Building the Schuemann Wing Planform
Robert Steinhaus
Many current high performance soaring gliders utilize the "Schuemann
Wing Planform" where a tapered glider wing shows a sharper second taper
toward the tip of the wing. Does anyone know of a web resource
(preferred) or book/magazine describing the Schuemann Wing Planform, its
advantages, and the specifics of how you construct it? Thanks!
The Boeing Company
Actually, the so called "Schuemann" planform is just an easy way for us RC
modelers to approximate the ideal elliptical planform. If you want to
strive for absolutely max performance, then a high aspect ratio elliptical
wing is needed. For most of us an approximation that employs easy to build
straight lines is satisfactory.
Schuemann was trying to solve a specific problem of tip stalling on the
predecessor to the Discus. He found that by sweeping the leading edge,
which induces a small amount of spanwise flow he improved the stall
characteristics of the wing, thus enabling the thermalling performance of
the Discus to be more competitive with other sailplanes in its class.
Robert Steinhaus <hone...@hotmail.com> wrote in article
<3529CA...@hotmail.com>...
Al Bowers
<waid.r...@pss.boeing.com> writes:
> Robert Steinhaus <hone...@hotmail.com> wrote:
> > Many current high performance soaring gliders utilize the "Schuemann
> > Wing Planform" where a tapered glider wing shows a sharper second taper
> > toward the tip of the wing.
> Actually, the so called "Schuemann" planform is just an easy way for us RC
> modelers to approximate the ideal elliptical planform...
> Schuemann was trying to solve a specific problem of tip stalling on the
> predecessor to the Discus. He found that by sweeping the leading edge,
> which induces a small amount of spanwise flow he improved the stall
> characteristics of the wing, thus enabling the thermalling performance of
> the Discus to be more competitive with other sailplanes in its class.
I think you sell Wil Schuemann a bit short. The history is a bit
different.
Wil had noticed two things. First was that flexible wings seemed to
extract more energy from disturbances while flying. Second was that
these flexible wings were able to zoom better than stiff wings.
In the course of trying to make his AS W-12 (an 18 meter open class
sailplane designed by Gerhard Waibel of Schleicher) into a 15 meter
span racer, Wil started by simply cutting the tip off. While this
resulted in a slightly lower aspect ratio than optimum, the loss in
performance was even more dramatic, especially in turbulent or gusty
air, or in zoom climbs against sailplanes with more flexible wings
(notable AS W-20s). this observation was confirmed, though to a
lessor degree by pilots of Schemp-Hirth Ventus sailplanes racing
against AS W-20s. The Ventus had a thinner airfoil (with a carbon
spar) and the -20 used a slightly thicker airfoil with a fiberglass
spar. In calm air, the ventus had a slight advantage (an intuitive
conclusion, IMHO). But when the air became gusty, the -20s would pass
the Venti in performance. Why?
Schuemann postulated (not entirely correctly) that a change in wing
planform could make up for a lack of flexibility in his truncated AS
W-12. Indeed, he did improve performance by making his lift
distribution more elliptical. But he also gained a bit more than he
should because of a trailing edge flow separation phenomena. The
swept leading edge delayed the stall at the ROOT of the wing. In
fact, the wing tended to let go (stall) all at the same time.
Interestingly enough, the Dornier company in Germany had made similar
observations, and their wing is also segemented straight taper with a
STRAIGHT trailing edge. BTW, Schuemann has never owned a Discus or
worked with Schemp-Hirth, in so far as I know.
Doing this with straight taper segements isn't necessary, though it is
easier to build. The SZ D-55 standard class sailplane from Poland
(design by Labuc) has a continuous curvature leading edge (and a
straight trailing edge) and is definitely a Schuemann planform. Also,
there is some evidence that curved wings in the vertical sense (the
old Hobie/Midwest Hawk) or polyhedral also has induced drag benefits.
A final note to all of this. Ulv Mai later found additional
information and did analysis for flexible wings in gusts. His
analysis showed that when the aeroelastic axis is aft of the
aerodynamic center of a wing (the rotation center is aft of the 1/4
chord lift line) that a flexible wing does, indeed, extract energy
from gusts. Definitely worth some thought if flexibility doesn't
undermine mission goals (like winch launch in F3B)...
References:
Schuemann, Wil: "A New Wing Planform With improved Low-Speed
Performance"; Soaring, February 1983.
Johnson, Richard H.: "A Flight Test Evaluation of the Discus";
Soaring; February 1986.
Strojnik, Alex: "the Schuemann View on Spanwise flow" (letter to the
editor "Soaring Mail"); Soaring, April, 1983.
Johnson, Richard H.: "A Flight Test Evaluation of the SZD-55-1";
Soaring; March 1992.
Jones, R. T.: "Minimizing Induced Drag"; Soaring, October, 1979.
Mai, H. Ulv: "The Effect of Aeroelasticity Upon Energy Retrieval of a
Sailplane Penetrating a Gust"; Technical Soaring, Vol 10, No 4.
Al Bowers
Dan
Al Bowers wrote in message ...