Issue putting KF wings on Avid

33 posts in this topic

Posted

Well, when I bought my Avid MKIV, the wings were built wrong, but I figured  for the price I paid, I could afford to buy some Kitfox parts and build KF wings to fit it. I've found that there are several issues that need to be addressed to make this work though.

 

The first issues were pretty easy to deal with:

 

Lift strut attachment - The KF spar brackets are all made to accept rod end bearings instead of the wider end of the strut tubing that the Avid has. I figured that the easiest solution is to just weld threaded fittings into the ends of the struts.

 

Flaperon location - Because the flaperons hang on the KF wings differently, they end up in a different spot relative to the support bearing on the fuselage. No problem, just modify the support bearing and linkages as needed.

 

Then the bigger issues:

 

Wing incidence and wing folding - Because of the differences in the KF airfoil and the Avid, the mounting points at the fuselage position the spars differently between the two planes. On the avid, the front spar carry through is about 1.4" lower than the rear (using the lower longeron as a reference for level). You could adjust the angle quite a bit by modifying the height of the tubes that the spar pins go through. Even if you do that though, the best I can get is about 1.7° more negative than what the KF has. Also, it appears that the flaperons will just barely clear the turtle deck when folding the wings. You could adjust it to get more clearance for the flaperons to be safe, but this would make the incidence angle even worse.

 

Has anyone dealt with this before? Larry, I know you had a friend that did this. Any info on what he did?

 

I'm thinking the only way to do this right is some pretty extensive cutting and welding...

 

Thanks,

Luke D.

 

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Posted

Luke,

    The Kitfox incidence was changed to make the tail higher in flight - You can cut and adjust your spar attach points to make it match the Kitfox as long as your struts fit.    There seems to be a lot of engineering done to change the wings, but anything is possible - just more headaches to go along with it.  I believe the rear spar on the Kitfox 4 wings sets differently in the rib - this has to also be considered.

    such is the nature of "true experimental" building.

Keep us posted.

EDMO

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Posted

Yeah, I've pretty much decided I just need to bite the bullet and make some pretty big changes to make it work. I will actually be solving a few problems at once, but it will add a bit more time to the project.

 

I measured the incidence on two late model kitfoxes (5 or 6 can't remember). They both have the same airfoil that I'm using, and both had the airfoil at about 0° of incidence. It's probably not really 0° (I know angle of attack is usually measured through the cord line). The way I measured should serve my purposes though. I just put a straight edge on the bottom of the wing (on the first rib next to the fuselage), and measured the angle. I compared that to the bottom of the door opening. They were almost exactly the same angle. I used a very sensitive digital level to do this.

 

Both planes also have the movable HS, and I asked the owners to set the trim for cruise. When I measured the angle it was about 0° also. That's pretty close to where my Avid HS is. I figure I'll be happier in the end if I get this right. I'm not very tall, so I don't want to sacrifice any visibility over the nose.

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Posted (edited)

Mine is about 1"+ higher on the rear spar mount, like yours - Since I have now modified my ribs to the Riblett type, I was wondering if it was worth the trouble to cut and reweld the top - 1"+ on 2 feet chord would give roughly 6" from cabin to tail, and I have increased the height of my Rudder about 10 inches.

     Since the Riblett airfoil has less downforce on the nose than the Eppler ribs, the incidence could be changed as you stated to give a higher tail position - but it might be easier to raise the front mount rather than to lower the rear mount and have problems with flaperon clearances.  You could put a filler strip at the top of the windshield.

     You need to make all the changes on the wing mounting before you put the threaded inserts in the top of the struts, so the struts are the right length to fit.  I made my inserts with 7/16 - 20 threads to fit my rod ends which are stronger than the 5/16 diameter originals.

Luke - Did you give the incidence of your horizontal stabilizer of the Avid?

