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.

[jtpitkin06]

Yes, the instructions in the SS7 manual recommend the 3/8 tubing snapped inside the trailing edge spanwise and bonded with Hysol.

The tubing is optional. I asked John McBean about it at Oshkosh.
Ans: If you shrink to 250, you don't need it. If you shrink to 350 you'll need the additional reinforcement to prevent trailing edge scallops.

I used aluminum tube and it really stiffened things up. It takes all the flimsy feeling out of the trailing edge. I was worried the thin trailing edge could be damaged easily without the tubing. Not any more. Had I known about arrow shafts I would have used 3/8 shafts in the same spanwise position bonded in place with Hysol. Arrow shafts are lighter and if bumped or banged will resist kinking and bending much better than aluminum tubing.

Spanwise installation inside the trailing edge will resist vertical displacement of the trailing edge as well as scallops.

John Pitkin
Greenville, TX

[MotReklaw]

I would like to see a picture of this with the adhesive/carbon fiber, or tubing method.... I'm having a problem picturing it in my feeble mind....

Mot, a visual learner....

[jiott]

The 3/8" tubing just snaps into the gap between the two edges of the trailing edge. On mine it was quite a tight fit and I had to kind of roll it in starting at one end. Deburr the sharp edges of the trailing edge first; it will greatly help snaping it in. Squirt the epoxy in the gap first with a syringe and the tubing will lay nicely in a bed of Hysol when it snaps in.

Jim

[Tom Waid]

A photo:

http://tropicaltuba.com/Kitfox%20Project/images/285.jpg

On my kit the short outboard trailing edges accommodated 3/8 tubing. The rest would only accommodate 5/16". Before ordering the tubing I did a trial fitting with drill bits. My guess is that the factory has multiple suppliers which explains the slight variation.

[HighWing]

My system simply used lengths of the carbon fiber tubes glued between the spar and trailing edge between each rib. It required a third less tubing and much less glue. They are straight running fore and aft (parallel to the ribs). I made up some aluminum fittings by punching holes in some thin aluminum (.016). then cutting them out to about a half inch by three quarter inch. By punching the holes it created a star shaped burr that fit into the end of the tube to stabilize it while the Hysol was setting and to separate the carbon fiber from the spar. One was placed on each end of the tube after cutting to a snug length. Hysol was placed between all surfaces. Again, my supports were not span wise, but were essentially a standoff between the ribs halving the distance between the trailing edge supports. I have no progress shots of that part of the build, but made up an aluminum part and attached a picture of that. The tube is slightly undersized as that is all I have right now. The trailing edge is straight as a string after shrinking.
Lowell

[MotReklaw]

Many thanks for the photos Tom and Lowell.
Now I see and will check out my aluminum stock and/or order some of the carbon fiber blanks.... found lots of them on EBay....

Thanks again,

[rogerh12]

Tom;

That aluminum tubing in the trailing edge, what type and wall thickness did you end up using (6061 @ .035)???

Thanks in advance

Roger

[Tom Waid]

My invoice says 6061T6 5/16 X .058

I bought it here (http://www.aircraftspruce.com/catalog/mepages/alumtube_6061t6.php)

Happy Homebuilding!!

[jtpitkin06]

6061T6 .0375 x .035

Nice fit, good strength with minimum weight.

The 3/8 size tube is a tight fit but you will need to work it a bit to get it in the trailing edge slot. Cut each piece about 2 inches shorter than the distance between the ribs so you don't pop the trailing edge bond loose from the ribs. Once inserted the 3/8 tube nicely fills the void for maximum strength with the greatest available cross section. There is just enough slop to allow the tube to rotate and smear the Hysol for a good bond. I found it easiest to rotate the tubes with an awl inside the end of each tube section at about a 45 degree angle.

John Pitkin

[rogerh12]

WOW, .058 ??

Well that auta do it all right.

Fortunately, AirParts in Kansas City is only 45 minutes from me, so I will just jot on over and pick up a few feet on the cheap (.035 has the best pricing). Plus, they have loads of surplus AN hardware too. You just can't ever have enought extra AN, can you?

