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  #11  
Old 04-24-2013, 12:57 PM
brad walton brad walton is offline
 
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As another refrence point, ever see a smooth golf ball?
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  #12  
Old 04-24-2013, 01:28 PM
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chepburn chepburn is offline
 
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Quote:
Originally Posted by tracy View Post
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.
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.
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  #13  
Old 04-24-2013, 03:07 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by chepburn View Post
The answer to your question is dependent on the airfoil section.
Exactly! You can't generalize the idea regarding all aircraft.
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  #14  
Old 04-24-2013, 03:56 PM
SHF SHF is offline
 
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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...
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  #15  
Old 04-24-2013, 06:10 PM
rv7charlie rv7charlie is offline
 
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And there's the story about Rutan scuff-sanding one of his creations (Catbird?) for a CAFE competition many years ago....
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  #16  
Old 04-24-2013, 06:48 PM
jetdriven jetdriven is offline
 
Join Date: May 2012
Location: Houston tx
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Quote:
Originally Posted by Bastien View Post
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
The reynolds numbers for sailboats and 500 MPH airplanes are similar. In fact, all of the racing boats have NACA or special airfoil keels.
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  #17  
Old 04-24-2013, 07:34 PM
cajunwings cajunwings is offline
 
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Default 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.
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  #18  
Old 04-25-2013, 08:12 AM
pa38112 pa38112 is offline
 
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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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