Help Save My Warped Wing - Advice Welcome!

With Larry Cunningham's generous help and excellent advice I have a tentative plan, but I'd also appreciate anything anyone would like to add.

The Long Story:

I have built most of a Tom Morris D-tube style wing that, up to leading-edge planking time, was very rigid and straight. It is like a new Millenium wing, except that rib slices have specific section contours and were built up into ribs before being rotated into the spar slots. The trussed spar, leading edge, and trailing edge are like those shown on p. 51 of the current Stunt News. 41 Ribs and l.e. stub ribs are spaced 1.25" apart ahead of the spar, while rib spacing behind the spar is 2.5", with diagonal bracing. All ribs are 1/16", as is the 6 lb/ft^3 sheeting.

I ended up diverting from my planned in-jig attachment with white glue, per Tom Morris, because I couldn't get the 6-lb sheeting into good contact with the ribs with Windex and an entire set of iron weights. The glue set up too fast, and the sheeting dried too fast. I ended up cracking the first sheet loose (pretty easy) and reattaching it with CA, doing some of the work out of the jig. I again sprayed Windex on the outside sheeting surfaces to soften and warp the sheeting toward the spar. I had to do this after tacking the sheeting dry to the l.e. spar (despite fears that this would induce a warp) since I couldn't keep the sheeting straight when damp (destroyed one trying).

The short Story:

I now have three of the four l.e. sheets attached (both bottoms and the upper right) and a twist that has the right tip leading edge almost 1/8" ( 1.4 degrees) low, when the center and other corners are jigged level without weights. The wing is so stiff that it takes over 20 oz of iron weights to hold the right tip trailing edge down when the leading edge is jigged up and level (some extra weights required at root too). Since this process was taking so long, I felt as though I had to jig the wing before applying the opposite sheeting - maybe a mistake.

I have not attached the upper right sheeting to the ribs yet (just to l.e. and spar), but contact with ribs is good so far.

Well, here are the questions:

1) What is generally the net effect of dampening wood with windex? It definitely swells when wet, but does it dry smaller or larger than its original size? A leftover piece bowed away from the once damp side seems to point to net expansion. This would influence strategy.

2) Should I finish attaching the upper right sheet to the ribs while in jig, but before again dampening anything or otherwise proceeding?

3) Should I sheet the upper left before trying to straighten the right wing half?

4) Should I dampen all, or just selected parts to try to jig the wing back into shape?

I considered dribbling CA over interior sheeting surfaces to stabilize the sheets in tension, but this seems extreme and heavy. There will also be a built up triangular tube (sheeted) trailing edge structure extending behind the current aft jigged spar; it might stabilize things some.

Or should I just give it up to lessons learned and try to figure out how to get Tom's instructions to work for me on some later project? This is a very labor intensive and - 'til now - well crafted wing with accurate airfoils. I really hate the idea of scrapping it.

All advice welcome - although I know that I've not glued wisely.

'just want to cover all the bases. Thanks for whatever you suggest.

SK

Serge,

Nearly any wooden structure can shaped with water. Block the wing with the warp twisted a little beyond straight, then spray it with water and let it dry. You should be able to eliminate the warp before proceeding with the construction. Next time use a better wing jig and complete your sheting before removing the wing from the jig. Also, fully sheeted wings are very warp resistant when complete, and they can be completed while in the jig in perfect alignment.

Thanks, Al.

I finally bit the bullet and sprayed the thing this morning, immediately putting it into the jig. After an early tussle, it relaxed nicely. Now, the amount of weight needed to hold everything level is less than half what it was, and the right t.e. tip is less than 1/32" high without weight (~1/64" with 3 oz weight). So I think I'm on the right track. Unless someone screams "Stop!", I guess I'll attach the remaining (left top) sheet, before attempting the final straightening...or is there ever a "final" straightening?

SK

P.S. Now I need to learn how to avoid the extra weight I put on by not controlling my CA application in the blind places. I added 1/8 oz yesterday and can't even vouch for the integrity of my work. There must come a time when one masters this. For now, I'm not holding mirror, flashlight, and CA applicator well simultaneously. But, as Ty says, "I'm getting better!"

Serg,

I'd spray it again. It shouldn't hurt the structure. get it straight before proceeding. If it is not immediately apparent, you will actually be molding the balsa structure to a new shape and it will assume that new shape with little tendency to return to a previous shape. get it straight, finish the sheeting, and mold it again if necessary after the sheeting is complete. Next time use a jig which permits gluing while jigged.

OK. I just sprayed it again and jigged the low corners up 1/16" higher.

Removal from the jig showed that it wasn't as much improved as I'd thought; the right front still drooped excessively, despite the lower weight needed on the t.e. with the l.e. jigged up. There's anhedral here that this twist is superimposed on. 'makes it hard to map out. I'd like to get the wing CL straight.

Thanks again

SK

When adjusting with weight, are you over-adjusting? You need to overcompensate and allow some leeway for springback.

Yes, this time - but only 1/16". The wing is really rigid, and much more than this lifts other corners too far for me to weight them all the way down to level. I'm hoping that I can twist it back enough. Thanks for the input.