EDMO

Edited by Ed In Missouri

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Posted

I don't really know a lot about the airfoil characteristics Ed. I'm sure what you said is correct. I didn't really want to take the time to dig into all that. I figured if I imitate the KF configuration as far as angle of attack and HS angle, it should get me pretty close.

 

I've decided to go with the second option you mentioned, and raise the front spar carry through tube. The only problem is that when you tilt the airfoil up more in the front, it makes the flaperon clearance worse for folding. To counteract that, I'm going to change the way the rear spar attaches to the rear carry through tube to raise the rear spar as high as I can. It looks like that will give my flaperons about 1/2" of clearance over the turtle deck.

 

While I'm  at it, I'm going to change the front diagonal brace tubes that go behind the windshield. Because raising the front tube would make those brace tubes even more upright (looking from the side) than they already were, I'm going to change to the Kitfox style of "X" brace. Because they go right to the top motor mount bolts, it will give them quite a bit more "rake". I'll post some pictures of my computer models when I get a chance.

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Posted (edited)

Luke - I wish you could have met Dave, he was only at OSH for Tuesday.  He's a wealth of practical knowledge about aircraft building.

 

He chose to completely convert his Avid C.  He retrofitted the Kitfox IV control mixer system along with the airfoil and symmetrical flapperons.  He didn't set his up to fold so that was not an issue for him, but the incidence and other wing settings were not changed either.  He built the wings right from the Kitfox IV builders instructions for washout and dihedral.

 

He did not raise the overhead, with all of the complications associated with that, but chose instead to lower and extend the footwell in order to make the cabin human sized.  

 

IMO, raising and changing the overhead is opening a pandora's box of issues that will effect the handling and flying characteristics of a great, proven aircraft design.

Edited by Av8r3400

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Posted

Larry, thanks for the details on Dave's plane. I wish I could have talked to him. I considered leaving off the wing fold capability, but I'm kind of convinced I should keep it now. I'm still thinking through my options. Right  now I'm still leaning toward raising the front tube. It seems like it would solve my problems, and not create any new ones (at least not many). It will probably cause issues with the struts, but they are an issue I have to deal with already.

 

Here is my current  plan in a nut shell:

 

1. Remove some of the front structure (those front uprights need to be extended).

post-760-0-08750100-1409462397_thumb.png

 

2. Make some particle board jigs using Solidworks to get the new structure true and square (also note the clamp on collars that would have temp tubes welded to them to hold the rear tube in place during welding).

post-760-0-63055000-1409462487_thumb.png

 

3. Here is what I would hope to end up with.

post-760-0-67918500-1409462522_thumb.png

 

4. Another view with all of the tubes in.

post-760-0-83592600-1409462792_thumb.png

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Posted

I have to ask, what is the design goal of raising the front carry through tube?

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Posted (edited)

Larry,

     He is trying to match the Zero degrees of wing incidence that the later Kitfoxes have with the Riblett wing.  I'm not convinced that it is necessary yet - I just read a chapter in my aircraft design book, and it says that most planes have about a positive 3 degrees of incidence???

     I am going to chew on this bit of jerky a while longer before making up my mind on the change of incidence - Thinking right now that with my heavy Soob and nosegear that maybe I need a little more up-force on the nose, so why lessen it by going to zero incidence - I think the original Avid/Fox incidence might just be OK with the new wings.

EDMO

Edited by Ed In Missouri

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Posted

The avid speed wing and the stol use the same mount points.. when riblet did the mods to the stol wing he did not change the aoa. .. I would be hard pressed to think that you need to change the aoa on your plane. Just my .02

:BC:

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Posted

Hey guys, thanks for the feedback. I only ran across this a few days ago, after measuring the two Kitfoxes. Maybe I need to keep looking at it some more to see if I missed something that I'm not taking into account.

 

To answer your question Larry, the reason I was thinking I might need to raise the front is because I can't replicate the angle of attack on the KF and make the flaperons clear the rear deck when folding the wings (I actually can't quite replicate it even if I didn't worry about folding the wings). There are a few other small side benefits when making this change as well, but they are not required.

