Warp in fuselage causing several issues...

[Geowitz]

Hi. Glad you guys are here. Lots of good info. Thanks!

I picked up a IV speedster kit and am up to the step of rigging the wings. Please bear with me as I try to explain this.

My fues is a little warped. I used string lines to find this out when my dihedral wasn't making any sense. The strut carry through tube is shifted about 5/8 inch to the right side of the plane. In other words, the top spar tubes and bottom strut tube are both parallel, but the bottom strut tube is racked to the right side of the plane 5/8 inch. Like a rhombus shape if you are looking at the front of the airplane. Therefore, to get the dihedral the same on each wing I had to move the left set of strut brackets 5/16 inch in and the right set 5/16 inch out making up for the 5/8 inch shift(5/16 + 5/16= 5/8). Dihedral came out at exactly 0.6 degrees on each wing. No problem really for flight, but due to the change in geometry when I fold the wings back the tips are not aligned. I know why this is happening, but I'm not sure if the flaperons will clear the top of the horizontal stab with the wings folded. The trailing edge of the left wing is about 6 inches above the stab and the trailing edge of the right wing is about 4 inches above the stab.

I need to figure this all out before I drill any holes.

How much clearance do I need for the flaperons over the horizontal stab? What are the suggested dihedral settings?

Other thoughts?

Thanks

[SkyPirate]

first ..is the plane covered?

[Geowitz]

No, not covered. I am literally up to the step in the instructions for the wing rigging.

I basically just need to know how much clearance I need at the trailing edge over the horizontal stab for the flaperon to clear. I may need to add more dihedral to get that clearance so I also need to find out typical dihedral.

[SkyPirate]

are you sure you don't want to fix the "tweak" first? then go from there?
it might not take too much to straighten the fuse first,.if it's 5/8" (.625) out ..that's only .3125 or 5/16" you need to tweak it to loose the 2 inch diference when folded,..that little bit you might be able to do without heat

[SkyPirate]

pick a spot in the center of the tail where you can attach a string and have a clear shot to the front lower cross section in line with the front door/ wing posts on the bottom,..mark center on that cross section and draw the string tight,.measure your strut mounts from the string ..each side,.then measure from strut mount to rear wing attach points each side,..if this is equal measurement then the rear wing mount and strut mounts positions are square,..this is a good thing,..means you just have to tweek the section between the wing connect points forward ..top or bottom,..depending on measurement.

[Geowitz]

I'd have to tweak it 5/8 inch by pulling on the bottom of the fuse in one direction and pulling on the top in the opposite direction. All while maintaining lateral stability. Pretty complicated rigging for my resources. I don't think that is a feasable option at this point if my horizontal stab clearance is fine or it can made to be fine with a little more dihedral.

[SkyPirate]

then your out 1.25" ? ..you take the ammount it's out and divide it,..that how much you tweak it,..too bad your not closer ..I'd give you a hand ..it's really not that hard to do ,..a couple heavy duty ratchet straps about 20 minutes of tweaking and your golden ,..

Chase
call me 573 435 6023

[t j]

Geowitz, My Classic 4 fuselage has one strut fitting 1/4 inch ahead of the other. I chased the rigging for several weeks trying to get the wing tips even when folded. I finally used a plumb bob to find the source of the problem.

I trust you have found this Service Letter. http://www.kitfoxaircraft.com/support/service_letters/sl25b.htm When I called Skystar about my wing rigging problem he pointed me to it.

Now to answer your question. When the wing is folded the leading edge of the flaperon hangs down 4 and 7/8ths inches below the trailing edge of the wing. Looks like you need more dihedral.

You also need to check two other places for wing folding clearance. The point where your mass balance weights will be installed. They need to clear the fuselage. Remember they will be hanging straight down as the flaperon will be vertical at this location. Also check the clearance at the flaperon control tube bushing attach tabs on the fuselage. The flaperon will remain horizontal as it swings over this location.

For what its worth. The skew in my fuselage results in 1/2 inch difference in the wing tips when folded. I have standard dihedral that is built into the rigging instructions (not a speedster) and that gives about seven inches from wing trailing edge to trailing edge of horizontal stab.

I spent a lot of time getting the rigging to a nat's eyebrow and it was well worth it. She flys hands off after first flight with just a slight tweek of the horizontal stab leading edge.

[SkyPirate]

not to blow my own horn ,,but I've built 14 planes working on 15th now ( my own design)..loads of race cars ,..worked in the sheet metal and fabrication field for 20 plus years,..was the R&D engineer/welder /fabricator for Jone & Lamson for 3 years,..repaired/rebuilt 100's of wreck cars too ..
certified AWS in every phase of welding

I wouldn't steer you wrong

Chase

[t j]

Geowitz, one more thing I just thought of to put a note in your manual about. Your jury strut installation may need to be tweeked. On one side the front top attach hole ended up more than half way up into the wing fabric. They can be made to fit below the fabric with a longer attach tab and the jury struts slightly off the vertical. That was my solution anyhow and no one has ever noticed the flaw.

[SkyPirate]

TJ ..it's not a flaw if it's an applied modification that works :) it's a solution

Chase

[Geowitz]

Thanks for the suggestions.

Here's what I did...

I binder strapped diagonally across the cockpit behind the seat as Skypilot suggested and was able to bring it closer to square. I didn't want to go any further because I bent one of the cross braces, but that's fixable. I'm gonna take out the rest of the difference(atleast relative to the wing) by welding one of the lower strut bracket holes closed and redrilling. That way I can get both wings set the same as far as dihedral and they'll match pretty close when they fold back.

TJ - Thanks for the info. I had not seen that particular service letter. Now I can redo the rigging with a lot more confidence.

