Thorp Air Command - T18.net

I was out working this weekend center punching every rivet on the outer portion of both S-18 wings that I'm repairing after the Lakeland tornado hit a few years back.

Anyhow upon closer inspection. There appears to be some damage to a short segment of the top spar cap on one of the outer portions of one wing. The apparent damage is in about the middle of the fourth bay out from the root of the outer wing panel. Or 1 and a half bays inboard of the wing tip.

It appears that the angle extrusion may be bent slightly down (inward towards the inside of the wing) and maybe some marks (scratches) on the cap extrusion itself. It does not seem extensive at all. But it is after all a spar cap we're talking about here! It appears it took a hit that impacted almost nearly "flat" on the wing. But slighly dinged up the cap.

So I'm interested in repair possibilities short of replacing the entire spar cap! But if that is what I need to do, well then, I guess that is what will have to be done. The other end of the spectrum is to just bend it to look good and sand out any scratches and go with it. But...

One repair possibility I was wondering about is if I should just bend the exterusion to make it as straight and corect as possible. Sand out the scratches on the extrusion to minimize any stress risers. And then add a splice, say three times the length as what I believe to be the damaged area. The splice I'm thinking about is to put in a doubler identical to the to how the spar cap is doubled or nested at/near the base or root of the wing.

I'm thinking the damaged areas is about 4 inches. So the splice I'm thinking would be for about 12 inches in length. So the splice would be held with the rivets from both the back of the web and the top of the cap would be re-riveted.

I don't see how I could cut the damaged portion out without damaging the web or being able to effectively deburr the cut ends of the spar extrusion that were to remain in place. And then replace that section along with the doubler idea. This just doesn't seem practical.

I will have to see if I can get good pictures and/or video and learn how to post them on this site to give a better reference as to the extent or non extent of the damage.

I'm not overly computer savy so I may need help walking me through how to get pictures or video uploaded to the site!

On the bright side. The rear spar I thought I was going to have to replace entirely on the other wing outer panel. I will "only" have to replace the outer section at the existing splice point.

Not saying a repair is not possible, but if it were me, I would replace the entire extrusion. FWIW.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Pretty much what John said.

Go to the AC43.13 for guidance on structural repairs. You'll see that the T18 spar design is not like anything for which repairs are shown.

DW

The rear spars that came with my project were spliced and I believe they came that way from Ken Knowles . I would have to look at the plans , but I believe a splice on the rear spar is permitted . As far as the main spar I vote with the other two and replace the top main spar cap rather than splice . When my butt is in the cradle I like have a solid piece holding me up at 12,000 ft ! We have structural engineers in the group and they may be able to weigh in here as to what may be permitted .

RB
NX115RX

Guys,

The spar cap at this point will be in compression most of the time, so a doubler, once it has been straightened just might be ok. I guess it depends on how bad the distortion is. The other factor is that it is near the outer portion of the wing where the bending stresses are lower.

Regards,

Jim

I will try to take some photo's tonight or very soon.

I went to my daughters first orcestra concert (she plays Chelo) last night. And I ended up running out of memory on my phone, so I have to download all my pictures off my phone onto a hard drive and start with empty memory for pictures and video's.

It may be a little hard to show at this point as the skin is still on the wing. Although a ripped out portion of the skin does reveal hopefully enough to see what kind of condition the spar cap is in.

Honestly it looks pretty minimal to me. And I also realized that it is towards the tip where the loads are less. I'm a ivil engineer. but not an aeronatutical or structural engineer. So what do I know. That is why I am seeking advice here.

On a seprate question. I bought last year a brand new ATS 3X rivet gun. Is a 3X sufficient to set the spar rivets, (both through the skin to the double thickness spar caps, and through horizontally the double spar caps) or do I need a 4X in order to set those rivets?

I realize it would probably be easier with a 4X, but is it possible to set with a 3X with just a lot more time rapping?

Those are 1/8" rivets? A 3x gun will drive those.
The spar splice might be possible, especially that far out the wing where loads are less, but the material thickness is not. I would consider an angle splice that has the angle at the aft edge of the repaired part. That would would form a flange to prevent a buckle of the original edge. maybe a 3/4x3/4x1/16th? $2.95 per foot at Aircraft Spruce.
Just a thought.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

This is my first attempt at uploading files to a post. I hope it works out.

There are 6 photo's showing the best I can the damage of the upper spar cap on the right wing. The sun and reflections may make this somewhat hard to discern. But I tried to do the best I could. If more or better pics are needed let me know and I will try to get more pics.

At this point the skin has not been removed. This section of the wing was banged up and the rib and skin torn away. Soon I will be drilling out all the rivets and remove the skin. At which point I may be able to get better pics. But I wanted to get something posted to start getting more input from all you Thorp guru's

I really want to know what (if anything) I should do to repair or replace due to this damage. Comments welcome!

Pic 1


Pic 2


Pic3


Pic 4


Pic 5


Pic 6

I would take a press and straighten it, sand the scratches out, prime and carry on, that is if the there is not stress(fracture) marks in the aluminum

Thanks Bill for such a quick response!

Is it possible to get the semi-assembled (removed wing skin) wing panel into a press?

Can I do some sort of dye penetrant to determine if there are any stress cracks? Or what method do I use to determine that there are no cracks.

Visually I see no cracks. But I have not used a magnifying glass. Just my eys along with reading glasses.

I would think you would not have anything to worry about after straightening it, but if it makes you feel at ease use dye penetrant . You put that kind of bend in the 530 angle in the fuselage.

Geez. That's nothing. Take a board and a hammer and whack it straight.
Cubes

If the Cubes persuader doesn't work I would just make a small shim to fill the depression contour and rivet it in place with the skin . Assuming you don't find any "cracks." You may actually do more damage trying to take out this divot .

RB

Before anyone else jumps on the bandwagon, I still stand by my "replace the extrusion" comment. Here's the deal... I believe that beating that spar cap back into shape would never be an approved repair - not by the FAA, not by any aircraft manufacturer, and I would hazard a guess that it would not be by John Thorp either. If I'm wrong about that I'll stand corrected. This is a major component of a main wing spar. True, it is towards the outer end of the wing, and therefore subject to less stress than if further inboard. It would possibly be plenty strong still, but do you know that? We go to extreme measures to avoid the slightest scratch or nick on spar components during manufacture because of the adverse effect on the fatigue strength & possibility of cracks forming in the structure. A spar cap that has been deformed has had it's yield strength exceeded, regardless of any surface damage. It's strength has quite likely been compromised. Bending the aluminum back again compounds the effect. I'm open to hearing a good reason, or analysis, about why I might be wrong about this. I think some of you guys are taking this issue too lightly, in my humble opinion. These are standards that have been developed over many years... many times learned the hard way.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Here here! I concur. Aluminum does not tolerate being bent and then unbent. Try it with some scrap sheet. You'll see.

D