 

I guess I'm just a little paranoid about putting all this work (and money) into building new wings, and having the plane not fly quite as nice as the plane the wings were designed for (KF). Honestly, when I hold the KF airfoil up to my fuselage, the angle just doesn't look right :o  The nose of the rib looks like it's pointing downward quite a bit. With washout, the tips will have an even lower angle of attack. I would expect the root to have at least a little positive angle of attack relative the the stock position of the HS. I haven't looked up the true chord line on the Riblett foil, but would assume that it runs from about the center of the LE radius to the tip of the TE. The angle of this airfoil when it's mounted on the KF is a lot closer to what I would have expected.

 

My thinking on this wing change was to not have to take the time to analyze the design too much. I just wanted to keep everything as close to the KF as possible because it's a known working configuration with this exact wing airfoil, flaperon airfoil, etc. I'll do some more looking at it. I appreciate everyone's input.

 

Thanks again,

Luke D.

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Posted (edited)

Luke,

     Did you check the sizes of the KF 5/6 cross tubes and the vertical tubes at each corner?  I think you will find that Skystar increased the diameter and wall thickness of these tubes for the increase in gross weight - The easiest thing you could do is to just add a 1.4" spacer under the front tube - If you make new cross tubes, you need to make sure the small upright attachment tubes of bushing metal are all exactly vertical, especially the rear tubes.   The boards you suggest would be good for tacking up the complete top before attaching to the cabin, but the top should be clamped to a steel welding table before finish welding so it don't warp.

     Make sure you don't change the door support tubes, or your doors wont fit......

As my flying buddy says, "You change the spinner on the nose, and it affects everything back to the rudder!". 

HAPPY BIRTHDAY Leni, and Bandit,

     Thanks for the info - I should have realized that Riblett only modified the ribs, and did not change the incidence on the early models - I really don't need more nose-down force by modifying the incidence - I can get more than I want by lowering flaps - but still calculating how much effect this would have.

EDMO

Added:   After thinking about this for 5 minutes, I realized that Skystar must have gotten the incidence wrong on the later models - Otherwise, why would they have to set the flaperons at a minus 3 degree angle to counter the nose-down force of the wings to get a better cruise?

- Guess Dean Wilson had it right from the first?

EDMO

Edited by Ed In Missouri

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Posted (edited)

Hi Ed, I didn't notice those tube sizes. I'll have a look next time.

 

I went searching for more info on this, and google turned some information on  the Team KF forum:

 

It will take the "Old Timers" here to verify this, but way back when the Model 2 was introduced, right around that time the factory made a change to the incidences that made it easier to see over the round cowl in flight. 


It helped that, but unfortunately it also killed some of the cruise speed. Let me guess...your Model 2 cruises about 75 mph? If so, you probably have one of the Model 2's with the incidence change. If you look back at old articles with pictures of Kitfoxes in flight you will see the 2's did in fact fly tail high. The incidence was changed back for the 3's, and ultimately Skystar got things figured out when the Model 4 was analyzed by some new engineers. 

Or at least that's the way I heard it... And it does kind of make sense because the 4's definitely fly the best of all the early models.

 

 

 

Edit: I should have clarified that this quote below is not related to the quote above. It is the measurement of a "Series 5" Kitfox. My understanding is that once Kitfox updated the design on the KF4, the wing geometry stayed pretty much the same from that point forward. Anyone correct me if I'm wrong on that.

 

 

The measurements as you requested, 37-15/16" fwd. as measured from a perpendicular from the bottom of the fuselage to the bottom of the Spar lug. Aft it is 37-7/8 from the bottom of the spar lug to the bottom of the lift strut attachment lug.

 

This second quote matches what I measured almost exactly. There are some other interesting comments on the KF forum, but these jumped out at me. I'm still reading some more over there.

