Smoothing aluminum skin

Alright guys this is not an RV but I am a an airplane modifier at heart. We are sending our Mooney 201 off to paint in 3 weeks. The wing is a laminar flow airfoil but the skin is low over the two stringers and the spar. I'd like to fill tbe skin to make it true to airfoil shape for laminar flow. Question is, how to execute it. Latest theory is strip, acid etch, alodine, prime with corlar strontium chromate primer. Then fill the low spots with West epoxy and glass micro. MGS is very hard to sand.

There's got to be a better way. How do I make my forward leading edge like a piano?

I have some time in a 231, and if the 201 and 231 have the same wing the issue will be flexing, oil canning, thermal contraction and low pressure forces on the top skin. However, if I were going to do what you're headed into I'd proceed as planned - the West epoxy sticks especially well to Corlar.

You won't be done after contouring the West/micro; you'll have pin holes to fill and then prime the West/micro areas - ugh. The thought of this is exactly why, after 5 composite airplanes, I'm going to build a metal airplane this time.

Too bad Mooney (or any one else for that matter) doesn't build wings as nice as Boeing

Smoothing skin

I don't think you will get an answer on this forum. Most RV builders buy new skins over the smallest dent ! And most don't paint or prep for paint, they leave that for the paint guy.
I think the answer should come from the guy doing the paint . I have seen hail damage, bird ,rock and hangar damage fixed with everything from a riveted patch to "Bondo " put on with a trowel .
Not sure the year of your Mooney but trying to make it look like a glass airplane is not the way to go .
Tom A

It's a 1977. We have sped it up ten knots but I'm looking for a couple more. The sheet metal on the wing is not true to the airfoil.

Byron, filling as you describe is more difficult than it seems. The aluminum substrate will flex under pressure in the spans between ribs and stringers, so the sanding block tends to cut more over the ribs and less in the spans. Same problem when filling low spots in a plywood wrapped leading edge. Close isn't too bad, but getting it optically flat will drive you crazy.

BTW, somebody filled the wings on the RV-1 at some point in it's life. Didn't make it look all that great.

Byron,
I worked on a Glasair with a friend years ago trying to get the wing perfect. We tried several methods of filling and sanding - none of them much better than when we started. The biggest problem we had was weight. Even with lightweight filler, it added up to more than we expected. In the end, we sanded almost everything off and left it. It wasn't perfect, but was pretty good.

On the next Glasair he built, we reduced the distance between the ribs and made sure they were perfect before skinning it out. They turned out great.

I am one of the few here that do not like the looks of the rivets and still considering filling all of them on my 9A. I have a complete empennage that I filled and primed and it looks great in my opinion, but it took a long time. I am not sure I want to spend that much time on it. Maybe I will follow Pete and get it wrapped!

Good luck on the Mooney. They are a nice plane!

Not to mention that it is a certified airplane and one can't just go messing with it...

rockwoodrv9 said:

Byron,

snip

I am one of the few here that do not like the looks of the rivets and still considering filling all of them on my 9A. I have a complete empennage that I filled and primed and it looks great in my opinion, but it took a long time. I am not sure I want to spend that much time on it. Maybe I will follow Pete and get it wrapped!

Good luck on the Mooney. They are a nice plane!

Click to expand...

The 1st RV4 that I purchased (flying) had the rivets filled just on the top of the wings, and a very nice paint job (got best metal homebuilt at SERFI about 18 years ago). Empty weight, IIRC, was around 930. Not Van's prototype weight, but still pretty good. It was pretty fast for the time, cruise was around 170 kts (slightly better than Van's specs) on 160 hp with a Warnke wood prop.

Absolutely nothing was done to the plane to make it faster, except filling the wing top skin rivets. It had around 700 hrs on it when I sold it, & no evidence of crazing/cracking (except on the fiberglass parts, of course).

Sure is tempting to fill the ones on the -7 I'm building...

Charlie

The real issue with using epoxy to skim coat your airplane is that it expands and contracts at a different rate than the underlying aluminum and may pop off over time.

If you insist on doing this, get some Aerodynamic Dent Filler. It has aluminum in it so it should expand and contract at the same rate as the underlying structure.

As for the RV builers who want to cover their rivets, it is your choice. However, in today's hyper competitive aircraft judging arena, you won't stand a chance. In researching an article I did for KitPlanes a few years back, the head EAA judge told me they want to see the rivet heads on aluminum airplanes. He went on to say that often the difference between winning and first runner-up is one rivet head missing from a line of rivets.

One other thought to keep in mind, when I'm asked by an RV novice what to look for when buying a flying RV, the first thing I tell them isbto compare the empty and weights to Van's number. If they can live with that, then it is time to look at build quality, engine, systems, etc.

Build 'em light!

I like look of rivets, I actually like to see them on metal airplanes. My situation is the skin is 1/16" low over two forward stringers and the spar. Now the spar is at 50% of MAC and I can live with that, but the stringers mess with laminar flow. Still struggling wih this. I wish I could post a pic to show it.

The plane has run 196.6 mph at the SARL air races. The paint will speed it up a couple MPH (old paint is in terrible shape) but there's also 3 MPH in that wing skin and I hate to leave that out. NASA did a study in the 80s where they smoothed the wing and got 3 MPH from it.

Skins

I think you're chasing your tail. It sounds like you would need to add significant weight to improve the flow. Probably a wash in the end and you run the risk of cracking paint. I would forget it.

jetdriven said:

I like look of rivets, I actually like to see them on metal airplanes. My situation is the skin is 1/16" low over two forward stringers and the spar. Now the spar is at 50% of MAC and I can live with that, but the stringers mess with laminar flow. Still struggling wih this. I wish I could post a pic to show it.

The plane has run 196.6 mph at the SARL air races. The paint will speed it up a couple MPH (old paint is in terrible shape) but there's also 3 MPH in that wing skin and I hate to leave that out. NASA did a study in the 80s where they smoothed the wing and got 3 MPH from it.

Click to expand...

Most Mooney's, including your J, use a different airfoils at the root and tip - are you sure the skin "depression" isn't an engineering economy in transitioning the thin skins from one airfoil to another? I know the 301 used a NLF airfoil, but the NACA 64-412 isn't considered to be true NLF and you may not be giving up much laminar flow.

Before starting down the contouring path I'd use the dirty oil trick to find out where laminar flow ends and separation begins along the span - it's free, easy to do and is very accurate.

dk