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Re: dimple tape

Subject: Re: dimple tape
From: Paul Boulet <possible2do@yahoo.com>
Date: Mon, 19 Aug 2002 07:52:38

thanks Ben...well thought out and well written.  Can you suggest a "sailplane 
store"
in the Los Angeles/San Fernando valley area?

Paul

 BCLERX@aol.com wrote:A method used by sailplane pilots involves painting a few
feet of wing with 
used (dirty) engine oil at several locations (tip, mid-span, root). Fly 
around at cruise airspeed for awhile and you can see in the oil flow pattern 
where laminar flow ends and the separation bubble starts. Place the 
turbulator strip slightly ahead of the separation bubble. The idea is to go 
---From laminar to turbulent flow and eliminate the separation bubble. You need 
to check the bottom of the wing too (though the bottom bubble will usually be 
further aft if one exists). You can place a few feet of turbulator strip on 
the wing and do another oil flow test and note the flow pattern aft of the 
turbulator compared to the adjacent "clean" section of wing to see if the 
turbulator really did reduce or get rid of the bubble. Those who understand 
high performance and sailplane aerodynamics will get the picture. Those who 
don't understand it will benefit by reading a book or two on sailplane 
aerodynamics.

If the new high performance sailplanes still need some "tweaking" with 
boundary layer turbulation, then surely the Europa can stand a bit of oil 
flow testing to check for separation bubbles, especially since the surface 
finish of a homebuilt can vary greatly between individual planes. Designers 
will say that a well designed airfoil doesn't need turbulation. Thats fine 
and dandy, but so far the perfect airfoil doesn't exist. My ASW28 (pretty 
much state-of-the-art racing sailplane) has turbulators on the bottom of the 
wing to aid in high speed, low AOA flight.

Several forms of turbulation can be used. A rough finish (sand wing with 400 
grit) is fine if you want the entire wing turbulent (which might be better 
than the laminar- separation bubble combination). More likely to improve 
performance would be a smooth wing to 50% chord with a well placed 
turbulator. Turbulator needs to be thick enough to penetrate the laminar 
boundary layer (figure on 15 mils). Glider supply stores sell "zig zag" tape 
for this purpose. You can make your own with the old style label maker tape 
cut in half with plinking (zig zag) shears, but the glider stuff looks nicer.

Sorry this email got so long. There are real performance gains possible, but 
I wouldn't worry about this until you clean up everything else first like 
fairings and speed kits. Make sure the first 50% of your wing is not only 
smooth, but no chordwise undulations (keep these to less than 4 mils). This 
means do the finish sanding with a long board (2 feet) in a chordwise 
direction. Don't even think about doing the final sanding in a spanwise 
direction. Hope this helps.

Ben Clerx
California, USA
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