On this topic of determining L/D from interthermal cruising, Dick
Johnson did a fascinating and underappreciated study in the late
1970's of airmass behavior between thermals. Dick flew in mostly blue
conditions and simply recoded his height loss versus distance covered
between thermals. Dick's results are reported in SOARING, June 1979.

Dick found that, on the average, the airmass between thermals has an
average sink rate related to the upgoing thermal strength. The
relationship Dick found was; the airmass sink is approximately 10% of
the lift strength.

I have always felt Dick's study explains why it is so difficult to fly
cross country (in a blue conditions) with a ship having an L/D less
than about 30:1. The probability of running into the next thermal
purely by chance becomes too low as the L/D drops.

For this discussion, I think Dick's study shows the inadvisability of
trying to deduce flight performance from interthermal measurements.

Bob

On Mon, 27 Dec 2004 20:43:41 GMT, "Papa3"
wrote:

Mark,

My first post came across as a bit glib - apologize for that. But, I
actually put a little thought into that subject recently while writing some
batch analysis specs for GPS log files. The problem is that a good glider
pilot will not encounter random vertical motions - even in cruise flight.
He/she will stack the deck in his favor, seeking out cloud streets or
connecting the best looking individual Cu. Thus, you can't just average
out the L/D over time on specific segments (e.g. cruise flight). What you
can do is deterimine which pilot does the best job of achieving highest L/D
on a given day. Several of the popular flight analysis programs do this
already.

I'd certainly be interested in any detailed ideas you might have.

P3

"Mark Zivley" wrote in message
om...
Obviously ridge flight would not be conducive, nor wave, but put enough
data together from cruises during thermal flights and I bet something
could be put together.

Papa3 wrote:

Mark,

How do you propose to isolate the impact of vertical air motion? For
instance, I can fly the ridge at 100Kts and maintain altitude (same for
wave
or cloudstreets). I'm sure Rolladen-Schneider (ahem, DG) would love to
publish the L/D of my LS8 from the average of my flights for a season:
"LS8, with a measured L/D of 800:1..."

Cheers,
Erik


"Mark Zivley" wrote in message
m...

We all know what the manufacturer's polars look like, but what about our
individual planes. Has anyone done any work to develop a program that
would look at some flight logs and determine what a particular glider's
actual polar is? At one point Ball was making a vario system that would
determine the aircraft's polar over time just by flying.

For someone who already had some algorithms for computing wind from
ground track drift during thermals could take this info and then be able
to back figure from GPS ground speed what the IAS was during a
particular phase of the flight. By isolating longer sections of cruise
flight at varios airspeeds it should be do-able. Question is, has it
been done.

Mark

extra "hot" in the address to delay the spammers...