L/D @ SPEED

[Mark Jardini]

Sailplanes have of necessity a polar with min sink at speeds where
circles can be small enough to center thermals, and L/Dmax only a few
knots higher. Ballast is added to raise the L/D max perhaps 10Kts
higher.

What might a wing look like that was built for optimum L/D @
80-100Kts? What kind of L/D might be possible?

(will this winter never end?)

Mark J

[[email protected]]

On Jan 28, 1:59*pm, Mark Jardini > wrote:
> Sailplanes have of necessity a polar with min sink at speeds where
> circles can be small enough to center thermals, and L/Dmax only a few
> knots higher. Ballast is added to raise the L/D max perhaps 10Kts
> higher.
>
> What might a wing look like that was built for optimum L/D @
> 80-100Kts? What kind of L/D might be possible?
>
> (will this winter never end?)
>
> Mark J

The wing you are talking about woudn't be a whole lot different than
current flapped ships with the flaps
in the first reflexed position. This is the target sweet spot for
these ships now.
UH

[DRN]

On Jan 28, 1:59*pm, Mark Jardini > wrote:
> Sailplanes have of necessity a polar with min sink at speeds where
> circles can be small enough to center thermals, and L/Dmax only a few
> knots higher. Ballast is added to raise the L/D max perhaps 10Kts
> higher.
>
> What might a wing look like that was built for optimum L/D @
> 80-100Kts? What kind of L/D might be possible?
>
> (will this winter never end?)
>
> Mark J

Antares 20E max L/D is already up around 70 knots.
Still, there are a lot of compromises to make these
modern gliders thermal well...

See ya, Dave "YO electric"

[[email protected]]

On Jan 28, 5:58*pm, DRN > wrote:
> On Jan 28, 1:59*pm, Mark Jardini > wrote:
>
> > Sailplanes have of necessity a polar with min sink at speeds where
> > circles can be small enough to center thermals, and L/Dmax only a few
> > knots higher. Ballast is added to raise the L/D max perhaps 10Kts
> > higher.
>
> > What might a wing look like that was built for optimum L/D @
> > 80-100Kts? What kind of L/D might be possible?
>
> > (will this winter never end?)
>
> > Mark J
>
> Antares 20E max L/D is already up around 70 knots.
> Still, there are a lot of compromises to make these
> modern gliders thermal well...
>
> See ya, Dave "YO electric"

If you think just in terms of wing loading, and ASW-27 needs about
14lbs/sq ft to get best L/D up to 80 kts and 18 lbs/sq ft to get it to
90 knots. Ignore aspect ratio, Reynolds number effects, etc and take
it back to wing area and you are talking about 62-79 sq ft. instead of
the 98 sq ft of the -27 and an aspect ratio of 31-39 instead of the 25
on the current -27. Without doing the math, say that the aspect ratio
effect outweighs the Reynolds number effect by a little bit and you
get a 75-80 sq ft wing and an aspect ratio just above 30.

I'm guessing with current construction techniques you could build it,
but without some new airfoil innovation it might not climb very well.

9B

[[email protected]]

On Jan 28, 9:05*pm, wrote:
> On Jan 28, 5:58*pm, DRN > wrote:
>
>
>
> > On Jan 28, 1:59*pm, Mark Jardini > wrote:
>
> > > Sailplanes have of necessity a polar with min sink at speeds where
> > > circles can be small enough to center thermals, and L/Dmax only a few
> > > knots higher. Ballast is added to raise the L/D max perhaps 10Kts
> > > higher.
>
> > > What might a wing look like that was built for optimum L/D @
> > > 80-100Kts? What kind of L/D might be possible?
>
> > > (will this winter never end?)
>
> > > Mark J
>
> > Antares 20E max L/D is already up around 70 knots.
> > Still, there are a lot of compromises to make these
> > modern gliders thermal well...
>
> > See ya, Dave "YO electric"
>
> If you think just in terms of wing loading, and ASW-27 needs about
> 14lbs/sq ft to get best L/D up to 80 kts and 18 lbs/sq ft to get it to
> 90 knots. Ignore aspect ratio, Reynolds number effects, etc and take
> it back to wing area and you are talking about 62-79 sq ft. instead of
> the 98 sq ft of the -27 and an aspect ratio of 31-39 instead of the 25
> on the current -27. *Without doing the math, say that the aspect ratio
> effect outweighs the Reynolds number effect by a little bit and you
> get a 75-80 sq ft wing and an aspect ratio just above 30.
>
> I'm guessing with current construction techniques you could build it,
> but without some new airfoil innovation it might not climb very well.
>
> 9B

Oh, and I'd be surprised if you could get the max L/D much above the
mid 50s on a 15 M ship. Of course, you'd want to cruise it well above
the max L/D speed. Out west you might be able to imagine a glider
that could win races optimized around 14 lbs/sq ft, but I'd think even
western conditions might prove insufficient to support a glider with a
best L/D design point of 100 kts, since that implies cruise speeds
pretty considerably into triple digits and stall speeds and circling
radii that are pretty big.

