Weight Control for Aircraft other than Fixed and Rotorwing
Some light aircraft utilize different methods of determining
weight and balance from the traditional fixed and
rotorwing aircraft. These aircraft achieve flight control
differently than the fixed-wing airplane or helicopter. Most
notable of these are weight shift control (WSC) aircraft
(also known as trikes), powered parachutes, and balloons.
These aircraft typically do not specify either an empty
weight center of gravity or a center of gravity range. They
require only a certified or approved maximum weight.
To understand why this is so, a look at how flight control is
achieved is helpful.
As an example, airplanes and WSC aircraft both control
flight under the influence of the same four forces (lift,
gravity, thrust, and drag), and around the same three axes
(pitch, yaw, and roll). However, each aircraft accomplishes
this control in a very different manner. This difference
helps explain why the fixed-wing airplane requires an
established weight and a known center of gravity, whereas
the WSC aircraft only requires the known weight.
The fixed-wing airplane has moveable controls that
alter the lift on various airfoil surfaces to vary pitch,
roll, and yaw. These changes in lift, in turn, change the
characteristics of the flight parameters. Weight normally
decreases in flight due to fuel consumption, and the
airplane center of gravity changes with this weight
reduction. An airplane utilizes its variable flight controls
to compensate and maintain controllability through the
various flight modes and as the center of gravity changes.
An airplane has a center of gravity range or envelope
within which it must remain if the flight controls are to
remain effective and the airplane safely operated.
The WSC aircraft has a relatively set platform wing
without a tail. The pilot, achieves control by shifting
weight. In the design of this aircraft, the weight of the
airframe and its payload is attached to the wing at a single
point in a pendulous arrangement. The pilot through the
flight controls, controls the arm of this pendulum and
thereby controls the aircraft. When a change in flight
parameter is desired, the pilot displaces the aircraft’s
weight in the appropriate distance and direction. This
change momentarily disrupts the equilibrium between
the four forces acting on the aircraft. The wing, due to its
inherent stability, then moves appropriately to re-establish
the desired relationship between these forces. This happens
by the wing flexing and altering its shape. As the shape
is changed, lift is varied at different points on the wing to
achieve the desired flight parameters.
The flight controls primarily affect the pitch-and-roll
axis. Since there is no vertical tail plane, minimal or no
ability exists to directly control yaw. However, unlike the
airplane, the center of gravity experienced by the wing
remains constant. Since the weight of the airframe acts
through the single point (wing attach point), the range
over which the weight may act is fixed at the pendulum
arm or length. Even though the weight decreases as fuel is
consumed, the weight remains focused at the wing attach
point. Most importantly, because the range is fixed, the
need to establish a calculated range is not required.
The powered parachute also belongs to the pendulumstyle
aircraft. Its airframe center of gravity is fixed at the
pendulum attach point. It is more limited in controllability
than the WSC aircraft because it lacks an aerodynamic
pitch control. Pitch (and lift) control is primarily a function
of the power control. Increased power results in increased
lift; cruise power amounts to level flight; decreased power
causes a descent. Due to this characteristic, the aircraft is
basically a one-air speed aircraft. Once again, because the
center of gravity is fixed at the attach point to the wing,
there can be no center of gravity range.
Roll control on a powered parachute is achieved by
changing the shape of the wing. The change is achieved
by varying the length of steering lines attached to the
outboard trailing edges of the wing. The trailing edge of
the parachute is pulled down slightly on one side or the
other to create increased drag along that side. This change
in drag creates roll and yaw, permitting the aircraft to be
steered.
The balloon is controlled by the pilot only in the vertical
dimension; this is in contrast to all other aircraft. He or she
achieves this control through the use of lift and weight.
Wind provides all other movement. The center of gravity
of the gondola remains constant beneath the balloon
envelope. As in WSC and powered-parachute aircraft,
there is no center of gravity limitation.
Aircraft can perform safely and achieve their designed
efficiency only when they are operated and maintained in
the way their designers intended. This safety and efficiency
is determined to a large degree by holding the aircraft’s
weight and balance parameters within the limits specified
for its design. The remainder of this handbook describes
the way in which this is done.
