Kitfox wings for Avid MK IV

[Luked 71]

I'm planning to build new wings for my MKIV project since I don't want to use the built wings that came with my kit (speed wings with excessive washout). Shortly after I bought my kit, I called Brett at Airdale to see about parts availability. He did call me back, but it took several days to hear back. Being the impatient type that I am sometimes, I thought I'd call Kitfox too.

 

I got a call right back from John McBean. He was very nice, and said that others have installed KF wings on Avids before. I'm pretty sure he quoted about $5000 for a full wing kit including lift struts. That was more than I was looking to spend, but I did ask if I could buy the wing construction manuals from him to see the individual parts in the KF wing. He said sure, and sent them out. I also asked if I could purchase individual wing parts, and he said that I could, but they would cost more per part that way.

 

I've been putting together a parts list for a partial wing kit to get a price, but I have a few questions about using these wings on an Avid:

 

Span - Should I stick with the 12' wing like the original MKIV wings. I'm thinking this would be the safest bet since everything else (carry through, struts, strut attach points) was engineered for 12' wings.

 

Gross weight - I read the "Wing Technical Data" thread, and it looks like the MKIV with the HH wings was good for 1150 (later changed to 1200?). The HH wings used .083" wall spars, and 7/8" struts. I've got the 7/8" struts that came with my kit, but the KF spars are .063". They do have the "I" beam stiffener though. I think later KFs use this same spar, and have higher gross weights. I'm thinking it should be pretty safe to set my gross at 1150. Does this sound reasonable?

 

Wing tips - This is more about personal taste I guess. At first I was not a fan of the Avid wing tip style, but it's starting to grow on me, and I think it is kind of part of the classic Avid look. I'm thinking of putting my old Avid tips on KF wings. Any issues with this?

 

Gas tanks - I'd like to have the option to run auto fuel. Does anyone know if the KF tanks can handle auto fuel without any issues?

 

Guess that's pretty much it. What do you guys think?

 

Thanks,

Luke D.

 

 

[Av8r3400 1178]

Span - You can build what ever you would like.  The Kitfox oem span would be fine.  They work nearly as well one rib-bay short - The Kitfox Speedwing.

 

Gross Weight - The same wing, .065" wall spar with 60" I-beam inserts holds a recommended 1550 gross on the Kitfox 5, 6, 7.  1200 is just fine.

 

Wing Tips - My friend built his Hybrid Avid-Fox using the Avid style, fiberglass, fabric covered tips.  They work just fine.

 

Gas Tanks - The new Kitfox tanks are ethanol resistant, from the same vendor that Brett/Airdale uses.  Ethanol resistant does not mean ethanol proof.  I don't believe any fiberglass is ethanol proof.

 

Flapperons - Something you didn't mention, I would say you should use Kitfox symmetrical flapperons with the Ribblett-airfoil Kitfox wings.  They work much better with that wing than the flat bottom Avid Flapperons.  Additionally the mounts and hangars for the Kitfox wings may not work with the Avid flapperons.

[Luked 71]

Thanks for the detailed answers. I forgot to mention the flaperons. I will need those too since the flaperons I have are only 9 feet long. I'll probably get KF parts like you suggest to be the most compatible with the wing.

 

I wonder what the KF tanks are made of. Must be vinyl ester. I've read that most vinyl-ester resins are ethanol resistence, but like you say not ethanol proof. I've read where some guys say that they have used auto fuel in vinyl-ester tanks for 20 years with no problem though.

[Av8r3400 1178]

It will always be best policy to avoid ethanol at all costs.  

 

That being said, I believe the new style tanks are the best bet when using auto gas that may or may not be contaminated by ethanol.

[EDMO 1078]

I had to laugh a little on the flaperon post - not at anyone here, but at Kitfox:  At some point with the Kitfox Classic 4 or 5, they changed from the flat-bottomed flaperons to a more symmetrical style that Harry Riblett recommended.  Then some model later, probably on the 6 or 7, they advertised a modification kit, "new, more efficient flaperons with the flat bottom"!  Go figure!   Kitfox owners were not impressed!

     You could save some money and use your old flaperons by extending the pivot tube out, but you would still have to modify your attachments to fit the later style Kitfox ribs.

     Cheapest route, but practical, would be to extend your wings, (or not), modify the rib bottoms to match the Kitfox Riblett style, and Keep your flaperons.   This would make up for some of the washout and give more lift and cruise.  I am doing that to my undercambered wings.

my 2c worth.

