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however, maximum flap extension gives the steepest
angle of approach and minimum speed at touchdown.
Maximum flap extension, particularly beyond 30 to
35°, results in a large amount of drag. This requires
higher power settings than used with partial flaps.
Because of the steep approach angle combined with
power to offset drag, the flare with full flaps becomes
critical. The drag produces a high sink rate that must
be controlled with power, yet failure to reduce power
at a rate so that the power is idle at touchdown allows
the airplane to float down the runway. A reduction in
power too early results in a hard landing.
Crosswind component is another factor to be
considered in the degree of flap extension. The
deflected flap presents a surface area for the wind to
act on. In a crosswind, the “flapped” wing on the
upwind side is more affected than the downwind
wing. This is, however, eliminated to a slight extent
in the crabbed approach since the airplane is more
nearly aligned with the wind. When using a wing low
approach, however, the lowered wing partially
blankets the upwind flap, but the dihedral of the wing
combined with the flap and wind make lateral control
more difficult. Lateral control becomes more difficult
as flap extension reaches maximum and the
crosswind becomes perpendicular to the runway.
Crosswind effects on the “flapped” wing become more
pronounced as the airplane comes closer to the ground.
The wing, flap, and ground form a “container” that is
filled with air by the crosswind. With the wind striking
the deflected flap and fuselage side and with the flap
located behind the main gear, the upwind wing will
tend to rise and the airplane will tend to turn into the
wind. Proper control position, therefore, is essential
for maintaining runway alignment. Also, it may
be necessary to retract the flaps upon positive
ground contact.
The go-around is another factor to consider when
making a decision about degree of flap deflection
and about where in the landing pattern to extend
flaps. Because of the nosedown pitching moment
produced with flap extension, trim is used to offset
this pitching moment. Application of full power in
the go-around increases the airflow over the
“flapped” wing. This produces additional lift
causing the nose to pitch up. The pitch-up tendency
does not diminish completely with flap retraction
because of the trim setting. Expedient retraction of
flaps is desirable to eliminate drag, thereby allowing
rapid increase in airspeed; however, flap retraction
also decreases lift so that the airplane sinks rapidly.
The degree of flap deflection combined with design
configuration of the horizontal tail relative to the
wing requires that the pilot carefully monitor pitch
and airspeed, carefully control flap retraction to
minimize altitude loss, and properly use the rudder
for coordination. Considering these factors, the pilot
should extend the same degree of deflection at the
same point in the landing pattern. This requires that a
consistent traffic pattern be used. Therefore, the pilot
can have a preplanned go-around sequence based on
the airplane’s position in the landing pattern.
There is no single formula to determine the degree of
flap deflection to be used on landing, because a
landing involves variables that are dependent on each
other. The AFM/POH for the particular airplane will
contain the manufacturer’s recommendations for
some landing situations. On the other hand,
AFM/POH information on flap usage for takeoff is
more precise. The manufacturer’s requirements are
based on the climb performance produced by a given
flap design. Under no circumstances should a flap
setting given in the AFM/POH be exceeded
for takeoff.
CONTROLLABLE-PITCH PROPELLER
Fixed-pitch propellers are designed for best efficiency
at one speed of rotation and forward speed. This type
of propeller will provide suitable performance in
a narrow range of airspeeds; however, efficiency
would suffer considerably outside this range. To
provide high propeller efficiency through a wide
range of operation, the propeller blade angle
must be controllable. The most convenient
Ch 11.qxd 5/7/04 8:50 AM Page 11-3
11-4
way of controlling the propeller blade angle is by
means of a constant-speed governing system.
CONSTANT-SPEED PROPELLER
The constant-speed propeller keeps the blade angle
adjusted for maximum efficiency for most conditions
of flight. When an engine is running at constant
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AIRPLANE FLYING HANDBOOK 飞机飞行手册下(11)
