How to simply determine the L/D of your glider

On 1/11/2011 3:13 AM, BruceGreeff wrote:
Then ask yourself whether it is significant.

Best L/D is just one number that has dominated marketing for gliders.
Like most things marketing it is subject to a lot of creativity....

Actual performance, how well a wing uses energy from vertical gusts, how
it climbs, how sensitive it is to contamination, whether it gets
distorted over time. All these will affect how far and fast you fly -
Best L/D is a useful "summary" but it is a generalisation and subject to
a deplorable level of hype and exaggeration.

snip

you would soon discover the vast difference in
achievable XC performance between the two.

This posting gets my vote for "best overall view of the situation".

I routinely exceed Schleicher's 50:1 claim for my ASH 26 E by 10% to
40%, flying 15 to 20 knots higher than best L/D. That's "Mean L/D" from
SeeYou statistics. It's easy in good conditions with plenty of lift,
cloud streets, or ridge lift. If the Mean L/D drops under 50:1, it's
almost always been a bad day with lift hard to find. So, I really doubt
this L/D statistic has any value for determining a point on your
glider's polar.

It is instructive to compare your statistics for the day to another
pilot flying a comparable glider. I've been surprised at how different
they can be, particularly the number of thermals taken, how fast they
cruise on average, and the percentage of circling times.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

It is instructive to compare your statistics for the day to another
pilot flying a comparable glider. I've been surprised at how different
they can be, particularly the number of thermals taken, how fast they
cruise on average, and the percentage of circling times.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Mean L/D is actually a very poor statistic. 1/ Mean D/L is a lot
better. Seriously now, they are very different. As you go through
lift, L/D passes through infinity and then becomes negative. 1/ Mean D/
L is much better behaved. Now, which one do our computers really
present???

John Cochrane

On 1/11/2011 7:57 AM, John Cochrane wrote:

It is instructive to compare your statistics for the day to another
pilot flying a comparable glider. I've been surprised at how different
they can be, particularly the number of thermals taken, how fast they
cruise on average, and the percentage of circling times.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Mean L/D is actually a very poor statistic. 1/ Mean D/L is a lot
better. Seriously now, they are very different. As you go through
lift, L/D passes through infinity and then becomes negative. 1/ Mean D/
L is much better behaved. Now, which one do our computers really
present???

The numbers the OP gets from his PDA, and what SeeYou provides on the
Statistic page, are labeled "L/D", so that's probably why we were using
them. SeeYou also provides "Mean L/D" separately for rising air (it's
negative) and sinking air. So far, no Mean L/D beyond about 80, so
infinity is safe from me!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On Jan 11, 8:43*am, Eric Greenwell wrote:
On 1/11/2011 3:13 AM, BruceGreeff wrote:

Then ask yourself whether it is significant.

Best L/D is just one number that has dominated marketing for gliders.
Like most things marketing it is subject to a lot of creativity....

Actual performance, how well a wing uses energy from vertical gusts, how
it climbs, how sensitive it is to contamination, whether it gets
distorted over time. All these will affect how far and fast you fly -
Best L/D is a useful "summary" but it is a generalisation and subject to
a deplorable level of hype and exaggeration.

snip

* you would soon discover the vast difference in

achievable XC performance between the two.

This posting gets my vote for "best overall view of the situation".

I routinely exceed Schleicher's 50:1 claim for my ASH 26 E by 10% to
40%, flying 15 to 20 knots higher than best L/D. That's "Mean L/D" from
SeeYou statistics. It's easy in good conditions with plenty of lift,
cloud streets, or ridge lift. If the Mean L/D drops under 50:1, it's
almost always been a bad day with lift hard to find. So, I really doubt
this L/D statistic has any value for determining a point on your
glider's polar.

It is instructive to compare your statistics for the day to another
pilot flying a comparable glider. I've been surprised at how different
they can be, particularly the number of thermals taken, how fast they
cruise on average, and the percentage of circling times.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

My mean L/D is always much better than that. I routinely make flights
of 150-250 miles with a net loss of altitude of 2000 ft or less.
That's an achieved L/D (if we want to persist in using that term out
of context) of about 450/1. Pretty impressive for a standard class
glider. The manufacturer only claims 44/1.

