How to simply determine the L/D of your glider

Hi Gang
Like you all I have questioned what the best L/D of my 3 (now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in reality
suspecting that real L/Ds would be lower than the manufacturers
published values. I have used a PDA for many years switching it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11 calculates
an average L/D which can be viewed with the stats for each flight.
Also one can straight edge any glide from a flight and calculate that
effective glide ratio. Especially on a non thermic day one can get a
feel for a gliders' performance. So what I have I found after crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43 (manufacturer claimed 51.5.
How the hell could DG claim a half percentage point in 51???? That
represents a 1% accuracy! What nonsense!) - the Stemme around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)
If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will find the measurements become
quite consistent which suggests that they might represent something
close to reality.
Dave

On Jan 10, 12:07*pm, jan wrote:
Hi Gang
* Like you all I have questioned what the best L/D of my 3 (now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in reality
suspecting that real L/Ds would be lower than the manufacturers
published values. I have used a PDA for many years switching it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11 *calculates
an average L/D which can be viewed with the stats for each flight.
Also one can straight edge any glide from a flight and calculate that
effective glide ratio. Especially on a non thermic day one can get a
feel for a gliders' performance. So what I have I found after crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43 (manufacturer claimed 51.5.
How the hell could DG claim a half percentage point in 51???? That
represents a 1% accuracy! What nonsense!) - the Stemme around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)
* If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will *find the measurements become
quite consistent which suggests that they might represent something
close to reality.
Dave

just out of curiosity I went through my OLC flights from last year.
Looking at the E value and averaging it between 9 flights resulted in
an average L/D of 20.25 for my Cherokee which I flight tested in
perfect smooth air conditions at 23.5. Sounds about right.

Like you all I have questioned what the best L/D of my 3 (now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in reality
suspecting that real L/Ds would be lower than the manufacturers
published values. I have used a PDA for many years switching it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11 calculates
an average L/D which can be viewed with the stats for each flight.
Also one can straight edge any glide from a flight and calculate that
effective glide ratio. Especially on a non thermic day one can get a
feel for a gliders' performance. So what I have I found after crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43

The above describes one way to develop what I think of as a 'working L/D' for
your glider, your soaring style and 'typical conditions.' I developed my own
w/o benefit of electronics/GPS/software, and it was something over 30:1 for my
never-detailed 15-meter glider, which model Dick Johnson measured at
37.something. Given the relative crudity of my measurements, the
routinely-in-motion atmosphere through which the measurements were taken, and
the fact I 'rarely' flew at max L/D, the disparity between numbers seems
entirely sensible to me. I'd hope (expect?) anyone desiring to fly XC does
something similar before heading out while boldly depending on their glide
computer to get them home sans risk of a landout. That noted, don't mistake a
lower number (and I *have* experienced 30+ mile glides in the PM exceeding
60:1 L/D) for manufacturers' 'brochuresmanship.' In the absence of controlled
test conditions, you are NOT evaluating your ship's max L/D.

(manufacturer claimed 51.5.
How the hell could DG claim a half percentage point in 51???? That
represents a 1% accuracy! What nonsense!)

Um...how the hell did whomever evaluated Robert Harris' former world altitude
record determine he soared to 49,009 feet? Nine feet?!? 9/49,000 = .018%
accuracy. Bogus? Not a bit; it simply 'fell out in the wash' when doing the
evaluative arithmetic. No need to impute sinister motives to anyone...

- the Stemme around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)

If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will find the measurements become
quite consistent which suggests that they might represent something
close to reality.

We're in agreement here, but don't mistake the numbers for anything other than
what your ship delivers, with your flying style, in 'typical conditions.' Then
be happy, don't worry!!! (Oh... and be prepared for the occasional landout,
too, I don't care WHAT your computer may have told you ' a while back!')

Regards,
Bob W.

