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Flying
Handbook Menu > Basic
Flight Maneuvers > Descent at Minimum Safe Airspeed > 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 theusual 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 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). [figure3-17]
figure3-17. L/DMAX.
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.
[figure3-18]
figure3-18. Best glide speed provides
the greatest forward distance for a given loss of altitude.
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 the airplane. Due to lack of experience, the
beginning student may be unable to recognize slight variations
of speed and angle of bank immediately by vision or by the pressure
required on the controls. Hearing will probably be the indicator
that will be the most easily used at first. The instructor should,
therefore, be certain that the student understands that an increase
in the pitch of sound denotes increasing speed, while a decrease
in pitch denotes less speed. When such an indication is received,
the student should consciously apply the other two means of
perception so as to establish the proper relationship. The student
pilot must use all three elements consciously until they become
habits, and must be alert when attention is diverted from the
attitude of the airplane and be responsive to any warning given
by a variation in the feel of the airplane or controls, or by
a change in the pitch of the sound.
After a good comprehension of the normal glide
is attained, the student pilot should be instructed in the differences
in the results of normal and “abnormal” glides.
Abnormal glides being those conducted at speeds other than the
normal best glide speed. Pilots who do not acquire an understanding
and appreciation of these differences will experience difficulties
with accuracy landings, which are comparatively simple if the
fundamentals of the glide are thoroughly understood.
Too fast a glide during the approach for landing
invariably results in floating over the ground for varying distances,
or even overshooting, while too slow a glide causes undershooting,
flat approaches, and hard touchdowns. A pilot without the ability
to recognize a normal glide will not be able to judge where
the airplane will go, or can be made to go, in an emergency.
Whereas, in a normal glide, the flightpath may be sighted to
the spot on the ground on which the airplane will land. This
cannot be done in any abnormal glide.
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