final glide estimates

Eric, regarding the rough air speed - I thought you
flew an ASH 26? If so the BGA datasheet (quoting the
'FAA Web') lists the rough air speed as 99 knots i.e
the same as the manoeuvring speed. For my 2005 Discus
2cT the speeds are both 103 knots. I think all gliders
have a rough air speed limit for conditions such as
'wave rotor, thunderclouds, visible whirlwinds or when
crossing mountain ridges' to quote my manual.

http://www.gliding.co.uk/bgainfo/tec...eets/ash26.pdf

Bagmaker - you seem analytically minded so have a look
at John Cochrane's excellent paper 'Just a Little Faster
Please'. The link won't post but if you Google +'john
cochrane' +soaring you will get his site.

John Galloway

At 06:36 11 March 2006, Eric Greenwell wrote:
bagmaker wrote:
Newbie help required!
Lets suppose I am on a shallow final glide, 60k out
in my 40:1 ship,
cruising at 60 knots. Recent thermals have been at
least 5 knots and I
am coming into some big lift. For the discussion rough
air Vne is 100
knots.

Best not to confuse 'rough air' speeds with Vne, which
is the 'never
exceed' speed. The 'rough air' limit (I'm not sure
this is used any more
- only my very old gliders had it) is quite a bit lower,
and the newer
gliders don't have it, but do state 'maneuvering speed',
lower yet.

And, practically speaking, you certainly don't want
to be going Vne when
there are thermals bigger than 5 knots in your way!
If the glider hangs
together, it will be a very rough ride.

What strength thermal should I take to increase finishing
speed to Vne

You need your gilder's polar to determine what strength
thermal is
needed for any McCready speed determination - nothing
else needed.

and how much (if any) time will this save me?

It will save you time, but without the polar, your
altitude above the
finish, and the distance to the finish, we can't give
you a number.

How do I estimate this at the time?

You don't have to estimate it. A chart of speed vs
McCready settings is
made up ahead of time. The less thinking you have to
do while flying,
the more time you can spend flying the glider.

What is the latest point on track
to take such a thermal?

I'm guessing, but I'd say about where you need to stay
in the thermal at
least one full turn to get high enough to use the 100
knot speed. Any
closer, and you'd waste time getting unneeded altitude.


--
Change 'netto' to 'net' to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download 'A Guide to Self-launching
Sailplane
Operation'

bagmaker wrote:

Lets suppose I am on a shallow final glide, 60k out in my 40:1 ship,
cruising at 60 knots. Recent thermals have been at least 5 knots and I
am coming into some big lift. For the discussion rough air Vne is 100
knots.
What strength thermal should I take to increase finishing speed to Vne
and how much (if any) time will this save me?

Assumptions:
1) 60 knots at curent altitude will get you there (just).
2) 60k is 60 nautical miiles(nm)
3) there is no wind - this keeps your example simpler
4) 40:1 is at 60 knots
5) glider sink rate at 100 knots is 6 ft/sec

So you know you will finish in 1 hour at 60 knots, current altitude.
At 100 knots, you will finish in 60nm(1 hr/100 nm) = .6 hr = 36
minutes.
But you need to know how long to climb so you can go 100 knots.
Your current altitude is 60nm/40 = 1.5 nm.
1.5nm(6000 ft/nm) = about 9,000 ft.
36min(60 sec/min) = 2,160 sec
2,160 sec(6 ft/sec) = about 13,000 ft of altitude needed.
This means you need to climb about 4,000 ft.
A 5 knot thermal will give you about 500 ft/min.
So it will take you 4,000ft(1min/500ft) = about 8 minutes to climb.
So total time is 8min + 36min = 44min, or 16 minutes faster than
1hour.

How do I estimate this at the time?

I wouldn't. As others have indicated, use of a flight computer is the
best way to go.

Regards,

-Doug

I should point out that the above is a bit of an oversimplification in
that the distance travelled through the air will be greater than the
ground track distance. So it would take a bit longer than 1 hour to
glide 60 *ground* nm at an *airspeed* of 60 knots. Also, it would take
proportionally longer than 36 minutes to cover the same ground from a
4,000 ft higher altitude. A more rigorous calculation would account
for this.

