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