How to simply determine the L/D of your glider

That certainly works, Matt. I've found it easier, though, just to go
to the set-up page on the SN-10 and select the polar values from
there. grin But you can also go to Paul Remde's site; he has polar
values for a large number of planes listed he http://www.cumulus-soaring.com/polars.htm

-John


On Jan 11, 10:56 pm, mattm wrote:
Well, I'll bite. Here's what I do for the planes I fly:

First, I've been fortunate enough to fly planes that have Johnson
reports
available. I know there are probably Akaflieg reports as well, but I
haven't
seen those. I carefully extract the data points from Dick's polar
charts and
correct them for my flying weight (unfortunately always considerably
higher
than Dick!). I input the adjusted values into my PDA (which just
wants
the sink rate at 3 airspeeds, rather than the numbers listed above).
Finally,
I set the Polar Potential via experiment. Typically I'll set it to
90% and then
see how well my final glides work out. If I have a bunch of altitude
left over
on a glide then I'm doing better, and I'll increase the potential. If
I tend to fall
below glideslope a lot then I'll decrease the potential. For the most
part I've
wound up with values around 90% or 92% (which probably means I need to
work harder at tuning up the planes I fly).

Essentially this is a refinement of the beginner approach to glide
slopes:
take the published value and divide by 2 as a safety factor. I divide
by something
closer to 1.1 and usually make it home just fine. The times I've had
to
break off have been because I was below glideslope to begin with.

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 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)
- "

Originally Posted by jan

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

Sounds like you have some very nice gliders, Dave! I'm pretty new to soaring, but isn't the manufacturers L/D really a "best case" kind of number, meaning when the moon is in the seventh house of aquarius and all the elements are aligned in ultimate perfection? I've heard the L/D can be dropped by all manner of things like water droplets on the wing, bug splatter, and the stern look of a angry red-tailed hawk. Still, it's good to know the real numbers. As it is, it looks like you can just estimate that reality is a percentage (all three of yours are between 82 and 84%) of dreamland L/D numbers from manufacturers. Also, and I'm just wondering here, since I don't really know, could it have something to do with the way you fly? Or how much weight you're carrying? CG location? Wouldn't there be many factors?

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

On Jan 13, 4:18*am, Sparkorama
wrote:
jan;759730 Wrote:
Hi
If you look at the polar of a glider it is usually quite flat for a
moderately wide band of air speed and the best L/D is also flat. The
best L/D is probably the single best measure of the performance of a
glider. So for XC flying it is desirable to have a reasonable estimate
of the best L/D. When Dick Johnson used to do his evaluations of
gliders he found after proper sealing with tape of any joins and
polishing of the wings and surfaces that mostly he was unable to
duplicate the manufacturers claim for best L/Ds. What I attempted to
show in my original posting was that it is fairly easy to obtain a
reasonably accurate value of L/D for your glider as you usually fly it
- not necessarily prep'd for best performance. I used to use a L/D of
35 for the DG800B and Stemme and a L/D of 25 for the SparrowHawk. I
have only landed out once in the SparrowHawk and rarely have had to
push the button for the iron thermal on the motor gliders. I hope this
is useful.
Dave


Sounds like you have some very nice gliders, Dave! I'm pretty new to
soaring, but isn't the manufacturers L/D really a "best case" kind of
number, meaning when the moon is in the seventh house of aquarius and
all the elements are aligned in ultimate perfection? I've heard the L/D
can be dropped by all manner of things like water droplets on the wing,
bug splatter, and the stern look of a angry red-tailed hawk. Still, it's
good to know the real numbers. As it is, it looks like you can just
estimate that reality is a percentage (all three of yours are between 82
and 84%) of dreamland L/D numbers from manufacturers. Also, and I'm just
wondering here, since I don't really know, could it have something to do
with the way you fly? Or how much weight you're carrying? CG location?
Wouldn't there be many factors?

--
Sparkorama

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