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How polished is your paint? - Skin Friction Drag

A

Aero_Octaveus

Well Known Member
When I took my car into the Autobody shop to get the bumper re-painted. I got on to the discussion of paint with the Shop Foreman. He says that at a professional level, a painter can get the paint too perfect when applying it to car to match the factory paint job. He said that factory paint jobs have a very slight orange peel to them and when painting a body panel to match, if the paint is applied they same way, it can look very different even if the color is matched near perfectly. Taking a very close look at a factory paint job on a car, I would agree that the paint does have a very slight orange peel to it. But compared to an restored show car where the paint is like a mirror. Wow there is a difference, but you have to look close!

With that said, the boundary layer over a wing can be as small as 1/100th of an inch, or 0.25mm thick. At this level one could assume that this sort of difference between a very slight orange peel as your would get on a factory car finish or home/shop painted finish, can produce more Skin Friction Drag then if the paint was "Perfect and Polished"

With the large amount of builders who have painted their own wings, has anyone noticed an increase in cruise just by having their plane repainted professionally with a "Perfect and Polished" paint job?

Or are the planes too slow for skin friction at this level to play a significant factor?
 
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Probably not much difference

How much faster does a polished plane fly compared to a painted one? Big difference is on the cowl design and engine cooling drag. There are a couple of threads showing 7 to 10 mph differences on cowl design. So I would say paint drag is a second order effect.
 
Check out the riblet tests

You might want to Google "3M riblet film" for some interesting information on skin friction drag.

There were tests done on an A320 some number of years ago. I also recall one of the America's Cup racing yachts had riblets on the hull (I think they won that year).

The Reynolds numbers of an A320 or a yacht in water are widely different than an RV, but you might find the information a bit surprising.
 
Sail plane pilots have debated waxed vs. unwaxed vs. very light crosshatch pattern on wing surfaces. My recollection is a light crosshatch sanding with 400 wet/dry sandpaper produced a measurable increase in the amount of laminar flow.
Recall also a smooth golf ball won't carry as far as a dimpled ball.
A little orange peel in the paint surface may actually be beneficial.
 
You'll get much bigger gains out of improving your engine cooling and doing a good job of fairing your gear than you will from the paint.
 
brad walton said:
Sail plane pilots have debated waxed vs. unwaxed vs. very light crosshatch pattern on wing surfaces. My recollection is a light crosshatch sanding with 400 wet/dry sandpaper produced a measurable increase in the amount of laminar flow.
Recall also a smooth golf ball won't carry as far as a dimpled ball.
A little orange peel in the paint surface may actually be beneficial.
Click to expand...

All true above, but don't start sanding yet. High performance gliders (fiberglass) are covered with gelcoat that is contoured and sanded by hand for hundreds of hours to an extreme tolerance of less than .003" deviation (wing).
Gelcoat and water/humidity don't mix and because it cost over 25K$ to refinish we wax it. Less than 3 do not because they believe the slight improvement in performance gives them an edge racing. So they sand with 4' aluminum sanding blocks and check the wings profile with a wave gauge to achieve <.003. A lot.
Everyone else waxes the **** out of it to ward off the devil.
Because smooth is sticky, we apply turbulator strips spanwise on the underside 'near' the trailing edge to help with airflow separation . Location is determined by wind tunnel testing. Why new glider = 160K$ +++

What improves our glider performance more, and I plan to apply this to my 14, is we do everything we can to reduce/eliminate/control where cockpit air escapes or leaks. Canopy and wing roots (via fuselage spar openings) escape are very high drag producers.
So, we seal the best we can by glueing material over openings, attaching flexible boots to pushrods , thin foam to canopy frames.
Most important is we give the air inside a place it wants to go, the tail. An exit as far back as possible in the belly so the escaping air does not impact a tail surface. A NACA vent perhaps. This means louvers in the aft cabin bulkhead.
Or go slow....less drag.

Hank
140268
Sailplane Racing
744
Dues
 
A few years back I purchased a Cherokee with the worst paint ever, sort of a badly oxidized almost satin finish, absolutely no shine whatever.
I had it painted (with Imron) and will never forget the difference it made in the plane, not only speed increase but gave it a "slippery" feel in the air as well.
 
Orange peel may not necessarily be a bad thing. I think myth busters applied a golfball like texture to a car to see if the gas mileage would increase due to the physics of a golf ball. The results were the car saved gas with golf ball like dimples. Even with the added weight.

Now how that translates to a smaller scale with orange peel like texture I don't know.
 
Lot of time

I spent a lot of time and effort getting the right size orange peel in my paint job along with some nats and other flying critters. At between 1000ft and 20 ft, she looks great. However at 1 ft, I get a craving for bacon and eggs to go with my orange skin. 😜
 
I suspect that the deformation of the skin in between the ribs causes higher drag than poor paint jobs can achieve.

Dave
 
Klaus Savier (LSE electronic ignition) has years of documented, instrumented testing of surface finish on his highly modified Varieze and LongEZ. NO ONE goes faster on less horsepower than Klaus; good luck in getting him to talk about what he's learned.

Anyone, however, can carefully inspect the finish of his planes, especially the canard, wings and winglets. Very interesting :)
 
There's been some interesting info on intake manifold design. I remember they did a test with a rough rasp on the bottom of the intake port(side to side ) and showed how it helped keep, fuel in suspension especially in carberated engines, in testing it was much better with the turbulence than the smoothed polished ports which was what was Done previously.

I realize this is very different than the flow over a wing I just thought it was interesting information.
 
