曝光台 注意防骗
网曝天猫店富美金盛家居专营店坑蒙拐骗欺诈消费者
designed to fly at an angle of attack to avoid stalls
(resulting from too high an angle of attack), and avoid
wing collapses (resulting from too low an angle of attack).
Each manufacturer specifically determines the
limitations so a proper angle of attack is maintained
throughout the flight operation range.
The basic design of the powered parachute is to fly
at a relatively constant speed which results in a constant
angle of attack. However, angle of attack can
change just as with any aircraft as when a gust of wind
Figure 2-23. High wing attachment point effects in flight.
2-13
dulum (the cart and occupants) moves forward of the
wing, the angle of attack increases, generating more
lift and more drag. The pendulum is the weight of the
CG under the wing which swings forward for this
transient situation due to pendulum effect.
Note that flare provides a temporary large increase in
the angle of attack (AOA) until the pendulum swings
back underneath the wing. This action thus returns the
wing to the normal stable flight configuration — the
cart (the total weight of the pendulum) under the center
of the wing. Therefore, a flare will only temporarily
add an increase to lift and drag. Once the pendulum
swings back down, the drag of the wing, and therefore
the reduced airspeed, will continue until the flare is
released.
Figure 2-26. AOA changes as wind hits the airfoil.
changes the direction the air is hitting the airfoil. [Figure
2-26]
The pilot can add weight or increase loads which may
also increase the angle of attack slightly.
Flaring Increases Angle of Attack
The flare (pulling down the trailing edges of the
wing—and thus lowering the trailing edge) increases
the angle of attack. [Figure 2-27] In a flare, the trailing
edges of the wing are pulled down (usually, as
both foot steering controls are pushed forward). This
is similar to lowering the flaps on an airplane: lift is
increased, drag is increased, and for a PPC, the angle
of attack is increased. The result is that the higher drag
wing slows down and thus the wing moves backward
relative to the cart. So as the total weight of the pen-
Figure 2-25. PPC stability after a side wind gust.
2-14
Porpoising Creates Variations in AOA
Another slight variation in the angle of attack is the
swinging pendulum action of the PPC when high
thrust engines provide strong and immediate full
thrust of the propeller. This extra thrust swings the
cart through the pendulum arc relative to its position
under the wing. This is why many times you will see
the PPC take off and porpoise until it stabilizes. This
is a good example of the dynamic pendulum effect.
As the propeller thrust swings the cart out front, the
cart peaks then swings back to center. The cart successively
swings back and forth, continuing to reduce
oscillations until it stabilizes in a climb. This porpoising
is most common with a high power engine. This
can be eliminated by using gradual throttle increases
so as not to create a dynamic pendulum effect entering
a climb.
Stalls: Exceeding the Critical Angle
of Attack
The critical angle of attack is the angle of attack at
which a wing stalls regardless of airspeed, flight attitude,
or weight. The drawings in Figure 2-28 show
airflow over a typical rectangular PPC wing. The first
shows a laminar, smooth, lift-generating airflow—one
that is typical when the angle of attack is within the
flight range. The second depicts an exceeded angle
of attack, turbulence and loss of the lifting force.
[Figure 2-28]
Figure 2-28. Wings stall due to an excessive angle of attack.
Figure 2-27. Flare on landing typically increases the angle
of attack.
Unlike a fixed-wing aircraft that takes constant awareness
of angle of attack to prevent a stall, the powered
parachute wing is designed by the manufacturers to
maintain a specified range of angle of attack and airspeeds.
It is resistant to stalls because for all practical
purposes, it is designed to fly at a constant normal operating
range. This range is maintained if the operator
flies within the operating limitations specified in the
POH. Flying the PPC within the limitations specified
in the POH and avoiding turbulence means you will
not exceed the critical angle of attack and stall the
wing.
However, situations that could contribute to a stall are:
• A large increase in wing drag (full-flare)
— which the PPC pilot controls by pulling the
wing back, thus increasing the AOA. (Note: A
full-flare is normally used and recommended
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：
Powered Parachute Flying Handbook动力伞飞行手册(19)