SK

On the other question you asked, my experience leads me to believe there is slight positive expansion, even after drying. There is a whole lot of small things I've seen over the years that lead me to believe this. Perhaps I am wrong, however. Interested in that question, so if I am wrong, I won't be offended if pointed out by someone more knowledgeable.
On a side note that is just interesting. Exterior concrete in a climate with cold winters and hot summers will actually slowly expand with age. I was told by an airport engineer that it never contracts quite as much as it expands. That's why on large areas, such as an airport tarmac, they will resaw the grooves every 15 years or so and reseal. At least that was what I was told when the airport bid for contracts for the work. The other concrete note I have is that the slower it cures, the stronger and longer lasting it is (generally). Once had a yard-nazi neighbor that had sprinklers on his newly poured driveway for about two months. Serious yard fanatic. But enough of my ramblings,.... back to balsa

LAST EDITED ON Dec-24-03 AT 11:24 PM (CDT)
 
My take: 1) Somethings wrong with your choice of balsa. Either it isn't 6 lb stock, or it's C grain or at least B grain. In any case, I would not rely on spraying, once started and finding that it didn't conform well. Get a piece of cotton, strips of flannel or toweling would be great. Soak a bunch of them in boiling water, and pull them out and slap them onto the sheeting with the edge glued down, pile on more and more, keeping it hot and wet. It will conform! When it's pulled down tight, weight, pin or clamp it into place and let it dry. I think that if you dry it with heat, it might shrink, like silkspan, but maybe not. That's what is happening with sunken areas between ribs, but it mostly shows up after covering and doping. Maybe it's just the covering pulling down the sheeting as the covering shrinks?

2) For gluing sheeting down, for large areas, you need to either use the pinhole technique with thin CA or use the "Iron On" method with Titebond. I think both have been described here before, perhaps by Al Rabe. In general, I would treat this type of de-warping as I would a repair, by jigging it into position and remove some wood, then replace it with scarf joints. Not easy, but probably more likely to yield good results. My opinion, anyway. Steve

http://www.nwskyraiders.com/index.html

I can't find the thread where I fought an Oriental wing the same way you are. however- when I put shear webs between ribs(grain vertical) with CA, the wing stayed straight when i took it out of the jig. When I "overshot" the warp and CAd it , it stayed "overshot". I ended up by CAing the shear webs with the wing straight. The only really straight wing I've been able to build is the present Impcat on the Rabe aluminum rods. By all means the sheeting must go on while the wing is on the rods/jig. Hope you get a good wing when all is finished.

LAST EDITED ON Dec-25-03 AT 10:25 PM (CDT)
 
Thanks for all the help, guys. Since I will probably not be working on the wing before returning from out-of-state holiday visits to relatives, I thought I'd report that the wing is now within 1/32" of being straight and has all four sheets attached.

I followed Al's advice to try to straighten it before applying the last piece and made some progress. However, this thing is so rigid that it seemed impossible to overshoot far enough on two further tries, and there was still anhedral and twist. Theorizing that shrinkage from a very wet application of the final sheet would pull and twist the wing in the right directions, I CA'ed the front edge dry to the l.e. spar, sprayed all sheeting heavily with windex, re-jigged it (with over-compensation), and CA'ed it to the spar. Although I failed to get the sheeting tight against 3 of the ribs, it dried in good contact. Results: the anhedral is gone, the worst corner is right, and the only jigged station that is off is the other low corner, which is now 1/32" too high instead (takes 13 oz to level it though).

So I'm leaning back and pondering whether I should try to remove the last 1/32" of tip twist at this stage (I can see the slight t.e. bow near the tip; it doesn't translate across the entire span). I also want to see how stable the change is. All in all, I feel a lot better about my wing and appreciate the help that has been offered. Thanks again, guys. 'hope everyone's holidays are going great too.

SK

Edit -update: The 1/32" expanded to almost 1/16" when left out of the jig for a while, and the original culprit corner dropped a little too. It was still far better than before, but I re-sprayed with windex and rejigged with over-compensation on the two corners, concentrating stress to bend at the t.e. bow. It is almost dry and showing indications of being better. But I won't know until tomorrow.

Serge, it almost makes one want to use foam exclusively! Not! Sounds like a wing I did about two years ago. I'll blame the warp for the crash. Heh, heh. So what if it did run out of fuel inverted at 20'??

How are you measuring your 1/32 or 1/16" out of true condition?

Currell-

Usually two ways for comparison:

1) Just center section jigged and weighted in 4 slotted blocks.

2) 8 of 12 jig blocks used (center, mid half spans)

I measure the amount of change in l.e. or t.e. height when tips are in and out of jig-block slots. I haven't really get much better than 1/32" accuracy, because the balsa has shrunk some and now is a bit loose in slots (I may shim with paper if/when all fit). With this looseness, the wing fits all but the left tip t.e. block. It moves about 1/32" down in order to fit against the top surface of the slot. I tried to get a micrometer reading on the amount of "slop" in the slot, but balsa compreesion makes that almost impossible. However, it seems to be no more than .007" - .010". A crude measurement with micrometer of the vertical block movement to accomodate that corner gave .032" (about 1/32"). So I think the thing's close to right. With center anchored, left wing is marginally high; so there's probably a little less than 1/32" of wash-out (negative aoa) there. Right l.e. is just marginally high compared to t.e. (was low, but both fit slots). So at this stage, I'll need a trim tab angled up on the right wing.

There's still trailing-edge structure to go, and as I typed this, things changed slightly already under the lights. So perfection may not be that relevant now, although I'll probably try once more to lower the left tip t.e. when I return next week leaving within the hour). By then I'll know for sure what set it has taken. As usual, advice is welcome.

Serge