On Jan 10, 11:07*am, jan wrote:
Hi Gang
* Like you all I have questioned what the best L/D of my 3 (now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in reality
suspecting that real L/Ds would be lower than the manufacturers
published values. I have used a PDA for many years switching it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11 *calculates
an average L/D which can be viewed with the stats for each flight.
Also one can straight edge any glide from a flight and calculate that
effective glide ratio. Especially on a non thermic day one can get a
feel for a gliders' performance. So what I have I found after crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43 (manufacturer claimed 51.5.
How the hell could DG claim a half percentage point in 51???? That
represents a 1% accuracy! What nonsense!) - the Stemme around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)
* If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will *find the measurements become
quite consistent which suggests that they might represent something
close to reality.
Dave

Do you really spend all your flight time at best L/D. You must fly in
really crappy conditions. I feel sorry for you.

Andy

On Jan 10, 12:13*pm, Andy wrote:

Do you really spend all your flight time at best L/D. You must fly in
really crappy conditions. *I feel sorry for you.

Andy

I was going to say something similar, but without the pathos.

The glide angle I most need to know is at around 80 knots dry / 90
knots wet. I rarely fly at best L/D as it is generally not useful for
X-C flying and certainly not for racing except under extreme
circumstances. Even when I am low and scratching I generally fly 10
kts above best L/D as the tradeoff between glide angle and forward
progress argues for biasing towards a higher speed in the flat part of
the polar.

When I am flying slow I don't count on achieving the quoted L/D. At
that flat a glide angle any air motion will blow the glide out of the
water. Think about a 50:1 glide over 25 miles - if you experience 500
fpm in sink for 60 seconds you will all of a sudden need an L/D of
62:1. Generally I won't fly a final glide at less that 3-4 knots
McCready. If I do that then I can set the computer to read out arrival
altitude and modulate my speed depending on whether the arrival
altitude is going up or down over time. My experience is that I need
to fly around 8-10 knots below the calculated speed to fly to arrive
at the intended altitude. With this technique it pretty much doesn't
matter how accurate the polar in my computer is.

While cruising on course my achieved L/Ds as calculated by SeeYou run
anywhere from the high 40s in to the 100s at cruising speeds of 85
knots and up. This is way above the polar so the polar's not really of
any use unless I'm trying to cross a big blue hole in which case I
revert to the technique in the above paragraph.

9B

On Jan 10, 10:07*am, jan wrote:
Hi Gang
* Like you all I have questioned what the best L/D of my 3 (now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in reality
suspecting that real L/Ds would be lower than the manufacturers
published values. I have used a PDA for many years switching it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11 *calculates
an average L/D which can be viewed with the stats for each flight.
Also one can straight edge any glide from a flight and calculate that
effective glide ratio. Especially on a non thermic day one can get a
feel for a gliders' performance. So what I have I found after crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43 (manufacturer claimed 51.5.
How the hell could DG claim a half percentage point in 51???? That
represents a 1% accuracy! What nonsense!) - the Stemme around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)
* If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will *find the measurements become
quite consistent which suggests that they might represent something
close to reality.
Dave

If you do the math you'll notice you have an 8 point or so discrepancy
in all 3 of your gliders performance results.
You think that's coincidental or maybe your testing technique?

At 18:07 10 January 2011, jan wrote:
Hi Gang
Like you all I have questioned what the best L/D of my 3
(now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in
reality
suspecting that real L/Ds would be lower than the
manufacturers
published values. I have used a PDA for many years switching
it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11
calculates
an average L/D which can be viewed with the stats for each
flight.
Also one can straight edge any glide from a flight and calculate
that
effective glide ratio. Especially on a non thermic day one can
get a
feel for a gliders' performance. So what I have I found after
crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43 (manufacturer
claimed 51.5.
How the hell could DG claim a half percentage point in 51????
That
represents a 1% accuracy! What nonsense!) - the Stemme
around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)
If you haven't done these seat of the pants measurements
with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights.
Interestingly
after a time and with practice you will find the measurements
become
quite consistent which suggests that they might represent
something
close to reality.
Dave


Simply put, you are comparing an average computed over all
the whole range of speeds you fly at in unstable rising/sinking
air.. with a single point of LDmax at a single speed in completely
stable air.

This doesn't work.

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

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