> What is recommended trail edge thickness to shoot for Aileron,
> Flap and Stabilator?
>
> Should I shoot for a square edge with a slight radius at the
> sharp edges, or should i make it a true radius?
>
> Thanks
> Ron Parigoris
Both above questions are covered in detail in Hoerner's companion books on
"Fluid
Dynamic Lift" and "Fluid Dynamic Drag". As a practical matter, the thickness
of the trailing edge on Europa's airfoils is constrained by how much surface
between top/bottom skins remains for secure bonding. At 1/8" thickness or less,
Hoerner just about says it's all irrelevant. So I believe that both thickness
and rounding of the edges so as to best permit paint to flow nicely and
more permanently adhere in service has no appreciable aerodynamic effect.
Nevertheless, in theory, a trailing knife-edge reduces interference drag.
Especially
for ailerons, a blunt thick edge, especially with sharp corners, increases
drag -- called base drag, but it also increases control effectiveness. This
is because of a pair "vortex streets" created in the dead space behind the
trailing edge, increasing drag but also promoting airflow attachment upon
aileron
deflection.
But not necessarily desirably. There is an aileron AD on my other plane, where
you can chop off the aileron trailing edge (to about 3/4" thick) to get out of
a perpetual 100-hr aileron disassembly/inspection routine. Don't ask the
relationship
of the two; a long story and makes sense to FAA!
What happens, though, is that the increased "figure of merit" vs. aileron lift
effects results two things. Small aileron deflections have reduced control
effect
about neutral; but increased aileron lift upon large deflections translates
to higher lateral stick forces. Europa's Flettner strips on the trim tabs
I think are an exaggerated derivative of the phenomenon, as is Jim Nelson's
observation
about aileron trimmers. But on my other plane, the result of complying
with the "hacksaw option A" in the AD is reportedly a total ruination of aileron
control feel, both sensitivity in cruise flight and control forces in
maneuvering.
Regards,
Fred F.
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