at one single speed. In your example you are correct that the ASH25 achieves
60:1, but only at, say, 55 knots. At 100 knots its L/D might be, for
example, 40:1 and will continue to decrease as the speed increases.

If you consult the polar curve for any glider you can see how the glide
ratio changes with speed. Lay a ruler from the origin to any point on the
polar and its slope will represent the glide angle at that speed. Divide the
IAS by the sink rate for that point on the polar to get the numerical value
of glide ratio at that particular speed. It will be pretty obvious that as
you move the ruler's point of contact along the polar, there will be a
single position where it just makes a tangent to the polar, where the
ruler's slope is at its most shallow. This represents the best achievable
glide angle and you can read off the speed where this occurs. If the graph
contain several polars at different wing loadings, you can see how the glide
angle is affected as the wing loading changes. You will find that the actual
best glide ratio is more or less the same for each wing loading, but will be
achieved at higher speeds for the heavier glider.

David Starer



Perhaps it's good to know why this is important, too.
In training and gliding, often in smooth air, you'll fly
the min sink speed or the best L/D speed and think that is
great.

In soaring, there may be lift in places, and hellacious
sink in places (caused by the ridge lee, rotor, divergence,
a cruel God, etc...). When in sink, you'll want to
get out of it in a shorter amount of time.

To answer how well you can escape the sinking air, look at
the L/D at the higher speeds (let's say at double the
best L/D speed). A really bad L/D at high speeds means
you'll be in bad sink, push the nose over to accelerate, and
notice the airspeed indicator hardly increase, while the
vario shows you going down a lot more.

This is called "lack of penetration." Fancy gliders
have negative flaps to reduce the sink rate at high speeds,
and ballast to make the best L/D speed a higher speed.

I noticed this most when flying a 2-33 with a heavy
passenger, vs. flying a 1-26 solo. Boy, the 1-26
doesn't speed up much, but just goes down if I push the
nose over. Fiberglass ships, especially with
ballast and negative flaps, accelerate very quickly
with little forward stick.

The 1-26, the PW-2, the Quicksilver MXIII Sprint
ultralight, and the Piper J-3 Cub are
"floaters", meaning they require very little lift
to stay aloft. On the other hand they "penetrate"
poorly, meaning drag and sink rate build up quickly
at higher airspeeds.

The other place this is important is in winds.
I got blown downwind from an airfield once at about
2000 ft, and didn't realize there was a 20-25 knot wind.
I really had to push the nose over a lot to get back to
the field and almost didn't make it back. Adding 15
knots over best L/D speed really increased the sink
rate a lot.