Trailing edge questions

[szicree]

On page 36 of my manual it says: "Trowel a thin fillet of adhesive inside and against the bottom sides of the trailing edge. Make sure to fill any voids between the rib capstrips, webs, and the trailing edge."

Are they saying to fill the triangle shaped space that occurs at each rib/trailing edge junction? They don't mean the entire length of the trailing edge, right? Am I filling these little voids in anticipation of the aft-most flap hanger bracket rivets passing all the way through the trailing edge? Also, doesn't that aft rivet look kind of unsightly from the bottom? Any advice and/or photos would sure help.

[HighWing]

Steve,
There was a recent thread that included some comments on fabric shrink temps. It was mentioned that on the Kitfox, Polyfiber recommended small incremental temp increases with checks to see if the longerons / trailing edges begin to sag in and if so stop there. The person posting that comment mentioned he didn't do anything with the trailing edges - as I recall - with very good results. With that said, on both of my Model IVs, I used carbon fiber arrow blanks between the ribs running between trailing edge and spar - Hysoled in place as stiffeners. I then shrunk to the recommended temps. On the first IV I failed, however to fill the space between the aft of the rib and the internal surface "V" of the trailing edge, and eventually, on the non flaperon hangar ribs, the glued only trailing edge came loose and fabric tension collapsed the trailing edge there into the space resulting in a rather ugly wrinkle on the fabric right at the rib. The thinking on the arrow blank idea was hopefully to save a bit of weight. I know of others who have glued in lengths of "hard" aluminum tubing where the manual says to apply the adhesive. I think if I was doing the adhesive thing, I would opt for some epoxy stiffened quite a bit with Micro Balloons (at least 25% by weight which will expand the volume about four times). "Micro" is very stiff and quite light weight. That is what I used this time between trailing edge and all ribs - and lots of other places to smooth things out a bit before covering and painting. It is used extensively on the composite airplanes for filling gaps and leveling surfaces.
Lowell

[jiott]

I wondered about that fillet statement myself and came to the conclusion that I needed to fill any void between the aft end of the ribs and the inside of the trailing edge so the fabric tension would not break the glue bond and collapse the trailing edge as Lowell had happen to him. This problem was a non-issue with me as I was able to pull the trailing edge up tight against the aft ends of the ribs and the edge remained perfectly straight, so there was no void to fill. I clamped them up tight against the ribs while the epoxy set up. In case you have to hold some of the trailing edge pieces back a little from the rib aft end to keep a straight line, then you would need to make sure that void was filled with epoxy. That was my take on the statement in the manual. Also be careful not to get too much epoxy into that vee shape or you won't be able to slip the flaperon bracket reinforcing plates into place on the sides of the aft part of the odd numbered ribs (don't ask how I know this).

Regarding the aft most rivets on the flaperon brackets, the factory assembly guy told me at the Arlington Air Show that they use a shorter rivet in that spot on their factory build SLSA so it doesn't show thru the bottom and look ugly.

Jim

[szicree]

Thanks for the info guys. Jim, I found on my wing that if I brought the trailing edge up against the ribs I would only measure about 11-1/4 instead of 11-3/8. Also, thanks for the heads-up about the flaperon bracket interfering with the goop.

[jtpitkin06]

Ooh!!! What a great idea, Lowell, using arrow shafts instead of Aluminum tubing for the trailing edge reinforcement. A good use of alternative materials. Light, strong, cheap and available at sporting goods and hobby stores.

Put that one in the builder's tips!

John Pitkin
Greenville, TX

[szicree]

If I understand correctly, the tubing recommended in the manual runs spanwise inside the trailing edge, whereas Lowell's idea puts the arrowshaft chordwise from trailing edge to spar. I think the arrowshaft would be significantly more effective at resisting bending.

[sundance]

When i built my model 5 outback there was no instructions to reinforce the trailing edge. So i used a can of expandable building foam insulation and filled the space in the trailing edge. Cut off the excess with a razor blade and coated the foam with epoxy glue. I heat shrank fabric to 350 degrees with [ NO ] bowing of trailing edge. It is very strong and lite. That was 11 yrs. ago it is still perfect.

[Av8r3400]

I know of several examples of this method (expanding foam).
All of these have worked fine, too.

[jrevens]

One possible issue with using urethane expanding foam in the aluminum trailing edge is corrosion over time with moisture. I say "possible", as I have no personal experience with it, but have read of that problem in the past... there was a Thorp T-18 that the builder filled some areas with that type of foam to avoid oilcanning issues he had. When the aircraft was sold, the new owner had some terrible corrosion that was discovered during a rebuild. I just throw this out as food for thought.

[MXJ]

Can this be the material that you are looking for?

http://airmetalfab.com/main.html?src=%2F#4,0

Then select the Flap Trailing Bulb