Thanks for info!!!


Roger

[HighWing]

Reading John's most recent post suggests that the challenge of fitting the aluminum tubing into the trailing edge might just be a sequencing issue. If the trailing edge was fitted and trimmed, then the tubing inserted and the trailing edge glued in with the same glue batch, it doesn't seem that there would be near the difficulty.

Maybe the manual is what needs the touch up. If the tube suggestion is made at the finishing stage, maybe it should be made at the wing build stage. Then again, with the optional quick build wings, maybe it should be included from the factory rather than challenging the builder with an out of sequence step.

[dginok]

I bought a set of pre-cut tubes from the factory for my quick-build wings. At first, I thought there must be some mistake -- I did not see how they could possibly fit in the trailing edge slot. I made it over that mental hurdle and got them to slide in with a little care and coaxing.

[jtpitkin06]

I suppose I should give a bit more background...

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.

The 3/8 size fills the trailing edge nicely but the opening (slot) in the edge metal is less than 3/8 inch. The slot must be flexed open a bit to get the tube inserted. Trying to force the tube in all at once is far too difficult. It's best to start at the midpoint between the ribs where the trailing edge will flex the most. Once you get a tube started use a twisting motion while pushing at the same time. The tube it will slide into the training edge and toward the a rib at the same time. When you have it about two thirds inserted just roll it in with your thumbs.

It's really not a difficult procedure. Brush some Hysol into the trailing edge, start the rib, twist and push. It pops right in. I did have one of my trailing edge-to-rib joints pop loose requiring a rebond. Not a big deal.

If I were starting from scratch I would insert the reinforcement tubes before the trailing edge is riveted or bonded to the ribs.

Two things improved after installing the tubes. I noticed an immediate improvement in the trailing edge stiffness in both fore/aft and up/down.

The negative is the tubes and Hysol add yet a bit more weight. Every little thing you install in the airplane adds up. A few ounces become pounds and pounds breed like rats in an airplane.

However, I really think this added weight is worth the penalty. The additional strength is a real plus. Where I see the real payoff is a reduction in hangar rash. The non-reinforced trailing edge can be bent or kinked very easily. Just a bit of rough handling or a bump can lead to some wing repair. With the added tubes, I think the trailing edge can take a lot more bumps and bangs without bending the flimsy metal trailing edge.

The thing I like about Lowell's use of arrow shafts is the yield point on metal tubes is far lower than the shafts. If a trailing edge reinforced with the carbon fiber shafts were to take a blow it would have to bend much further before the trailing edge would have a permanent kink.

Lowell installed the reinforcement differently but he achieves a lot of resistance to scalloping when the fabric is shrunk. Because the distance to the spar is less than the distance between ribs, he gets additional stiffness with less weight. There is more than one way to beef up the edge.

John Pitkin

[rogerh12]

Speaking of adding aluminum;

I purchased 12 foot aluminum tubes to be used instead of the wood lateral stringers, the ones the fabric wraps around. They might add a bit more weight, but won't break when someone knee's the side of my plane like the wood ones can. Replaced my wood floor boards with aluminum too.

I guess I am on an alumunim kick, but wood is so, well.... wooden !!!

I think I will just glue/roll some exra aluminum tubes into the trailing edge, sounds easy enough. I figure I will just knock off 50 lbs to make up the extra wieght before I go fly (I will just stop eating for a while).
Roger

[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

[Peter B]

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

Did you use the arrow shafts span-wise in the trailing edge between the ribs or did you place them chord-wise (from the spar to the trailing edge) half way between each rib?

Peter B
Natchez, MS (HEZ)

[Flyingsnowdog]

Span-wise, I rolled them in the channel with Hysol

[Esser]

What sized arrow shafts did you use? Aluminum arrow shafts have a standardized sizing chart across the board but carbon shafts are sized by spine is it's hard to convert the size if ordering online

[Flyingsnowdog]

I do not have packaging any more, 500 series seems to ring a bell, anyway it is the biggest diameter they had at the sporting goods store. I know nothing about arrow shafts except they are very light and rigid.