Structural analysis on spars

[CoClimber 33]

I have a Kitfox 2 that is seeing extensive modifications.  Today's discussion is about the wings and struts.  I want to increase the gross weight from 950 to 1200 pounds when I apply for my airworthiness certificate.  The fuselage has been modified and I am now considering the wings.  I have the old spar that has the web extruded right into the tube.  The model 4 1200 has the strut moved out 10" but I am reluctant to drill out the strut attachment ant move it out.  

I understand that the load on the strut is a function of the square of the cantilevered section of the spar.  Currently the distance is 88" from the strut to the wing tip.  If I remove one rib bay that goes to 70".  88 squared / 70 squared =  1.58.  So, 1.58 times 950 pounds = 1500 pounds.

Is my logic correct?  If I remove the outermost bay and leave everything else the same, the wing that was rated for 950 pounds should see the same forces at the strut brace when loaded to 1500 pounds.

[Good old number 29 158]

The angle of the strut may still be a limiting factor. Clipping the wing may allow higher loads on the wing, but the lift strut will still be pulling at the lower attach point at a steeper angle. It’s better to have the longer strut with a shallow angle pulling straighter out on the lower attach bracket. Have you considered making a larger strut/spar attach bracket that will attach at the original mounting point and extend ten inches farther out to the new attach point? You did the spar and lift strut carry through tubes? I’d love to see pictures of the project. 

[CoClimber 33]

Interesting point but I don't know if the angle change is significant enough.  The angle will change from 23.8 degrees to 21.6 if I move the attachment out 10 inches.  Ithink the reason the attachment point was moved out was to reduce the amount of cantilevered wing.  It was moved out 10 inches and I will move my tip in 18".

Another consideration is that the fuel load has been moved from in front of the panel to the wings themselves.  That is 156 pounds that don't have to be supported by the fuselage.

I sat down with a mechanical engineer today and we looked at the loads through the wing struts.  I think I should go ahead and get beefier struts. We calculated that there is a 1500 pound tensile load on the struts and that each strut has a yeild strength of 5000 pounds, conservatively.  This load changes to 1337 pounds if I clip the wings.  I don't know where the center of lift is on these wings but if the load is distributed equally between the tubes, They would fail at 5Gs with a 1.5x safety factor (actual failure at 7.5Gs).  

All this math is fun but, ultimately, unforgiving.

[lostman 198]

His makes my tired head spin, but I want to know what you did to the fuselage as well to make up the difference. 

[Good old number 29 158]

Here’s a link to an article that details fairly well the changes from the first Kitfox to the Model 4-1200. When I first bought my plane, I called Kitfox and asked about increasing the gross weight, and John told me basically that it couldn’t be done. From a liability standpoint- who cares, that’s wrong. It can be done. But, to go clear to 1,200 is a lot of work. Needs stronger landing gear. Those lift struts went to 1” diameter .049” thickness. Drag/antidrag tubes are thicker and have steel attach brackets. Thicker lift strut carry through tube. Bigger and thicker carry through tubes for both spars. The 1200 got a 10 inch taller rudder than the Model 2, with four inches more chord. Same four inches more chord on the elevator, and two inches longer chord on the flaperons. All things to consider. Not trying to talk you out of it, by any means. Just listing some of the changes Kitfox made. I absolutely think you should do it. I kind of wish I’d swapped for the extruded stiffener in my Model 1. Easy to get that extra hundred pounds. Go for it. Keep us posted, too. We’ll all be interested to see how it goes.

 

http://avidfoxflyers.com/applications/core/interface/file/attachment.php?id=19834

[Allen Sutphin 705]

Good article on the changes. I think I'll stay with the designed gross weight on my model 2.  More weight mean less performance or more horsepower needed. Mine is already heavy enough. A 50 lb gr. wt. increase would be enough for me, a hundred would be super. But just not worth the work, to me, anyhow.

[CoClimber 33]

To answer some of the comments, on the carry throughs, I cut and drilled out the small vertical tubes that connect the spars to the carry throughs, welded in smaller tubes inside the carry throughs, then welded in new vertical tubes.  Hope that makes sense.  On the strut carry through, I welded a 1" strap below the existing tube.  It is in tensiion so a strap works well therre.  The landing gear has been completely replaced with cabane mains and a nose wheel.  The 65Hp 582 is swapped with a 912UL.  With the 912, nose wheel, and 26 gallon wing tanks vs the 9 gallon fuselage tank, the airplane needs a boost in GW to remain useful.

Happy New Year everyone.  I think I'm going to go make my wife some coffee.   Then clip those wings. 

[Buckchop 479]

Im very interested in this subject also. Question tho, if u clip the wings, u mean cut some off the end of wing to make it shorter??  If u do that doesnt that make less wing area for lift? Ur tryin to get ur gross weight up and less wing area will pickup less right? or wrong? i dont know. 

I know when i bought my black kitfox#0021 it weighed 530lbs, that its done at #950 gross with a 532, it weighs in at 505lbs now, and when i bought my blue Kitfox#0038 it weighed 488lbs that its done at #850 gross with a 503, it weighs in at 492lbs now.  Id love to up my blue kitfox to 950-1000gross.  But how much does that really matter other than on ur paperwork? 

