L/D of Virgin Atlantic GlobalFlyer

In article ,
Marc Ramsey wrote:

Bruce Hoult wrote:
And yet they said yesterday that if he ran out of fuel he could glide
200 miles from 50,000 ft. Which is 24:1, assuming nautical miles.
Which makes one suspect that the 37:1 is with the engine idling, not
dead.

If someone was going to tow you out to sea in your Standard Cirrus,
would you determine how far you can glide back based on the
manufacturers advertised 37:1 glide ratio, or perhaps something a bit
more reasonable like, say, 24:1?

From 50,000 ft? If I was going for a world record and had a parachute,
people standing by to pick me up, and a wealthy backer? I would expect
that the probability of sink the whole way would be vanishingly low and
that working on 32:1 or 33:1 would be pretty safe but that there would
be a pretty good chance of using what atmospheric variation was
available to manage a good bit better than the glider's raw glide angle.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

Bruce Hoult wrote:
In article ,
Marc Ramsey wrote:

Bruce Hoult wrote:
And yet they said yesterday that if he ran out of fuel he could glide
200 miles from 50,000 ft. Which is 24:1, assuming nautical miles.
Which makes one suspect that the 37:1 is with the engine idling, not
dead.
If someone was going to tow you out to sea in your Standard Cirrus,
would you determine how far you can glide back based on the
manufacturers advertised 37:1 glide ratio, or perhaps something a bit
more reasonable like, say, 24:1?

From 50,000 ft? If I was going for a world record and had a parachute,
people standing by to pick me up, and a wealthy backer? I would expect
that the probability of sink the whole way would be vanishingly low and
that working on 32:1 or 33:1 would be pretty safe but that there would
be a pretty good chance of using what atmospheric variation was
available to manage a good bit better than the glider's raw glide angle.

It was pretty late in the day when he landed at Bournemouth and would
have been later still at Manston. Good glider or not, I wouldn't count
more than still air, with maybe a bit of added sink, at that time of day.

BTW I agree it would be interesting to see how it performed as a glider.
Let's see now:
- restricted vis - I wouldn't want to share airspace with it
- it might run a cloud street, but could it turn tight enough to
core a thermal?
- with a design cruise of 250kts, it may be too fast to use
anything but wave or ridge lift.
- Anybody fancy running the Appalachians in it? A two hour 750
should be on in theory.

--
martin@ | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot

In article ,
Martin Gregorie wrote:

BTW I agree it would be interesting to see how it performed as a glider.
Let's see now:
- restricted vis - I wouldn't want to share airspace with it
- it might run a cloud street, but could it turn tight enough to
core a thermal?
- with a design cruise of 250kts, it may be too fast to use
anything but wave or ridge lift.
- Anybody fancy running the Appalachians in it? A two hour 750
should be on in theory.

The 250 kts is TAS, at altitude. IAS VNE is 170 knots, similar to
modern gliders.

We also know that empty it has about the same wing loading as typical
gliders, and so presumably about the same stall speed and turning
circle. Note that fully fuelled, at which it has six times the wing
loading it has empty, best powered climb is at 100 knots dirty, 130
knots clean. The corresponding speeds with empty tanks will be 2.4 -
2.5 times lower, or 40 knots and 53 knots. So min sink empty probably
lies somewhere around this range.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

did he run bug wipers??

Bruce Hoult wrote:
In article ,
Martin Gregorie wrote:

BTW I agree it would be interesting to see how it performed as a glider.
Let's see now:
- restricted vis - I wouldn't want to share airspace with it
- it might run a cloud street, but could it turn tight enough to
core a thermal?
- with a design cruise of 250kts, it may be too fast to use
anything but wave or ridge lift.
- Anybody fancy running the Appalachians in it? A two hour 750
should be on in theory.

The 250 kts is TAS, at altitude. IAS VNE is 170 knots, similar to
modern gliders.

OK, not as fast as I thought, then, but could still be fun in wave or on
a ridge.

We also know that empty it has about the same wing loading as typical
gliders, and so presumably about the same stall speed and turning
circle.

I'd agree about the speed, but doesn't turning circle also depend in
control surface size and deflection range?

Note that fully fuelled, at which it has six times the wing
loading it has empty, best powered climb is at 100 knots dirty, 130
knots clean.

Not good to winch with full ballast, then.

--
martin@ | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot

Martin Gregorie wrote:
I'd agree about the speed, but doesn't turning circle also depend in
control surface size and deflection range?

No. More control will just enable you to get into and out of a turn faster,
but it has no influence on the turn radius itself.

-Gerhard
--
Gerhard Wesp
Zuerich, Switzerland
+41 (0)76 505 1149 (mobile)
+41 (0)44 668 1878 (office)

Gerhard Wesp wrote:
Martin Gregorie wrote:
I'd agree about the speed, but doesn't turning circle also depend in
control surface size and deflection range?

No. More control will just enable you to get into and out of a turn faster,
but it has no influence on the turn radius itself.

I'd agree for ailerons and rudder, but I think that in a steeply banked
turn elevator deflection can limit the turn radius. Of course that's
only the case if there's not enough deflection to cause a stall at the
turning bank angle.

I'd further suggest that if there's not enough aileron and rudder
deflection for fairly crisp manoevering it all becomes academic if the
plane can't turn sharply enough to get into the thermal in the first place.


--
martin@ | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot

Bruce Hoult wrote:
In article ,
Marc Ramsey wrote:


Bruce Hoult wrote:

And yet they said yesterday that if he ran out of fuel he could glide
200 miles from 50,000 ft. Which is 24:1, assuming nautical miles.
Which makes one suspect that the 37:1 is with the engine idling, not
dead.

If someone was going to tow you out to sea in your Standard Cirrus,
would you determine how far you can glide back based on the
manufacturers advertised 37:1 glide ratio, or perhaps something a bit
more reasonable like, say, 24:1?


From 50,000 ft? If I was going for a world record and had a parachute,
people standing by to pick me up, and a wealthy backer? I would expect
that the probability of sink the whole way would be vanishingly low and
that working on 32:1 or 33:1 would be pretty safe but that there would
be a pretty good chance of using what atmospheric variation was
available to manage a good bit better than the glider's raw glide angle.

Maybe they were factoring in a headwind.