EDMO

Edited 8 Apr 2014 by Ed In Missouri

[Av8r3400 1178]

My friend who built the hybrid used the latest Kitfox flapperon design.  It is in fact the symmetrical one.  This was what Riblett suggested with the IV and later design airfoil.  All of the 4, 5, 6, 7, and SS Kitfoxes I've seen have them too...

 

I've seen plenty of modified Avid "flat bottom" wing ribs - otherwise known as the Avid Speedwing.  One of the worst flying wings ever.  Point of fact.

[EDMO 1078]

Kitfox even offered to sell owners the "new, flat-bottomed flaperons" to replace their symmetrical ones at one time.  This was met with disgust by owners who knew about the earlier design.

This "Riblett modification" is like the later Kitfox ribs, and not flat-bottomed like the Avid Speedwing.

So how were these Speedwing ribs that you saw, modified?

EdMO

Added:  Kitfox struts wont connect to Avid fuselage fittings - at least I know that the earlier models were not the same.

Edited 6 Apr 2014 by Ed In Missouri

[Luked 71]

Yeah, good point about avoiding ethanol. Still it's nice to know your tanks won't desolve if you get some in there. The KF tanks sound like to way to go though.

 

Ed, I think the current wings just have too much twist to correct very easily. From what I measured, they had a little over 2" of twist, and these are speed wing length. If I extend them, the twist will get worse. I thought about trying to tear them apart to re-use what ever I could, but the more I thought about it, the more I started thinking I probably wouldn't end up salvaging that many usable pieces.

 

I don't know much about the history of the KF flaperons (I think I might have read that story about KF changing back and forth on the airfoil though), but I have the latest build manual for the SS. It's kind of interesting when you look at the CAD drawings in the manual. They definitely look pretty symetrical. I wouldn't say that they are perfectly symetrical though. If you put a straight edge on the diagram from the leading edge to the trailing edge, the chord line goes below the center of the torque tube by a fair amount. On the other hand, it's sure not a flat bottom like my Avid flaperons either.

[Av8r3400 1178]

The Speedwing came "modified" right from Avid.  They were't totally flat, they had a very slight positive camber to them, and they work for shit, long or short span.  As you already know.  Anyone who has flown this wing compared to the under cambered or the Kitfox-Riblett wing will attest to this - if they are honest.

 

As to your modified, extended Riblett copy, I wish you the best of luck with that.  I hope it works for you.

 

For his hybrid (heavily modified Avid C Fuselage, Kitfox IV+ wing), my friend built his own lift struts.  He used one adjuster per wing, not the four used by Kitfox or the one used by Avid.  If I were to build my own struts, I would use streamlined tubing, which would negate the need for farings, wood or plastic.  Like what Just uses for their planes.

Edited 6 Apr 2014 by Av8r3400

[dholly 718]

... and they work for shit, long or short span.  As you already know.  Anyone who has flown this wing compared to the under cambered or the Kitfox-Riblett wing will attest to this - if they are honest.

 

Hey Leni and Jim C, Larry just called you guys liars again!

 

Larry, all bullshit set aside, I would respectfully say you need to temper such all-encompassing blanket condemnations. Many, many variables all come into play... weight, engine, wing twist yada yada, but the way you blast this airfoil you'd think every speedwing was one dry fart short of dropping from the sky. As I understand, the speed wing airfoil performed as Dean envisioned on the early, lightweight Avid models. Problems seem to be most common when folks ask too much of the airfoil. Perhaps Joey or another of Deans friends here could get the scoop straight from the Boss next time they see him?

 

I would only add that over the years I've spoken to or read comments from folks who used speed ribs on long spars and by and large give good reviews. I know some have a bucketload more brains than I ever will, and I certainly don't consider them all liars. Here were a couple comments off the yahoo list I saved for the numbers:

 

 

I put the wing tip extensions on my MK4 speedwing very early in it's construction. By looking at the finished wing, it does appear to have an excessive amount washout, but it's an illusion.

My manual has instructions on building both the hauler wings and the speed wings (Section II, Chapter 4). In the case of the Hauler, the spar length is 144" and the washout 2" over that length (.1666" of washout per foot). The speed wing spars are 108.5" long with washout of 1.5" over that length (.1658" of washout per foot). Since the extension kit lengthens the Speed wing to that of the Hauler, the washout at the tip is the same as the hauler, it's still .166" per foot.

