Bill,

Your question is not daft, it just sounds as though you've been listening to
"experts" who tend to abbreviate the terminology in a way that might be
confusing to outsiders.

L/D is indeed the lift/drag ratio. It's possible to show by drawing the
diagram of forces acting on the glider that this ratio is equal to the ratio
of airspeed to sink rate. This is true for any speed since the drag changes
as the airspeed changes.

When you hear people talking about a glider's L/D or glide ratio, what they
usually mean is the best or highest value of this ratio that can be achieved
by a particular glider. An L/D or glide ratio of 33 is shorthand for 33:1,
i.e. the glider will travel forwards 33 times as far as it descends in a
given time. It's important to appreciate that this will occur at one
specific speed, known as the best glide speed or Max Glide. What this speed
actually is depends on the wing loading for any particular type, but is
typically about 50-55 kts for most modern gliders and increases as the wing
loading increases. The maximum achievable glide ratio is basically an
inherent property of any particular type though you can achieve small
alterations by moving the centre of gravity and other subtle modifications.
The important thing to understand is that a particular L/D is only achieved
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

"Bill Gribble" wrote in
message .. .
Completely daft newbie question that I'm hoping will have a simple
answer.

Mention is frequently made of the term L/D, which I know is an
abbreviation of Lift / Drag. For example, the L/D cited in an advert for
a Ka6 recently was "L/d 33"

If L/D is the Lift/Drag ratio, why are the figures accompanying it
frequently not ratios? For instance, in the above, what is the "33"?

Is L/D used as the best Lift/Drag ratio the glider in question can
achieve? How does this translate in real terms? Does it describe the
best glide speed, or relate to the glide ratio in any way?

The glide ratio (eg. An ASH25 has a glide ratio of 60:1) seems a great
way of describing certain aspects of the performance of a glider. Yet
most the specifications I read describing gliders don't give this figure
in quite such a straight forward way. Is this information somehow
derived from other information given? Or is it too variable to generally
provide as a generic statistic (eg. One ASH25 might have a 60:1 ratio,
another less at 45:1)?

Am I managing to make any sense, or am I completely confusing myself (in
which case, no worries - more time, exposure and experience will
eventually rattle all this out for me, I'm sure)?

--
Bill Gribble

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