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Bob Axsom

Well Known Member
At very high speed I have had the wing seal pull out on the upper surface and beat against the fuselage and wing. No damage occurs, just black marks on the surface. This has happened to others as well and glueing the seal to the closure strip with 3M black adhesive solved the symptom until my pull up after crossing the timing line at the end of the AirVenture Cup race this year. The plane was probably well over the 180sh "red line" so it shouldn't be a problem in normal operations but that doesn't stop my curiosity from getting activated. There are two lines of screws in the upper surface attachment and the front end wraps around the leading edge so even though the inboard end is unsupported there is some stiffness designed into the closure strip installation. Especially at the lowest pressure point on the upper camber (where the screw line stagger occurs). Inspite of that the rubber flies out and the loose section is roughly from the high point of the airfoil all the way back to the trailing edge of the seal. I have sealed everything on the bottom of the wing in this area so I suspect that at some speed the pressure differential on the strip "sucks it up" in an arc about the attach point and the rubber seal is expelled. My thought is it probably occurs around mid-chord and pulls the back end out with it. The best fix I can think of off the top of my head is a thicker 2024 closure strip. Any thoughts?

Bob Axsom
 
You might try remaking the aluminum strip and narrowing the gap between the strip and the fuselage side. Perhaps that will hold the rubber gap seal more tightly against the fuselage side?
 
I don't think so

I don't think so. When I install the closure strip a LOT of pressure is required to compress the rubber and hold the dimpled holes in enough alignment to get the screws in and run them down progressively from the rear all the way around the leading edge and back to the fixed lower wing skin interface with the fuselage. It is impossible to align the holes without something like the screws themselves or clecoes to sustain the side pressure - it is a high pressure fit. However, let's say that even greater compression force was a fix for the rubber coming out, the question remains what are the physical actions that result in the rubber coming out? Or in other words what forces cause the failure.

Bob Axsom
 
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Pull-out Experience

Bob,

I agree with your second post.

I had the same problem with my -7, but can't say it was speed related because it could take 2-10 hours for the seal to pop loose. Painting didn't happen until 75 hours, so I used some electrical tape spaced about 6" to keep it in place meanwhile; I wasn't going to gum up a to-be-painted surface with adhesive. Only the aft 12" was taped to start with, but the seal still popped out mid-cord. It never pulled out forward of the spar because the compound curve of the seal makes it resistant to pull-out (and insertion!) After painting, it was spot glued every 3". So far so good for 65 hours.

My fairing-to-fuse gap is an even 3/16, halfway between the 1/8 and 1/4 Van's calls out (if I remember correctly). 3/16 preloads the seal with a small bend radius which distorts the U-channel so that it's not really gripping the fairing. The preload is sort of pushing it out to begin with. It might stay in place if the gap were wider and the rubber only slightly bent up against the fuse, but I'm not going to experiment at this point.

What is your fuse-to-fairing gap?

John Siebold
 
It's pretty small

It has been a while since I set it up so I just don't know. I usually go for the end that says I can cutoff more later if I have to. You know the rationale. Next time it pops out I try to remember to measure it but for now as long as the adhesive holds...

Bob Axsom
 
too close

This sounds crazy to me but I'll ask, could the gap be so small that it is being squeezed or pried up and than off. The body flexes and wing flex could just be extruding it off? Long shot. G
 
In my case it is directly related to speed

Good thinking. You may have something there but in my case it is directly related to speed. At 180 kts if I don't have some good adheasive on it the seal will come out. Below that it is OK and I never had it come out below 170kts even without adhesive. The cause seems dynamic with no progressive component.

Bob Axsom
 
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