SNX-W07 Wing "twist" / misalignment help

[NWade]

Hello Fellow Builders -

I could use help and suggestions! After a nearly-flawless RH wing build, I started in on the LH wing assembly & skinning and ran into a problem. Below is an email I sent to Sonex Tech Support a few days ago. But since they're shut down for the Holidays I am tossing this out to all of you for ideas:
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The wing-build has progressed well up until the point at which I have pilot-drilled the upper-aft skin to the LH wing. Before beginning (and again during progress), I made sure that the spars were twist-free, the wing & ribs were perfectly square, and that the distance from the main spar middle tooling-hole to the rear spar attachment point was set at 19", per the instructions. The upper aft wing-skin was set flush with the trailing edge of the rear spar upper flange along the entire length of the wing (and heavily clamped into place that way).

Pilot-drilling the ribs proceeded smoothly, and the ribs required almost no adjustment in order to get the centerline showing through the wing-skin's laser-drilled pilot holes. I checked the wing-skin overlap with the main spar and it appeared to be about 1/16" shy of meeting up with the forward edge of the main spar cap. This didn't bother me as my RH wing had an identical situation and turned out fine. So I began drilling through the wing-skin into main spar - working from the center of the wing both inboard and outboard. At the point I was about 85% done (last 2 rib bays at either end), I began to notice that the holes near the wing root were creeping towards the main-spar web, and the holes near the wingtip were creeping away from the main-spar web. At first I thought I was drilling at a different angle to the skin and that accounted for the difference; but as I neared completion and the skin was truly sitting flush against the spar caps I realized the problem: The wing skin was not quite square to the main spar!

At the tip of the wing, the upper aft skin extends forward of the upper spar cap by a few thousandths of an inch. At rib #5 (where the main-spar cap stops being parallel with the spar web) the upper aft wing skin is short of the cap by 1/16". And at the root of the wing, the upper aft skin is 'short' by about 3/32".

Here are photos showing the wing skin overlap of the LH main spar:
https://plus.google.com/photos/10396415 ... 8413352722
https://plus.google.com/photos/10396415 ... 6774219794

Of course I discovered this while drilling holes in the wing spar - so if I have to re-do this I will need to build up a whole new main spar for the wing as the spar caps now have holes in them in the wrong locations... Heartsick, I've spent a couple of hours looking at the wing from every angle and taking every measurement I can. And from what I can tell, there is NO "twist" in the main spar or rear spar. There is also no problem with the wing ribs or variations in the distance between the main spar web and the rear spar web. Ribs are still square to the main spar, the 19" distance to the rear spar attachment is still good, etc.

Finally, we looked at the rear-spar in a different direction, and found what I believe to be the source of the problem: The outboard section of the rear spar (beyond the splice plate) was removed because I thought it was improperly dimensioned. But after removing it I discovered that it was correct all-along and re-riveted it to the splice plate. Somehow in doing this it got slightly out of square with the inboard section of the rear spar. Viewed with the wing in "flying position", the LH rear spar tip is 3/32" higher than the root end of the rear spar.

Here are photos of the rear spar misalignment:
https://plus.google.com/photos/10396415 ... 6310897650
https://plus.google.com/photos/10396415 ... 6058358866

I assume that this misalignment effectively reduces the upper camber of the wing at the outboard tip, and that's allowing the skin to overlap the spar slightly further out at the tip. This is similar to a small amount of wing twist ("washout" as we would have called it back in my model-aircraft days). But I don't have an easy way to calculate the amount in degrees, to get an idea of how much of a concern this will be.

I know some Sonex builders wind up with a "heavy" wing and take care of balancing the aircraft by slightly drooping a flap or an aileron... Do you think this will be an acceptable work-around in my case, to correct this slight change in wing-shape at the outboard end?

Lastly: I'm mildly concerned about the bend in the rear spar causing the ailerons to bind due to having a non-straight hinge. Any thoughts or suggestions? Should I try to re-make the outboard end of the rear spar (to get it parallel with the rest of the rear spar), then re-drill the skin to it?
(Obviously, I'd have to accept that the trailing edge of the wing skin may not be flush with the trailing edge of the rear spar section)
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Thanks in advance for any tips or ideas on how best to correct this,

--Noel Wade
Sonex #1339
(TD, Turbo, Center-Stick, Acro Ailerons)

[falvarez]

Ok...it looks like you have a lot going on so you definitely will want to get the Sonex folks to provide the final guidance...

Here's a couple of things that I've seen first hand...first, it's fairly easy to replace a wing skin. So keep that in mind as you look for solutions. Also, I've know builders that upgraded to 5/32 CherryMax rivets to attach the skin to the spar to help make up some of the difference causing the twist.

Most importantly, just keep in mind, there is a solution to this...don't get discouraged!

[planeolbob]

I had a similar problem with my wing. I had to replace the rear skin to "Relax" the wing. Here is how I did it:

https://www.flickr.com/photos/planeolbo ... 661866595/

Read all the notes listed on each photo and buy some long drill bits.

