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square foot when 1,000 pounds total weight for a two-seat
WSC aircraft with two people is supported by a 200 square
foot wing. If fl ying the same wing with one person and a
lighter total weight of 500 pounds, the wing loading would
be 2.5 pounds per square foot. In the small, high performance
wing of 140 square feet loaded at 1,000 pounds, wing loading
would be 7.1 pounds per square foot.
Gliding fl ight is fl ying in a descent with the engine at idle
or shut off. For example, use a glide ratio of 5, which is fi ve
feet traveled horizontally for every foot descended vertically.
Glide ratios vary signifi cantly between models.
WSC Wing Flexibility
The WSC wing retains its rigid airfoil shape due to rigid
preformed ribs called battens, which are inserted from the
root to the tip along the span of the wing (similar to ribs
for an airplane wing) and a piece of foam or mylar running
along the top side of the leading edge to the high point, which
maintains its front part of the airfoil shape in between the
battens. [Figure 2-11]
Some WSC double surface wing designs use a rib similar
to a PPC wing that attaches to the lower surface and the
upper surface to maintain the wing camber in addition to
the battens.
Even though the airfoil sections are rigid, the WSC aircraft is
called a “ fl ex wing” for two reasons. First, it is designed so
the outboard leading edges fl ex up and back when loaded. The
fl exing of the outboard section of the wing also allows load
relief because the tips increase twist and decrease AOA—the
greater the weight, the greater the fl ex and wing twist. This
fl exing allows the WSC aircraft to automatically reduce loads
in unstable air, providing a smoother ride than a rigid wing.
Since the wing fl exes and reduces the load for a given angle
of attack at the root chord, WSC aircraft cannot obtain loads
as high as those obtained by a rigid wing. This fl exing of the
outboard leading edges also assists in initiating a turn.
Second, the wing is designed to fl ex as it changes twist from
side to side for turning, historically known as wing warping.
WSC wing warping is similar to what the Wright Brothers
did on their early aircraft, but they did it with wires warping
the wing. The WSC aircraft uses no wires and warps the
wing by shifting the weight, which is covered in Chapter 3,
Components and Systems.
This fl exibility is designed into the wing primarily for turning
the aircraft without any movable control surfaces like the
ailerons and rudder on an airplane.
2-7
L = CLV2
2
S
L = Lift (pounds)
CL = Coefficient of lift
(This dimensionless number is the ratio of lift
pressure to dynamic pressure and area. It is
specific to a particular airfoil shape and, above
the stall, it is proportional to angle of attack.)
V = Velocity (feet per second)
= Air density (slugs per cubic foot)
S = Wing surface area (square feet)
Figure 2-13. The lift equation.
Weight Lift
Thrust CG Drag
Relative Wind
Flightpath
Figure 2-12. The four basic forces in level fl ight.
Forces in Flight
The four forces that affect WSC fl ight are thrust, drag, lift,
and weight. [Figure 2-12] In level, steady WSC fl ight:
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.
3. The sum of all moments equals zero.
Note that the lift and weight forces are much greater than
the thrust and drag forces. A typical example for many
WSC aircraft is that the lift/weight forces are fi ve times the
thrust/drag forces.
Thrust—the forward force produced by a powerplant/propeller
as it forces a mass of air to the rear (usually acts parallel to
the longitudinal axis, relative wind, and fl ightpath).
Drag—the aerodynamic force acting on the wing and carriage
in the same plane and in the same direction as the relative
wind.
Lift—the aerodynamic force caused by air fl owing over the
wing that is perpendicular to the relative wind.
Weight—the force of gravity acting upon a body straight
down and perpendicular to the Earth.
During level fl ight, these forces are all horizontal and vertical.
During descents or climbing, these forces must be broken
down into components for analysis.
Dynamic Pressure (q)
Both lift and drag are a direct result of the dynamic pressure
of the air. Dynamic pressure (q) is created from the velocity
of the air and the air density. An increase in velocity has a
dramatic effect on dynamic pressure (q) because it increases
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Weight-Shift Control Aircraft Flying Handbook(18)
