High altitude flutter - Vne

wrote:
On Jan 3, 9:00 am, Eric Greenwell wrote:
Darryl Ramm wrote:
On Jan 2, 10:52 pm, Allan wrote:
Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft -
just what you need for those high-speed wave flights!
But whats the L/D at 160 & 200 Knots? 10 & 5 ?
Allan
L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot
headwind and you've got +5 knots of up in wave you won't care...
I can't wait to see what magic Greg Cole pulls off with the Duck Hawk.
Darryl has it right - the limit for high speed wave flights is Vne, not
L/D. But, extrapolating from an ASW 27 B polar gives ~20:1 at 160 knots.
Attempting to extrapolate to 200 knots is pointless, so we'll have to
wait for Windward Performance to publish a curve.

Just to finish the thought - you only need 8 knots of lift to maintain
altitude at 20:1 and 160 kts.

I am skeptical about 20:1 at 160 knots. I think the best Dick Johnson
flight test was the ASH-26 at 9 pounds wing loading, and it was below
20:1 at 120 knots. A more realistic figure might be 10:1 at 160 knots,
which would require 16 knots of lift to maintain altitude.



I've never had a Vne issue in thermal
soaring, even at 18,000', though I have been occasionally concerned
about it in strong lift approaching cloudbase. Even a great
cloudstreet is unlikely to have sustained lift that strong so if you
are keeping decent clearance from cloudbase you can usually let your
altitude vary rather than running up the airspeed. I could easily
imagine it being more of an issue in wave, particularly for those
folks running wave under IFR above 18,000'.

Wouldn't the 200 kt Vne be from sea level up to some limited altitude?
If so, you probably don't need to figure the L/D at 200 kts IAS for
wave flying - you'll be flying no faster than Va in wave above the
upper teens to low twenties (depending on how high the 200 kts is good
for), so 8 kts of up will be the strongest lift in which you'll be
able to hold altitude (versus 6-7 kts for, say, an ASW-27).

I will need to get used to the idea of flying that fast in a glider
that weighs 300 lbs empty.

9B

On Jan 3, 4:34*pm, Greg Arnold wrote:
wrote:
On Jan 3, 9:00 am, Eric Greenwell wrote:
Darryl Ramm wrote:
On Jan 2, 10:52 pm, Allan wrote:
Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft -
just what you need for those high-speed wave flights!
But whats the L/D at 160 & 200 Knots? 10 & 5 ?
Allan
L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot
headwind and you've got +5 knots of up in wave you won't care...
I can't wait to see what magic Greg Cole pulls off with the Duck Hawk..
Darryl has it right - the limit for high speed wave flights is Vne, not
L/D. But, extrapolating from an ASW 27 B polar gives ~20:1 at 160 knots.
Attempting to extrapolate to 200 knots is pointless, so we'll have to
wait for Windward Performance to publish a curve.

Just to finish the thought - you only need 8 knots of lift to maintain
altitude at 20:1 and 160 kts.

I am skeptical about 20:1 at 160 knots. *I think the best Dick Johnson
flight test was the ASH-26 at 9 pounds wing loading, and it was below
20:1 at 120 knots. *A more realistic figure might be 10:1 at 160 knots,
which would require 16 knots of lift to maintain altitude.

Good catch. My curve fit to the factory polar of my ASW-27 shows 20:1
at 150 kts and max gross, but that seems wildly optimistic. Looking at
a couple of final glides in relatively smooth air show less than 20:1
at 120 kts.

So the summary point would be that it would have to be one heck of a
wave day or you'd have to have an IFR flight plan to fly higher to
really need much above 150 kts of Vne on a cross country flight - at
least from a performance perspective.

9B

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.

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.
--
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

Greg Arnold wrote:

L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot
headwind and you've got +5 knots of up in wave you won't care...
I can't wait to see what magic Greg Cole pulls off with the Duck Hawk.

Darryl has it right - the limit for high speed wave flights is Vne, not
L/D. But, extrapolating from an ASW 27 B polar gives ~20:1 at 160 knots.
Attempting to extrapolate to 200 knots is pointless, so we'll have to
wait for Windward Performance to publish a curve.

