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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:

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.

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."

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

These just sold for high bid of $92 plus $14 shipping - Rather stupid when you could have bought them for $50....

EDMO

Okay guys, sorry about that. I know I've probably gained the well deserved reputation as the over analyzer on the forum   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)

Next is Jim's Kitfox 1.

Angle = 2.61°(negative)

Next is Jim's Kitfox 4

Angle = 1.04°(negative)

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)

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.

Thats a good idea Leni.  I think I'll go try it.

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?

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

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?

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

I wrap mine around two leads so I can test the ignition system and watch for RPM drop between the two.  A 500 RPM swing is pretty big, but without a prop on it I can see where you wouldn't really hear that much of a difference in it with no load on it.

You can put it around any spark plug wire you want.  They are all firing at the same rate, so that being the case I'd try another wire and position untill it works correctly.  I use a tiny tach with about 4 wraps with no issues.  It sounds like you are picking up interference from somewhere else.  

Hello Guys:

I just finished up installing a Tiny Tach on my avid, and the manual recomends 3 to 4 wraps around the #1 ignition cable; however, I ran a test, without prop inside of my garage, just to test it, and is giving me readings of 1400 up to 1900 rpm at idle, judging by the engine sound, there is no changes, I try 2 and even 1 and is not stable, I even tried it without wraps, and even though is more stable is still jumping.

any of you have a good advise of how, or what am I doing wrong? or this supposed to be like that??

Regards

Dimi3

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

That's true about model airplanes. You can make just about anything fly on a small scale  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.

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?  

EDMO

HAPPY BIRTHDAY LENI, CHRISTINE, AND NUMBER ONE - U2 BANDIT!

EDMO

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.

Happy birthday guys.

If it makes you feel any better, Leni, I'll always be older than you...

Happy Birthday Bandit! 

Yesterday was my baby girls 1st birthday.... Kinda sucked that I am 700 miles away at work for her 1st one, but I will make up for it when I get home the 9th

Not sure I will be in any shape to fly the following weekend as we are going to have a Bday party for my wife, my daughter and myself on the 12th  

Thanks Mr.  Yep I grew another year older today but I still refuse to grow up!

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

If the forum software is accurate, it looks like it's your guy's birthdays. Hope you're doing something fun. Bandit, it was nice to meet you at Oshkosh. Sorry, your first name slipped my mind (I have a hard time with names until their stuck in my mind). Leni, hope to meet you too sometime. Anyway, have a good one guys!

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   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.

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