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Wing Rack Question

dwollen89

Well Known Member
I constructed my wing rack and hung the wing skeleton for the RV7. I hung the plumb bobs at the extreme ends of the main spar and rear spar per the plans. I measured between the two points and adjusted the angle of the supports until they were the same at each end.

The spar is close to perfect level left to right. However, when I put the level front to back, they are not level. In fact, they are opposite of one another, which to my simple thinking would indicate a twist.

My question is, are the plumb marks the only important thing to make sure there is no twist in the wing? Do I need to worry about what the level says? I know some say the prepunched kits today eliminate twist, but I also want to be diligent.

Any thoughts?
 
Wing jig

I built a wing jig. Easy to adjust the skeleton and clamp it. To answer, ideally a plumb bob hung from each end should have an equal measurement.
I recommend checking as the work progresses. I noticed the measurements changed a tiny bit and had to be readjusted.
Level matters too. You want the spars level to each other and straight. Depending on how things are set up, the skeleton can sag in the middle.
True, prepunched kits should eliminate errors but it doesn't hurt to add a level of confidence.
 
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Thanks for the response Larry. I always appreciate your input. I actually looked at your builder's log pics before I made my wing rack.

I'll level the spar again and re-hang the plumb bobs later today to re-check. I used the fishing line method described in the plans when I used the jack to take the sag out of the center. The skin holes seem to line up well with the ribs while putting clecos in. I'm probably overthinking it....like usual.
 
Twist

I had some twist built into my -7 spar when it was at rest in the jig. While it was easy enough to straighten it with a small amount of hand pressure it would spring back when released. Our spars don't really have any torsional stiffness. So I made a note to myself to be extra sure the twisted section was pulled straight when applying the skins.

The skins/rear spar themselves did all the straightening I needed without any extra effort.

But, it is possible to induce some error by not following a strategic rivet drive pattern. So when you have some light twist already there it is important to watch how you set the skin rivets and check plumb frequently if you want a truly straight wing.
 
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On the RV-4..

I built a -4, and I'm not infinitely familiar with the -7, however, I suspect they are similar. on my -4 build, the spars are supported at the ends in the jig (leveled in both directions) and the main spars have an accurately located (by the builder) tiny hole at the inboard and outboard end (last rib bay). The rear spar has a similar hole, but larger (#30ish). A piece of thread is suspended through the upper hole and secured with a knot, and a small plumb bob/ fishing weight is attached to the bottom end through the #30 hole. During the build up through installation of the upper skin, the threads are kept in check to stay centered in the #30 holes, thus preventing twist. The rear spar is moved as needed to keep alignment. All this process is in the build manual.
 
String

Using cleco clamps I run a taunt string from one end of the spar flange to the other end. The string is spaced about 1/2” from the flange and runs along the spar flange rivet line.

If the string is really tight I can easily verify if there is a bend or a bow in the spar. If the string is closer or further away from the flange in any location it could indicate a bend. If the string doesn’t run uniformly along the rivet line the spar could be bowed. I use the string as an indicator and a jack screw to raise the center of the span until it’s level.

If you use a string level on the line it should be level along the length of the spar. If there is a twist one corner of the spar will be higher or lower and you may see it in the string level.

I check the string each morning and periodically while I’m working on the wing. Just a quick visual verification.
 
I appreciate all the responses. There is a great group of people and a wealth of knowledge here.

After clecoing on the top skins, I took the clamps off the spar ends. Then I moved the spar out closer to the ends of the wing rack supports. Moving them away from the pivot points/vertical posts gave more authority to the adjustable aluminum angle supports. I then re-leveled the spar left to right and front to back, getting the level perfect. I hung a plumb bob on each end of the wing spar and rear spar and made a mark on the floor. I measured the difference and it was within 1/16" between the ends, which I consider margin of error.

I'm sure that clecoing on the skins helped get everything squared away. I have a new appreciation for those building the pre-pre punched hole kits.

I hung the bottom skins and everything went on with ease. I'll be diligent about checking and rechecking before I actually start to rivet skins on.
 
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