Building/sealing flat metal tips for racing

[Bob Axsom]

My semi-flat fiberglass wingtips with rounded edges were a bust and I wave been advised that sharp edges are necessary to get the desired speed increase. This is different than a tip plate in that the tip does not extend beyond the wing surface. The trick as I see it is achieving that sharp edge and not have any air gaps between the upper and lower wing surfaces and the flat metal tips.

I believe I have thought this through enough that I can do it. What I now plan is cutting a 1/2" wide strip off of the edge of the 6' sheets of 0.016" 2024T3 to be used for an inner flange. The sequence is critical to doing the job right I think.

1 - "C" clamp the spliced strip inside the wing skin, hard against the inside of the leading edge, outboard edge precisely aligned with the wing skin outboard edge.

2 - Start at the top front of the wing and carefully drill a #30 hole in the flange strip through the center of the forward wing tip mounting hole which is already sized and dimpled for a #8 flathead screw (that's what I mount my existing 8 wing tips with) and 1/8" clecoe it in place. Then move to the next hole aft and repeat until the last one is done then do the bottom in the same way. Mark for wing tip and orientation and remove the flange strip.

3 - Saw 120 1" brackets from 6061T6 sharp corner 1/16" thick 3/4"x3/4" angle (break sharp edges, etc., etc.) and "C" clamp them to the inside of the flange strip with the outboard bracket vertical surface precisely aligned with the flange outboard edge and drill a #30 hole in the horizontal surface using the flange hole as a drill guide. Mark every bracket and the flange with a corresponding number so every bracket has a specific installation location.

4 - Using a non-floating plate nut as a drill guide, clecoe the bracket, flange and platenut (oriented for&aft) together with a 1/8" clecoe the drill the first platenut mounting hole with a #40 drill. Clecoe the stack with a 3/32" clecoe and drill the other mounting hole. Repeat the process until all of the holes are drilled then disassemble for cleanup, deburring etc.

5 - Enlarge the screw holes in the flange and brackets to clear the wing skin dimples (1/4" to 3/8") dimple the platenut mounting holes in the thin flange and countersink the mating 3/32" platenut mounting holes in the brackets for the 3/32" rivets. Take apart debur the enlarged holes.

6 - Rivet the FLOATING platenuts to the stacked flange and brackets.

7 - Clecoe the flange assemblies back inside the wing.

8 - Cut oversize tips from the large sheets of 0.016" 2024T3 aluminum ...

9 - At this point the bracket present a fairly large target but I need to refine the process for drilling holes through the tips into the brackets and clecoeing as I go. There are several ways to do this but I have to work it out yet.

10 - Remove the tip and flange assembly from the wing, dimple and countersink and debur the tip to flange bracket mounting holes and rivet the tips to the flange assembly.

11 - Use RTV to seal the inside of the interface between the flange and the tip.

12 - Reinstall and progressively mark, cut, try and repeat until the flat internally sealed sharp edged tip exactly matches the surface of the wing skin.

Bob Axsom

[Tom Martin]

Alternate method
1. fix fluted/notched angle to wing first. Use 3/4 by 3/4 by .032. You might have to take one side down to 5/8" depending on how much wing skin over hang you have.
2. place end piece over end of wing, make it larger then required, and drill into flange
3. rivet flange to end piece
4. install and trim to exact size.

[Bob Axsom]

I have some unstated requirements that are driving my approach especially the ones that require complete interchangeability with all of my other wing tips and no alteration of the of the painted wing. I haven't been able to get out to the hangar to measure the distance from the last rib to the edge of the wing skin - I'm expecting the need to cut off the inboard edge of the brackets as you mentioned. I have all of the material available so I will go with that but I agree thinner brackets would be better. I also didn't mention something that I'm sure you assumed - I am going to reinstall my end plugs in the ailerons and only extend the flat tip back far enough to cover the outboard end of the wing and aileron interface.

Bob Axsom

[jsharkey]

...to mold or otherwise form sharp corners on composite tips.

[rvmills]

Bob,

Couple thoughts, as I've been considering this project as well. These are the thoughts of a rookie of course, so caveat emptor!

I've been thinking along the lines of Tom's suggestion, and you could probably put floating nutplates in a that angle AL flange, so you could press in and tighten them up to make the tip flush along the entire span. Not sure if there's a rub there, in that with a continuous flange there might be some binding and some resulting gaps between the wing skin and tip material. Your independent flanges might do better, though it is a much more complex project.

Mike Thompson has an alternative approach. I looked at Mike's tips at the Taylor 150 and asked him about the flat plate tip-to-wing surface interface, and how he mounts his tips to the wing.

Mike's tips are 3" in span, and have a removable flat plate tip. Think of it as a wing-shaped cyllinder on the end of his wings, with the flat plate covering the outside of the cylinder. You've probably seen them, but thought I'd throw it out here. His design does add the 3" to the span, but you could probably make them 1" or 2" and acheive good results.This design does offer a couple nice features.

First, his 3" tips go on and off just like most normal tips (same flange design), and just like yours and mine...so that makes them fully interchangeable with your other tips. The flange on the inner part of the tip would probably be easy(er) to make as well.