Thanks

[SkyPirate]

keep us posted George,..the one tube I'd cut close to the weld and reweld it once you got it in the right place and straight,.. I know that you will be much more satisfied and proud of your plane knowing that it is right,..there is nothing worse then trying to explain why things don't line up,.especially to a potential experimental aircraft pilot. or a GA pilot that doesn't like experimentals,..it's allot better if they can't find anything wrong to dispute,..you won't regret it .
Chase

[HighWing]

I find this topic fascinating. Lots of advice and in my opinion most of it not very good. I am on my second Model IV, the first one a march 1993 kit and the new one out of 1992. For what it's worth the first one had 900 hours on it over nine years, with friends with 1500 hours and more on theirs.

I am in the process of building a one off engine mount so my 1992 fuselage happens to be leveled with the engine on a stand in the position I want it to assume in flight. It was easy to drop a line from the aft carry through spar attach fitting and check the measurements stated in the first email.
Mine is exactly the same as those mentioned - 5/8" difference.

What this means to me is that every Model IV out there was built in exactly the same jigs to the exact same dimensions. The rigging issues have been common from day one, hence the original manual suggestion that after the dihedral is established that the wings be folded and then the lift strut attach bracket location tweeked to make a more pleasant wing geometry when folded. Subsequently, the Service Bulletin was issued; prompted, as I understand it, by builders questioning why the in flight geometry was compromised simply to make it more aesthetic with the wings folded. In my first project, I adhered to the service bulletin.

Chase mentions his 14 aircraft built, well I would like to remind everyone that this is a Kitfox Model IV and in the 16 years I have been following the lists and forums, this is the first time I recall that someone has dertermined the actual geometry of the wing attatchment points on the fuselage. The other 2000 builders simply built to the instructions and flew their airplanes.

I hope this doesn't sound critical of the original poster as his question was absolutely legitimate, but for the life of me I don't understand advising fixing something that has not been a problem in 17 years and likely a million flight hours. I flew with a group of Model IV owners (flights of six to ten) putting many hundreds of hours on our airplanes together and although issues arose at times, this partiular issue never came up.

For what it's worth, knowing now that my fuselage is crooked, I plan on ignoring it and building strictly according to the manual and flying it, just like I did the first time.

Lowell

[SkyPirate]

Hi highwing,..since the model 4 was introduced kitfox has also changed hands,..I can't say any jig's were re worked since kitfox has changed hands because I do not know,..
but I can say on the kitfox I owned the geometry was perfect,..it's all to do with what you as an individual is willing to except,..if a repair/modification is to be done to bring thing's into allignment ,.it's best to do it before the plane is finished/covered,..personally I do not except anything less then perfection,..and to me it sounds like George has the same frame of mind,..to some 5/8" doesn't sound like allot ,.. it obviously bothered George as it would have me if it were my plane.

An example ..building race cars,..you can have one that is 5/8" out of "track" some dirt cars are purposely set with 4 to 5 inches out of "track",..but race cars are made to turn in one direction ,..
You mentioned tweaking it after dehydral was set,..so what's the difference? difference is when he get's the geometry right to begin with ,... then tweaking is not required. nothing wrong with starting with a square airframe.
..it will reduce the ammount of adjustment needed to allign things in the end.
and when he swings the wings to fold position,..they will still allign
I just wish I lived closer to George,..and I could have helped him do the fix,. it is a relatively quick fix.

[Geowitz]

Thanks to everyone for their thoughts.

While I understand the theory of just following the directions and letting it be I'm glad I did what I did. It was kind of scary, but Skypirate helped me realize wanting the dihedral right when it is in flying form as well as folded was important for many reasons to me. I learned a lot about my plane in the process as well.

Tonight after a little more work I brought the fuse within 1/16 inch of square. One tube behind the seat needed to be shortened and will be rewelded. Tonight I reset the wings by going back to my original strut bracket marks and added 3/16 inch for just a tad more dihedral. When I put the smart level on the wings both were exactly at 0.9 degrees of dihedral. Good sign. Washout was basically right on as well and at the moment of truth the wings folded back with the tips within 1/4 of an inch. Hardly noticable. You can't argue with success. I will probrably leave the lower strut holes alone.

[leptronjohn]

Hi Guys,
my model 4 is a 92 and was from about the period that Dan sold the company the Phil. now with that said, it did not take Phil long to see the jigs were worn. new jigs were put together then. the problem is nothing new to the aircraft industry, and not a big deal.
My plane is out of wack as is the plane you are talking about and i elected to build it so that with the wings folded no one would know the story. I figured if the fuse hung my seat a bit out of plum I would never really care and it has not bothered me in 10 years.
I left the wings high enough to leave a couple inches with the flaperons folded up, that seems like plenty.
john

[Geowitz]

Just to clarify. I KNOW the plane will fly many happy hours the way it came from the factory. I really do appreciate all of the opinions. That's what I asked for and it's what I got.
That being said - I wasn't totally comfortable with that and I was prepared originally to try and accept it and just follow the directions, but when it was able to be simply fixed early on in the construction process why would I let my dihedral be based upon the comsmetics of the wing folded back? I'm sure the instructions can be followed most of the time in this regard, but if I had leveled the folded wings on my specific airframe the difference in dihedral in flying form was visually obvious. It looked like crap. It was easily noticable. This fix isn't for everyone. Every frame will have differences. Mine were enough that it bothered me.

Also, the center of gravity works left and right as well. You've got a big heavy pendulum hanging on a wing and if the wing has to fight the pull of gravity on the fuse to equalize the wing you're gonna be needing trim. Some will say that's what trim is for, but you can't argue with less needed being better. Especially when a small deviation multiplies into a big deviation down the road as the build progresses.