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Paint vs Speed

T

tracy

Well Known Member
Friend
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.
 
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.
 
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.
 
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.
 
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!
 
DonFromTX said:
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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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.
 
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 :D
 
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.
 
tracy said:
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.
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The answer to your question is dependent on the airfoil section. Some sections have a 'weak' laminar boundary layer that 'holds on' too long, then, at a particular angle of attack, will form a large separation 'bubble' before transitioning to turbulent flow. Thus, higher drag, since the boundary layer aft of this bubble is much thicker than it should be. However, if that section is 'roughed up' before where the bubble normally occurs, the laminar flow area will transition to turbulent flow further forward on the wing section, and the bubble is avoided....thinner boundary layer aft...less drag.
Wings that used the 23012 section could exhibit this bubble behavior, so adding roughness would help those aircraft.

RV wings don't have this bubble issue...so polish away.
 
Have you ever felt a sharks' skin? I know NASA has been experimenting with 'rough' surfaces on aircraft skins. Apparantly the roughness reduces the thickness of the turbulent layer because some of the disturbed air is allowed to disappear into the 'valleys' in the rough surface. I think indeed a lot depends on the speeds involved...
 
And there's the story about Rutan scuff-sanding one of his creations (Catbird?) for a CAFE competition many years ago....
 
Bastien said:
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 :D
Click to expand...

The reynolds numbers for sailboats and 500 MPH airplanes are similar. In fact, all of the racing boats have NACA or special airfoil keels.
 
Scuff or buff

Long ago had friends that raced Hobie Cats. The hard core guys would scuff their hulls from the waterline on down with fine scotchbrite.
 
The reason for scuffing boats is to trap micro air bubbles. This is to take advantage of the fact that the friction coefficent is lower air to water than it is fiberglass to water. The same could be true for an airplane (less friction air to air than air to metal/paint), but I would think the effect would be much more subtle.
 
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