Bob (been there) Mika

[NWade]

I had a similar problem with my wing. I had to replace the rear skin to "Relax" the wing.

Bob (been there) Mika

Thanks, Bob! I appreciate the knowledge (and comfort/sympathy).

What I don't know is whether your solution would work for me, since my main spars now have holes drilled in one side that are not quite parallel to the shear-web. Its not the aft wing skin I need to worry about - its the forward wing skin. That's because I'm not sure it will "wrap" properly and get close-enough to aligning with the holes in the main spar on both the upper and lower surfaces (since they won't quite be parallel to each other).

I took the skin off and actually measured the holes in the upper main spar cap. At rib #2 the holes in the main spar are 24/64" forward of the center of the spar cap. At rib #8 they are 25/64" forward of center, and at rib #11 they are 27/64" forward of center.

NOTE: I also took a bunch of measurements of holes in the rear spar and elsewhere and am confident that this was all caused by the outboard rear spar section - and it can be re-made so I can hang the aileron with a straight hinge.

So the big question is: Will this mis-match (i.e. non-parallel holes) between the upper and lower the main spar caps make it impossible to fit the forward wing skin? I think the upshot is that if I wrap the forward wing skin such that the pilot-holes line up at the main spar on both top and bottom, the mismatch will give the wing just over 1/32" more chord at the outboard tip. Essentially, the wing will have a very slight inverse taper; but over a 10-foot span it doesn't seem horrible.

Still, I'm open to all thoughts and suggestions! Thanks again,

--Noel

[planeolbob]

Well, since I can see your wing if you can install the forward skin without the wing wraping then you can install enough of the aft skin to the point that it just starts to wrap. Then back off a cleco or two and do the drilling of the skin from the other side like I did. note that I had the skin clamped to the spar as I drilled. this gets the skin installed with the wing flat. After you do the edge at the spar then you can do the rib holes.

Fool around with a 4x3 sheet of alum. and see how it works. A few sheets of alum is better to waste than your spar. Go slow.

Bob

[lpaaruule]

Here's my 2 cents worth.

Since you've narrowed down the source of your twist to the rear spare misalignment at the splice plate, if it were MY wing I'd be TEMPTED to do the following.

1) Remove the rivets from the splice plate to allow that portion of the rear spar to "float"
2) remove the clecos from the main spar starting from the point perpendicular to the splice place out to the tip. Do this on the top and bottom
3) Verify that there is no longer any twist
4) Observe how much offset there is in the holes in the main spar at the tips versus the holes in the skins. Is it close enough that a #30 hole could be drilled without being oblong/oval? If so, you may have gotten off easy.

If this works, then you'd just have to contend with modifying, or replacing the splice plate, and end of the rear spar.

Of course, I've been contacting Sonex tech support about once a week recently, so I'm no expert!

[Pickleman]

Install the lower skins with clecos on the ribs and rear spar to hold the wing straight and in alignment. bend a leading edge skin that overlaps the trailing edge by a couple of inches. Use a blind hole transfer tool to transfer the pattern of the holes in the spar to the leading edge skin. Use ratchet straps to hold the shin on while drilling. Trim skin to final size. Easy Peasy! One of the Bingalis books shows how to make a tool to transfer holes that you can't see. If this is clear as mud, email me, I will send you my phone number, and I will try to explain further. Where is the project?

Andy

[NWade]

Andy -

Thanks. I'm near Seattle, WA.

Making a new fwd wing skin was a possibility; but I loathed the idea of having to "freehand" cut the ends and somehow transfer over the pilot holes from the kit fwd skin that I have (which is pre-bent). Even making a new one per the plans (from scratch) would have been a fair amount of added work.

Luckily, the Sonex tech support folks looked over my photos and thought it was safe to proceed essentially as-is (after I re-made the outboard rear spar section, to get the trailing edge straight). They seemed to believe that any twist would be small and could probably be fixed with a drooped aileron or flap.

I went back to the factory-recommended method for working on the wing: lay it on a table and shim under the main spar and rear spar to eliminate twist. With lots of careful measurement and alignment I got the wing to go together seemingly straight. I think the nose of the airfoil (in front of the fwd ribs) may vary slightly in shape from root to tip due to the skin being pulled further aft near the root end (because of the main spar holes I drilled at the beginning of this saga). However, its extremely minor and not visible to the naked eye. I'll certainly take some protractors to the wing once its on the airplane and see if I can find any measurable twist.

Thanks again, all!

--Noel
Sonex #1339
TD, Center-stick, Turbo, Acro-ailerons

[hickej]

You may be too far along for this trick, but I used a laser level in a closed hangar to align my wings as I riveted the skins. I was able to 'shoot' the level line through the wing and move the leveler from one side to the other to confirm no twist. As I did the riveting, I would adjust the shims to keep it true as I pulled more and more rivets. I was very pleased with the results.