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method of losing altitude is to descend with partial
power. This is often termed “cruise” or “enroute”
descent. The airspeed and power setting recommended
by the airplane manufacturer for prolonged descent
should be used. The target descent rate should be 400 –
500 f.p.m. The airspeed may vary from cruise airspeed
to that used on the downwind leg of the landing pattern.
But the wide range of possible airspeeds should
not be interpreted to permit erratic pitch changes. The
desired airspeed, pitch attitude, and power combination
should be preselected and kept constant.
DESCENT AT MINIMUM SAFE AIRSPEED—A
minimum safe airspeed descent is a nose-high, power
assisted descent condition principally used for clearing
obstacles during a landing approach to a short runway.
The airspeed used for this descent condition is recommended
by the airplane manufacturer and normally is
no greater than 1.3 VSO. Some characteristics of the
minimum safe airspeed descent are a steeper than normal
descent angle, and the excessive power that may
be required to produce acceleration at low airspeed
should “mushing” and/or an excessive rate of descent
be allowed to develop.
GLIDES—A glide is a basic maneuver in which the
airplane loses altitude in a controlled descent with little
or no engine power; forward motion is maintained by
gravity pulling the airplane along an inclined path and
the descent rate is controlled by the pilot balancing the
forces of gravity and lift.
Although glides are directly related to the practice of
power-off accuracy landings, they have a specific
operational purpose in normal landing approaches, and
forced landings after engine failure. Therefore, it is
necessary that they be performed more subconsciously
than other maneuvers because most of the time during
their execution, the pilot will be giving full attention to
details other than the mechanics of performing the
maneuver. Since glides are usually performed relatively
close to the ground, accuracy of their execution
and the formation of proper technique and habits are of
special importance.
Because the application of controls is somewhat different
in glides than in power-on descents, gliding
maneuvers require the perfection of a technique
somewhat different from that required for ordinary
power-on maneuvers. This control difference is
caused primarily by two factors—the absence of the
usual propeller slipstream, and the difference in the
relative effectiveness of the various control surfaces
at slow speeds.
The glide ratio of an airplane is the distance the airplane
will, with power off, travel forward in relation to
the altitude it loses. For instance, if an airplane travels
10,000 feet forward while descending 1,000 feet, its
glide ratio is said to be 10 to 1.
The glide ratio is affected by all four fundamental
forces that act on an airplane (weight, lift, drag, and
thrust). If all factors affecting the airplane are constant,
the glide ratio will be constant. Although the effect of
wind will not be covered in this section, it is a very
prominent force acting on the gliding distance of the
airplane in relationship to its movement over the
ground. With a tailwind, the airplane will glide farther
because of the higher groundspeed. Conversely, with a
headwind the airplane will not glide as far because of
the slower groundspeed.
Variations in weight do not affect the glide angle provided
the pilot uses the correct airspeed. Since it is the
lift over drag (L/D) ratio that determines the distance the
airplane can glide, weight will not affect the distance.
The glide ratio is based only on the relationship of the
aerodynamic forces acting on the airplane. The only
effect weight has is to vary the time the airplane will
glide. The heavier the airplane the higher the airspeed
must be to obtain the same glide ratio. For example, if
two airplanes having the same L/D ratio, but different
weights, start a glide from the same altitude, the heavier
airplane gliding at a higher airspeed will arrive at the
same touchdown point in a shorter time. Both airplanes
will cover the same distance, only the lighter airplane
will take a longer time.
Under various flight conditions, the drag factor may
change through the operation of the landing gear
and/or flaps. When the landing gear or the flaps are
extended, drag increases and the airspeed will
decrease unless the pitch attitude is lowered. As the
pitch is lowered, the glidepath steepens and reduces
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AIRPLANE FLYING HANDBOOK 飞机飞行手册上(34)