My buddy has a Avid-B STOL wing, we both have the same engine and my plane is about 40-50lbs heavier. The difference in performance between our planes is very small. He climbs a little faster then me and I fly a little faster than him. But overall, I think the extended tips got me very close to STOL performance.

Dan H.

 

And from a -double gasp- Sube powered extended speedwing flyer:

 

 

Brian,

Your kit is almost certainly a C model. Your engine choice will work just fine with the stretched fuselage. I have a late C model that has all the Mark IV upgrades plus the stretch. I have extended speedwings and a Stratus EA-81 Subaru for power. I cruise on average at 100-110mph ground speed depending on how much fuel I want to burn. 3.5 gallons an hour gives me 100mph most any time. The only time I find the engine a little heavy is on the back side of a loop ) . Get 'er built and have a blast. I've been flying mine now for 15 months and have 262.7 hours on the hobbs. Just did a weekend camping trip flying 1.9 hours, averaged 98 mph groundspeed (landed at two other airports to visit on the way) and burned 6.6 gallons of fuel. I don't think the Stratus or the Franklin is "way too heavy" for the plane to still be a real performer. BTW, I don't have any weight added to the tail either. The only thing back there is the tail feathers and an ELT.

Andy F.

 

No comment on the Franklin power, but I would take either of those other planes over no plane! Would I want short speed wings on an over gross B, C or even on a Mk-IV w/ 65hp or less? Hell no. Would I want a speedwing with excessive washout? No. Would I ever want a speedwing with extentions? Maybe. Would I build a long spar with speed ribs to heavy hauler specs and standard or less twist than called for in the manual? Yep, exactly what I did for my Avid+. No regrets.

 

However, knowing what I do know now, I probably would have un-glued the three ribs that were installed on one wing by the previous owner when I bought the kit. He had set twist at 1-1/2", less than the 2" called for a long spar wing in the manual, but I would have done 1-1/4" and maybe even less. My KF 4-1200 washout is only 1". Either way, with an 80hp 912 I think it'll hit Vne and perform just fine.

 

As always, YMMV.

[Av8r3400 1178]

With all due respect, Doug I base that opinion off first hand observation of three different planes. A 582 short wing, a 582 long wing and a RR 670 extended wing (short wing with inserts).

The best of the three was the 582 long wing. At 600# empty it would do okay solo, but any more than a light load it was a slug, even at our low altitude. Takeoff, climb and handling was inferior to a HH wing. It would go a little faster, but not as fast as a similar loaded Kitfox IV wing, flown along side my plane. (Yes I understand the power difference)

The short wing was a fatality looking for a place to happen and was quickly sold. I think it got scrapped.

The 670 just was never reliable enough to make a showing. When it did run and fly the performance was no different than the 582 long wing. It also was sold and has never flown again to my knowledge. That owner now has a 582 powered Kitfox IV and is happy as a clam.

These planes were all brought to my home airport for the resident LS-CFI (Steve) to do owner checkouts in.

Edited 6 Apr 2014 by Av8r3400

[Luked 71]

I don't have any experience with the Avid speed wing airfoils, so I'll leave that to others who do.

 

EDMO PMed me with some things to look out for on the struts and strut attach points though. After talking about this with him, I went to the airport where my wings are to check some things. I thought I would just add what I found. Most of the experienced Avid guys probably know all this, but it may help someone else like me down the road.

 

Note: all of my references to the KF manual are from the two that I have which are:

 

Kitfox Super Sport wing Assembly / REV A / Date:05/11

Kitfox Super Sport wing Assembly Supplement / REV A / Date:05/11

 

Strut attach location - I measured my pair of short wings, and the struts attach at 80" (from end of tube to bolt hole). This is also what the Avid manual shows for HH wings (144" spar tubes). The KF manual shows 95" at the front spar, and 94.5" at the rear spar (162" spar tubes).

Strut attach bracket lug spacing - Avids only have a rod end on one of the four struts, the three without rod ends are just slightly wider than the strut tube at the end. Mine appear to be 1" wide. The spacing on my strut attach brackets also appears to be about 1" wide. The pictures in the KF manual look like they have a narrower spacing. It's just a little wider than the rod end, so that a washer can go on either side of it.

Right now I'm thinking of modifying the ends of my struts to accept rod ends so that I can use them. I figure that I can build my wings to HH dimensions, and everything should line up that way. Also, the shorter HH wing length should keep everything basically stock structurally. This seems like the simplest solution.

