Non-removable empennage gap cover?

roee · #1 10-31-2010, 04:24 PM

This is referring to the aluminum strips that close the gap between the bottom of the horizontal stabilizer and the sides of the fuselage. The plans call for these to be installed with #6 screws (drill & tap the longerons), and a rubber channel at the intersection between the top of the gap cover and bottom of the HS.

There are a few things about this arrangement that I don't like so much... but never mind that. The question is this: I don't see any good reason why this strip needs to be removable. Am I missing something? Removing these strips doesn't provide practical access to anything, since the gap is so narrow. Better access to the same space is available through the removable top empennage fairing (the fiberglass piece on top of the HS). So what's the point of having the bottom gap covers removable?

Instead of the callouts on the plans, what I'm thinking of doing is this: Rivet the gap covers directly to the fuselage sides instead of using the #6 screws. Leave only a very narrow gap, say about 1/32", between the top of the gap cover and the HS, just wide enough to ensure that there can be no metal-to-metal contact there under vibration. Then fillet the intersection with a bead of proseal instead of the rubber strip. Seems to me simpler, more robust, lighter, and probably aerodynamically cleaner.

Can anyone think of a compelling reason not to do this?

Thanks,

TX7A · #2 10-31-2010, 04:57 PM

Roee,

I fitted & riveted mine on to stay. I think you'll find that many others have done the same. Some have extended the empennage fairing to serve as the gap seals. Some have used proseal to attach...no rivets.

Vern · #3 10-31-2010, 05:54 PM

Some folks adjust the elevator shim thickness while test flying. No big deal to drill out though.

Mel · #4 10-31-2010, 06:26 PM

Mine have been riveted on for over 17 1/2 years.

jsharkey · #5 10-31-2010, 06:38 PM

Riveted - and no seal yet.

Jim Sharkey
RV-6

roee · #6 11-01-2010, 12:48 AM

Thanks everyone for the responses. Sounds like it's a good way to go then.

Quote:

Originally Posted by Vern

Some folks adjust the elevator shim thickness while test flying. No big deal to drill out though.

Good point, Vern. And even if the HS incidence does need some slight adjustment, I don't think I'll even need to drill out the gap fairing. I can trim it more in-place if needed. Or if the adjustment is the other way, then it'll just open up the gap slightly, also no big deal.

I'll just wait on the proseal fillet at the intersection until after the first few test flights when I'm satisfied that the HS incidence is ok. For those first few test flights I can tape it or just leave it open (sounds like some even leave it open permanently, so it shouldn't be a problem).

Thanks again.

Bob Axsom · #7 11-01-2010, 03:20 AM

I did re-glass the upper flanges to actually conform to the shape of the horizontal stabilizer and extended slightly around the leading edge. The bottom closure strip with the screws and rubber have worked well so far and I do not plan to change them but I'll keep an open mind. I have been flying our RV-6A for almost 7 years and I have been considering raising the leading edge slightly to possibly improve the speed using a shim method that allows return to previous setting. However, I need to determine my pitch trim setting at maximum speed in level flight to assure that I'm not forcing the use of more trim with the change before I work on that. I am a little curious about your choice of proseal instead of some other RTV - what do you see as the benefit of this material?

Bob Axsom

roee · #8 11-01-2010, 04:17 AM

Bob,

I thought of proseal for the fillet because I suspect that it will have stronger adhesion and better longevity than silicone-based RTV (that's just what my gut says, I could be wrong). But also, I plan on painting the airplane sometime after it's already flying, and proseal is paintable, whereas silicone RTV is problematic for paint.

In your case, since you like to continuously experiment and fine-tune for speed, and especially if you may want to go back and forth between one configuration and another for racing vs. your other flying, then you may very well be better off with the removable strip and rubber channel. That's definitely one advantage in favor of the removable strip. [Note however that changing the HS incidence will also affect the fit on your empennage fairing (the composite top piece), so there's a little more work there.]

In my case, I hope that my HS incidence will be set near-optimally from the get go (I rigged it precisely per Van's specs), or at most have to be adjusted slightly during Phase 1 flight test, and then never be adjusted again. So in that case I just didn't see much benefit to the gap strip being removable. And I must admit, my motivation for making it non-removable has less to do with aerodynamics and more to do with just structural robustness and simplicity. I suspect that the difference in drag and weight will be negligible one way or the other (at least for those of us not looking for tenths of a knot

).

P.S. Bob- when/if you do experiment with your HS incidence, I'll be very curious to hear your results, out of academic curiosity if nothing else. I would think there could very well be a discernible difference in max speed due to trim drag if it's significantly off. What do you think would be the ideal condition? I would imagine HS, elevator, and trim tab all in-trail with each other? Or maybe not?.. Of course whatever it is, I would think the HS incidence you end up with would be optimal only for one particular flight condition, density altitude, power setting, and W&B. But I digress... Fascinating topic!

Rick6a · #9 11-01-2010, 04:22 AM

Roee,

I am the very first RV builder to install the lower empennage fairing by bonding it in place with proseal. I did not see the need to add a fillet and 5 years of operational service later, the method of attachment is as secure as ever.

See post #3 for detail photo of the installation here:

http://www.vansairforce.com/communit...257#post416257

roee · #10 11-01-2010, 04:35 AM

Looks good, Rick. And I don't doubt that it's secure. That proseal is tough stuff!

About the fillet though, that is the one component that I did think to include for aerodynamic reasons, not structural. Without the fillet, even a very close fit between the gap strip and the HS will not be air tight. And there will likely be a pressure differential across it, so air will flow through the gap and contribute to drag. I figured that making a fillet to seal the gap is easy enough.

There is also probably some optimal radius for the fillet that minimizes drag, but what that is is beyond me, and probably makes a very small difference anyway. So I figured I'd just make a minimal fillet (probably a small popsicle stick end radius) to seal the gap.