High altitude flutter - Vne

Interesting subject needs new thread.

It strikes me that a glider designed to soar in weak thermals is well
off its design point when running fast in a wave XC or, for that
matter, in strong thermal conditions. Ballast helps, of course, but
the basic airframe aerodynamics are wrong for extreme conditions.
Flutter shouldn't be allowed to become the limiting condition.

There were hints that Klaus Ohlman's Nimbus 4DM was modified to
increase Va and Vne for the record flights. It was never made clear
just what those mods were. With today's records being set in
extremely strong conditions, an increase in Va and/or Vne seems like
it would be more important than any increase in the already high L/D
max.

bildan wrote:
Interesting subject needs new thread.

It strikes me that a glider designed to soar in weak thermals is well
off its design point when running fast in a wave XC or, for that
matter, in strong thermal conditions. Ballast helps, of course, but
the basic airframe aerodynamics are wrong for extreme conditions.
Flutter shouldn't be allowed to become the limiting condition.

There were hints that Klaus Ohlman's Nimbus 4DM was modified to
increase Va and Vne for the record flights. It was never made clear
just what those mods were. With today's records being set in
extremely strong conditions, an increase in Va and/or Vne seems like
it would be more important than any increase in the already high L/D
max.

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!

--
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

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

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

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.

--
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

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

That would be mighty strong wave to have you consistently going up at an
airspeed of 200 knots.

On Jan 3, 9:46*am, Greg Arnold 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

That would be mighty strong wave to have you consistently going up at an
airspeed of 200 knots.

I've listened on channel 9 while a westbound United flight crew was
surprised they were climbing in
wave near Denver with a 160 kt headwind on the nose. Net speed had to
be well over 200kt and wing
loading well over 10.75 lbs/sqft!

The big problem with high altitude and wave flight is the constriction
of your flight envelope. At Minden, I
discovered ambudant ballast in a G-102 during a wave flight pre-take-
off check. Ship had been flown
the day before by a petite pilot (I'm about 200 lbs with wave flight
gear). Calculated my flight envelope at
the ballasted weight after the flight - not much between stall and
flutter.

Made me appreciate the A12 and U2 pilots who often had +/- 3 kts or
less [Shadow Flights by Curtis Peebles, pg 96].

Also made me appreciate my insistance on going through my check-list
even tho the tow plane was ready!

On Fri, 2 Jan 2009 17:10:34 -0800 (PST), bildan
wrote:


There were hints that Klaus Ohlman's Nimbus 4DM was modified to
increase Va and Vne for the record flights. It was never made clear
just what those mods were.

It's got fully balanced control surfaces.

Andreas Maurer wrote:

There were hints that Klaus Ohlman's Nimbus 4DM was modified to
increase Va and Vne for the record flights. It was never made clear
just what those mods were.

It's got fully balanced control surfaces.

Which you usually don't want on gliders, because it makes the controls
heavier (see DG-1000 vs. Duo).

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'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