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Filling sheet metal before paint?

E

Echo Tango

Well Known Member
Other than the cosmetic advantage of having no discernible "rivet lines" or imperfections (see: errant rivet gun) and a perfectly smooth skin, are there any speed/drag reasons to fill the skins with some light, sand-able compound to achieve a perfectly smooth surface?

I kind of like the idea of a composite-looking airframe and I was just curious if anyone has done this or decided not to do it. I'm finishing up my HS right now and the skin is pretty nice, but not perfect :eek:
 
Don't waste your time IMHO. Any minor drag reduction achieved is at least partially offset by the weight added. Not a good trade-off for time for reward received. YMMV
 
rbibb said:
Don't waste your time IMHO. Any minor drag reduction achieved is at least partially offset by the weight added. Not a good trade-off for time for reward received. YMMV
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I don't think the weight would be more than an extra pound or so, but I see your point. I still might do it for cosmetics as I prefer that "minimalist" look, but I was just curious if anyone had any experience with skin friction drag numbers or how little they might actually matter.

ive heard grumblings that countersunk rivets themselves are a waste of time at RV speeds, but i would put them in anyway just because they look nice :)
 
I have heard of people doing this before, however, should you ever have to make a repair or replace a skin, you will never find where your rivets are to drill them out.

If you want the composite look, you could try a Glasair but who would want to do that :)
 
There was a time when filling was in vogue. How many noticed that someone did it to the RV-1?
 
If it cracks because of flex, you're going to ruin your paint job. There's an RV-8 on my field with a really nice paint job that almost looks "composite" thanks to being well done.

Not to beat a dead horse, but the RV-7 break up in Canada was partially attributed to the flutter margins being changed by the fact that it was filled and smoothed with filler.
 
Filling

I filled my rivets lines. Thought i did a good job. Painter didnt like it and sanded it all off and redid it nicer. Plane before paint after i did filler 1080. Plane after all stripped, refilled properly and nicely painted 1100.

As for the breakup, that report made it almost sound like there was a LOT of filler on tail, also that plane was being flown WAY outside of the envelope. I dont think a little filler had much to do with that.
 
Not quantified!!!!

Sig600 said:
Not to beat a dead horse, but the RV-7 break up in Canada was partially attributed to the flutter margins being changed by the fact that it was filled and smoothed with filler.
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There has been nothing to quantify this at all. There was wording that this "may" have contributed but nothing for sure.

If this is truly the case there would be RV's falling out of the sky with all the folks that have filled parts
 
Echo Tango said:
Other than the cosmetic advantage of having no discernible "rivet lines" or imperfections (see: errant rivet gun) and a perfectly smooth skin, are there any speed/drag reasons to fill the skins with some light, sand-able compound to achieve a perfectly smooth surface?
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I was told to fill the stiffener rivet line on the front portion of my wing, the one between the spar, and the leading edge.

This was from the guy who designed the airfoil for VANs---Steve Smith, a member here at VAF.

Reason being, when the airfoil was designed, he did not expect the factory would add the stiffener, and its rivets to the shape of the airfoil. He was thinking it may improve the performance a minor amount.

This is specific to the ten..........dont know if any of the other models would benefit from it.
 
anecdotal evidence

My 1st -4 (purchased; not built by me) had the wing top skins filled & not much else done to make it 'clean', & it was very fast for the HP (170 kts @ 8.5-9gph w/160 hp carb'd O-320 & Bernie Warnke wood prop). It was 7 or 8 years old with ~700 hrs when I sold it, & never had any problems with the filler cracking. No idea what the builder used for filler.

A trick I read about (but haven't tried yet) is to fill the rivet dimple rings by wiping a bit of thinned proseal/flamemaster tank sealant over the finished rivet with a finger tip. Idea is that the only thing left is a skinny ring around the rivet head. I doubt that the plane would gain a pound if you did every rivet that way.

Drag of 'universal' rivets: I'm pretty sure that there are studies available showing the difference, but look just look at certified planes. Even the factories use flush rivets on the leading edges on faster planes like Bonanza's, etc. I doubt that they would spend the extra cost in labor if they didn't get a return on the investment.

Charlie
 
Sig600 said:
If it cracks because of flex, you're going to ruin your paint job. There's an RV-8 on my field with a really nice paint job that almost looks "composite" thanks to being well done.

Not to beat a dead horse, but the RV-7 break up in Canada was partially attributed to the flutter margins being changed by the fact that it was filled and smoothed with filler.
Click to expand...

that's all certainly possible, but i don't buy it
 
rv7charlie said:
My 1st -4 (purchased; not built by me) had the wing top skins filled & not much else done to make it 'clean', & it was very fast for the HP (170 kts @ 8.5-9gph w/160 hp carb'd O-320 & Bernie Warnke wood prop). It was 7 or 8 years old with ~700 hrs when I sold it, & never had any problems with the filler cracking. No idea what the builder used for filler.

A trick I read about (but haven't tried yet) is to fill the rivet dimple rings by wiping a bit of thinned proseal/flamemaster tank sealant over the finished rivet with a finger tip. Idea is that the only thing left is a skinny ring around the rivet head. I doubt that the plane would gain a pound if you did every rivet that way.

Drag of 'universal' rivets: I'm pretty sure that there are studies available showing the difference, but look just look at certified planes. Even the factories use flush rivets on the leading edges on faster planes like Bonanza's, etc. I doubt that they would spend the extra cost in labor if they didn't get a return on the investment.

Charlie
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I think I will fill sparingly, just to smooth things out a bit, like you said. No need to dump 20lbs of filler onto a plane... just break up the rivet lines a bit.


I'd be very interested in reading a study on universal head rivets, but I have searched and not really found anything substantial versus their countersunk brethren, as far as drag is concerned.
 
RV7Guy said:
There has been nothing to quantify this at all. There was wording that this "may" have contributed but nothing for sure.

If this is truly the case there would be RV's falling out of the sky with all the folks that have filled parts
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Don't shoot the messenger, tell the Canadian Gov't you think they're wrong.
 
rv7charlie said:
A trick I read about (but haven't tried yet) is to fill the rivet dimple rings by wiping a bit of thinned proseal/flamemaster tank sealant over the finished rivet with a finger tip. Idea is that the only thing left is a skinny ring around the rivet head. I doubt that the plane would gain a pound if you did every rivet that way.Charlie
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Yes, The main purpose of doing this is to seal the edge of the rivet to the skin so the paint can bridge that area with a good base. You can still see the rivet and the dimple, but the paint is strong there.

Squeege on the proseal in all directions over the rivets...... then wipe clean with mek or your choice of thinner. It will leave the rivet edge sealed.
 
when my plane was built, not by me, it was faired out except the wings and bottom of the fuselage. came in at 1,100 and looks like glass. i suppose doing the wings would give more of a performance gain though. he spent a lot of time doing it!
img0644ov.jpg
 
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