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Match wits with Van's Engineering!

DrillBit

Well Known Member
Woe is me, but might as well give the VAF brain trust's mechanical engineering neurons a tickle over the long weekend.

Due to a tear in the spacetime continuum, the trailing edge of my -9A's right flap was bent during pushback into the hangar when an unauthorized tool chest suddenly materialized in its path. (That's my story and I'm sticking with it! :eek:)

The T.E. distortion was limited to the most-outboard bay. (See the poor picture--in all senses of the adjective.) AEX wedge bent downwards, double-flush TE rivets in the vicinity distorted, and up visible gaps between the skins the wedge, and the outboard rib opened up. The bottom skin creased to a greater extent than the top skin in the picture. Every other part (especially the ribs) is undamaged. Nevertheless, I don't think trying to unbend the TE is likely to succeed, aesthetically or structurally.

view


All right. Denial, bargaining, depression, and all that duly observed, comes acceptance and a plan to move forward: Instead of de-riveting the top and bottom skins and replacing them and the wedge entirely, I came up with this idea for a repair, and sent it up to the mothership for their opinion. (I know, the dwg shows the outboard end of a left flap, so please apply your mental mirror as required. Neither the dimensions nor details of the depicted ribs are to scale.)

view


I'm not sure about the number of rivets for the joint plate. AC 43.13-1B (if I'm reading table 4.9 correctly) calls for 5.2 3/32 rivets per inch along a lap joint between 0.020" 2024-T3 sheets. That seems like a lot more than one sees in other lap joints on the RV-9A (e.g.: the wings' inboard to outboard lap joint; forward fuselage to tailcone joint). I'm also not sure about additional reinforcement of the butt joint between the new and original AEX wedge. Structural epoxy between the top skin, wedge, and bottom skin, say? I suppose a ~12" long "vee" of .020 applied to the trailing edge wouldn't be much of a disturbance to airflow over the TE. Could be match drilled to the TE rivet holes in the original skin, then dimpled in assembly with the underlying counter-sunk AEX wedge and dimpled skin serving as the female die.

Van's support got back to me Friday, 31 August, but the Chief Engineer was already out for the Labor Day holiday. They suggested two other options: (a) buy the parts to build an all new right flap; (b) buy a "factory second" QB flap. Evidently, there are cosmetically unacceptable but completely airworthy QB flaps in stock. Mr. Lock himself does the pricing, however.

So, while helpless laughter may break out at KUAO and its environs Tuesday morning, what say ye VAFers: is this an acceptable repair, and if not, what could make it so?
 
The weak point created by your repair may be the spot where there is a discontinuity in the trailing edge wedge piece.
 
The weak point created by your repair may be the spot where there is a discontinuity in the trailing edge wedge piece.

Indeed, that's my second concern.
...I'm also not sure about additional reinforcement of the butt joint between the new and original AEX wedge. Structural epoxy between the top skin, wedge, and bottom skin, say? I suppose a ~12" long "vee" of .020 applied to the trailing edge wouldn't be much of a disturbance to airflow over the TE. Could be match drilled to the TE rivet holes in the original skin, then dimpled in assembly with the underlying counter-sunk AEX wedge and dimpled skin serving as the female die.

The "vee" would be putting extra skin, top and bottom, in either side of discontinuity, with ~ 6 rivets to either side. If the "vee" would have to be several gauges larger than .020, then one might have to put the same "vee" on the left side to even things out, roll-moment-wise?
 
Just build a new flap. It will take less time and a lot less cursing.

If you repair, you have a constant reminder to live with.

Carl
 
I'm not sure about the number of rivets for the joint plate. AC 43.13-1B (if I'm reading table 4.9 correctly) calls for 5.2 3/32 rivets per inch along a lap joint between 0.020" 2024-T3 sheets. That seems like a lot more than one sees in other lap joints on the RV-9A (e.g.: the wings' inboard to outboard lap joint; forward fuselage to tailcone joint). I'm also not sure about additional reinforcement of the butt joint between the new and original AEX wedge. Structural epoxy between the top skin, wedge, and bottom skin, say? I suppose a ~12" long "vee" of .020 applied to the trailing edge wouldn't be much of a disturbance to airflow over the TE. Could be match drilled to the TE rivet holes in the original skin, then dimpled in assembly with the underlying counter-sunk AEX wedge and dimpled skin serving as the female die.
Keep in mind that the flaps carry a lot of load. The flaps are the only control surface that are deflected steeply into the airstream and remain there. Other control surfaces are deflected into the airstream for only a few seconds at a time.
 
