Page 35 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

14 Bu il d Y o ur O w n Q u a d c o p t e r

Figure 2.1 Wright 1903 Flyer shown with flight forces. (Courtesy of NASA)

center-of-gravity concept. CG can be thought of as an imaginary point within an aircraft
where it could be suspended in a perfectly balanced position. In the real world, a CG point
is used to determine if an aircraft is stable; if the entire payload including the airframe, fuel,
passengers, and cargo is within prescribed design limits; and consequently, if the aircraft is
safe to fly. A quadcopter CG may be thought of as the point within the copter where a string
could be attached to suspend it in a perfectly balanced state. Naturally, one would expect the
CG to be collocated with the physical center of the quadcopter. If the CG is located off center,
it would tend to make the quadcopter unstable, perhaps to the point of being uncontrollable
if it were located too far from the physical center. This is always something to consider when
attaching devices to the quadcopter. For instance, attaching a camera module close to the
outboard side of a motor boom, which might make sense for a better field of view, would
probably upset the CG to the detriment of the quadcopter flight stability.

Flight Axes
In order to fully understand airplane flight dynamics, it is necessary to discuss three physical
axes and the three rotations associated with those axes. Figure 2.2 shows a light general-
aviation (GA) airplane with its longitudinal axis running fore and aft through the fuselage.
The lateral axis is perpendicular and in the same plane as the longitudinal axis and runs
through the wing, intersecting with the longitudinal axis at the CG. The third axis, called the
vertical axis, is perpendicular to the other two and also goes through the CG. The three

Force Description
Weight The downward force acting upon the aircraft due to Earth’s gravity.
Lift The upward force created by the rapid passage of air over and under the airfoil
(wing).
Thrust The forward force created by the rotating propeller pushing air backward.
Drag The backward force created by wind resistance due to the fuselage shape and
non-streamlined appendages.

Table 2.1 Four Aerodynamic Flight Forces