Bent flaperon push-pull tube.

[rwaltman]

As mentioned elsewhere, a friend tried to fold the wing in my KF-IV without disconnecting the flaperon first. He realized the problem and caught the wing before it moved "too far", but not fast enough to avoid all damage.

The p/p tube and the flaperon control horn were bent. I don't know yet what is the condition of the lower attachment. Looks OK when looking from the top of the fuselage, need to remove the seat to inspect it better. There is a little play on the flaperon mixer tube on the side of the bent tube, (left), none in the opposite side. It may have been there before.

I am not concerned about the tube itself, that is easy to replace. But for the tube to be bent like that it means that whatever is connected to it in both ends had to apply a large force, most likely in ways it was not designed to.

The question is, what should I check, beyond the obvious bent/not-bent cracked/not-cracked condition?
What are the chances the stress damaged something in the flaperon axis/hinges, in the control tubes, etc.?

Roberto.

[Dutch]

Hi Roberto.

Looking at your pics. the p/p tube and horn tabs do not appear too be bent very much at all. The tube is the same as the flap tube running under your seat, and as you notice it has a considerable bend to it to clear the seat bottom. which was done when it was installed. I bent mine several times to get the right clearance. so straightening it out should present no problem. same with the control horns. The area to check is the hanger at rib #1 plus the rib itself also is the flaperon shaft itself not bent I think the small amount of play in the mixer assembly was probably there before. As the pins that it rotates on are really beefy. As a side note not to hijack your tread. Your last pic. shows what appears to be a fuel drain. I am at that point in my build and have wondered how I would go about it. If you dont mind I wonder if you could post a few closeup shots of that area (both inside & outside).

Thanks Dutch

[jtpitkin06]

I’m sure there are lots of experts that can help, but here is where I would look for damage.

If the wing is folded while the flaperon push/pull rod is still connected the rod will wedge itself against a fuselage diagonal brace. One should check for any damage or indentations on the fuselage diagonal. If there is a kink or dent it will require straightening and possibly a scab patch.

Inspect the push pull rod for indentations. You can order a new rod, but your rod looks like it has a gentle bend that can be straightened. If it has a kink, I would get a new one.

The flaperon control horn was obviously bent. It may be possible to straighten the horn. Inspect the control horn for cracks at the welds after straightening.

The flaperon torque tube took a load nearly parallel to the trailing edge. The torque tube may be bent where it enters the flaperon. Check the alignment and smoothness of operation. If bent, it will be a difficult repair because the torque tube is the flaperon spar. You may need to repair it and insert an internal sleeve.

The last area to look is the flaperon brackets and ribs. With that much force applied it may have broken a rib near the trailing edge or bent the brackets. Also inspect the trailing edge for alignment and separation from the ribs. Puckering or wrinkling of the fabric near the trailing edge is an indication to look a little deeper.

That should just about do it. I’m sure there will be some other suggestions. It gives a heads up to the rest of us to always supervise the wing folding closely. It might be a good idea to put a placard next to the forward spar clevis pin for those times when we forget.


“Caution: Remove turtle deck and disconnect flaperon control rods prior to folding wings.”

Let us know what you find.

John Pitkin
Greenville, TX

[rwaltman]

the p/p tube and horn tabs do not appear too be bent very much at all.

The horn was straightened before taking the picture, and I couldn't get the camera in a position that would show the tube in it's worst profile. But, yes.

I bent mine several times to get the right clearance. so straightening it out should present no problem. same with the control horns.

Good to know.

The area to check is the hanger at rib #1 plus the rib itself also is the flaperon shaft itself not bent

OK.

last pic. shows what appears to be a fuel drain. I am at that point in my build and have wondered how I would go about it. If you dont mind I wonder if you could post a few closeup shots of that area

That's the fuel line going from the header tank to the front. In my plane it is routed along the left side of the fuselage. Will take a few pics. (I'll be able to post them only on Saturday evening.)

Thanks,

Roberto.

[rwaltman]

...
The flaperon torque tube took a load nearly parallel to the trailing edge. The torque tube may be bent where it enters the flaperon. Check the alignment and smoothness of operation. If bent, it will be a difficult repair because the torque tube is the flaperon spar. You may need to repair it and insert an internal sleeve.
The last area to look is the flaperon brackets and ribs. With that much force applied it may have broken a rib near the trailing edge or bent the brackets. Also inspect the trailing edge for alignment and separation from the ribs. Puckering or wrinkling of the fabric near the trailing edge is an indication to look a little deeper.
...

John, thanks for the details. I did inspect the flaperon as you and Dutch recommend, and did not see anything unusual.
I'll do it again this Saturday, and will ask somebody else to do the same. Eyes #3 and #4 may see something mine did not ...

Roberto.

[Dave S]

Although the push-pull tube doesn't look that whacked out, I would consider the threaded ends of the push-pull tube could potentially have been subject to some considerable torque forces........the length of the wing has a lot of leverage on a 3/16" stud. A point to consider as a crack would be pretty hard to see on the threads.

Just a thought; but, the person inspecting the part is the only one in a position to assess this.

Good luck,

Dave S
KF7 Trigear
912ULS, Warp Drive

[rwaltman]

I would consider the threaded ends of the push-pull tube could potentially have been subject to some considerable torque forces...

I thought the same. So I already ordered a new tube + ends from Kitfox Aircraft.

Roberto.

[HighWing]

A friend and neighbor - managing engineer at Aerojet - came by one day and we talked about 4130 steel. He mentioned that it is capable of taking a 60% deformation before loss of strength. One thing that was mentioned, that might justify the replacement of the control tube is the threaded ends. It was rumored that in the old days, the threaded ends were sometimes socket head cap screws which are an entirely different material. That discussion came up when some of the threaded extensions on the horizontal stabilizer braces would break. The thought then was that the cap screws would harden with the heat of welding, and the factory did not routinely normalize the welds. I personally would be comfortable straightening the wings on the flaperon horn and any other bent pieces. The stick forces on the flaperon controls are not that heavy. Also with that in mind, a friend told me once - he always trailered the airplane from home to the airport - that he once forgot to connect one flaperon before flight. He discovered it after preparing the airplane to be put back on the trailer. He said, with a smile, that he had no idea anything was wrong during the flight.
Lowell