Thin Airfoil and Climb Performance

Some time ago there was a discussion about influence of (thin) airfoil on
climb performance of sailplane. I am not a regular reader of the group, so
my post is slightly delayed.
Influence of airfoil characteristics on climb performance and final
cross-country speed was the main objective studied during design of Diana-2
sailplane. The paper on this subject was presented at OSTIV Congress and
published in Technical Soaring. The paper can be download now from
http://www.dianasailplanes.com/Tech_Soar_KK.pdf
May be someone will find it interesting.
In my opinion there is nearly no room for further improvement of sailplane
performance measured by e.g. max. L/D. But there is a relatively large
space for improving final cross-country speed by better utilization of
thermals. Diana-2 was just an attempt to perform this.

Krzysztof Kubrynski

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

Paul -

You're missing out on a key point: Changing materials to a lighter
structure does not change the aerodynamics of the wing.

It changes the wing-loading - but that's no different from adding or
removing water ballast. As long as the wing SHAPE remains the same, a
different weight/wing-loading simply shifts the polar. It doesn't
create an improvement.

Wing efficiency is affected by the shape of the airfoil, the wing
planform (and how the two add together to become a total 3-d package),
the smoothness of the skin/surface, the surface-area/skin drag, and
numerous other smaller factors.

Existing composite materials can be made to follow complex curves and
result in an extremely smooth surface, so changing to a fancier
material does nothing to improve the efficiency of the wing.

The reason that Pre-Preg and other fancy/costly exotic materials
aren't commonly in use is because they aren't NEEDED to achieve
aerodynamic optimums. Fancy materials are mostly used for
manufacturing reasons, to carry higher loads, to have more specific
stiffness in a particular load direction, etc. Aerodynamics doesn't
enter into the equation.

Small, incremental advances in overall performance have been made over
the last 20 years; but they are pretty tiny in comparison to the
performance jumps that were seen from the 50's to the late 70's (when
glide ratios doubled or tripled). We're down to the point where we're
fighting physics every step of the way in order to see any
improvements. And in order for theoretical gains to be realized in
actual flight, the tolerances are getting so tight that normal
manufacturing techniques cannot be applied. And with tolerances that
tight you also end up fighting the fact that the air gliders move
through is not sitting still. At some point the turbulence and
natural air movement disrupts the pristine theoretical predictions of
how the air will behave as the sailplane passes through it, rendering
supposed performance gains null (and in some cases causing an actual
performance penalty - see the part of the Diana 2 paper where they
talk about lift minimums in certain airfoils at high Cl).

Take care,

--Noel

P.S. Note that in the paper that they talk about little improvement
in basic airfoils or passive boundary layer control. They leave the
door open to performance improvements via active boundary layer
control methods (turbulators, Sinha's "de-turbulator" tape, etc). The
trick to active boundary layer control is in making it both reliable
and efficient over the wide speed band and aerodynamic conditions that
sailplanes fly.

On May 5, 9:23 am, "noel.wade" wrote:
Paul -

You're missing out on a key point: Changing materials to a lighter
structure does not change the aerodynamics of the wing.

I did not miss the original point, I just don't agree with it. Clearly
you miss mine though. If structure does not change the aerodynamics of
a wing, then why are modern hot ships still made of wood, or metal?
The fact of the matter is that the aerodynamic optimum will never
agree with the structural minimum, but the more that materials
advance, the closer to that optimum designers can get. It most
certainly does affect the areodynamics of a wing, so long as the wing
is designed specific to the materials it is made from.

It changes the wing-loading - but that's no different from adding or
removing water ballast. As long as the wing SHAPE remains the same, a
different weight/wing-loading simply shifts the polar. It doesn't
create an improvement.

Huh? Wing loading? Again, you are way off my point. Advances in
structure ALLOWS changes in shape, which most certainly can lead to
improvement. Any racing ship will have the ability to change it's wing
loading with ballast, but since we're on the subject of wing loading,
by using prepreg your dry weight can be significantly lower, thus
'possibly' allowing a wider range in wing loading and thus a ship
adaptable to a wider range of conditions.