9B

[Peter Higgs]

Hi All, I once read an article by a 'boffin' at Cranwell, who stated that
Vx was always 84% of Vy. In other words the two speeds are
mathematically linked... Something like the inverse of the 4th root.

I can understand that the induced drag increases by the inverse square, as
the speed decreases, so there must be one further square root to take into
effect in the equation.

So if you have a high Vy then your Vx will also be high, and the glider
will not turn in the thermals very tightly. Unless you can modify the
wing shape, with flaps, for the two different types of flight.

Pilot Pete

[flying_monkey]

I saw a glider with the Sinha (sp?) deturbulator tape this last
weekend. The owner was explaining the theory of how that works, and
it seems to revolve around the turbulent stream hitting exactly at the
location of this tape, which would explain why it works only at that
speed. I thought about this question just after talking to him: Why
not coat the whole aft 2/3 of the wing with the deturbulator material,
then you would cover all the possible reattachment points? Of course,
as a scientist, I wonder about the validity of any test where the
proponent says that "You have to take each test individually, not
aggregate them together." Sounds like anecdotal evidence to me.

But, assuming that this would really work, then you could have what
the owner described as a "induced drag only" wing, and it L/D would
depend only on fuselage drag. So, a wing like this on a really
streamlined fuselage (Diana?) might see high L/D speeds.

Just my $0.02,
Ed

[[email protected]]

On Jan 29, 10:27*am, flying_monkey > wrote:
> I saw a glider with the Sinha (sp?) deturbulator tape this last
> weekend. *The owner was explaining the theory of how that works, and
> it seems to revolve around the turbulent stream hitting exactly at the
> location of this tape, which would explain why it works only at that
> speed. *I thought about this question just after talking to him: Why
> not coat the whole aft 2/3 of the wing with the deturbulator material,
> then you would cover all the possible reattachment points? *Of course,
> as a scientist, I wonder about the validity of any test where the
> proponent says that "You have to take each test individually, not
> aggregate them together." *Sounds like anecdotal evidence to me.
>
> But, assuming that this would really work, then you could have what
> the owner described as a "induced drag only" wing, and it L/D would
> depend only on fuselage drag. *So, a wing like this on a really
> streamlined fuselage (Diana?) might see high L/D speeds.
>
> Just my $0.02,
> Ed

I share your skepticism - there may in fact be some favorable boundary
layer effects associated with this type of surface treatment - I
recall America's Cup boats used something that seemed similar about a
decade ago (yes, it's a different fluid...). Nevertheless, the idea of
totally eliminating form drag is overreaching by a lot methinks.

I remember Helmut Reichman won the 1978 Worlds in France, flying the
Akaflieg Braunschweig-built SB-11 which had slotless fowler flaps that
could increase the wing area by about 25-30%. The design opted for
improved low-speed performance as the 15M glider had an aspect ratio
of around 17 with the flaps fully deployed. Deploying the flaps took
some cranking I recall. That's all fine for Europe I guess, but this
discussion does raise for me the idea of a glider that uses the same
concept to give you climb comparable to existing gliders with a 10-15
knot higher cruise speed. It could be really interesting out west
where the thermals are strong enough that a few ft/min of lower
minimum sink would be a lot less advantageous than a higher inter-
thermal cruise speed. Now if you could just fit all that mechanism
into a 30:1 aspect-ratio wing with a 12% thickness ratio and still
have room for ballast (can you add Epsom salts to water ballast?).

9B

[[email protected]]

On Jan 29, 1:27*pm, flying_monkey > wrote:
> I saw a glider with the Sinha (sp?) deturbulator tape this last
> weekend. *The owner was explaining the theory of how that works, and
> it seems to revolve around the turbulent stream hitting exactly at the
> location of this tape, which would explain why it works only at that
> speed. *I thought about this question just after talking to him: Why
> not coat the whole aft 2/3 of the wing with the deturbulator material,
> then you would cover all the possible reattachment points? *Of course,
> as a scientist, I wonder about the validity of any test where the
> proponent says that "You have to take each test individually, not
> aggregate them together." *Sounds like anecdotal evidence to me.
>
> But, assuming that this would really work, then you could have what
> the owner described as a "induced drag only" wing, and it L/D would
> depend only on fuselage drag. *So, a wing like this on a really
> streamlined fuselage (Diana?) might see high L/D speeds.
>
> Just my $0.02,
> Ed

NO Winter is cold enough to have this topic come up again. Let it die
the death it deserves.
The only think that has no form drag is a thing that has no size.
UH