High altitude flutter - Vne

But a 200 knot redline would sure make for an impressive flyby!

Older aerobatic savvy gliders had redlines of 350 km/h (SZD Kobuz,
Lo-100), 380 km/h (Mü-28) and even 450 km/h (DFS Habicht). Somehow
sometimes somebody decided that in 99.9% of the flights this was not
necessairy and that for 99.9% of the pilots a better L/D at lower speed
was more disirable because it fits better the requirements for average
weather conditions.

Greg Arnold wrote:

I am skeptical about the accuracy of that curve! Didn't they claim
best L/D of 48?

Maybe we don't have the same curve? The one I got from their download
page has a line labeled E=48.


The point I was trying to make, maybe not very well, is that their claim
of 48 probably is unreasonably optimistic (it was measured at 45.6 by
the Akaflieg). Thus, the rest of their polar curve probably also is too
optimistic.

A curve with 5% error is plenty good enough for our wild guessing about
the likely performance of a nominally similar glider, for which we don't
even have the calculated curve and is designed for speeds way beyond the
curve we are using

Greg Cole hasn't shared any secrets with me, but it's possible he's
chosen to use airfoils and flap settings that favor very high speed
flight more than Waibel chose to do when he designed the ASW 27. And, it
is a higher aspect ratio wing (30:1 versus 25:1). Also, he's not using
winglets, which are generally considered a drag at high speeds.

Someone mentioned the wing won't hold enough water to get to max wing
loading, which is true. I believe Greg is planning a fuselage tank to
make up the difference, like the early ASW 27s had to use.

--
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

I sure wish he'd change the name from Duck Hawk to something a little
more pleasent sounding,
It sounds a little too much like Duck Fart.

Gary Boggs

www.nwskysports.com

On Jan 4, 8:00*am, Eric Greenwell wrote:

Someone mentioned the wing won't hold enough water to get to max wing
loading, which is true. I believe Greg is planning a fuselage tank to
make up the difference, like the early ASW 27s had to use.

A fuselage tank would help, but...

The max gross weight is listed as 860 lbs, which gets you to only 10.7
lbs/sq ft - more than pound per sq ft lower than the -27. I estimate a
wing that size can carry about 25 gallons of water. If you add a 5
gallon tank in the fuselage and a 165 lb pilot, chute, instruments, O2
tank, etc. you only get to 9.7 lbs/sq ft. To get to max gross you'd
either need a 245 lb pilot (might be a tight squeeze in that cockpit)
or a 15 gallon fuselage tank. I'd be surprised if you could put that
much water in a fuselage that size - unless it goes in the pilot's lap
- ;-)

If the empty weight is more like 400 lbs than the listed 300 lbs you
could get to max gross more easily - but the minimum wing loading
would be 7.8 lbs which could be high (the three-view doesn't show
flaps - or ailerons). With flaps it would be like thw -27. But missing
the target empty weight by more than 30% might have other
consequences.

Alternatively, we can all go on a Krispy-Kreme diet.

Eric, you mentioned using the 55.6 kg/m^2 curve in one post and the
36.6 kg/m^ in another, which is a big difference. The right number for
trying to get to an expected loading for the Duckhawk is more like 50,
but as you correctly pointed out extrapolating has inherent
inaccuracies and the assumption that one glider will perform about
like another at the same wing loading includes a fair amount of
heroism too.

It's mostly nit-picking now as I think where we got to is any glider
that can get to ~150 kts ought to be fine for making the most out of
all but the most extreme lift conditions (sustained wave/streeting 10
kts) and/or altitudes above 18,000'.

9B

P.S. Irrrespective of any of the above - the use of pre-preg
construction is a great innovation for sailplanes and it's fantastic
to see a US manufacturer taking full advantage of it.

Guys - Using a quadratic to estimate high-speed polar is
really not realistic. The airfoils used on the gliders you
are discussing suffer massive lower-surface separation
over ~110knots depending on wing-loading.

You really need measurements (not extrapolations
of wishful polars ignoring low-CL separation)...

Hope that helps,
Best Regards, Dave

DRN wrote:
Guys - Using a quadratic to estimate high-speed polar is
really not realistic. The airfoils used on the gliders you
are discussing suffer massive lower-surface separation
over ~110knots depending on wing-loading.

You really need measurements (not extrapolations
of wishful polars ignoring low-CL separation)...

Hope that helps,
Best Regards, Dave


Ah ha! My point exactly.

