On 5/22/2013 11:20 AM, soartech wrote:
What is lost without them?

For the truly anal, mostly sleep! :-)
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One point of glide?

It depends...!
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What about sink rate?

It depends...!
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Is this just for racers? My glider does not have them.
Does anyone have any facts on this?

Facts?!? Now you've done gone and changed a relatively simple question to a
genuinely complex one!

If you're OK with conclusions drawn via observational extrapolation and the
application of "common sense" (of which the development of airplanes -
including sailplanes - has a long and rich history), gap seals are obviously
not *required* for effective aerodynamic 3-axis control.

What they *are* good for is aerodynamic efficiency and improving control
effectiveness, most commonly detectable at lower-end speeds. True for gliders
and powerplanes. This was well known in the powerplane (e.g. racing) field by
the mid 1930s.

Sealing control gaps tends to make the sealed controls act more like a part of
the larger appendage (e.g. main wing, horizontal stabilizer/elevator, vertical
stabilizer/rudder), than a separate, trailing, appendage, by reducing
(eliminating?) "cross-talk" between the higher-to-lower pressure side of the
fixed/moveable assembly. With a(n impermeable) seal in place and ignoring tip
effects, cross talk can't begin until the trailing end of the moving portion,
as opposed to beginning at the gap itself. As with everything aerodynamic,
there's books-full of complexities associated with the above claim, but the
claim is accurate.

As noted in other threads, poorly installed/worn gap seals can cause more
problems than well-installed ones fix...something to bear in mind if the
absence of gap seals on your bird bugs you. (Their absence never did on the
Schweizers I owned...come to think of it, the last glider I owned that had
[external vinyl tape] gap seals was in 1981!)

It's a safe generalization to presume - in the absence of solid information to
the contrary - that EVERYthing having to do with gliders' external surface
configurations not explicitly determined by some underlying structural
consideration, is done for (at a minimum) aerodynamic drag reduction purposes.
Gap seals (say on a tube-n-rag Schweizer) being a case in point...

Bob W.