How to simply determine the L/D of your glider

L/D numbers are the things dreams are made of. *Max L/D is much like
breast size, it gets a lot of attentions but the important thing is
what you do with it.

Ron-


now THAT'S funny!!!

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 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