Why do we want to continue using the terms L/D, and best L/D, out of
context?

Andy

On 1/11/2011 8:06 AM, Andy wrote:


My mean L/D is always much better than that. I routinely make flights
of 150-250 miles with a net loss of altitude of 2000 ft or less.
That's an achieved L/D (if we want to persist in using that term out
of context) of about 450/1. Pretty impressive for a standard class
glider. The manufacturer only claims 44/1.

An extreme example of why just flying around doesn't yield very good
polar measurements.

Why do we want to continue using the terms L/D, and best L/D, out of
context?

I do it out of habit, history, and continued use of the term by the
manufacturers and other pilots. I don't know why the manufacturers
continue to use it, but I'm going to guess: habit, history, and
continued use of the term by their customers.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "

On Jan 12, 9:30*pm, Eric Greenwell wrote:
On 1/11/2011 8:06 AM, Andy wrote:



My mean L/D is always much better than that. *I routinely make flights
of 150-250 miles with a net loss of altitude of *2000 ft or less.
That's an achieved L/D (if we want to persist in using that term out
of context) of about 450/1. *Pretty impressive for a standard class
glider. *The manufacturer only claims 44/1.

An extreme example of why just flying around doesn't yield very good
polar measurements.



Why do we want to continue using the terms L/D, *and best L/D, out of
context?

I do it out of habit, history, and continued use of the term by the
manufacturers and other pilots. I don't know why the manufacturers
continue to use it, but I'm going to guess: habit, history, and
continued use of the term by their customers.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "

I don't think the misuse has anything to do with manufacturers, if you
mean glider manufacturers. They use the term best L/D in its proper
context. The fidelity of the number is a separate discussion.

I think a lot of the blame for the misuse of "L/D" lies with SeeYou
flight analysis software. There are far too many people that believe
anything that appears on their computer screen.

In any case the knowledge of the glide angle achieved in any segment
of a flight is meaningless without taking into account the wind
velocity and average ground speed for the flight segment. If those
and the glider polar are known then something can be deduced about the
behavior of the air mass for that flight segment. Similarly if they
are known, and the air mass movement is known, then something can be
deduced about the glider polar.

So how about it SeeYou people. Can you please stop using the term "L./
D" out of context?

Andy

I think something may have gotten lost in the translation. The
discussion I thought was the L/D value used as part of the required
flight computer data required to establish the correct polar for the
glider. The fact that it will change based on a number of variables
doesn't mean it is a meaningless value. Ideally the other variables
are also taken into consideration by the flight computer either by
manual input or sensors.

None of the PDA / flight computers use best L/D directly (OK I know that
is not the right term but it's convenient)

In general - to be usefully able to predict performance they all try to
match actual performance against a polar curve (L/D graph) - which they
calculate by taking at least three points on the polar and doing a fit
to these points. Clearly the maximum value is significant so they want
the speed and quantum of minimum sink at measured minimum sink + a
higher number (preferably in the cruise speed range) + a sink rate at
minimum speed or close to it. Then the resulting graph sort of relates
to the actual performance - it gets complicated and bumpy for ships with
flaps, and some airfoils have kinks and bulges in their graph.

So in all cases the polar curve gets estimated - it is a model - all
models are false, some models are useful. In this case the polar model
is a useful approximation of glider performance under standard
atmospheric conditions, at a specific wing loading and speed.

How well that matches to your aircraft, your conditions and your flying
style varies. But at least the flight computer can give you a place to
start.

On 2011/01/13 11:40 PM, Gary Evans wrote:
I think something may have gotten lost in the translation. The
discussion I thought was the L/D value used as part of the required
flight computer data required to establish the correct polar for the
glider. The fact that it will change based on a number of variables
doesn't mean it is a meaningless value. Ideally the other variables
are also taken into consideration by the flight computer either by
manual input or sensors.