Edited by Luked

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Posted (edited)

Luke,

     That measurement only shows 1/16 inch difference - not what I measured - my model 2 measured about 1 1/2" higher on rear crossover - and that should make it "nose high" - NOT "nose-low" as the writer stated.  It also made the tail lower, and this was changed to make the tail higher by Kitfox on the new Riblett wing for the model 4.   Could be that as the engines got heavier, that the original incidence was better than what Kitfox had modified, and that is why they use a negative flaperon angle fro cruise?

      Wasn't life much simpler when we were building model airplanes?   :lol:

EDMO

Edited by Ed In Missouri

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Posted

That's true about model airplanes. You can make just about anything fly on a small scale :P Of course we weren't riding in side to see how crappy they really flew!

 

I edited my post above. I think I confused the issue the way I did the quotes. They were from two completely different threads, and not related to each other. I also didn't mention that the measurements quoted were from a later KF.

 

From everything I have measured and read, the later Kitfoxes had the bottom surface of the airfoil almost parallel with the lower longeron, or lower door sill. This measurement is not technically the true "angle of incidence", but only for comparison. That is pretty much settle in my mind. The main questions seem to be:

 

- How much difference will a couple of degrees of incidence make?

- Exactly how do you compare the angles of attack on airfoils that have different characteristics (pitching moment, chord line location, etc)

 

I thought it was interesting that it seems a small change in incidence made a difference in cruise speed and forward visibility.

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Posted (edited)

I thought about changing the incidence in my wings to match the KF4 when I started this project - but since I was using the undercambered Eppler ribs, I decided to copy the KF2 - The KF4 was designed to use the 582 engine and engines have gotten a lot heavier - I am not going to change that now that I have modified the ribs to the Riblett pattern, and extended the leading edges, because it will still be nose-heavy.

EDMO

Edited by Ed In Missouri

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Posted

Well I guess I'll jump into this as well.  Did some measuring on fuselages today.  Kitfox 1 and 4, and Avid C and MK IV.  Yes I know I have to many unfinished projects!  Anyway, I measured from the top of the front and rear wing carry throughs straight down to the bottom of the fuselage.  Avid C and MK IV were 36" front, and 37 1/2" rear.  Kitfox 1  36 3/4" front and 38" rear.  Kitfox 4 was 38 1/2" front, and 39" rear.  Not sure if this helps or not, but that is what I got.  Jim Chuk

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Posted

As I said, Dave didn't change any of the incidence on his Avid C fuselage when he went to the Kitfox IV wing and it flys very, vey nicely.

I think you guys are way over analyzing this.

What fuselage are you starting with, Luke?

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Posted (edited)

Jim,

     The other thing to take into account is that the Kitfox 4 rear spar sits in a different position than the early ones - that would also make a difference.   Yes, it has probably been over-analyzed, but lots of fun and informative as to the differences - Bet they all fly!

EdMO

Edited by Ed In Missouri

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Posted

As I said, Dave didn't change any of the incidence on his Avid C fuselage when he went to the Kitfox IV wing and it flys very, vey nicely.

I think you guys are way over analyzing this.

What fuselage are you starting with, Luke?

 

:withstupid:   :lol: 

 

:BC:

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Posted

Okay guys, sorry about that. I know I've probably gained the well deserved reputation as the over analyzer on the forum  :blush:  I guess I can't help myself! In my defense, I've been down this road before on another homebuilt project and when I jumped in without fulling checking things out, I ended up second guessing myself down the road, and re-doing things a different way. Anyway, thanks to everyone for your patience and suggestions.

 

Hey Larry, I'm starting with an Avid MK-IV. What model did you friend have? Also, if he didn't need to fold his wings, he could have a higher angle of attack because the traililng edge could hang down more.

