final glide estimates

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.

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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----------

Bruce Hoult wrote:
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.

That seems to suggest the pilot can damage the glider, but turbulence
cannot. Perhaps that's not unreasonable, given the very low Vne,
suggesting the flutter issues were not handled as well as the airframe
strength.

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Eric Greenwell - Washington State, USA

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

I have been watching this thread for a while now.

The term "rough air" seems somewhat subjective. Review of the various
aerodynamic manuals in my personal library does not give me a clue to what
gust load is used in rough air limit calculations or its' relationship to
the V-n diagram.

I am sure there is a precise definition which is used in to determine
performance limits. Could anyone provide the gust load definition of "rough
air?"

Respectfully,

Wayne
http://www.soaridaho.com/Schreder

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

Andy Blackburn wrote:

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

Hi Andy,

Yes. Of course you are right. Thanks for running the check.

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

9B

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'