How to simply determine the L/D of your glider

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