Turbo69bird said:
There's been some interesting info on intake manifold design. I remember they did a test with a rough rasp on the bottom of the intake port(side to side ) and showed how it helped keep, fuel in suspension especially in carberated engines, in testing it was much better with the turbulence than the smoothed polished ports which was what was Done previously.

I realize this is very different than the flow over a wing I just thought it was interesting information.
Click to expand...

Automotive found the same many years ago with good injected fuel atomization, the polished ports were better. So, like with many things, it depends. If 90% of your drag was due to surface finish effects, then orange peel would be a factor, but just doing the basics ( Paser:Speed with Economy ) will get us much farther with each gallon.

OMG! Did you check the price on that link? My $20 copy is now much more treasured.
 
Fascinating

frghtdg said:
All true above, but don't start sanding yet. High performance gliders (fiberglass) are covered with gelcoat that is contoured and sanded by hand for hundreds of hours to an extreme tolerance of less than .003" deviation (wing).
Gelcoat and water/humidity don't mix and because it cost over 25K$ to refinish we wax it. Less than 3 do not because they believe the slight improvement in performance gives them an edge racing. So they sand with 4' aluminum sanding blocks and check the wings profile with a wave gauge to achieve <.003. A lot.
Everyone else waxes the **** out of it to ward off the devil.
Because smooth is sticky, we apply turbulator strips spanwise on the underside 'near' the trailing edge to help with airflow separation . Location is determined by wind tunnel testing. Why new glider = 160K$ +++

What improves our glider performance more, and I plan to apply this to my 14, is we do everything we can to reduce/eliminate/control where cockpit air escapes or leaks. Canopy and wing roots (via fuselage spar openings) escape are very high drag producers.
So, we seal the best we can by glueing material over openings, attaching flexible boots to pushrods , thin foam to canopy frames.
Most important is we give the air inside a place it wants to go, the tail. An exit as far back as possible in the belly so the escaping air does not impact a tail surface. A NACA vent perhaps. This means louvers in the aft cabin bulkhead.
Or go slow....less drag.

Hank
140268
Sailplane Racing
744
Dues
Click to expand...

Hank, T
This entire discussion is fascinating. I have a very fast RV-4, yet a big leak at speed on the aft canopy. I believe it is from a small deformation at speed, or perhaps a (negative drag) lifting force from the canopy itself.
Either way, I wanted to reduce it because it is noisy to the point of hearing the radio becomes a challenge, and it lets rain in. I have pondered the effect on speed for quite some time, and the former physics guy within me was intrigued.
I've also studied other aspects of high drag areas during the build of my (previous KR-2). I used the wing roots from Howard Hughes's racer. Great attention to detail did make a noticeable difference at speed.

Fly well,
Daddyman
Dues paid >2017
 
Speed Increases

My first RV-6 is now faster than when I had it. I had a very slick paint job on it and will all the speed mods and fairings being perfect, it would do 220 on 180hp, with the older style Hartzell prop (non-blended airfoil). The current owner is getting about 224-225 confirmed by 4 way runs and two GPS units. It has the same engine (with 800hrs on it) and nothing new. So why does it go faster?
My guess is that Doug keeps it in pristine condition and waxes it constantly. He says you cannot put a rag on the wing.....it will slide right off. My thought is he is filling in all the small irregularites in the paint, thus decreasing drag.
Back in the day, a Spitfire was polished and waxed and got 17mph gain from just that!

Keep them buffed up and the bugs off!
 
Seems I've read this somewhere in the past..... I don't know if it's been mentioned but as your cg moves aft, (to a point), you decrease drag. Not enough to see a speed increase but I'm guessing there would be less power required, (probably not enough to measure), for the same speed thus using a little less fuel.
 
Less downforce on tail

That part I do know. A CG that is aft will result in a plane that is slightly faster as less downforce is needed at the tail (A CG that is more forward would require a little more downforce on the tail, and hence more drag).

Very interesting, I'm not entirely sold on the golf ball theory...I mean it works for golf balls, but does it work for planes. I imagine if it did, we'd see "Speed" dimples on all the planes at Reno. Although the 3M riblet might be an interesting idea. I know speedo was experimenting with a similar ribbed design of their swimsuits for competitive swimming mimicking the same type of ribbed shape that sharks skin have under microscope.

Super clean and super smooth seems to be the order of the day and now I'm sold on waxing! I never thought to concentrate on the fairings on the fuselage and canopy. Well I suppose its like weight....the sum of ALL parts big and small is most important.

Has anyone worked on intersection fairings for the wings? I don't see many RV's with them??
 
One thing to keep in mind with paint vs polished is paint adds a considerable amount of weight. I painted a rocket for a guy a few yes ago with ppg base clear each coat I applied was 1 1/4 gallons and with a 7 coat total that's 8 3/4gals I was surprised how much paint it took. Didn't have a before weight so not exactly sure how much weight was added. I realize only 80% only makes it on the plane and some solvents dry out but I would bet 6 or 7 lbs per gallon stayed on the plane
 
Little Al said:
One thing to keep in mind with paint vs polished is paint adds a considerable amount of weight. I painted a rocket for a guy a few yes ago with ppg base clear each coat I applied was 1 1/4 gallons and with a 7 coat total that's 8 3/4gals I was surprised how much paint it took. Didn't have a before weight so not exactly sure how much weight was added. I realize only 80% only makes it on the plane and some solvents dry out but I would bet 6 or 7 lbs per gallon stayed on the plane
Click to expand...

True, but I wonder how much this affects speed? The paint weight shifts CG aft, which might actually cause an increase in airspeed?
 
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