Udo Rumpf wrote:
The performance will be reduced, if the indentation is more than the
allowable tolerance, which can vary a little from airfoil to airfoil.
Just look at the top surface for now only.
If you would assume an indentation of, for example, of .004"
over a distance of 1/4", a transition from laminar to turbulent flow would
follow.
The same .004" over 2 or 3 three inches may not have any negative effect.
In case of a spar shrinkage, the imperfection would be located
approximately at
40% chord. Your glider in optimum climb has a laminar transition at
approx. 65%
on the top surface. If this early transition happens at 40% due to the
imperfections,
the climb will be negatively affected.
Since pressure and friction drag values in climb are relatively small,
compared to induced and interference drag, the difference in climb between
a perfect wing and one that has imperfections may not show up readily.

In fast cruise the bottom surface becomes very important.
The ASW27 wing is designed to have laminar flow to 90% chord.
If the same imperfection holds true as above on the bottom surface,
over half of the bottom wing becomes a turbulent boundary layer wing
and your performance will suffer substantially.
The drag relationship in fast cruise changes and the pressure and
friction drag become dominant and induced drag is a much smaller portion
of the total. Now laminar flow will show its stuff and the airfoil of the
ASW27 will shine.

Make comparison flights with an other ASW27 which is known to perform well.
Also read Dick Johnson's articles on laminar flow.
I hope this helps to make a decision.
Regards

I second Udo's suggestions. After 9 years, my ASH 26E had accumulated a
number of dimples and waves, but after flying next to a new ASH 26 E, I
realized the performance difference was imperceptible. So, I'm waiting
for more serious problems before I do anything about the wing imperfections.


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Eric Greenwell
Washington State
USA