On Jan 4, 10:49*am, DRN wrote:
Guys - Using a quadratic to estimate high-speed polar is
really not realistic. The airfoils used on the gliders you
are discussing suffer massive lower-surface separation
over ~110knots depending on wing-loading.

You really need measurements (not extrapolations
of wishful polars ignoring low-CL separation)...

Hope that helps,
Best Regards, Dave

Yup - I think we're all agreed on that. The best masurement I had was
some logs of fast final glides that showed 25% worse than the
quadratic polar extrapolation - it's rough at best, but some
significant haircut is in order.

The exercise was really focused on whether it was theoretically
advantageous to design a glider for super-strong conditions and high
altitudes. The initial discussion centered on Vne limits, but your
point raises the issue of whether an airfoil designed for cruise at
much higher speed would have an advantage. The general trend in
sailplane design has been to optimize around higher cruise speeds, but
I would observe that racing pilots have been slowing down in cruise
for a host of tactical and strategic reasons, so I guess I don't see
much advantage.

You post made me think of a separate question for you Dave - does the
SN-10 give a haircut to the factory polars above 110kts? I find that
I consistently need to fly 10+ knots slower than the dialed-in
McCready speed to make most final glides work out (sorry I don't have
an SN-10). Also, it would be neat if there were a way for computers to
"learn" the polar of a glider based on actual versus expected
performance - even if it were just for the final glide portion of the
flight. There may well be too many unknown variables to actually do
it. I wonder if there could be a "learn mode" where you take a couple
of high tows on a calm day and do some long runs at 3-4 different
speeds.

9B

On Jan 3, 7:24*pm, Eric Greenwell wrote:
wrote:

* Wouldn't the 200 kt Vne be from sea level up to some limited altitude?

I don't know how Greg sets the Vne, but my ASH 26 E uses the IAS for
Vne(IAS=146) from sea level to 10,000', and then uses TAS for
Vne(TAS=170) from point on. So, at 18,000', the Vne is down to ~120
knots IAS, but still ~170 knots TAS.


My -27 also has Vne "flat rated" up to 10,000' at 151 kts. I would
guess that means that you have a bunch of extra flutter margin at 151
knots at sea level - not that I'm suggesting anyone try it.

9B

On Jan 3, 7:24*pm, Eric Greenwell wrote:
wrote:
Just to finish the thought - you only need 8 knots of lift to maintain
altitude at 20:1 and 160 kts.

At 160 knots indicated at 18,000', the TAS would be 216 knots. You'd
need ~11 knots of lift (216/20). I don't know how common that is along
the Sierras. We don't get it in Washington State, but possibly along the
Oregon Cascades.

True - good catch - I believe Vne is likely to still be higher than Va
at that altitude - depending on how they certify it. I think we've
agreed that the L/D would likely be lower than 20, so you're talking
about 15 knots or more, which makes my point even more strongly - you
are unlikely to be able to make practical use of the extra Vne to make
XC speed under just about any expected soaring circumstance.

If they can build the higher Vne into the design for a minimal weight
penalty then, what the heck, they might as well. Otherwise, they still
have a fair amount of weight advantage to play with resulting from
their design and construction techniques. Of course weight translates
to wing loading or the need for wing area to achieve low speed
performance, so presumably they would want the strength for some other
reason.

9B

On Jan 4, 2:35*pm, wrote:
You post made me think of a separate question for you Dave - does the
SN-10 give a haircut to the factory polars above 110kts? *

No, polars are typically fit for the region of "normal" flight,
not for blown final glides or high-speed wave...
Don't know if any instruments try to model high-speed
polars really accurately.

I find that
I consistently need to fly 10+ knots slower than the dialed-in
McCready speed to make most final glides work out (sorry I don't have
an SN-10). Also, it would be neat if there were a way for computers to
"learn" the polar of a glider based on actual versus expected
performance - even if it were just for the final glide portion of the
flight. There may well be too many unknown variables to actually do
it. I wonder if there could be a "learn mode" where you take a couple
of high tows on a calm day and do some long runs at 3-4 different
speeds.

9B

A "learn mode" is not practical. The *only* way to get reasonably
accurate polar information is by parallel measurement using a
super-well-calibrated reference glider. To my knowledge, this
is *only* done periodically by the Idaflieg group.

See: Judah Milgram, "Flight Testing at the 1998 Idafleig Meet",
SOARING, April 1999 , page 34, available he
http://skylinesoaring.org/AUTHORS/

Hope that helps !
Best Regards, Dave