As a learning opportunity, what's to be lost by drilling out the affected rivets and playing around with straightening? Find an A&P in your EAA chapter who's done 'body work' on certified planes to walk through it with you.
 
If you still have the building surface available for it, I would build a new flap since you're not painted yet. Will likely be faster, and will certainly be stronger.
 
Build a new flap and hang the damaged one on your wall of shame.

If you still have the building surface available for it, I would build a new flap since you're not painted yet. Will likely be faster, and will certainly be stronger.

Looks like I am the 3rd person with the same opinion.

Three people are thinking the same thing about RV aircraft construction and repair.
 
Personally, I'd try to unbend the damaged section. The photos don't look too bad, and there are tools meant to remove such damage to sheet metal in the automotive realm.

If it bothers you that much sure, buy a whole new flap. To me... that would be a waste of money.
 
Flap damage

I would certainly start by removing some rivets to see if it will smooth out. Get a opinion from a sheetmetal guy. Building another is always a option.


Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
You have to take whatever course of action that will make you happy. This is probably a 70-80 thousand + airplane that your probably going to sell one day not a beater car to pound dents out of. You could have just about had a new flap built in the time used on these posts. The next owner will thank you....
 
One option is to replace the skins and trailing edge, keeping the spar assembly.

Dave

Already did that for the right aileron, which—ahem—didn’t escape the ravages of the rogue tool chest either.

I appreciate the conservative advice—rebuild from all new parts, or just the top and bottom aft skins and wedge, since no other parts were damaged. Sound advice it is, leaving no doubt as to airworthiness.

And I also appreciate the adventurous calls to see what happens with an attempt at straightening. If I got a cosmetic QB second right flap to replace the original, I’d have a stormy day project to work on instead of flying.

But folks, the point of the OP was whether a repair that discarded all crumpled components, but retained as much of the original aluminum as possible was, well, possible? I’m really not excited about re-doing all 8+ feet of a -9 flap (and my building surface for the flaps was recycled long ago), which means I’m lazy and/or immoral for sure. But the engineering question posed was, could one make an acceptable repair from a << 8 ft length of AEX and a 1x4 ft sheet of .020 Alclad. Cheaper than two full skins and AEX, with overlength ship charges to boot.

Cost (along with Schedule and Performance) IS the engineering triangle. I’m curious whether it could be done... but won’t ignore whether it should be, no question.
 
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8" of new trailing edge and opps rivets where there are elongated holes will have that looking almost as good as new, and it would be structurally sound.

It's your knowledge that it is less than perfect will bug you until you fix it until it is completely good as new.
 
The mantra when I worked for a NASA contractor: "Better, faster, cheaper. Pick two.";)

I'm not sure that "Better" and "Cheaper" will play well together. I think a more practical statement would be to "Pick One"!
 
I'm not sure that "Better" and "Cheaper" will play well together. I think a more practical statement would be to "Pick One"!

Well seeing as how the gummint was involved, I would say that you certainly could achieve "better" and "cheaper" by removing them - and likely get it faster too.
 
The mantra when I worked for a NASA contractor: "Better, faster, cheaper. Pick two.";)

My engineering baptism (having entered industry with an academic Pee Aitch Dee) was similar: "Fast, cheap, and good: pick two." The corollary is one is in fact constrained to optimize between the two chosen attributes. Arguably, as other posters have pointed out, the final choice is one out of three.
 
Denouement

For the record:

  • Not unexpectedly, Vans Engineering pointed out the weak butt joint between the original wedge and new wedge.
  • Vans tech support encouraged: a) buying an surplus QB right flap; b) buying a surplus dinged, but airworthy, QB right flap; c) replacing the the aft top and bottom skins and wedge; d) building an entire new flap from parts.
  • I tried b), but the dinged flap had already been sold and inventory wasn't updated. There were two other dinged flaps to choose from, but, after some back and forth about The Ghost Flaps of Aurora...
  • ...and the no-ding QB flap was a mere 15% of an AMU more expensive than than dinged flaps...
  • ...I bought the no-ding QB flap, which was delivered yesterday, installed today, and repairs are now complete.

Turns out the right flap had a bit of twist in it. The new-to-me right QB flap is straight as an arrow, and the flaps are dead nuts aligned with neutral ailerons, left and right.

The Deity clearly had a Plan: let not His servant Kurt fly untrue. :D
 
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