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
|