Rubber wing root gap seal

APACHE 56 · #1 03-20-2011, 05:08 PM

I find myself being out smarted by the most simple of building tasks; the wing root rubber gap seal installation. It is particularly troubling coming so late the building process as I should be really ?smart? by now?lol.
The RV8 builders book calls for a 3/16 gap and typically when Van is that specific they mean ?3/16? not 1/8 or1/4. Well 3/16 just doesn?t work as it tends to pucker the metal fairing meaning the gap has to be widened. The issue? Perhaps none in that I?ll just keep enlarging the gap a 32nd at a time until I can get a good channel and have the fairing lay flat. My concern is that I?m missing some obvious trick to get the rubber channel to make a good ?U? and achieve a flat metal fairing. Trick? No trick? Keep trimming?
Thanks,
Don

lrfrey · #2 03-20-2011, 05:37 PM

I have a 7 not an 8 but there should not be any difference. I don't recall the dimension I had in the instructions but 3/16 sounds right. Anyway, I made it exactly as in the plans because I did not want it to come out in flight. It was tough to get installed initally but I finally managed.

Start laying it in and it will "go astray" soon. At that point I used a plastic credit card to slide between the fuse and the strip to nurse it in place as they say as I went along. Up/Down/Left/Right. It will go in.

After 15 hours on my plane it has not come out.

It does not "form" easily or perfectly at first but in a few hours it will, warm Wx helps too.

Are you sure you don't have it up-side down? That won't work (I know).

Good luck.

Larry

Rick_A · #3 03-20-2011, 05:58 PM

The seals need to push up hard against fuselage so that the inboard part of the seal makes a 90 deg turn to push horizontally against the fuselage. They look better when you've got a good upward turn and they stay in place better.

I had to re-make my aluminum root fairing because I make the gap just a little too big and the seals did not fit tight. It can be quite the challenge getting the rubber seals in place for the first time. You'll be cursing yourself for removing that last 1/32nd if you make the gap too big.

Even though mine are quite tight now, I had a problem this afternoon that one of the seals came loose in flight. I may try just a tiny bit of weatherstrip adhesive when I put it back in.

APACHE 56 · #4 03-20-2011, 06:26 PM

Thanks guys. 3/16th it is!

johnfurey · #5 03-20-2011, 07:16 PM

I just did that process yesterday. I decided on .100 gap. I installed the metal fairing then "painted" a sripe on them with blueprint dye (painting a line with a heavy felt marker also works. I then made a little square block from UHMW, wood or metal would work fine. Drill a hole so a small rod fits tight then grind the end of the rod so it will mark the painted line at whatever gap you decide you want then just run the block along the side of the fuselage while applying enough pressure to scribe the line on the fairing. You will have a perfect fine line to work to. It's almost a two man job to install them but when done mine looked GREAT

Sid Lambert · #6 03-20-2011, 07:23 PM

When installing, leave the screws just a tad loose until the entire seal in. If you tighten the first screw it will make the aluminium piece buckle and stick up between just about every screw position.

elippse · #7 03-20-2011, 07:54 PM

Is it physically possible to install the seal with the lip turned down against the fuselage, rather than up? The reason I ask is if it is possible, then the higher pressure inside the wing will hold the seal tightly in place just as you do with the plenum seal inside the cowling. You always want to turn a seal in toward the higher pressure.

Bob Axsom · #8 03-21-2011, 01:33 AM

This is one of the parts of building the plane that demands a craftsmanship approach. A lot of hands on working and fitting get it tight and right. It does have conform to the application demands over time and it presents little trouble removing and reinstalling the strip with the rubber in place during the annual condition inspections. No matter how well you do it, it will blow out at high speed as in a pull up after crossing a finish line in a race. If you keep it under 170kts you probably will never see it but in spite of your best intentions you will eventually go faster and under the right conditions it will blow out starting at the rear on top of the wing. The rubber will spiral in the slipstream and beat against the fuselage and wing. It will not do any damage but it will leave black rubber skid marks where it does the beating that you will want to clean them off. It happened to me a couple of times before I applied the yellow 3M weatherstrip adhesive to the edge of the closure strip/rubber interface only (none on the fuselage/rubber interface) - problem solved. I have continued to modify the plane for more speed (now at 184+ kts) but the problem has not recurred in four years of SARL racing.

Bob Axsom

flion · #9 03-21-2011, 06:17 AM

Quote:

Originally Posted by elippse

Is it physically possible to install the seal with the lip turned down against the fuselage, rather than up? The reason I ask is if it is possible, then the higher pressure inside the wing will hold the seal tightly in place just as you do with the plenum seal inside the cowling. You always want to turn a seal in toward the higher pressure.

Actually, the pressure varies along the root. At the leading edge, the outside pressure is much higher than inside. Generally, it makes a better seal curving outward than it would curving inward.

elippse · #10 03-21-2011, 08:30 AM

Quote:

Originally Posted by flion

Actually, the pressure varies along the root. At the leading edge, the outside pressure is much higher than inside. Generally, it makes a better seal curving outward than it would curving inward.

Afraid I have to disagree with you. At the stagnation point the pressure is the highest, but immediately behind that the pressure is the lowest as the air flows over the small radius leading edge, just as Coanda has shown, and Bernoulli, too! Have a look at Abbott & Doenhoff's Theory of Wing Sections where on p.78 they show the top and bottom surface pressure distributions. These are on a laminar-flow airfoil, and the 23013.5 on the RV has a really big low pressure spike just aft of the LE on the top surface because of its 230 mean-line, and even on the bottom. That's also why the hood on a car, left unlatched, will lift right up and wrap itself around a windshield. The upward curve at the front of the hood has a very low pressure, not high pressure as some NASCAR announcers think.