Second, his flat plates on the ends are interchangeable as well, and Mike has tried some different designs, including some that had mini-fences. He attaches the flat plates to the tips with (I believe) 4 screws (maybe more). However, in looking at them, the plates were tight and flush to the wing...no air gaps that I could see.

The benefit of the 1", 2" or 3" exrtension is that the flat plates are easier to swap in and out, or so it appeared. You might give Mike a shout to get more detail. I know he's your RV Blue adversary, but I'm sure he'll give you the straight scoop!

I'll be watching you develop this with great interest, as I've been trying to sleuth out how to do some variable flat plate designs as well.

Best of luck with the design and development!

Cheers,
Bob

[Bob Axsom]

I got to spend a little time at the hangar yesterday working on a better solution for the fuel drain fairings. With the half hour left over I decided to remove the right wingtip and take a look at what I have to deal with to implement this "simple" conceptual design. The problems are several. The skin extension past the last rib is not consistent; the tip screw mounting holes are too close to the end of the wing skin to install the platenuts on the angle brackets without relief; the end of the wing skins may not be parallel and in line with the direction of flight etc. I had mentally worked out solutions to all of the observed problems but it requires a lot of detail work. Thinking of how to get that done before the race in Plainview, Texas on June 5th and deal with my responsibilities at home I came up with a new idea: install the previous flop semi-flat tips which are already interfaced with the wings, mark the desired cut lines, cut off everything outboard of the mount interface straight line, install the flat metal tips on the old residual mount with angles & rivets between platenuts and seal the interface on the inside leaving the sharp edge flat tip on the end of the 21' wing. I plan to fly with one new metal flat tip and one currently demonstrated fastest 3" streamlined tip. The yaw result should indicate the tip configuration with the least drag.

Bob Axsom

[Bob Axsom]

I removed the right 3" streamlined racing wing tip and checked the end of skin alignment with respect to the leading edge using a large square. It is very close to square with the leading edge slightly longer than the trailing edge - at this point I and calling it close enough to perfect that it is not to be messed with.

I installed the right semi-flat tip on the wing and marked where the skin ends on it. The I removed it and applied masking tape as a marker using the edge as the cutoff point. I carefully cut off the the portion of the tip that extended beyond the wing skin using a die grinder and cutoff wheel. Then I reinstalled it on the wing to assure that I had good alignment.

I decided to make brackets for attaching the flat metal plate to this fiberglass flange out of 1/16x1/2x1/2 6061T6 aluminum angle. I cut these using a metal blade in my small Craftsman bandsaw (wear safety glasses). These generally go between the platenuts on the flange and the spacing I used for those platenuts depended on the demands of the curvature of the airfoil. Some of these are very close together and some are widely spaced so the bracket sizes vary widely - some are only wide enough for one rivet and some have to share the platenut mounting rivet (I try to avoid drilling out the existing platenut mounting rivets so as not to affect the tip to wing mounting hole pattern even slightly but I have resorted to it once so far). This part of the job is very critical and vulnerable so I am methodically working through it on bracket at a time. So far I have 16 done and more than 30 to go. It makes a good activity to replace eating and reading when I wake up in the middle of the night.

I plan to do just this one and fly it with the 3" streamlined racing tip on the other wing to verify that it is worth building the other flat metal tip with sharp edges.

The approach looks very good. More later.

Bob Axsom

[Bob Axsom]

I plan on starting the work to attach the 0.016" tip to the tabs on the fiberglass flange tomorrow.

Bob Axsom

[Bob Axsom]

I test flew this evening with the new flat metal tip on the right wing and the 3" streamlined racing tip on the left wing. The results were inconclusive. I expected a yaw to the left, which I had on the climb out as usual but was it more than usual - well maybe but it was not significantly more if any at all. When I leveled out the ball stayed pretty much centered with no effort from me. I ran a speed test but I was rushed by the coming sundown and my unlighted plane in racing configuration so I did not have max RPM until the second leg and it was not leaned properly until late in the run so the slow 177.9 KTAS is not meaningful. The work time on this "simple wing tip" is much more than I expected. The wing tip fits very well and it is trimmed perfectly flush with the wing skin's outer surface. There is a 2024T3 0.090"x1"x8" stiffiner riveted inside the tail of the tip. There is a balsa wood plug in the end of the aileron. Conclusions: If there is any gain in speed it will probably be small. I will not complete the other one before the race in Plainview, Texas on June 5, 2010. I will build the other tip and carefully test both on the airplane in the future. Below are the two wing tips in today's flight test.

Bob Axsom

[elippse]

Boy!, You are some experimenter! You didn't say at what altitude you did these tests, but the effectiveness of these tips, which has to do with the Oswald efficiency factor, relates to the effective span of the wing which has to do mostly with the induced drag. As such, you will have a positive benefit from these tips at all altitudes and speeds, but the higher or slower you fly, the more it will show up. So maybe in comparing these with the rounded tips you'll see a slightly lower stall speed and a little more speed at higher density altitudes.