 

Thanks  a ton Jim for your measurements. I made some drawings of what you measures. I'm sorry the quality is so bad. I don't have a good way to change my drawings into clear images. You should be able to read them if you click on them to enlarge. I indicated angles that have the front spar lower then the rear as negative angles.

 

 

I'll start with the Avid MK-IV that you measured. This what I have, and the measurements you got are exactly the same as mine (that's a good sign). The Model C was the same too.

Angle = 3.13°(negative)

post-760-0-94684500-1409531720_thumb.png

 

Next is Jim's Kitfox 1.

Angle = 2.61°(negative)

post-760-0-29444200-1409532228_thumb.png

 

Next is Jim's Kitfox 4

Angle = 1.04°(negative)

post-760-0-45893900-1409532289_thumb.png

 

For comparison, I'm  including measurements from a member on the TeamKitFox forum. He measured a different way than Jim, but I drew the picture showing his measurements, and then added measurements done the way Jim did them for comparison. Also, here is a quote showing how he measured:

The measurements as you requested, 37-15/16" fwd. as measured from a perpendicular from the bottom of the fuselage to the bottom of the Spar lug. Aft it is 37-7/8 from the bottom of the spar lug to the bottom of the lift strut attachment lug.

It's my understanding that this was a Kitfox model 5. It matches very closely to what I measured on  another kitfox that I'm pretty sure is a model 5 or model 6.

Angle = .13°(positive)

post-760-0-03491200-1409532442_thumb.png

 

 

Thanks again Jim. This information is very interesting. It shows quite a few differences between various models.

 

Also, if anyone sees a mistake I made, please let me know. I did these in a bit of a hurry.

 

Thanks,

Luke D.

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Posted

Luke,

      This topic could even get deeper if I started quoting design book info - incidence, also called "Longitudinal Decalage" can affect the angle of lift for takeoffs, and the loss of lift after landing, as well as cruise and visibility issues.  

  Not even going there - All these birds fly - maybe a tiny bit differently.

EdMO

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Posted

When ever I start getting too wonkey with technical details on my Kitfox build, Dave tells me,

 

"C'mon man, it's not a church we're building, it's just an airplane."

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Posted

Ed, believe it or not, I'm trying to keep this pretty simple by not getting into analyzing airfoils, etc. (over my head anyway). I just want to use an existing and proven configuration that I know will work.

 

I hear you Larry. I can get kind of obsessed with some details at times, and I might need to take a step back to keep them in perspective. I'll think on this one a little more.

 

One more picture for now and I'll call it good. This (picture below) is basically what I see when I hold a KF rib up to my plane. I can't shake the feeling that the angle of attack is too negative. Measuring a Kitfox seems to agree (this rib on the Avid is almost exactly 3 degrees more negative than the way it's mounted on the Kitfox). Will the plane fly okay? Probably so (with proper adjustment of the HS). Will it be more nose up in cruise than I would like? Will wings fold okay? Still trying to decide those questions... Here's the picture:

 

post-760-0-11986900-1409534498_thumb.png

 

Note: please ignore the flaperon hanger. I was just playing with some ideas. Nothing near final. The main point is that it is in the same location relative to the wing as the stock KF flaperon.

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Posted

In talking to dean wilson, he was pretty much trying to make the plane idiot proof with the massive washout and conservative cg range. Hence the aoa making it damn near impossible to get into a deep stall and a forward cg that makes you run out of elevator long before you run out of wing. This was intentional as the original thought was it's a plane that some guy is going to buy, build, then learn to fly in. I think for the most part it's been more experienced pilots flying them, hence the wings gradually getting less and less washout in them. I know I wish I didn't have all the washout in mine.

I guess you could also liken this as the main difference between a pa-12 and an 18... the 18 has about 2deg. More aoa at the fuse than a 12. Better stol, less cruise.

At the end of the day, I'm sure no matter what you do, it's going to fly great!

Oh, and you may have just surpassed joey on the square rooter claim to fame hahahaha

:BC:

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