Accurate polar data is very difficult to measure. Although we have
made many attempts over the years to measure actual glider performance, we have found that
even with the best instruments and the most careful measurement methods, the level of scatter
in the measured data is very high, and in fact eclipses the differences in performance between
one model of glider and another, or even between one class of glider and another.
It has been our policy for 20 years not to publish claims of specific
L/D performance, because of the problems cited above with measuring the data accurately and
because the use of published performance claims by pilots to compare one glider to another
inevitably leads to performance "inflation."
However, due to the large number of pilots now using advanced instrumentation
incorporating speed to fly calculations, we are receiving more and more requests for polar
data. As a result, we have generated estimated polars for each of our gliders. The data is
based on a large number of measurements of both relative and absolute performance of a large
number of different gliders over many years. This data is not a claim of performance, and
is not intended to be used to compare one glider's performance against another's. (While we
feel that this data gives an accurate relative comparison among the models named, it would
be useless to compare one of these models with a model not on this list, using data for that
model obtained from another source.) We have tried to state realistic data so that the data
will be useful for speed to fly calculations. The data represents what we expect for a pilot
hooking in at 130% of the minimum recommended pilot weight (as listed in the owner's manual
or operating limitations placard). Theoretically, to a first approximation, L/D will not change
with changing pilot weight, and the true airspeed and descent rate for a given L/D / airspeed
data point shown, at any pilot weight other than 130% minimum recommended, will be equal to
the square root of the ratio of total weight (pilot plus glider) times the speed and descent
shown. (In other words, if the placarded minimum pilot weight is 140 lbs, the polar data is
for an assumed pilot weight of 140 * 1.3 = 182 lbs. If the glider weight is 65 lbs, the total
weight for which the polar is assumed is 247 lbs. If you are using a pilot weight of 210 lbs,
then the total weight is 275 lbs. At each L/D / airspeed data point indicated, to compute
the new speed and descent for that new pilot weight, multiply the speed and descent rate shown
by the square root of the ratio of 275 divided by 247, or 1.055.)
For a very clear explanation and discussion of glider polar theory,
the webpage "Glider Performance for
Dummies" by Jim D. Burch is hard to beat. The examples are all based on sailplanes,
but the theory applies to hang gliders (and paragliders, for that matter). | 
