Elevator Covering Problem

[jiott]

As I mentioned before, I am just starting the covering process on my SS7 and I started with the elevator because it is a fairly simple, small, flat item. Now I already have a problem!

I got the fabric glued on all the edges just fine, on both upper and lower surfaces. Then I did the first heat shrink at 250 and find that all the wood ribs that are not up against a metal tube (every alternate one) have buckled a small amount and I have not even done any heating above 250 degrees. The ribs that are against a metal tube have remained straight. The manual did not call for any rib stiffeners on these unsupported ribs, like it does for the rudder and stabilizer. What did I do wrong and what should I do now?

I could just leave alone and not do any more heat shrink above 250; probably the tapes that go over each rib will mostly cover up the snakey ribs underneath. I just bugs me because I know it will show somewhat, and I worry because I was not able to shrink to the recommended 350 degrees.

Help,
Jim

[jdmcbean]

Page 4 and page 8 of the horizontal tail section both mentioned the use of rib stiffeners. Regardless, you can remove the covering and place stiffeners and then recover or you'll need to except what you have. Easier to fix now.. It will bother you later and you won't want to fix it aft it's painted.

[HighWing]

I fully agree with John. To redo the covering now is a minor glitch and will take some time, but it will pale with the time you will spend looking at something that bothers you and wish you had redone it when it was easier. And quite frankly, If you leave it as is, I fully expect that you will be the only one to ever notice the glitch, but you will. You will see it every time you look at your airplane, especially when you are showing it to someong. And very likely, you will point it out to them - trust me, I know. I find that in situations like this, if I set it aside for a while and proceed to the next step (horizontal stab, maybe), there is less stress doing it over a bit later - kind of like a new project. Patience is a virtue I learned while building Kitfoxes - just look in my trash bin.
Lowell

[t j]

jiott, the good part is you will find that with fabric covering it is very easy to fix your mistakes. I too started with the elevator and fouled it up the first try.

[szicree]

To be fair, there is actually nothing in the manual indicating stiffeners on the elevator. In fact, my kit had only enough stiffener material for the v-stab, h-stab, and rudder.

[Dave S]

To be fair, there is actually nothing in the manual indicating stiffeners on the elevator. In fact, my kit had only enough stiffener material for the v-stab, h-stab, and rudder.

Until I re-read this entire thread - it didn't remember right away that the kits have some differences - model-to-model. I went back and looked at my construction notes/photos; and, yup - no wood ribs on my elevator either - but, then - the elevator with the trim tab may be built a little different anyway. Ours is assembled without wood ribs in the elevator and there is no observable distortion - but the elevator is built differently. See attached photo. I am not an expert on the new configuration; but, I am wondering if wood ribs are even necessary anyway with either version since it is a flat surface? - Possibly there is an extremely small amount of deformation of the trailing edge tube on shrink coupled with the wood ribs that may account for what you have observed which would remain un-noticable if the wood ribs were absent????????? A couple thousandts of an inch of distortion of the tube is probably all it would take to cause the effect you have observed.

For what it is worth - I ripped the fabric off my elevator the first time because I didn't like the way the fabric contoured between the elevator spar and the side/trailing edge tube - did some foam work and liked the final result better (see second attached photo)

I totally agree with the posts that say to take your time and you will be happier with the end result - building is like that.

Sincerely,

Dave S
KF7 Tribear
912ULS Warp

[jiott]

Page 4 and page 8 of the horizontal tail section both mentioned the use of rib stiffeners. Regardless, you can remove the covering and place stiffeners and then recover or you'll need to except what you have. Easier to fix now.. It will bother you later and you won't want to fix it aft it's painted.

I'll move on to the horizontal stabilizer and probably come back and redo the elevator.

John, you should revise the manual when you get a chance; page 4 says "some of the ribs" speaking of the whole tail section. Page 8 says "cut rib stiffeners as needed" but the picture clearly shows stiffeners only on the unsupported stabilizer ribs and none on the elevator ribs. Also there was not enough stiffener material in the kit to do the elevator, therefore when I puzzled about this I concluded none were needed on the elevator. I accept it as my mistake, but would like to see it clarified for future builders.

Am I correct in thinking that if I remove the fabric and put in stiffeners I will be able to shrink to 350?

Jim

[HighWing]

John,
Your last question - "Am I correct in thinking that if I remove the fabric and put in stiffeners I will be able to shrink to 350?" - brings up some thoughts. In the old days with the flat elevator and the light weight fabric, the whole airplane was to be shrunk to 300°. There were arguments on this as the Polyfiber manual called for 350° and of course they were the experts. The factory recommendion was due to the longerons becoming wasp waisted at the higher temp as they didn't have the bracing and bulk to withstand the tension of that high a shrink temp. The flat elevator had no ribs and 300° was the norm there as well. Your question suggests that the higher temps, in your case, are rib related. Maybe so, but if you decide to shrink to lower temps, I would suggest that you shrink to at least 300°. I just don't think 250° would give sufficient tension on the fabric. In my present project, I used foam ribs with a single ply laminate of light weight fiberglas and shrunk to 300° per the original manual's recommendations with no rib warpage.
Lowell

[jiott]

I am curious about this whole fabric tension vs. temp thing. It seems to me that tension is tension no matter how it is obtained. Lets say I am a super fabric installer and somehow am able to glue the fabric onto the frame, elevator or fuse or anything, with a huge amount of built-in tension. If I could do this then it seems there would be no need to apply any heat at all. Or more likely, I get the fabric glued on with some very moderate amount of built-in tension; then I would only need to apply a small amount of heat-say 250 to achieve the proper tension.

The reason I ask this is because I think part of my problem with the elevator, other than not putting rib stiffeners, is that I tried to do too neat of a job in gluing the fabric on. It was laying very smooth with almost no wrinkles before I did the first shrink at 250. When I applied 250 it became extremely tight and buckled the unstiffened ribs as I said earlier. The elevator is flat even with the wood ribs there is no airfoil profile like there is on the H & V stabilizer and rudder, therefore it was quite easy to get the fabric glued on very flat and smooth. I probably should have laid a 3/4" stick across the ribs to give the fabric some "belly" when gluing the edges. The polyfiber book says to apply the fabric like a well made bed sheet, but not an army cot. I thought I was doing this, but later noticed a Polyfiber statement that says to leave 1" slack in the fabric. Nicely made bed sheet vs. 1" slack belly-I am frankly quite confused.

Anyway, this is why I am asking for covering tips to help out a first timer like me. I know if I keep trying I will eventually get it figured out, but it takes me most of a day to glue the fabric on just one side of a piece like the elevator and I hate to keep wasting that much time to start over.

Jim

[Dave Holl]

I thought that the temperature used controlled the taughtness of the fabric,within set parameters the higher the temp the tighter the fabric, this is what I found doing the wing.i thought that glueing the fabric to the wing etc snuggly was to ensure that there was enough for correct shrinkage of the fabric ie if too slack the fabric would shring the full amount without getting to the correct tightness?