[briand]

I dimpled my aileron spar last night and got some serious bowing (it looks like a pre-fluted rib now). Should I worry?

 

[PJSeipel]

Anything you dimple will bow a little bit because you're stretching the metal. I definitely would not flute a spar. How bad is it? If you lay it flat on the table and can just barely tell it's bowed, then I'd keep it. If it looks like a banana, then somethings wrong and I'd replace it. YMMV.

PJ

 

[Jamie]

I personally don't know if it's possible to overdimple something. Dimple dies will not bend the metal more than necessary for a rivet to sit flush in them. Once the dies close all the way, you're just pressing metal.

As mentioned above, dimpling does stretch the metal and there is almost always some very slight arching of parts. It's impossible to think that there will not be a fluting effect at all on the metal. You're bending the metal by dimpling it...plain and simple. You'll especially notice this on the longer parts. The more holes there are, the more 'flutes' there are and the more bend you're going to have over the length of the part.

How about sitting your spar on a flat surface with the bow in the center and tell us how much it bowed? This could help you quantify the problem.

 

[briand]

Pnuematic. Theres a little less (edit) than a half inch space in the middle when its layed web down(edit). I'll measure when I get home. It doesn't take too much pressure to push it flat. I'm wondering if these spars should be c-sunk instead of dimpled. There was a little bit of a bow in the part to start with, maybe a little less than a quarter inch.

 

[bullojm1]

I was told that underdimpling will cause bowing in skins - don't see why it wouldn't apply to spars also. Here is a link to an example:


http://caura.dyndns.org/gallery/How-Not-to-Dimple

 

[briand]

Its 0.35" high in the center and its with the web facing down.

Up:


Down:


Up:

 

[Mark Burns]

Build on!

Briand,
I seem to remember my aileron spars being a little bowed after dimpling.
If it pushes down flat with just a little pressure like you say, then the skins should pull it straight just fine.

Mark

 

[briand]

I went ahead with the aileron, when I primed it I could definitely see some marks on one side of the 3/32 dimples (where the dies compressed a little more), which I can not feel with my fingers before or after primer . I'm thinking it was worse on the top of the spar because of the lower surface area of the 3/32 dies than the 1/8 dies. I also checked my setting on my squeezer and I had about .035 more shimming on the dies than what I used when dimpling the emp. spars. I guess I thought more was better and I would get a crisper dimple. I was wrong.
I will check my die gap prior to dimpling every time and think a .010 preload on the dies should work, I might go for a .015 or .020 on the 1/8" dies though.