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