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  #1  
Old 12-11-2016, 02:55 PM
Aero_Octaveus Aero_Octaveus is offline
 
Join Date: Aug 2014
Location: Edmonton, AB
Posts: 216
Default How polished is your paint? - Skin Friction Drag

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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Last edited by Aero_Octaveus : 12-11-2016 at 02:58 PM.
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  #2  
Old 12-11-2016, 03:50 PM
BenNabors BenNabors is offline
 
Join Date: Mar 2014
Location: Alabama
Posts: 127
Default 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.
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  #3  
Old 12-11-2016, 03:51 PM
KatanaPilot KatanaPilot is offline
 
Join Date: Feb 2015
Location: Locust Grove, GA
Posts: 621
Default 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.
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  #4  
Old 12-11-2016, 04:20 PM
Aero_Octaveus Aero_Octaveus is offline
 
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Location: Edmonton, AB
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Default

3M riblet film. Never heard of it, but it seems really interesting!

http://m-selig.ae.illinois.edu/pubs/...ilsRiblets.pdf

Now all I need is Aircraft Wraps to wrap my wings in it!

Can't imagine it being cheap though
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RV-7A - Emp and Wings Done. Working on fuse

http://garetsrv.blogspot.ca/
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  #5  
Old 12-11-2016, 04:51 PM
brad walton brad walton is offline
 
Join Date: Jul 2007
Location: Cypress, TX
Posts: 526
Default

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.
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  #6  
Old 12-11-2016, 04:53 PM
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rmartingt rmartingt is offline
 
Join Date: Nov 2006
Location: Savannah, GA
Posts: 1,029
Default

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.
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  #7  
Old 12-11-2016, 05:05 PM
RVDan RVDan is offline
 
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Location: Frederick, MD
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This video demonstrates that a slightly rough surface can be lower drag than a smooth surface.

http://techtv.mit.edu/collections/if...of-drag-part-1
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  #8  
Old 12-11-2016, 07:42 PM
Paul K Paul K is offline
 
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Location: Grand Rapids, MI
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Whatever! Mine looks fast! Can't emagine not having a hot rod finish look.
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  #9  
Old 12-11-2016, 07:51 PM
Bill Dicus Bill Dicus is offline
 
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Location: Shorewood, WI (Milwaukee area)
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Default Fluid flow

Fascinating. More to come?
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  #10  
Old 12-11-2016, 08:31 PM
rockwoodrv9 rockwoodrv9 is offline
 
Join Date: Dec 2011
Location: Meridian ID, Aspen CO, Okemos MI
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Quote:
Originally Posted by Paul K View Post
Whatever! Mine looks fast! Can't emagine not having a hot rod finish look.
Paul's does look fast! I hope my paint looks half as good as his.
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