Page 21 - Understanding Flight

CH01_Anderson PFS2 7/25/01 8:55 AM Page 8

8 CHAPTER ONE

Straight-and-level flight

Lift

Thrust Drag

Weight

Fig. 1.8. The four forces on an airplane in straight-and-level
flight.

The Four Forces

There are four forces associated with the flight of an airplane. These
forces, illustrated in Figure 1.8, are lift, weight, thrust, and drag. In
straight-and-level flight (not changing speed, direction, or altitude)
the net lift on the airplane is equal to its weight. We say net lift
because, for a conventional airplane design, the horizontal
stabilizer pulls down, putting an additional load on the
The Wright brothers’ first flight
wings. The thrust produced by the engine is equal to the
was less than half the length of
drag, which is caused by air friction and the work done to
a Boeing 777-300.
produce the lift.

Mach Number

One important parameter in describing high-speed flight is the Mach
number. The Mach number is simply the speed of the airplane, or
speed of the air, measured in units of the speed of sound. Thus, an air-
plane traveling at a speed of Mach 2 is going twice the speed of sound.
The speed of sound is fundamental for flight because it is the speed of
communication between the airplane and the air, and between one
part of the air and another. As will be seen in later chapters, as the
speed of an airplane approaches Mach 1 there are dramatic changes in
its performance. One change that affects performance is that the air