Dented Tubing - Wing Strut - Repair? 1

Maybe i can help you with the first part to keep you safe, but would have trouble keeping you financially solvent...

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.

Thanks for the prompt and helpful reply!

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?

I did my homework - much easier than I expected - which is usually a bad sign for me.

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!

Meigsy - before you launch off and cut and hack, have you determined the extent of the dent? Depth, smoothness, length/width? Are there jury struts that would provide buckling support for your lift struts, reducing effective column length?

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.

You fly it - you fix it. Sort of a life and death decision.

REPLACE IT

If my calcs are right - a 9' section weighs about 20 lbs - less than $100 I would think

"easy-to-find 1100F soft rivets rather than 2117-T's that I cannot seem to find"

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

Many thanks to all for the help. I think I understand now that what I'm 'repairing' is the lost critical buckling load, which is so tremendously lower than the tensile strength. Given that there are indeed jury struts, I would in all probability be fine sleeving it, riveting appropriately, and moving on.

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!

When I was working on rocket motor cases, we faced many small mfg defects some of which were dents and weld offsets. Generally these were in weld zones in pressure vessel and skirts. We engaged Dr. P. Bijlaard for the solution of the problem. His specialty was in plates and shells with particular focus on the local effects of anomalies. If you are curious, check the net.

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

You indicate that it is a "fat ultralight". The time is past to legally register and certify this machine as an Experimental Light Sport. You are dealing with an aircraft that is not legal to operater.

Thanks for the guidance - when I calculate with the jury struts, I find that sleeving over the dent is more than adequate.

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!

There are 2 exemptions for Experimental Light Sport airplanes. As long as the application for registration was made prior to the deadline (and you are a member of the organization that holds the exemption), they can get an airwothiness certificate.

The whole light sport rule is a little strange compared to regular airplanes.

In the repair you referenced, what were you going to do in the area of the dent as far as repair fasteners go? Were you going to install the splice fasteners on either end of the dent?

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.