How to simply determine the L/D of your glider

Then ask yourself whether it is significant.

Best L/D is just one number that has dominated marketing for gliders.
Like most things marketing it is subject to a lot of creativity....

Actual performance, how well a wing uses energy from vertical gusts, how
it climbs, how sensitive it is to contamination, whether it gets
distorted over time. All these will affect how far and fast you fly -
Best L/D is a useful "summary" but it is a generalisation and subject to
a deplorable level of hype and exaggeration.

So - Real world performance is affected a lot by wing loading, and
profile chatacteristics. Your best L/D is a nice easy number to compare
relative performance, but it is a measure in a flight regime that you
will very seldom occupy. (minimum weight, still air, cool dense air,
slow flight) In the real world it is high wing loading wherever
possible, as fast as prudent and turbulence (aka lift) is good.
Predictably the published L/D is sometimes a poor indicator of overall
performance. The DG600 is a classic example.

So - an example
My Std Cirrus has a best L/D of say 36. I have cleaned her up, sealed
everything and made her as good as she gets. However, one just about
never flies at 95km/h - so my achieved L/D in her varies between 22 and
32. What she does well is climb, particularly in rough air.
What she is bad at is dolphin flying - that wing is very rigid so no big
AoA changes please or you are out of the drag bucket and it all goes
downhill...
What she is absolutely awful at is contamination - particularly water on
the wings converts the glide performance to Ka8 standard.

The Kestrel with it's 19m wing is magnificent at 1:44 at 97kmh. Real
world final glides get me 40. But notethat the polar is quite steep at
higher speed. So in low to moderate speed flight she is very efficient,
and will happily run at very hight L/D numbers. But on a strong day you
have the problem that performance deteriorates fast over say 170km/h. On
a weak day the Kestrel will thrash a Ventus (which also has best L/D of
1:44), but if the average climb value gets above say 3m/s - the Ventus
disappears into the distance. Better climb and same "best" L/D count for
nothing when it comes down to a drag race. Here it is wing loading and
how flat the polar is.

An extreme Example
The Bergfalke II/55 has a best L/D of 27 at around 82 km/h - the Blanik
L13 has about the same 1:28 but at a more usable 90km/h. Now, while two
seat contests were won in the 70s with the Bergfalke 3- you don't REALLY
want to go XC in either of these ladies. But if you were enthusiastic
enough to attempt it - you would soon discover the vast difference in
achievable XC performance between the two.

On 2011/01/11 11:02 AM, Chris Wedgwood wrote:
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


--
Bruce Greeff
T59D #1771 & Std Cirrus #57