Wing efficiency is affected by the shape of the airfoil, the wing
planform (and how the two add together to become a total 3-d package),
the smoothness of the skin/surface, the surface-area/skin drag, and
numerous other smaller factors.

Precisely. And by using prepregs a designer can go with a less
compromised foil since the material is so much stronger, not to
mention they have the ability to be smoother (and stay that way!),
much less affected by weather/heat (they are not room temp cured so
at normal runway temps are not even close to getting soft and you will
never be complaining about the dreaded 'spar hump' for that matter).
This all means nothing of course if the design blows...


Existing composite materials can be made to follow complex curves and
result in an extremely smooth surface, so changing to a fancier
material does nothing to improve the efficiency of the wing.

As can prepregs, although it is a good bit more work. However
'fancier' materials allow the designer to make a wing that looks a lot
more like the airfoil they really wanted to use, instead of the one it
had to be mutated into to allow for the safe amount of structure.

The reason that Pre-Preg and other fancy/costly exotic materials
aren't commonly in use is because they aren't NEEDED to achieve
aerodynamic optimums. Fancy materials are mostly used for
manufacturing reasons, to carry higher loads, to have more specific
stiffness in a particular load direction, etc. Aerodynamics doesn't
enter into the equation.

Absolutely incorrect. The reason they are not used is because their
significance is not widely recognized enough for customers to expect
any different. Most big manufacturers have their entire infrastructure
built around traditional wet layup. In fact some of them even were the
original pioneers of these now widely utilized, current industry
standard materials and techniques. They are not about to fire all
their workers, sell off all their equipment and revamp their entire
existence around this "new" material, and anything short of that would
yield no results. It is a completely different ballpark, and there is
only one manufacturer who is setup for it, although it is now becoming
commonplace to be used in flight control surfaces for obvious
aerodynamic reasons (flutter?).

Small, incremental advances in overall performance have been made over
the last 20 years; but they are pretty tiny in comparison to the
performance jumps that were seen from the 50's to the late 70's (when
glide ratios doubled or tripled). We're down to the point where we're
fighting physics every step of the way in order to see any
improvements. And in order for theoretical gains to be realized in
actual flight, the tolerances are getting so tight that normal
manufacturing techniques cannot be applied. And with tolerances that
tight you also end up fighting the fact that the air gliders move
through is not sitting still. At some point the turbulence and
natural air movement disrupts the pristine theoretical predictions of
how the air will behave as the sailplane passes through it, rendering
supposed performance gains null (and in some cases causing an actual
performance penalty - see the part of the Diana 2 paper where they
talk about lift minimums in certain airfoils at high Cl).

I agree that the performance leaps will probably never be as rapid as
the transition from wood/metal to glass period (which was essentially
due to "new fancy materials" allowing the structures to come much
closer to the aerodynamic optimums and thus allowing the new
aerodynamic advances of the time to enter the equation, leading to
more aerodynamic research...), and I am not saying otherwise. I AM
saying though, that L/D WILL continue to improve, especially at higher
speeds. The initial L/D's will be slowly creeping up while the polars
are getting flatter. As new materials come along, so do new design
possibilities that were once ruled out due to structural
considerations. Not to mention possible advances in aeroelastic skins
(morphing wing/fuse shapes) and as mentioned de-turbulation. I guess
it comes down to the simple fact that I just think we have more to
look forward to in the future than others may.

Paul Hanson

PS. In the late 50's, the world's leading aerodynamicists dismally
predicted L/D's around the 40 mark were thought to be "the plateau"
designers and ships would get stuck at. Luckily new designers came
along that hadn't heard the rules so were not bound by them...

On May 5, 10:26 am, sisu1a wrote:
On May 5, 9:23 am, "noel.wade" wrote:

Paul -

You're missing out on a key point: Changing materials to a lighter
structure does not change the aerodynamics of the wing.

I did not miss the original point, I just don't agree with it. Clearly
you miss mine though. If structure does not change the aerodynamics of
a wing, then why are modern hot ships still made of wood, or metal?
The fact of the matter is that the aerodynamic optimum will never
agree with the structural minimum, but the more that materials
advance, the closer to that optimum designers can get. It most
certainly does affect the areodynamics of a wing, so long as the wing
is designed specific to the materials it is made from.

