Dan G wrote:
On 28 Mar, 01:59, Brad wrote:
As a guy who has flown thick airfoil ships and currrently owns a ship
with a 17% airfoil, I am curious what kind of performance in climb I
might see with a 14% airfoil section.

Someone on here mentioned a presentation by Loek Boermans at the
recent SSA conference, where he stated that modern, thin, laminar-flow
sections don't climb as well as they theoretically should in bumpy
gusty thermals.

I attended Boerman's lecture. The problem isn't thin, laminar flow
airfoils in general, but some specific designs over the last two decades
or so that have a "flat spot" in the lift coefficient (Cl) curve as the
angle of attack (AOA) approaches stall. Normally, the Cl increases with
increasing AOA, but in the flat spot region, it remains constant (or
nearly so) even as the AOA increases. Past this region, Cl begins to
increase again with AOA at the usual rate.

While thermalling, the glider's AOA will be near this region. If a gust
increases the AOA enough to enter the region, the climb rate is reduced
momentarily. By the top of the thermal, repeated gusts mean the glider
hasn't climbed as quickly as it might have.

In gusty conditions, the climb rate can be improved by thermalling a bit
faster, so this region is avoided (gusts can't increase the AOA enough
to enter the region). Because the "flat spot" is wider in the
thermalling flap setting and diminished or not present with more
negative flap settings, using the neutral flap setting in gusty
conditions will also avoid (or at least improve) the situation.

--
Eric Greenwell - Washington State, USA
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