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Pitch angle is the angle the PPC wing chord makes
with the horizontal plane. Pitch angle is what you can
see. Many pilots confuse the pitch angle, which you
can easily see and feel, with the angle of attack which
may not be as perceptible. [Figure 2-4] For example,
the pitch angle in an engine-out glide could be minus
8 degrees, in level flight 10 degrees above the horizon,
and in a climb it could be 28 degrees above the
horizon. These are significantly different angles you
easily see. Pitch angles are covered in greater detail
in Chapter 6.
Deck angle is the angle of the cart’s lower frame
(from the front wheel to the rear wheels), to the landing
surface. The deck on the lower part of the conventional
cart frame can be used to visualize deck angle.
An imaginary line between the front and back wheel
axles can also be used on unconventional carts.
Figure 2-1. Aerodynamic terms of an airfoil.
Leading edge
Chord line
Lower camber
Upper camber
Trailing edge
Relative wind
Flight path
Relative wind is the direction of the airflow with respect
to the wing; it is usually parallel to and opposite
the PPC flight path. Relative wind may be affected
by movement of the PPC through the air, as well as by
all forms of unstable, disturbed air such as wind shear,
thermals, turbulence, and mountain rotors. When a
PPC is flying through undisturbed air, the relative
wind is parallel to and opposite the flight path.
Angle of attack is the angle between the relative wind
and the wing chord line. [Figure 2-2]
2-2
Planform is the shape or form of a wing as viewed
from above. The PPC wing comes in two wing planforms:
rectangular, and elliptical. [Figure 2-5] The elliptical
planform leading and trailing edges are curved
to form an elliptical shape when viewed from the top
or bottom. These two shapes have unique flying characteristics.
Rectangular wings typically produce more
drag, are lower-performance, and do not move fore
and aft, relative to the cart, as quickly as elliptical
wings. These characteristics are more obvious when
the wing is inflating, during pitch changes, and when
flying in turbulence. Rectangular wings are therefore
more stable and require less effort to fly. Elliptical
wings are higher-performance and more efficient due
to less drag. Elliptical wings react more quickly with
changing conditions and require greater pilot experience
and skill during inflation, in turbulent air, and
with abrupt throttle changes.
Aspect ratio is the wingspan divided by the average
chord line. A PPC with a common 500-square foot
rectangular wing (about a 38-foot wingspan) and with
a typical mean chord line of 13 feet, would have an
average aspect ratio of about 3. This relatively low
aspect ratio is less efficient at producing lift. An elliptical
wing with the same 500 square feet and a 45-foot
wing span and an 11-foot average chord would have
an aspect ratio of about 4. The PPC wing is similar to
airplane wings in that the aspect ratio will differ with
the specific design mission for the aircraft. Generally,
Figure 2-2. Angle of incidence.
Figure 2-3. Angle of trim and center of pressure in gliding
flight.
rectangular wings have lower aspect ratios and lower
efficiency than the higher aspect ratio and higher efficiency
elliptical wings. Generally, a high aspect ratio
wing, compared to a low aspect ratio wing, produces
higher lift at lower angles of attack with less induced
drag. [Figure 2-6].
2-3
Wing loading is a term associated with the total
weight the ram-air wing must support. Wing loading
is found by dividing the total weight of the aircraft, in
pounds, by the total area of the wing, in square feet.
Wing loading is found by dividing the weight of the
aircraft, in pounds, by the total area of the wing, in
square feet. For example, the wing loading would be
2.0 pounds per square foot when 1,000 pounds—a
common weight for a two-seat PPC with two people
— is under a 500-square foot wing. If flying with one
person the aircraft weight might be 700 pounds and
the wing loading would decrease to 1.4 pounds per
square foot.
Gliding flight is flying in a descent with the engine at
idle or shut off.
Powered Parachute Wing
Pressurization and Flexibility
The powered parachute has two distinctive modes: (1)
inflated, it is a ram-air wing with a curved arc—
a recognizable
airfoil shape; and (2) deflated, it is a canopy
that is either lying flat on the ground or packed into
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Powered Parachute Flying Handbook动力伞飞行手册(13)
