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the distance traveled. With the power off, a windmilling
propeller also creates considerable drag,
thereby retarding the airplane’s forward movement.
Although the propeller thrust of the airplane is normally
dependent on the power output of the engine,
the throttle is in the closed position during a glide so
the thrust is constant. Since power is not used during a
glide or power-off approach, the pitch attitude must be
adjusted as necessary to maintain a constant airspeed.
The best speed for the glide is one at which the airplane
will travel the greatest forward distance for a
given loss of altitude in still air. This best glide speed
corresponds to an angle of attack resulting in the least
drag on the airplane and giving the best lift-to-drag
ratio (L/DMAX). [Figure 3-17]
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3-17
Any change in the gliding airspeed will result in a proportionate
change in glide ratio. Any speed, other than
the best glide speed, results in more drag. Therefore, as
the glide airspeed is reduced or increased from the
optimum or best glide speed, the glide ratio is also
changed. When descending at a speed below the best
glide speed, induced drag increases. When descending
at a speed above best glide speed, parasite drag
increases. In either case, the rate of descent will
increase. [Figure 3-18]
This leads to a cardinal rule of airplane flying that a
student pilot must understand and appreciate: The pilot
must never attempt to “stretch” a glide by applying
back-elevator pressure and reducing the airspeed
below the airplane’s recommended best glide speed.
Attempts to stretch a glide will invariably result in an
increase in the rate and angle of descent and may precipitate
an inadvertent stall.
To enter a glide, the pilot should close the throttle and
advance the propeller (if so equipped) to low pitch
(high r.p.m.). A constant altitude should be held with
back pressure on the elevator control until the airspeed
decreases to the recommended glide speed. Due to a
decrease in downwash over the horizontal stabilizer as
power is reduced, the airplane’s nose will tend to
immediately begin to lower of its own accord to an attitude
lower than that at which it would stabilize. The
pilot must be prepared for this. To keep pitch attitude
constant after a power change, the pilot must counteract
the immediate trim change. If the pitch attitude
is allowed to decrease during glide entry, excess
speed will be carried into the glide and retard the
attainment of the correct glide angle and airspeed.
Speed should be allowed to dissipate before the pitch
attitude is decreased. This point is particularly
important in so-called clean airplanes as they are
very slow to lose their speed and any slight deviation
of the nose downwards results in an immediate
increase in airspeed. Once the airspeed has dissipated
to normal or best glide speed, the pitch attitude
should be allowed to decrease to maintain that speed.
This should be done with reference to the horizon.
When the speed has stabilized, the airplane should
be retrimmed for “hands off” flight.
When the approximate gliding pitch attitude is
established, the airspeed indicator should be
checked. If the airspeed is higher than the recommended
speed, the pitch attitude is too low, and if
the airspeed is less than recommended, the pitch
attitude is too high; therefore, the pitch attitude
should be readjusted accordingly referencing the
horizon. After the adjustment has been made, the
airplane should be retrimmed so that it will maintain
this attitude without the need to hold pressure on the
elevator control. The principles of attitude flying
require that the proper flight attitude be established
using outside visual references first, then using the
flight instruments as a secondary check. It is a good
practice to always retrim the airplane after each
pitch adjustment.
A stabilized power-off descent at the best glide speed
is often referred to as a normal glide. The flight
instructor should demonstrate a normal glide, and
direct the student pilot to memorize the airplane’s
angle and speed by visually checking the airplane’s
attitude with reference to the horizon, and noting the
pitch of the sound made by the air passing over the
structure, the pressure on the controls, and the feel of
Increasing Lift-to-Drag Ratio
Increasing Angle of Attack
L/Dmax
Figure 3-17. L/DMAX.
Best Glide Speed
Too Fast
Too Slow
Figure 3-18. Best glide speed provides the greatest forward distance for a given loss of altitude.
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AIRPLANE FLYING HANDBOOK 飞机飞行手册上(35)
