Thin Airfoil and Climb Performance

sisu1a wrote:


In my opinion there is nearly no room for further improvement of
sailplane performance measured by e.g. max. L/D.


Wow, how many times has THAT old line embarrassed someone in the past?
(rhetorical..)

Perhaps this may be true for CURRENT traditional materials and
established dogma, but besides Windward Performance (who now has the
DuckHawk turning from a 'paper airplane' to a flying machine) who
makes eintire gliders out of PREPREG carbon, you will only find an
occasional aileron or rudder made out of this VERY UNDERUTILIZED
spaceage material. Considering that prepregs have appx double the
strength of the traditional wet layup construction that just about
every composite glider is made with (including the lovely Dianna 2), I
see much room for improvement in ALL aspects of soaring performance
enhancement. And this is even with materials that have already been
discovered (but as mentioned, other than some fancy trim parts have
been studiously ignored by 'most' designers).

In case the significance of what double the strength to weight ratio
means when applied to sailplane structures is not grasped by someone
out there, it is this: The structural minimum just got that much
closer to the aerodynamic optimum. Personally, I see no end to
improvements since our materials will continue to advance, thus
continuing to push the all important structural minimum closer to that
(also ever advancing) theoretical aerodynamic optimum.

How about once carbon nanotube fabric is available, then in prepreg
form? Still no room for improvement then?

Paul Hanson

"Free your mind and your a$$ will follow"--George Clinton


I wrote ... that in my opinion there is nearly no room for further
improvement of sailplane performance measured by e.g. max. L/D having in
mind real life. All know that there it is possible to decrease drag and
increase L/D by active boundary layer control (e.g. suction or, more
interesting, active devices for fluctuations damping). Such solutions are
possible, but at least in the next 30 years I do not expect to see
sailplanes utilizing such technology. Such situation is because of: costs,
complexity and inconvenience in daily use.
We can imagine soaring competitions as F1 racing: new sailplane every two
contests, budget of a few hundred M$, many people in service... I am sure
that L/D ratio over 100 could be reached in less than two years. But it
will never happen. Even simpler solutions (but costly) as Eta or Sigma
seams to be impractical... So we must live in the real world.
Better materials could improve slightly characteristics (applying thinner
airfoils) - but at higher lift coefficients we can have problems. Adaptive
wings (especially near the leading edge) with elastic skin (or applying
smart materials) can help - and I am sure such solution will be utilized in
a relatively short period (I am also interested in this). But high speed
drag of current airfoils (having about 80% extend of laminar flow) leads
really to nearly no room for improvement - smooth flat plate at zero angle
of attack have usually lager drag. Reducing cockpit dimensions is an
acceptable solution in F1 cars but not in sailplanes - most pilots prefere
comfort.
On the other hand we can examine a large collection of real flight logs in
order to find correlations: mean_net_climb_in_thermals -
cross_country_speed. Result is very interesting. Diagrams, such as those
presented in my paper - extracted from Diana-2 flight logs, but performed
for other sailplanes show that results for different, but modern high
performance sailplanes, are very similar. Cross-country speed of open class
sailplanes are very similar to that of racing class sailplanes (slightly
better in lower average_net_climb and slightly worse in larger
average_net_climb. Of course all know, that longer wings leads to better
results at the same thermal conditions. I am sure such result is due to
better utilization of thermals by longer sailplanes - so at the same
thermal conditions they have higher net climb rate. So this is the most
convenient way to improve final cross-country performance. Of course
aerodynamic design should be performed as good as possible - and I tried to
do that in the case of Diana-2. I am sure, that it is still possible to
improve aerodynamics of Diana-2, but we must remember, that only new wing
was designed/manufactured. All other elements are nearly exactly the same
as in Diana-1 - and they are rather incorrect in respect to aerodynamics.
Really I am not fully satisfied with Diana-2 aerodynamics and I am sure the
next design should be (much) better. But much better can be measured by
3-4% of the drag and much higher costs.

Krzysztof