How to simply determine the L/D of your glider

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.

So - Real world performance is affected a lot by wing loading, and
profile chatacteristics. Your best L/D is a nice easy number to compare
relative performance, but it is a measure in a flight regime that you
will very seldom occupy. (minimum weight, still air, cool dense air,
slow flight) In the real world it is high wing loading wherever
possible, as fast as prudent and turbulence (aka lift) is good.
Predictably the published L/D is sometimes a poor indicator of overall
performance. The DG600 is a classic example.

So - an example
My Std Cirrus has a best L/D of say 36. I have cleaned her up, sealed
everything and made her as good as she gets. However, one just about
never flies at 95km/h - so my achieved L/D in her varies between 22 and
32. What she does well is climb, particularly in rough air.
What she is bad at is dolphin flying - that wing is very rigid so no big
AoA changes please or you are out of the drag bucket and it all goes
downhill...
What she is absolutely awful at is contamination - particularly water on
the wings converts the glide performance to Ka8 standard.

The Kestrel with it's 19m wing is magnificent at 1:44 at 97kmh. Real
world final glides get me 40. But notethat the polar is quite steep at
higher speed. So in low to moderate speed flight she is very efficient,
and will happily run at very hight L/D numbers. But on a strong day you
have the problem that performance deteriorates fast over say 170km/h. On
a weak day the Kestrel will thrash a Ventus (which also has best L/D of
1:44), but if the average climb value gets above say 3m/s - the Ventus
disappears into the distance. Better climb and same "best" L/D count for
nothing when it comes down to a drag race. Here it is wing loading and
how flat the polar is.

An extreme Example
The Bergfalke II/55 has a best L/D of 27 at around 82 km/h - the Blanik
L13 has about the same 1:28 but at a more usable 90km/h. Now, while two
seat contests were won in the 70s with the Bergfalke 3- you don't REALLY
want to go XC in either of these ladies. But if you were enthusiastic
enough to attempt it - you would soon discover the vast difference in
achievable XC performance between the two.

On 2011/01/11 11:02 AM, Chris Wedgwood wrote:
Read one of Dick Johnsons flight tests where he describes how
difficult it is to accurately measure LDmax, then ask yourself
why he does not use your technique...

Chris
www.condorsoaring.com


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

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 13, 2:40*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.

You are right something has become lost in this discussion. The OP
stated "Like you all I have questioned what the best L/D of my 3 (now
1)
gliders...were in reality suspecting that real L/Ds would be lower
than the manufacturers published values". He goes on to discuss how
measurements of glide performance during cross country flights showed
that his measured L/D did not match the claimed best L/D for his
gliders.

Several people have pointed out that the achieved glide angle on a
typical cross country flight has little relationship to best L/D since
XC flights are typically not made at best L/D speed and cross country
flights are typically not made in a motionless air mass.

It should be obvious that the ratio of distance flown to altitude lost
on a complete XC flight, or on any segment of an XC flight, is not a
measure of glider performance but a measure of the combination of
glider performance, air mass characteristics, and pilot skill. It is
my opinion that using the term L/D to describe this ratio is
misleading. Also, comparing this number with a manufacturer's claimed
best L/D tells you absolutely nothing about the validity of the
claimed best L/D value unless the primary objective of the flight, or
flight segment, was to fly at best L/D speed.

The title of the thread is "How to simply determine the L/D of your
glider". Several experienced XC pilots have pointed out that using
reported L/D data from typical XC flights is not a valid way to
determine the L/D of a glider.

None of the above denies that useful performance data can be derived
from making constant airspeed speed runs in a still air mass. None of
the above denies that a flight computer or post flight analysis
software can be useful in processing the data. None of the above
denies that the value of the ratio (distance flown)/(altitude lost)
achieved in a properly conducted flight test or by computer modeling
is properly described as "L/D". None of the above denies that the
maximum value of that ratio is properly described as "Best L/D".

Andy