On 2/1/2016 7:24 PM, Blake Seese 3Y wrote:
My PIK 20B has very close fitting ailerons, but no
mylar seals on them. My question is, are gap seals required top and bottom,
just bottom, just top? I think the laminar flow is off the wing by the time
it gets back to the ailerons on top?

From a cross-flow perspective (i.e. low-pressure-side to high-pressure-side
of the wing), and assuming the seal works perfectly, it makes no difference
where the seal is, but - the devil being in the details - top-only mylar would
be more likely to be sucked/pushed-away-from the surface than
similarly-attached bottom-only, the bottom being the higher pressure side.
This assumes leading-edge-only attachment of the mylar, of course. Good old,
crude, sticky vinyl tape attached along both edges may be less prone to such
separation, but even then it's easy to rationalize the "tape crinkle bump" on
the bottom is less likely to be measurably draggy than on the top (and
arguably will turbulate, woo hoo!), if you opt for a single-sided installation.

"Roger that," on "close fitting ailerons" but air's gonna move from low
pressure to high by whatever paths are available, so it's definitely better
from a theoretical viewpoint to block such passages if they offend you for any
reason. Whether so doing will have measurable effects? Check out Dick
Johnson's (numerous) flight test reports done to his(?) PIK-20 over the course
of several years...easily found in the online "Soaring" mag archive. That
said, I never bothered to seal my Zuni's control surfaces gaps (same airfoils,
for all practical purposes); doing so wasn't sufficiently good for my soul.
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Udo Rumpf said The Pik20 and the LS3 use both the FX67-15%&17%

In various thicknesses, that was the flapped section of choice in those "1st
generation glass" days: PIK-20; Nimbus II; Glasflugel 604; Jantar 2B; Janus;
Vega; Zuni; Nugget & almost certainly others.
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So what is the proper way to measure 52% chord?

Straightforwardly done with a 3-dimensional T-square for a PIK-20
(conveniently manufactured with a straight leading edge, essentially
perpendicular to the fuselage's vertical center plane). You can make one from
2 pieces of wood, doing your best to make the straight bit that picks up the
L.E. of the wing, 90-degrees out of plane to the one laying flat against the
bottom of the wing and extending at least to the T.E.

I seem to remember the PIK-20 has a single crank at the inboard of the aileron
on the trailing edge. Because of the crank, you'll need to locate dots
(turbulator tape positions) along 3 straight lines along the side of the wing
of interest (the bottom for turbulator tape). Spanwise, locate one line
immediately outboard of any wingroot fairing, one at the T.E. crank, and one
as conveniently near the wing tip as possible. This is one of those "lay
things out the best you can, be as precise as reasonably possible, and call it
good" tasks, given the uncertainties of turbulator physics. Each line should
be perpendicular to the L.E., from L.E. to T.E.

0.52 of the in-plane, straight-line distance from the L.E. aft to the T.E. is
52% of the chord along each line. Connect the 52% dots with spanwise straight
lines and you have your turbulator tape position....if 52% is what you want.
(Working from memory, that seems "somewhat" forward of what I seem to remember
gleaning from old "Soaring" magazines.)

If you're really picky, chord is measured as if the wing planform is
2-dimensional, so measuring *along* a "sharpied" line from L.E. to T.E. will
introduce a slight fore-n-aft error. It's most accurate to measure along the
T-square's chordwise piece, from the front board aft to the T.E., at each L.E.
location of interest.

Some might choke on the crudeness-as-described methodology, but it should at
least give you an idea of what you're trying to accomplish, and will almost
certainly "be good enough for government work."

Have fun!

Bob W.