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Dip in wing above the spar

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Schmleff

Well-Known Member
Joined
Jan 2, 2006
Messages
2,714
Location
Waupaca, WI / USA
I did not build my wings. The spar is shallower than the ribs which leaves a dip in your wing skin above the spar. I have spent the last two days trying to fill that dip with micro and have it not look all wavy.

I believe there is some mention of this somewhere as its not the first time I have heard it. I would somehow shim the top and bottom of the spar so that it came out with the proper shape once skinned. I did not notice is so much on the bottom but its very pronounced on the top of the wing.
 
Hi Jeff, I have same in my wing. After I observed this, this is an item I am looking at on every Sonerai I meet or see on pictures. Even Monnet's prototype has this, as it was seen in an article in Sports aviation, long time ago. (I may try to look it up again, and mail You the picture) Depending at what attitude the plane is in, combined with the correct (casual) angle the picture is taken from, this mishap shows up clearly.
After I observed same "feature" on mine, I tried to find out how comes, and these are my findings: Incorrectly technique of dimpling the spar. Once You apply the dimpling, as explained in the plans, by use of the suggested tool, the spar flange aft edge bents upwards, due to stress induced by the dimpling procedure. (Just put a straight edge on the spar flange before and after the dimpling, and you'll see the difference). Once the skin is put on, this shows up in an amplified way, as the skin tries to follow this induced curvature for some distance. If I should do it again, I would devellop a kind of an improved dimpling die, or have the spar analysed if it could be simply countersunked.
Best regards,
Des.
 
Dip in wing above spar

Has anyone shimmed between the top of the spar channel and wing skins to attempt getting rid of the infamous "dip" in the wing skins above the spar? I'm thinking of doing this. I know others have used bondo to fill in this "dip" after wing construction.

At this point, I have drilled through the skins, spar channel, and ribs and have a wing clecoed together, and I'm beginning to see the genesis of the "dip".
 
Mark,

Somewhere in your wing build instructions it mentions that you may have to insert a shim between the spar and skin to smooth it out (or use bondo/filler). You will have to determine how thick it needs to be for your situation.

In the RV wing I made some years ago, I used a countersink, rather than dimple the main spars. However, those were AN3 rivets (with a 100o countersink), for which the .040 spar is thick enough to do this. For the 120o pop rivets used by the Sonerai, the spar is just not thick enough to countersink without widening the hole. So the upshot is you will have to dimple skin, shim and spar.

I suppose you could countersink through the shim and partly into the spar, but I don't know if this is an acceptable practice, and I don't have my reference books at hand to check. I suspect it is not, but maybe someone else can answer that question.

The Other Bill
 
I adjusted my ribs so that they appeared more flush on the top side, and I used micro-lite on the bottom....Ed
 
I believe I may have solved this problem by accident or totally messed up my ribs , one of the two.When I made my form blocks I sanded just a little too much.At the spar they are right at 6 1/4 .I made a nose rib and a trailing rib out of some scrap aluminum I had laying around and clamped them together (just for a prototype).It appears that the spar flange will fit snuggly against the skin and not make a dip because of the slightly underrsized ribs.they are about 1/16 off.or 1/32 per side.I ordered the .25 aluminum from aircraft spruce 2 days ago so once I form the ribs out of the right material and begin attatching them to the spar I will post an update .Hopefully it fixed this and did not cause any other unforseen issues.
 
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