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  #1  
Old 04-24-2013, 09:08 AM
tracy tracy is offline
 
Join Date: Dec 2012
Location: chattanooga,tn
Posts: 231
Default Paint vs Speed

I recently purchased an RV 8 and love the airplane. It looks very nice but I noticed during the prebuy that the paint was rough to the touch like real fine sandpaper. I have been polishing the leading edges to keep bugs from sticking and they are now smooth to the touch. This rv is roughly 5kts faster than the other rv8s with equivalent 0360 motors. I was told this by the previous owner but was skeptical. My question is this, does a rough surface give an aircraft more speed? If so, why would anyone make theirs buff smooth? I do not want to polish the rest of the aircraft if it slows it down.
I have flown with 3 0360's, 1 io360, that can't keep up with me, so I can't blame design flaws on their part.
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  #2  
Old 04-24-2013, 09:14 AM
SHIPCHIEF SHIPCHIEF is offline
 
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Location: Seattle
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Default

Sometimes you get a fast one, but not me.
I've had one plane (not RV) that was known to be the fastest of it's type (locally) but everything else I've had can't seem to make 'book' speed.
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  #3  
Old 04-24-2013, 09:38 AM
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DonFromTX DonFromTX is offline
 
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I bought Cherokee 140 once that had a paint job that was as rough as sandpaper. I flew it about a year until I got around to having it recovered in Imron. The difference in "feel" and flying was amazing, not to mention increased speed at the same throttle settings.
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  #4  
Old 04-24-2013, 10:27 AM
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hydroguy2 hydroguy2 is offline
 
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Location: Townsend, Montana
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if rough surfaces were faster, there would be NO shiny planes at Reno
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  #5  
Old 04-24-2013, 10:31 AM
brad walton brad walton is offline
 
Join Date: Jul 2007
Location: Cypress, TX
Posts: 526
Default Maybe

I recall from my sailplane days that there was some comparative testing done that showed light crosshatch pattern sanding of the wings with 400 or 600 wet/dry sandpaper outperformed the same aircraft with a good wax of the wings. I can't cite you to the source, just from my memory. The theory was improved boundry layer performance.
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  #6  
Old 04-24-2013, 11:09 AM
deek deek is offline
 
Join Date: Mar 2013
Location: Flathead Lake Montana - 8S1
Posts: 334
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The surface finish on the airframe and prop of Klaus Savier's Delaminator (O-200 powered Varieze) would suggest smooth but 400-600 grit finish is best for speed...but I like shiny!
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  #7  
Old 04-24-2013, 11:10 AM
tracy tracy is offline
 
Join Date: Dec 2012
Location: chattanooga,tn
Posts: 231
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Quote:
Originally Posted by hydroguy2 View Post
if rough surfaces were faster, there would be NO shiny planes at Reno
It is still shiny and you can't tell it's rough unless you feel it.
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  #8  
Old 04-24-2013, 11:45 AM
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Neal@F14 Neal@F14 is offline
 
Join Date: Aug 2007
Location: Wichita Falls, TX
Posts: 2,182
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Quote:
Originally Posted by DonFromTX View Post
I bought Cherokee 140 once that had a paint job that was as rough as sandpaper. I flew it about a year until I got around to having it recovered in Imron. The difference in "feel" and flying was amazing, not to mention increased speed at the same throttle settings.
Same story here, bought a Cherokee 140 with terrible paint (but new engine and prop) and after it was stripped and repainted with Acry-Glo polyurethane, it picked up about 5-6 mph. Adding a LFS nose strut fairing picked up another 2-3 mph.

The speed difference on the RV-8 in the OP's post is most likely attributed to being well-rigged, good wheel pant alignment, and perhaps even lower aircraft weight than the -8s he's comparing it to.
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  #9  
Old 04-24-2013, 12:02 PM
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Bastien Bastien is offline
 
Join Date: Jun 2010
Location: Vannes, FRANCE
Posts: 140
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Put a drop of water on a 400/600 sandpaper scuffed surface. The drop with spread. Do the same thing on a perfect polished surface the drop will not spread. Not the same viscosity as air but that's a starting point
My father do a lot of sailing and every year he does the antifouling paint. Just after painting, top speed with engine only (0kts wind) is 7.2kts. After scuffing with 600 grit paper top speed is 7.5kts.

It is still hard for him to hear that I'm going 25 time faster than his boat
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  #10  
Old 04-24-2013, 12:29 PM
David-aviator David-aviator is offline
 
Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
Default Smooth probably is better

I do believe with 2 identical airplanes, one with laminar flow, one without, the laminar flow machine will have less drag and be faster.

It certainly was true with the canard EZ's. Rain would trip the laminar flow, the aircraft (at least with mine) would descend about 500 fpm.

If the wing is not otherwise laminar flow, perhaps sand paper will make for less drag and more speed, but I am inclined to doubt it. If the sand paper is removing huge dead bugs, maybe so, but I doubt it.

I will venture that the extra speed you see is the result of an engine producing more HP, not all 0360's produce the same power for a number of reasons including prop efficiency.
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