The other option would be to make new struts to the KF dimensions, and build the wings to KF dimensions. Or, I could buy KF struts, and see if I can modify the bottom end to work with my Avid fuselage connection. This option seems more complicated, but might mean a little better STOL performance with the slightly longer wings.

Thoughts?

 

One of the strut attach points

 

Measurement from end of spar tube to strut attach point.

[EDMO 1078]

Luke,

      I believe my Kitfox wings have the strut attach at  85 inches - will check on that for you.

sometime, I think, that these makers change things a little to keep you from using your old parts???

EDMO

My early Kitfox book says 86 5/8.   I don't think it makes much difference as long as you go by the book and set up your struts with the temporary brace and your dihedral is correct before riveting the attach fittings.

Added:   My Kitfox 4 book says 96.75 for the long wing, and the Speedster is another inch longer, and I believe that is because the shortwing don't have as much dihedral, and struts only come in one length. 

They must have made the struts longer for some reason.

Avid Magnum gives no dimensions, only to set up with temporary support and locate attach plates.

From an amateur-engineering viewpoint - it looks like somewhere around 80 inches would be half-a-wingspar if you add the tips to the long wing.  And, 3/4 of the lift is on the inside 1/2 of the wing.

 As you make struts longer, the diameter has to increase because there is more bending potential, IMO.

Dean designed this originally, Denny and Skystar and McBean Kitfox  have all made changes - now who is right?

Edited 7 Apr 2014 by Ed In Missouri

[akflyer 1427]

I am late to the party on this one.. but as I have said many times in the past.  Flying my HH and my buddies MK IV extended speed wing, I could get the speed wing in and out just as short as my HH and RPM for RPM his was about 5-7 MPH faster.  Additionally, his roll rate was better than mine.  I have thought many times about building new wings to kitfox length using the Avid speed wing rib and only 1/2" total washout. 

 

I have not flown the KF IV airfoil yet but will someday so right now I can only compare the AVID extended speed wing to that of an Avid STOL profile.

 

I do NOT think it fair to have 3 different guys flying 3 different planes and call any type of contest STOL, spot landing etc a fair comparison to base any type of conclusion off of.  How many times has Paul Clause kicked the ever loving shit out of other cub drivers at valdez.  I know you can take the exact same cub and 10 different cub drivers and get 10 different take off distances and landing distances.  I am not saying that one pilot is inferior to the other, but to have a real comparison that you can hang your hat on, you need the same guy flying the same plane with enough time in each one to be comfortable with it.

 

I can get in Bobs kitfox and get it in and out considerably shorter than he does.  Bob is a good stick, I just fly a little closer to the edge than he does at times

 

There is NO WAY IN HELL I will take the 180 into the same lakes or strips that the previous owner would take it in his sleep.  Or that My buddy Mark would take it into.  I just cant or better yet wont compete with some guy that has 20,000 hrs in type and flew it for a living.  Give me a couple hundred more hours in it to learn just exactly how it wants to be flown in various weather conditions and weights and I will be singing a different tune, but today, I would not begin to think I could compete with them.

 

One thing that really stands out is just how fast the trees on the other end of the lake come at you when your looking for 90 MPH before you get out of ground effect and pull back on the yoke.  In my avid I will break water around 42 MPH and climb out at 50 all day long.  In the 180 the first float rolls out around 55-60 and both out around 65, hold in ground effect and dump a notch of flaps, look for 90 then pull back.. Two totally different worlds

 

[Luked 71]

Luke,

      I believe my Kitfox wings have the strut attach at  85 inches - will check on that for you.

sometime, I think, that these makers change things a little to keep you from using your old parts???

EDMO

My early Kitfox book says 86 5/8.   I don't think it makes much difference as long as you go by the book and set up your struts with the temporary brace and your dihedral is correct before riveting the attach fittings.

Added:   My Kitfox 4 book says 96.75 for the long wing, and the Speedster is another inch longer, and I believe that is because the shortwing don't have as much dihedral, and struts only come in one length. 

They must have made the struts longer for some reason.

Avid Magnum gives no dimensions, only to set up with temporary support and locate attach plates.

From an amateur-engineering viewpoint - it looks like somewhere around 80 inches would be half-a-wingspar if you add the tips to the long wing.  And, 3/4 of the lift is on the inside 1/2 of the wing.

 As you make struts longer, the diameter has to increase because there is more bending potential, IMO.