Regards,

-Doug

At 15:36 11 March 2006, Doug Hoffman wrote:
I should point out that the above is a bit of an oversimplificatio
n in
that the distance travelled through the air will be
greater than the
ground track distance. So it would take a bit longer
than 1 hour to
glide 60 *ground* nm at an *airspeed* of 60 knots.
Also, it would take
proportionally longer than 36 minutes to cover the
same ground from a
4,000 ft higher altitude. A more rigorous calculation
would account
for this.


0018 and .0022 seconds longer respectively - I think
we can safely call that false precision.

9B

At 15:36 11 March 2006, Doug Hoffman wrote:
I should point out that the above is a bit of an oversimplificatio
n in
that the distance travelled through the air will be
greater than the
ground track distance. So it would take a bit longer
than 1 hour to
glide 60 *ground* nm at an *airspeed* of 60 knots.
Also, it would take
proportionally longer than 36 minutes to cover the
same ground from a
4,000 ft higher altitude. A more rigorous calculation
would account
for this.


0018 and .0022 minutes longer respectively - I think
we can safely call that false precision.

9B

John Galloway wrote:
Eric, regarding the rough air speed - I thought you
flew an ASH 26? If so the BGA datasheet (quoting the
'FAA Web') lists the rough air speed as 99 knots i.e
the same as the manoeuvring speed. For my 2005 Discus
2cT the speeds are both 103 knots. I think all gliders
have a rough air speed limit for conditions such as
'wave rotor, thunderclouds, visible whirlwinds or when
crossing mountain ridges' to quote my manual.

You're right, John - I should've checked my manual. Does anyone know how
the Rough Air limit is determined? Is it always the same as the
maneuvering speed limit in gliders? It's been a long time, but I recall
the rough air limit being different (significantly higher) than the
maneuvering speed for my Std Cirrus.

--
Change "netto" to "net" to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download "A Guide to Self-launching Sailplane
Operation"

In article ,
Eric Greenwell wrote:

You're right, John - I should've checked my manual. Does anyone know how
the Rough Air limit is determined? Is it always the same as the
maneuvering speed limit in gliders?

No it's not. Our Janus, for example, has maneuvering at 92 knots, but
both rough air and Vne at 119 knots.

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

Yes 119 knots for both rough air and VNE for the Standard
Cirrus with a manouevring speed of only 81knots.

A quick scout through the BGA datasheets seems to show
a trend that for modern single seaters the rough air
and manouevring speeds are the same but for some older
singles and current deep spar two seaters the rough
air is higher. I don't know how the speeds are determined.

http://www.gliding.co.uk/bgainfo/tec...datasheets.htm




At 16:30 11 March 2006, Eric Greenwell wrote:
John Galloway wrote:
Eric, regarding the rough air speed - I thought you
flew an ASH 26? If so the BGA datasheet (quoting
the
'FAA Web') lists the rough air speed as 99 knots i.e
the same as the manoeuvring speed. For my 2005 Discus
2cT the speeds are both 103 knots. I think all gliders
have a rough air speed limit for conditions such as
'wave rotor, thunderclouds, visible whirlwinds or
when
crossing mountain ridges' to quote my manual.

You're right, John - I should've checked my manual.
Does anyone know how
the Rough Air limit is determined? Is it always the
same as the
maneuvering speed limit in gliders? It's been a long
time, but I recall
the rough air limit being different (significantly
higher) than the
maneuvering speed for my Std Cirrus.

--
Change 'netto' to 'net' to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download 'A Guide to Self-launching
Sailplane
Operation'

This is the basic method to set maneuvering speed:

The Maneuvering Speed is the minimum speed at which the wing can
produce lift equal to the design load limit. (in positive G's) Below
this speed the wing can not produce enough lift to overstress the
aircraft, no matter what angle of attack is used.

Rough air speed is generally set so that a certain limit of turbulence
will not over stress the aircraft.

Todd
3S

John Galloway wrote:

A quick scout through the BGA datasheets seems to show
a trend that for modern single seaters the rough air
and manouevring speeds are the same

Yes, but I don't believe it's really for technical reasons. I rather
suspect that the manufactorer defines it deliberately that way to keep
things simple for the pilot. (Quick test: Do you know whether the green
arc defines maneuvring speed or rough air speed?)

Stefan