On Jan 14, 2:07*am, BruceGreeff wrote:
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

Exactly. L/D/speed are used to establish one point on the polar curve.
I don't think anyone knows how accurate flight computers are in
predicting performance but they are probably more helpful than looking
out the canopy and guessing.