Eric Greenwell wrote:
Bill Daniels wrote:
Spoilers, even when closed, disrupt the upper wing
surface in the most critical area - flap only wings are cleaner.

And yet, on a well built glider, you can carefully smooth and seal the
spoilers so there is no leakage or surface unevenness, test the glider,
and it isn't any better. My recollection from Johnson's tests is the
most critical area on the wing is the first few inches of the nose (bug
problems), and that the boundary layer is thick enough at the spoilers
that they do not affect the flow on the typical racing glider.

One might also consider how much "disruption" in air flow a Holighaus or
Waibel would tolerate! Even if I'm right about the airflow disruption,
it doesn't negate the principal advantages of flaps, but I think most
pilots would prefer a combination of the two.

There may also be performance issues related to designing a modern flap
only glider: because modern airfoils use such narrow control surfaces, a
flap only glider might not have enough glide path control. I was
astonished at the difference in glide angle between my ASW 20 and my ASH
26 E, even though they employ the same flap system for landing: the ASW
20 descended much more steeply than does my ASH 26. Even though part of
that is likely due to the higher wing loading on the 26, the flap chord
on the 20 is almost double that on the 26.

--
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Eric Greenwell
Washington State
USA

True,
I am building a new wing and I was hoping to include landing flaps
only.
The airfoil was not given me the results I desired with the larger
flaps. I had to reduce the flap size to 16% and install spoilers. The
flap/aileron arrangement will similar to the DG800,
May real preference would be the ASW20/26/27 set-up but I decided to
keep it simple
As for surface aerodynamics the spoilers are always past the laminar
transition. Smooth and sealed spoiler boxes have no ill effect.
In my case the max transition on the top surface will be at 66% and
that is where the box will be installed.
Regards
Udo