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chapter 2

Quadcopter

Flight Dynamics

Flight Basics

I will begin this chapter with an introduction to basic flight principles that are applicable to
any aircraft with wings. It is important for you to have this knowledge in order to understand
how a quadcopter flies and the differences between its flight characteristics and those of a
normal aircraft.
Figure 2.1, which is from NASA, shows an iconic Wright 1903 Flyer with all four
aerodynamic flight forces that simultaneously and continually act upon it. The four forces
shown in the figure are further described in Table 2.1.
These forces are universally applicable to all aircraft—from the Wright Flyer to the
modern F-35 Joint Strike Fighter (JSF). How an aircraft responds to these forces determines
whether it is climbing, diving, in level flight, or turning.
The quadcopter design is different from that of regular aircraft in that it has no wings,
and thus, cannot generate any lift force. Instead, it depends solely on thrust forces created by
the motors attached at the end of each of its booms. Additionally, the upward and forward
velocities traveled by a quadcopter are small enough that drag forces are not really a factor.
As a consequence, there are only two principal forces affecting the quadcopter: thrust and
weight. Now weight is a fixed force that can be changed only by design or by altering the
payload. This leaves only thrust as the sole control force for a quadcopter. However, thrust
is nearly directly proportional to the rotational speed of the motors, which means that
controlling the motor speed totally controls the flight path of the quadcopter. When the
rotational speeds are all equal and sufficiently fast, then the quadcopter will rise straight
up into the air. A vertical flight path was essentially the only flight path available to the early
De Bothezat helicopter discussed in Chapter 1. Varying the rotational speeds of one or more
of the quadcopter motors is the only way to alter the quadcopter’s flight path. Altering the
quadcopter flight path would be a most daunting proposition for a human pilot who would
have to rely on his or her sense of balance and then somehow translate that sensation to
actual motor speed changes. It is easy to understand why manned multirotors remained an
unachievable goal until the advent of automated flight-control techniques.
Also important to quadcopter flight dynamics are “two additional flight principles
of balance and center of gravity (CG), which are directly related to weight, one of the
fundamental flight forces. Weight must be properly distributed in order for any aircraft to fly
safely. Determining safe weight distribution starts with the basic aircraft design and uses a

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