Dented Tubing - Wing Strut - Repair?
Dented Tubing - Wing Strut - Repair?
(OP)
I'm rebuilding a 'fat' ultralight, and discovered a dent in one of the wing struts. Replacement tubing is expensive (to me) to ship at 9' in length, and it's a shame to waste an otherwise 'good' part.
I looked at AC43.13 and only found help on splicing tubing on longerons/webs.
My thought is, since the cross sectional area has not changed (it's dented, not split in two), I wouldn't need the 24,000 lbs of shear capacity the AC43.13 splice would seem to provide...assuming my math is right...if I sleeved the dent as though it were a split. I could then use easy-to-find 1100F soft rivets rather than 2117-T's that I cannot seem to find.
The point of the repair is to transmit the wing strut stresses across the dent - I'm open to suggestions on how to do this, economically.
Can anybody help a chemical engineer stay safe and financially solvent?
I looked at AC43.13 and only found help on splicing tubing on longerons/webs.
My thought is, since the cross sectional area has not changed (it's dented, not split in two), I wouldn't need the 24,000 lbs of shear capacity the AC43.13 splice would seem to provide...assuming my math is right...if I sleeved the dent as though it were a split. I could then use easy-to-find 1100F soft rivets rather than 2117-T's that I cannot seem to find.
The point of the repair is to transmit the wing strut stresses across the dent - I'm open to suggestions on how to do this, economically.
Can anybody help a chemical engineer stay safe and financially solvent?





RE: Dented Tubing - Wing Strut - Repair?
A sleeve repair would reinstate the tubings strength and instability capacity, though you need to be able to transmit the forces into and out of the sleeve either side of the dent, the soft rivets may not be capable of this load.
If you want to obtain the maximum loads applied to the strut, then you could obtain the strength allowables of the tube, and also then calculate its critical buckling load, theis would then leads you to the loads that have to be transferred into and out of the splice.
You probably want to make sure that the part is ok, or the repair is good if its going to fly.
Oh, the stresses arn't transmitted across the dent, its the loads.
RE: Dented Tubing - Wing Strut - Repair?
For the strength of the tube, I used the cross sectional area of 0.1618 sq.in. and 42000 psi for the TENSILE strength of 6061-T6, and get 6765 lbf. This would be the force the strut would experience just before it fails, the wings fold up over my head, and I get to experience a deceleration like never before, right?
If I use readily-available 1/4" 1100F rivets, with a shear strength of 9000 psi, I get 16 required to match that 6765 lbf. More is better, so why not 18 in the beautiful pattern shown in AC43.18?
And yes, the loads are transmitted - I'm more of a distillation guy than a mechanical guy, and it shows.
I am currently clueless on calculating critical buckling load, but now I have a homework assignment for tonight!
When I calculated the strength of the 2117-T riveted splice in AC43.12 for 1.5" tube, I get 24,249 lbf. That's a factor of safety of 3.5-ish, if my math is correct.
Since the inner tubing is not cut, SOME of the applied loads WILL be being transmitted through the dented inner tube, right? The point of the repair is to transmit the loads, compressive and tensile in such a way that the 'imperfect cylinder' where the dent is doesn't cause it to crumple and fail, not a desired outcome.
Can we count on the inner tube to provide ANY of the required strength? Is a safety factor of 3.5 really necessary in this case?
RE: Dented Tubing - Wing Strut - Repair?
For critical buckling load,
For 1.5" x 0.035thick, 6061-T6 Al, cylinder, simply supported,
Area moment of inertia = 0.043 in4
Young's Modulus = 1.00E+07 psi
L = 109 in.
Fcr = 359.2 lbf
Good thing there are 4 of them working together!
RE: Dented Tubing - Wing Strut - Repair?
You may be able to call it good given that you haven't lost material... Or maybe the fishmouth repair is the way to go. Remember to consider the joint strength of the rivet(ie adjusted for material thickness), not just its static strength.
I recall explaining to a group of type club members why they couldn't just use aircraft cable in lieu of tubular struts. Took a while, but I got through to them.
RE: Dented Tubing - Wing Strut - Repair?
REPLACE IT
If my calcs are right - a 9' section weighs about 20 lbs - less than $100 I would think
RE: Dented Tubing - Wing Strut - Repair?
Any aircraft parts house has 2117-T4 rivets sold under the MS20470AD-size-length part number.
You probably confuse the parts house when you call out a material spec rather than a mil-spec.
John Schwaner
www.mechanicsupport.com
RE: Dented Tubing - Wing Strut - Repair?
However, I was able to locate a straight, used strut tube nearby, so my ultimate repair will be replacement, which will help me sleep better.
Incidentally, I did specify MS20470AD8-32, and have yet to find any, aside from an offer to make a special production run at $250, for more rivets than I would hope to use in 3 lifetimes. I'll keep looking, maybe set some aside just in case my question becomes pertinent again.
Again, many thanks to all!
RE: Dented Tubing - Wing Strut - Repair?
Many of the defects were reworked with satisfaction, but it was a frustrating business. The customer was not friendly when I called. We claimed to have an analytical basis for all our claims for acceptance, but he was not convinced. In one case we had an offset weld on the thrust skirt, so we ran an axial test at Huntsville. We passed, but the natives were not friendly.
If the prevailing load on the strut is lift load, and the dent is a small fraction of the size of the section, I wouldn't be concerned. Other considerations are depth of dent vs. section, smoothness of dent, etc. Good luck.
EAA Tech Counselor
RE: Dented Tubing - Wing Strut - Repair?
RE: Dented Tubing - Wing Strut - Repair?
And yes, the aircraft is registered, no worries there. The "fat ultralight" was more to create a mental picture than to describe the legal status. I believe that for EAA members who registered prior to the deadline, there is/was an extension for airworthiness inspection. A bit off topic, but thanks for your concern. My annual is due soon!
RE: Dented Tubing - Wing Strut - Repair?
The whole light sport rule is a little strange compared to regular airplanes.
RE: Dented Tubing - Wing Strut - Repair?
Also, that's a pretty long rivet and in open tubing. Deformation of the shank, where the rivet bends inside the open area, is always a concern. Even with AD rivet, if you've never worked a repair like this, you might need a "few" extra to get it right!
I wonder if blind rivets wouldn't be better for a repair like this.