Just to finish the thought - you only need 8 knots of lift to maintain
altitude at 20:1 and 160 kts.

I am skeptical about 20:1 at 160 knots. I think the best Dick Johnson
flight test was the ASH-26 at 9 pounds wing loading, and it was below
20:1 at 120 knots. A more realistic figure might be 10:1 at 160 knots,
which would require 16 knots of lift to maintain altitude.

You can get the ASW 27 B curve from Schleicher's site and draw your own
line. It's guess, because the curve only goes to 210 kph, but the curve
I drew "looks right", using the 55.6 kg/m2 line as the starting point.
I'm sure it's not 10:1, but certainly between than 15:1 and 20:1. The
ASH 26 is not a good choice for comparison because it has the same
aspect ratio as the 27, and only goes to 45 kg/m2 (the 9 pounds you
mentioned).

--
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

wrote:


Wouldn't the 200 kt Vne be from sea level up to some limited altitude?
If so, you probably don't need to figure the L/D at 200 kts IAS for
wave flying - you'll be flying no faster than Va in wave above the
upper teens to low twenties (depending on how high the 200 kts is good
for), so 8 kts of up will be the strongest lift in which you'll be
able to hold altitude (versus 6-7 kts for, say, an ASW-27).

I will need to get used to the idea of flying that fast in a glider
that weighs 300 lbs empty.

I agree, but I saw the spar stub mockups when I was at Windward
Performance in June. It's comforting that they are massive. Part of the
reason for the light weight is the wing is only 80 square feet, compared
to 99 on the ASW 27. High temperature cured pre-preg allows
significantly lighter weight than wet layups done by hand, too.

--
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

Eric Greenwell wrote:
Greg Arnold wrote:

L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot
headwind and you've got +5 knots of up in wave you won't care...
I can't wait to see what magic Greg Cole pulls off with the Duck Hawk.

Darryl has it right - the limit for high speed wave flights is Vne, not
L/D. But, extrapolating from an ASW 27 B polar gives ~20:1 at 160
knots.
Attempting to extrapolate to 200 knots is pointless, so we'll have to
wait for Windward Performance to publish a curve.

Just to finish the thought - you only need 8 knots of lift to maintain
altitude at 20:1 and 160 kts.

I am skeptical about 20:1 at 160 knots. I think the best Dick Johnson
flight test was the ASH-26 at 9 pounds wing loading, and it was below
20:1 at 120 knots. A more realistic figure might be 10:1 at 160
knots, which would require 16 knots of lift to maintain altitude.

You can get the ASW 27 B curve from Schleicher's site and draw your own
line.


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




It's guess, because the curve only goes to 210 kph, but the curve
I drew "looks right", using the 55.6 kg/m2 line as the starting point.


Can you extrapolate that way? Just as unflapped ships suffer a
noticeable fall off in performance above 80 to 85 knots, flapped ships
must have a similar speed at which their wings aren't working very well
(120 knots?).


I'm sure it's not 10:1, but certainly between than 15:1 and 20:1.


When the Duckhawk comes out, we will put you 15 to 20 nm out over the
ocean at 6,000', and see if you want to fly at 160 knots towards the coast!


The
ASH 26 is not a good choice for comparison because it has the same
aspect ratio as the 27, and only goes to 45 kg/m2 (the 9 pounds you
mentioned).


It is the best data point we have, I believe.

On Jan 3, 7:36*pm, Eric Greenwell wrote:

You can get the ASW 27 B curve from Schleicher's site and draw your own
line. It's guess, because the curve only goes to 210 kph, but the curve
I drew "looks right", using the 55.6 kg/m2 line as the starting point.
I'm sure it's not 10:1, but certainly between than 15:1 and 20:1.