It changes the wing-loading - but that's no different from adding or
removing water ballast. As long as the wing SHAPE remains the same, a
different weight/wing-loading simply shifts the polar. It doesn't
create an improvement.

Huh? Wing loading? Again, you are way off my point. Advances in
structure ALLOWS changes in shape, which most certainly can lead to
improvement. Any racing ship will have the ability to change it's wing
loading with ballast, but since we're on the subject of wing loading,
by using prepreg your dry weight can be significantly lower, thus
'possibly' allowing a wider range in wing loading and thus a ship
adaptable to a wider range of conditions.

Wing efficiency is affected by the shape of the airfoil, the wing
planform (and how the two add together to become a total 3-d package),
the smoothness of the skin/surface, the surface-area/skin drag, and
numerous other smaller factors.

Precisely. And by using prepregs a designer can go with a less
compromised foil since the material is so much stronger, not to
mention they have the ability to be smoother (and stay that way!),
much less affected by weather/heat (they are not room temp cured so
at normal runway temps are not even close to getting soft and you will
never be complaining about the dreaded 'spar hump' for that matter).
This all means nothing of course if the design blows...

Existing composite materials can be made to follow complex curves and
result in an extremely smooth surface, so changing to a fancier
material does nothing to improve the efficiency of the wing.

As can prepregs, although it is a good bit more work. However
'fancier' materials allow the designer to make a wing that looks a lot
more like the airfoil they really wanted to use, instead of the one it
had to be mutated into to allow for the safe amount of structure.

The reason that Pre-Preg and other fancy/costly exotic materials
aren't commonly in use is because they aren't NEEDED to achieve
aerodynamic optimums. Fancy materials are mostly used for
manufacturing reasons, to carry higher loads, to have more specific
stiffness in a particular load direction, etc. Aerodynamics doesn't
enter into the equation.

Absolutely incorrect. The reason they are not used is because their
significance is not widely recognized enough for customers to expect
any different. Most big manufacturers have their entire infrastructure
built around traditional wet layup. In fact some of them even were the
original pioneers of these now widely utilized, current industry
standard materials and techniques. They are not about to fire all
their workers, sell off all their equipment and revamp their entire
existence around this "new" material, and anything short of that would
yield no results. It is a completely different ballpark, and there is
only one manufacturer who is setup for it, although it is now becoming
commonplace to be used in flight control surfaces for obvious
aerodynamic reasons (flutter?).

Small, incremental advances in overall performance have been made over
the last 20 years; but they are pretty tiny in comparison to the
performance jumps that were seen from the 50's to the late 70's (when
glide ratios doubled or tripled). We're down to the point where we're
fighting physics every step of the way in order to see any
improvements. And in order for theoretical gains to be realized in
actual flight, the tolerances are getting so tight that normal
manufacturing techniques cannot be applied. And with tolerances that
tight you also end up fighting the fact that the air gliders move
through is not sitting still. At some point the turbulence and
natural air movement disrupts the pristine theoretical predictions of
how the air will behave as the sailplane passes through it, rendering
supposed performance gains null (and in some cases causing an actual
performance penalty - see the part of the Diana 2 paper where they
talk about lift minimums in certain airfoils at high Cl).

I agree that the performance leaps will probably never be as rapid as
the transition from wood/metal to glass period (which was essentially
due to "new fancy materials" allowing the structures to come much
closer to the aerodynamic optimums and thus allowing the new
aerodynamic advances of the time to enter the equation, leading to
more aerodynamic research...), and I am not saying otherwise. I AM
saying though, that L/D WILL continue to improve, especially at higher
speeds. The initial L/D's will be slowly creeping up while the polars
are getting flatter. As new materials come along, so do new design
possibilities that were once ruled out due to structural
considerations. Not to mention possible advances in aeroelastic skins
(morphing wing/fuse shapes) and as mentioned de-turbulation. I guess
it comes down to the simple fact that I just think we have more to
look forward to in the future than others may.