Dean designed this originally, Denny and Skystar and McBean Kitfox  have all made changes - now who is right?

 

Hey Ed,

 

Thanks for all the extra info. I just realized you added more. The more the better. It's all interesting. Everything you said makes sense to me. My thinking is not so much who is right, but what configurations are well documented, have been around a while, and have a successful track record in the field. I'll probably just go with a fairly conservative approach (stock MK IV structure). I don't want to be wondering if things are going to hold together down the road. 

 

 

 

I've got one more question for any Kitfox experts. I saw some pictures of a KF wing being built, and it had a fiberglass leading edge. Is this standard, or an option? The build manuals I have don't have any mention of this that I noticed. I was hoping to keep things simple and light. In other words, just fabric on the leading edge. Thanks for any info you can give.

 

I spoke with Kitfox yesterday, and they told me to email my list of parts for pricing. Just waiting to hear back now. They are probably a little busy winding down form SnF.

 

Luke D.

[EDMO 1078]

Luke,

     I should have added to "when struts get longer, the diameter needs to increase to prevent bending" - (and Gross weight increases.)   I would be interested in the price of the flaperon hinges and the rib attach fittings when you put your list in.

     The fiberglass front covering is optional - weight increases, but forgot how much.   I think the 12 inches of  1mm plywood on top, and the 6 inches on bottom, plus the glue, may have added 6 lbs - but forget now.  Anyway, I needed it because I extended my leading edge 4 inches.

     Let me know anytime you have questions.

EDMO

[dholly 718]

[edit] looks like Ed beat me to it!

 

I've got one more question for any Kitfox experts. I saw some pictures of a KF wing being built, and it had a fiberglass leading edge. Is this standard, or an option? The build manuals I have don't have any mention of this that I noticed. I was hoping to keep things simple and light. In other words, just fabric on the leading edge. Thanks for any info you can give.

 

Depends which model KF and exactly how you define "leading edge".

 

KF1-3 had no leading edge treatment to my knowledge. The Riblett airfoil first used on the KF 4 includes an extruded PVC leading edge extension, many retrofitted this extrusion to earlier KF and also Avid Flyers. Currently, KF offers the 'Laker Leading Edge' some kind of composite sheeting that covers and smooths the first 1/3 or so of the top airfoil surface, and is used in conjunction with the PVC extrusion. Aside from the PVC extrusion, I think 4-7 models simply depend on a copious amount of both upper and lower nose ribs. At one time, I do believe KF tried a fiberglass leading edge top sheet without the extrusion on early models but nixed it due to excessive oil-canning during wing flex. Same with Avid, but they used an aluminum leading edge top sheet which was in the Mk-IV construction manual.  

Edited 9 Apr 2014 by dholly

[EDMO 1078]

Luke,

     I forgot about the plastic leading edge attachment - I call it a Spar-Saver - well worth putting it on, IMO.  This is not a covering for the top - just the front of the spar.  It comes about 6 to 8 inches longer than the spar, so you can blend it in to tips.

EDMO

[Luked 71]

Thanks guys. I assumed it must be an optional thing. I included the PVC extrusion in my list.

[EDMO 1078]

Luke,

     Before you get ready to glue the plastic leading edge in place, you want to make sure you have it right side up - there is a difference.    I copied the drawing out of the book and made some templates to use to get it at the proper location along the wing.

If you will PM your address, I will send you one to use as a pattern to make more - You need about 6 or 8, depending on how particular you are.

      Best way I have found is to use these to line up the plastic, and go on the bottom side of the plastic and tape securely all along the bottom - then remove the guides, bend the plastic down a little, and put the glue on - then put the guides back on and tape everything until the 9460 sets up - like next day.

     Hope this helps.

EDMO

Edited 9 Apr 2014 by Ed In Missouri

[Luked 71]

Thanks for the tip Ed. I'll refer back to this when I get to that point. Wouldn't want to get it upside down. I think I have a full scale drawing of the leading edge in the manual I got from Kitfox, so I could probably make the templates myself like you did.

 

Luke D.

[EDMO 1078]

Just to add a little info to this old topic - I only recently learned that the Kitfox 4 and later symmetrical flaperons, and I guess the "new and improved" flat-bottomed (rejected by most builders) flaperons, are about 2 inches longer chord than the ones for models 1, 2, 3.   That's about a 20% increase in flaperon area and should be a lot more responsive.  EDMO

Edited 23 Aug 2017 by EDMO