Flight Training - July 2013 - 39

-2

-2
Wind
-20

Weight

-1

-1
20

100

120

-20

Max L/D
(best glide)

Sink rate, knots

Tailwind
2

100

120

Airspeed, knots

Airspeed, knots
Headwind

Minimum sink

2
3
4

POLAR GRAPH
USED OFTEN in soaring, a polar graph shows the relationship between speed and descent rate. A tangential line drawn from the axis intersection

to the graphed line is the maximum lift/drag point, or best glide speed. Drawing a new line for a headwind or tailwind shows how the relationship
changes. A heavier airplane will move the graph down and to the right, showing that best glide speed increases with an increase in weight.

power, there probably isn’t a need to
establish best glide speed.
When the engine does suddenly fail, it
can make sense to immediately establish
a best glide speed. If the failure happens
close to the ground after takeoff, establishing the speed will probably involve
pitching down. Higher up, it will require
pitching up. But to what speed? Airplane
manuals usually specify best glide at
gross weight. If you’re lighter that day,
the best glide speed will be slower. How
much lower depends on the airplane
and the weight. Regardless, it’s safe to
establish the book speed as a base reference and tweak slightly from there. In
an emergency, we’re not trying to win a
competition. We’re just trying to get it on
the ground safely.
And safety really is the key. Although
we practice establishing best glide speed
regardless of the distance between the
airplane and the landing site, pitching to
reach it sometimes won’t be necessary.
Remembering that best glide is only the
speed that gets us the most distance; a field
4,000 feet below and only two miles away
doesn’t require a long glide. When you
consider that best glide speed and a safe
power-off final approach speed usually
are within 20 knots of each other, and the

airplane is in a low energy
state, establishing best glide
may actually be less safe.
The other situation in
which powered pilots may
use best glide speed is on
final approach while trying
to make the runway. Here,
the polar graph comes into
focus. Throughout training you may have heard
to increase your approach
speed by half a headwind
component and decrease it
by one-quarter of the tailwind component.
A quick look at the graph shows why. The
graph is also a perfect illustration of why
stretching the glide by increasing pitch
is, at a certain point, a very bad idea. As
Knauff says, “There’s a strong tendency
to hold back on the stick to make it stay
in the air. If you understand [the polar
graph], you realize it doesn’t make sense.”
It turns out that, according to Knauff,
the final glide to the airport is when glider
pilots will use best glide speed. The one
place where it would seem logical to use
it—between two thermals, or areas of
lift—turns out to be a waste. In his world,
as in the powered pilot’s, best glide is
pretty slow. If glider one flies between two

SOARING EXPERT
Tom Knauff is a soaring pioneer
who literally wrote the book on
the ins and outs of the sport. His
series of books provide a foundation of soaring knowledge, and
insight into his incredible success
in the world of competitive and
record glider flying. Learn more
online (www.eglider.com).

thermals at best glide, and glider two flies
it 10 knots faster, glider one will arrive at
the thermal higher than glider two. Over
the same elapsed time glider two will gain
more altitude because it has been spiraling
up in the new thermal for a longer period.
In other words, consider the objective
before robotically picking a speed. For
Knauff, it’s to stay in the air the longest,
go the farthest, or race a course the fastest. For a pilot who’s just experienced an
engine failure, maybe it’s to land in a close
field (a safe approach speed), or perhaps
it’s to fly farther and land in a better one
(best glide).
Ian J. Twombly is editor of Flight Training magazine.
JULY 2013 FLIGHT TRAINING

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http://www.eglider.com

Flight Training - July 2013

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