[CoClimber 33]

I took 17" of of each wing, from 174" to 157".  Lift is a function of wing area, coefficient of lift, and airspeed squared.  It is that last bit that is important.  Not counting the effects of lift from the fuselage,  I reduced my wing area by 10%.  If the lift was a linear function of speed, we would have to go 10% faster to get the same lift.  If the stall is at 40, we would now have to go 44.  However, because it is the square of the speed,  the stall goes to 42.

Remember too that I am going from 65 to 80hp. 

[Buckchop 479]

Mmmm i do like the idea, of incresed weight, im still on the edge of the pond not ready to jumpin on the shorting of wings, even tho i shortend mine on #0038 by removing the 3.6bs fiberglass wing tips, lost 6"-7" on each wing but over 7lbs of weight, but im a simple man and no damm good at math. Hahahahahaaa hope it works for u. Ill keep up on this subject.

[CoClimber 33]

The reason I am going to all this effort is because I couldn't get an airworthiness certificate unless I could prove it was 51% homebuilt.  I figured that if I was going to tear it down and build it back up, I might as well build in the Model 4 improvements.  It has a 12" taller rudder, differential controls, model 4 ribs and flaperons, 5" wider fuselage, wing tanks allowing me to have a full size panel and no be limited to the space provided in the 9 gallon tank that came with it.  I also switched to nose wheel because I am a student pilot, insurance is cheaper, and I don't expect to ever fly somewhere without a proper runway.  I can always convert it back.

I also love to fabricate so I'm really enjoying the process.  I thought I could have the left wing assembled today but I can't find my box of latex gloves. It's the little things that get you.

[dholly 718]

pics

[1avidflyer 1983]

The reason I am going to all this effort is because I couldn't get an airworthiness certificate unless I could prove it was 51% homebuilt.  I figured that if I was going to tear it down and build it back up, I might as well build in the Model 4 improvements

Maybe I'm wrong on this, but I don't think it is ever questioned that a Kitfox is considered to be 51% homebuilt.  I believe the FAA has a list of kits they consider to be meeting the 51% rule, and Kitfox is one of them.  If you can get your DAR to sign off on the build, the FAA isn't going to argue about it  JImChuk

[CoClimber 33]

I went through three DARs and this is the best I could do.  I don't really mind because I am enjoying the process and I will have the airplane I want.  I will also be able to get a repairmans certificate.

[Allen Sutphin 705]

I went through three DARs and this is the best I could do.  I don't really mind because I am enjoying the process and I will have the airplane I want.  I will also be able to get a repairmans certificate.

Sounds like you have a plan coming together. And hope you end up with just what you want. Warning! If you enjoy it , you will be a repeat offender. So be warned, you'll be building or modifying something the rest of your life.

[TJay 1041]

Sounds like one heck of a big project. May I ask what size the spar carry through tubes are on your plane?  And what size you slid inside? And where are all our pictures?

To get 1200 lbs weight in it to test fly is gonna be tough. I had a hard time getting 1050 in mine for test flying.

[skelly 12]

To answer some of the comments, on the carry throughs, I cut and drilled out the small vertical tubes that connect the spars to the carry throughs, welded in smaller tubes inside the carry throughs, then welded in new vertical tubes.  Hope that makes sense.  On the strut carry through, I welded a 1" strap below the existing tube.  It is in tensiion so a strap works well therre.  The landing gear has been completely replaced with cabane mains and a nose wheel.  The 65Hp 582 is swapped with a 912UL.  With the 912, nose wheel, and 26 gallon wing tanks vs the 9 gallon fuselage tank, the airplane needs a boost in GW to remain useful.

Happy New Year everyone.  I think I'm going to go make my wife some coffee.   Then clip those wings. 

This is very interesting as I have a potential issue with 2 of those vertical tubes on my KF2.

May I ask how you lined up all 4 tubes and if the rear ones are directly above the lift strut attachment point holes ?

Cheers,

Skelly.

[TJay 1041]

To answer some of the comments, on the carry throughs, I cut and drilled out the small vertical tubes that connect the spars to the carry throughs, welded in smaller tubes inside the carry throughs, then welded in new vertical tubes.  Hope that makes sense.  On the strut carry through, I welded a 1" strap below the existing tube.  It is in tensiion so a strap works well therre.  The landing gear has been completely replaced with cabane mains and a nose wheel.  The 65Hp 582 is swapped with a 912UL.  With the 912, nose wheel, and 26 gallon wing tanks vs the 9 gallon fuselage tank, the airplane needs a boost in GW to remain useful.

Happy New Year everyone.  I think I'm going to go make my wife some coffee.   Then clip those wings. 

This is very interesting as I have a potential issue with 2 of those vertical tubes on my KF2.

May I ask how you lined up all 4 tubes and if the rear ones are directly above the lift strut attachment point holes ?

Cheers,

Skelly.

The easiest way is just drill them out and slide a new bushing in and weld it up.  The only way it will move on you is if you have a cheep wobbly drill bit. You could also make a jig that attaches to the rear spar that lines up with the front spar that you can drill through but that sounds like a lot of work.