Trailing edge questions

[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

[jiott]

I can't see how the foam can help much at all, since it has almost no bending strength. If the trailing edge stayed straight with the foam I suspect it would have also stayed straight without the foam.

Jim

[Dorsal]

I think the foam will make for a nice composite structure and could improve stiffness, might be concerned about the corrosion though if it is not sealed.

[HighWing]

[QUOTE=jrevens;21919]One possible issue with using urethane expanding foam in the aluminum trailing edge is corrosion over time with moisture.

This brings back memories. With my first Kitfox, with the thought of saving weight, I made up thin fiberglass skins with the intent of filling them with two part Urethane foam for the lift strut fairings. A guy came by the hangar, saw what I was doing and mentioned the use of foam in a wing construction he was familiar with - maybe the same airplane - and the consequent corrosion of the aluminum. Not being that much of an experimenter, I removed the fairings and installed the heavy PVC firings. I did, though, make up a test section using 4130 steel, and the foam under the fiberglass skin. To check for all possible variables, I masked the tube and prepped it to provide parallel surfaces with three surface treatments. One strip I sand blasted, one I left bare (black patina), and one was painted with the zinc chromate epoxy primer. The plan was to remove a section of the skin and foam periodically and check for corrosion on the three types of surfaces. For my part and with the 4130 steel, corrosion is not an issue, as in the succeeding 15 years, there has been no corrosion on any of the prepped or unprepped surfaces. I was comfortable using the foam this time, but under thin aluminum skins. If corrosion becomes an issue, it won't be on the safety critical part.

A comment on the Jiott question. With the configuration of the V shaped trailing edge, If shrinking pressure exceeds the strength of the trailing edge and it begins to deform, the V will begin to open up essentially moving toward a flat surface with minimal warp resistance. The Urethane foam, at least the two part variety is very adhesive and will tend to resist any movement in the V. The addition of a thin layer of epoxy to the foam will increase the resistance. It is much like the foam in some of our fiberglass parts - enclose a speedbump shaped strip of foam with a layer of glass on a thin flat layup and it becomes more stiff. Another example might be the foam boards with glass or carbon fiber skins that that are used extensively in the composite airplanes.

For me, the foam idea is a winner with reservations due to the corrosion issue. I would think experience over time should answer that question.
Lowell

[Peter B]

I built my wings last summer before going to Oshkosh without the reinforcement. At the end of the chapter in the build manual it refered to the reinforcement as optional and there was no material in the kit. When I visited the Kitfox Booth I asked J McB about the tubing and he gave some tips about inserting the reinforcement after my trailing edge was already installed. Evidently it is done all the time.

John Pitkin

I just got my 7SS about 2 months ago (July 2013) and I can't find anything in the build manual mentioning anything about an optional reinforcement of the trailing edges. Wonder if they took that section out?? I am just at the stage of installing the trailing edges now and it obviously would be much easier to do the reinforcement now as opposed to later after they are already bonded and riveted in place. I'm going to take a look at some arrow shafts at the sporting goods store before I make a decision on the material for the reinforcement.

Peter B
Natchez, MS (HEZ)

[Flyingsnowdog]

I think it mentions it in the covering section, I used the arrow shafts and it worked out nice.

[n85ae]

I didn't do anything to mine, and 10 years later my wings look like new ...

Regards,
Jeff

N85AE, Series 5, IO-240B