Paul Hanson

PS. In the late 50's, the world's leading aerodynamicists dismally
predicted L/D's around the 40 mark were thought to be "the plateau"
designers and ships would get stuck at. Luckily new designers came
along that hadn't heard the rules so were not bound by them...

Paul,

You have some nerve.....when Mr. Kubrynski is talking most of people
familiar with his work are listening....well, maybe you need to become
aerodynamicist and prove us all wrong....the field is wide open.

Jacek
Pasco, WA

sisu1a wrote:
On May 5, 9:23 am, "noel.wade" wrote:
Paul -

You're missing out on a key point: Changing materials to a lighter
structure does not change the aerodynamics of the wing.

I did not miss the original point, I just don't agree with it.

Point/counterpoint snipped...

Perhaps it may be useful to imagine using the material 'Unobtainium' to
design sailplanes having zero structural restraints/considerations (e.g.
infinitely thin airfoils, tailbooms, vertical stabs, whatever...)
influencing its aerodynamics...in other words, imagining 'the perfect
aerodynamics' designed to extract soarable energy from the atmosphere
while still enclosing a pilot.

Such a plane will probably be instantly recognizable to our eyes today
as a 'sailplane'...simply because the design will still have to deal
with physics...but my money is on it looking 'different' too. Maybe the
Bruce Carmichaels and Al Bowers of the world already have such ship in
their back pockets waiting for the material to arrive! (Remember that
'wonderfoil' designed by [I think it was] someone at Douglas or Boeing
with startlingly high 2D L/D values for its time [late '70's/early
'80's?]...but which couldn't be built for lack of Unobtainium?)

Regards,
Bob W.

Such a plane will probably be instantly recognizable to our eyes
today
as a 'sailplane'...simply because the design will still have to deal
with physics...but my money is on it looking 'different' too. Maybe the
Bruce Carmichaels and Al Bowers of the world already have such ship in
their back pockets waiting for the material to arrive! (Remember that
'wonderfoil' designed by [I think it was] someone at Douglas or Boeing
with startlingly high 2D L/D values for its time [late '70's/early
'80's?]...but which couldn't be built for lack of Unobtainium?)

Regards,
Bob W.

Thanks Bob, perhaps your clarifying and endorsing of my point will
hold more water than my untrained/unprofessional/non-aerodynamicist
opinion will (Bob Whelen IS an aerodymicist as well as a published
author for those who don't know). In quite typical fashion, your post
cuts right to a point that was attempted but with far less clutter.

Jacek, relax. I'm not knocking Polish gliders or aerodynamics FWIW, I
love Diana 2, and own an SZD-59 that I'm quite happy with. I'm just
saying I think there is more hope on the horizon than predicted, thats
all. We ALL have things to look forward to, it's a good thing.
Mebelieves the Dianna 2 represents the plateau for traditional wet
layup (or very very close to it at least) and is a wonderful machine.
My drool has had to be wiped off of the wings of the Diana 2 at the
convention on more than one occasion. Once we do have "Unobtanium"
though, there will be better ships yet! I'm sure the Pols' will be in
on it too! ; )

With Respect,
Paul Hanson

sisu1a wrote:

If structure does not change the aerodynamics of
a wing, then why are modern hot ships still made of wood, or metal?

When the shift from wood and metal to fiberglass occurred, a huge factor
was cost: it was much cheaper to build a glider to the tolerances
required in molded fiberglass than the other materials, and it retained
the shape better. At that time, you could build a lighter aluminum
glider of the same performance, but it was a constant effort to keep the
airfoil correct. I don't know if this is still true for carbon fiber
versus aluminum; regardless, I think the cost would still favor the
molded construction.

--
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

On May 5, 9:23*am, "noel.wade" wrote:

Existing composite materials can be made to follow complex curves and
result in an extremely smooth surface, so changing to a fancier
material does nothing to improve the efficiency of the wing.

Maybe someone will come up with an affordable combination of new
materials and new processes that will give us sailplanes that keep the
complex curves intended by the designer for more than a few months
after delivery.

Andy