocate point of view to show possible pitfalls or problems.

[rogerh12]

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.

[Dave F]

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

[Dave F]

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

[NotSoAmish]

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.

[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.

[Av8r3400]

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.

[3benny3]

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