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a bag.
Figure 2-4. Gliding and climbing pitch angles.
Figure 2-5. Planform view of a PPC inflated wing:
rectangular and elliptical.
Note: Chapter 7, Takeoffs and Departure Climbs, will
detail the methods of getting the uninflated canopy
laying on the ground turned into a flying wing. Since
the aerodynamics of the PPC do not start until the
wing is completely inflated, this chapter will assume
each reference to the PPC wing is to an inflated ramair
wing already in the shape of an airfoil.
The powered parachute ram-air wing retains its airfoil
shape due to the air pressurizing the inside cells
via the relative wind airflow being rammed into the
front openings of the canopy—thus the term “ram-air
wing.” The pressure inside the wing is much higher
than the outside top and bottom because the dynamic
pressure from the relative wind is converted to static
pressure to pressurize the wing. The greater the speed,
the greater the pressure inside the wing and the more
rigid the wing. The cell openings are designed to be
perpendicular to the relative wind to achieve maximum
pressure from the relative wind. This static internal
pressure harnessed from the relative wind is
called dynamic pressure (q), and is determined by the
velocity squared times the air density factor. [Figure
2-7] Note the dynamic air pressure converted to static
pressure at point A is constant throughout the wing
points B and C. This static pressure is always greater
than the pressure outside the wing at points X and Z.
Cross-port openings are placed in the ribs of each cell,
connecting the adjoining cells. These cross-ports are
dispersed throughout the wing (with exception to the
outboard side of the end cells) to maintain positive
pressure throughout. The pressure is constant inside
2-4
the wing because the dynamic pressure hitting the
opening is the same for each cell and the speed is
the same. The cross-ports aid the complete wing in
becoming pressurized during inflation and maintaining
the pressure throughout the wing in turbulence.
[Figure 2-8]
The inflatable wing airfoil generally remains a consistent
shape as designed by the manufacturer. However,
pilot control of the wing to make a turn significantly
changes the relative aerodynamic qualities of the PPC
wing by pulling down the trailing edge similar to a
flap on an airplane. [Figure 2-9]
Figure 2-7. Dynamic pressure.
Figure 2-8. Cell openings and cross-port view.
Figure 2-6. Aspect ratio comparisons for wings with similar areas.
Faster speeds from smaller wings or more weight create
a higher pressure in the wing resulting in higher
control forces because of the higher internal pressure.
Forces in Flight
Like all aircraft, the four forces that affect PPC flight
are thrust, drag, lift, and weight. [Figure 2-10] In
steady PPC flight:
1. The sum of all upward forces equals the sum of
all downward forces.
2. The sum of all forward forces equals the sum of
all backward forces.
2-5
Figure 2-9. PPC wing flexibility in flight.
Figure 2-11. The lift equation.
Figure 2-10. Level flight forces.
3. The sum of all moments equals zero.
THRUST – the forward force produced by a powerplant/
propeller as it forces a mass of air to the rear (usually
said to act parallel to the longitudinal axis).
vs.
DRAG – the aerodynamic force acting on the airfoil
lines and cart in the same plane and in the same direction
as the relative wind.
LIFT – the aerodynamic force caused by air flowing
over the wing that is perpendicular to the relative
wind.
vs.
WEIGHT – the force of gravity acting upon a body.
Lift
Lift opposes the downward force of weight and is produced
by the dynamic effects of the surrounding airstream
acting on the wing. Lift acts perpendicular to
the flight path through the wing’s center of lift. There
is a mathematical relationship between lift, angle of
attack, airspeed, altitude, and the size of the wing. In
the lift equation, these factors correspond to the terms
coefficient of lift, velocity, air density, and wing surface
area. The relationship is expressed in Figure 2-11.
This shows that for lift to increase, one or more of the
factors on the other side of the equation must increase.
Lift is proportional to the square of the velocity, or
airspeed, therefore, doubling airspeed quadruples the
amount of lift if everything else remains the same.
Small changes in airspeed create larger changes in
lift. Likewise, if other factors remain the same while
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Powered Parachute Flying Handbook动力伞飞行手册(14)