Based on a look at some actual flight logs I'd guess that at redline
(151 kts) the ASW-27 would get something like 16:1. A quadratic curve
fit to the factory polar shows 20:1. If you extrapolate out to the
aforementioned 160kts (above redline for the -27) the quadratic curve
off the factory polar yields 18:1, My guess at reality would be more
like 14:1 at that speed (and 11.5 lbs/sqft). We'll see, but I doubt
the Duckhawk will have the same performance as the -27 at equivalent
wing loadings and I also doubt it'll be able to get to much above 10
lbs/sq ft wing loading because it starts out 230 lbs lighter empty and
the wing is too small to carry enough water ballast to get above 10
lbs. Soooo, if I had to bet I'd guess an L/D below 15:1 rather than
above 15:1 at max gross and 160 kts.

Even 20:1 is enough to cruise in strong lift without gaining altitude,
so I think the main point has been established - that you probably
don't gain much by trying to optimize a design for wave XC in the 15
meter class.

9B

wrote:
On Jan 3, 7:36 pm, Eric Greenwell wrote:
You can get the ASW 27 B curve from Schleicher's site and draw your own
line. It's guess, because the curve only goes to 210 kph, but the curve
I drew "looks right", using the 55.6 kg/m2 line as the starting point.
I'm sure it's not 10:1, but certainly between than 15:1 and 20:1.

Based on a look at some actual flight logs I'd guess that at redline
(151 kts) the ASW-27 would get something like 16:1. A quadratic curve
fit to the factory polar shows 20:1. If you extrapolate out to the
aforementioned 160kts (above redline for the -27) the quadratic curve
off the factory polar yields 18:1, My guess at reality would be more
like 14:1 at that speed (and 11.5 lbs/sqft). We'll see, but I doubt
the Duckhawk will have the same performance as the -27 at equivalent
wing loadings and I also doubt it'll be able to get to much above 10
lbs/sq ft wing loading because it starts out 230 lbs lighter empty and
the wing is too small to carry enough water ballast to get above 10
lbs. Soooo, if I had to bet I'd guess an L/D below 15:1 rather than
above 15:1 at max gross and 160 kts.

Even 20:1 is enough to cruise in strong lift without gaining altitude,
so I think the main point has been established - that you probably
don't gain much by trying to optimize a design for wave XC in the 15
meter class.

9B


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

Greg Arnold wrote:

You can get the ASW 27 B curve from Schleicher's site and draw your
own line.

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.

http://www.alexander-schleicher.de/s...spekte/27E.pdf


It's guess, because the curve only goes to 210 kph, but the curve I
drew "looks right", using the 55.6 kg/m2 line as the starting point.


Can you extrapolate that way? Just as unflapped ships suffer a
noticeable fall off in performance above 80 to 85 knots, flapped ships
must have a similar speed at which their wings aren't working very well
(120 knots?).

That's certainly part of the problem trying to extrapolate. A bigger
problem is I put my dot on the graph paper in the wrong place when I
calculated from the 36.6 kg/m2 curve, so now the curve looks like a 14:1
glide at 160 knots IAS.

I'm sure it's not 10:1, but certainly between than 15:1 and 20:1.

14:1 is the new 15:1!

When the Duckhawk comes out, we will put you 15 to 20 nm out over the
ocean at 6,000', and see if you want to fly at 160 knots towards the coast!

Apparently, I would slow down a bit sooner than I first anticipated,
doing the last nm in ground (water?) effect

--
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

Eric Greenwell wrote:
Greg Arnold wrote:

You can get the ASW 27 B curve from Schleicher's site and draw your
own line.

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.



http://www.alexander-schleicher.de/s...spekte/27E.pdf


It's guess, because the curve only goes to 210 kph, but the curve I
drew "looks right", using the 55.6 kg/m2 line as the starting point.


Can you extrapolate that way? Just as unflapped ships suffer a
noticeable fall off in performance above 80 to 85 knots, flapped ships
must have a similar speed at which their wings aren't working very
well (120 knots?).

That's certainly part of the problem trying to extrapolate. A bigger
problem is I put my dot on the graph paper in the wrong place when I
calculated from the 36.6 kg/m2 curve, so now the curve looks like a 14:1
glide at 160 knots IAS.

I'm sure it's not 10:1, but certainly between than 15:1 and 20:1.

14:1 is the new 15:1!


And if their polar curve has some "salesmanship" in it, and allowing for
a possible drastic dropoff in performance at high speeds, even 14:1 is
too high.