--
Bruce Greeff
T59D #1771 & Std Cirrus #57

On Jan 14, 2:07*am, BruceGreeff wrote:
None of the PDA / flight computers use best L/D directly (OK I know that
is not the right term but it's convenient)

In general - to be usefully able to predict performance they all try to
match actual performance against a polar curve (L/D graph) - which they
calculate by taking at least three points on the polar and doing a fit
to these points. Clearly the maximum value is significant so they want
the speed and quantum of minimum sink at measured minimum sink + a
higher number (preferably in the cruise speed range) + a sink rate at
minimum speed or close to it. Then the resulting graph sort of relates
to the actual performance - it gets complicated and bumpy for ships with
flaps, and some airfoils have kinks and bulges in their graph.

So in all cases the polar curve gets estimated - it is a model - all
models are false, some models are useful. In this case the polar model
is a useful approximation of glider performance under standard
atmospheric conditions, at a specific wing loading and speed.

How well that matches to your aircraft, your conditions and your flying
style varies. But at least the flight computer can give you a place to
start.

On 2011/01/13 11:40 PM, Gary Evans wrote:

I think something may have gotten lost in the translation. The
discussion I thought was the L/D value used as part of the required
flight computer data required to establish the correct polar for the
glider. The fact that it will change based on a number of variables
doesn't mean it is a meaningless value. Ideally the other variables
are also taken into consideration by the flight computer either by
manual input or sensors.

--
Bruce Greeff
T59D #1771 & Std Cirrus #57

Exactly. L/D/speed are used to establish one point on the polar curve.
I don't think anyone knows how accurate flight computers are in
predicting performance but they are probably more helpful than looking
out the canopy and guessing.

Hi,

Actually, SeeYou Mobile does measure and display your "current L/D" (over
the ground) based on the measured sink rate and distance over the ground
over - averaged over some period of time (20 seconds?). On a low wind day
it could (perhaps) be used to determine the L/D at specific airspeeds. You
could fly approximately upwind at a given speed for a minute, read the L/D,
then do a 180 degree turn and do it again for another minute. Write down
both numbers and average them. Then do it at a different airspeed. Of
course, your altitude could throw off the numbers a lot. The L/D over the
ground would always been correct, but the set airspeed would not be the same
as your speed over the ground if you are at 10,000 feet (for example).

The "Current L/D" window in SeeYou Mobile is very powerful because you can
compare it with the required L/D to see how you are doing on the way to a
goal. I remember being impressed with it one day while flying in the
Minnesota Soaring Club's SZD Junior. The Junior is a wonderful glider, but
it has thick wings and doesn't penetrate wind very well. I was fighting a
20+ knot headwind to get to my destination and could see the destination
clearly. I noticed that my required L/D was onlly 20. The Junior can
theoretically perform at a 35:1 glide ratio, but with the strong headwind my
measured "Current L/D" was 12. I liked that the number was an actual
measured performance number, not an estimate based on a previously measured
wind and the entered approximate polar data. I knew for a fact that I
needed another thermal - and I found one. Cool feature!

Best Regards,

--
Paul Remde
Cumulus Soaring, Inc.


"BruceGreeff" wrote in message
...
None of the PDA / flight computers use best L/D directly (OK I know that
is not the right term but it's convenient)

In general - to be usefully able to predict performance they all try to
match actual performance against a polar curve (L/D graph) - which they
calculate by taking at least three points on the polar and doing a fit to
these points. Clearly the maximum value is significant so they want the
speed and quantum of minimum sink at measured minimum sink + a higher
number (preferably in the cruise speed range) + a sink rate at minimum
speed or close to it. Then the resulting graph sort of relates to the
actual performance - it gets complicated and bumpy for ships with flaps,
and some airfoils have kinks and bulges in their graph.

So in all cases the polar curve gets estimated - it is a model - all
models are false, some models are useful. In this case the polar model is
a useful approximation of glider performance under standard atmospheric
conditions, at a specific wing loading and speed.

How well that matches to your aircraft, your conditions and your flying
style varies. But at least the flight computer can give you a place to
start.

On 2011/01/13 11:40 PM, Gary Evans wrote:
I think something may have gotten lost in the translation. The
discussion I thought was the L/D value used as part of the required
flight computer data required to establish the correct polar for the
glider. The fact that it will change based on a number of variables
doesn't mean it is a meaningless value. Ideally the other variables
are also taken into consideration by the flight computer either by
manual input or sensors.

--
Bruce Greeff
T59D #1771 & Std Cirrus #57