Thin Airfoil and Climb Performance

On Mar 29, 4:35*pm, Eric Greenwell wrote:
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.

Thanks Eric. Which are the specific designs? The likes of the LS7/
DG600/ASW24?

On Thu, 27 Mar 2008 18:59:40 -0700 (PDT), 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.

If you are talking about the Wortman FX67-K170: The FX 62-K131 of the
ASW-20 from the same era climbs at least as well as the former.

The very thin airfoils of current gliders (e.g. ASW-27) climb like
homesick angels.
Bye
Andreas

On Mar 27, 9:59 pm, 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.

Thanks,
Brad

I have not yet been outclimbed in my Antares, with
its a very thin airfoil and 9lb/ft2 min wingloading,
when flying against newer 18-meter, 15-meter, and
standard designs at lower wing-loadings. Of course,
they don't catch me after the first thermal, so my
experience is limited ;-)

Different gliders have different capabilities, but
your thermalling skill and ability to fly the particular
glider as it demands will play a huge part.

Climb performance is only part of the equation,
unless you are limiting yourself to floating about
the home drome. XC performance requires
good run performance, climb performance,
handling, and performance in transitions.

Ask about the particular machine you're lusting
after and perhaps a more specific answer
can be had !

Hope this helps,
Best Regards, Dave "YO electric" (see August
Soaring cover)

On Mar 29, 11:43 am, Eric Greenwell wrote:
Eric Greenwell wrote:
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.

I should add that designers were aware of the flat spot years ago, but
did not appreciate the problems it could cause in gusty conditions.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes"http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

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If I understood Loek correctly, "gusty" isn't perhaps the right
description. Better "high frequency changes in vertical velocity,
causing momentary increases in required CL" ? Its not something
you'd necessarily feel as a pilot if I understood correctly.
Eric ?

wrote:
On Mar 29, 11:43 am, Eric Greenwell wrote:
Eric Greenwell wrote:
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.
I should add that designers were aware of the flat spot years ago, but
did not appreciate the problems it could cause in gusty conditions.


If I understood Loek correctly, "gusty" isn't perhaps the right
description. Better "high frequency changes in vertical velocity,
causing momentary increases in required CL" ? Its not something
you'd necessarily feel as a pilot if I understood correctly.
Eric ?

I don't recall the presentation well enough to say for sure, but my
impression is the pilot would be aware that it's not smooth air,
probably enough that the pilot would be making some control corrections.
It would be better to ask Loek, or someone like Dan Somers or Mark
Maughmer, who are also aware of it.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Eric,
what is the title of Loek Boerma's presentation?
Is it available in paper form?
Udo


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
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes"http://tinyurl.com/y739x4
* * * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" atwww.motorglider.org

Udo wrote:
Eric,
what is the title of Loek Boerma's presentation?
Is it available in paper form?
Udo

I don't recall the title. It might be published somewhere. The OSTIV
journal would be the first place I'd check.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org