Page 123 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 110

110 CHAPTER FOUR

The French Voisin brothers In the following sections you will be introduced to three
founded the first airplane modes of dynamic motion that are the easiest to understand
building company in 1906, and may be most familiar to you.
despite the fact they had never
Phugoid Motion
successfully flown.
Have you ever thrown a paper airplane and watched it follow
a flight path that climbs and slows and then descends and speeds up,
as illustrated in Figure 4.8? This type of motion is common to all air-
craft and is given the name phugoid motion. Phugoid motion is a trade
between kinetic and potential energy, that is, speed and altitude. It
occurs at a constant angle of attack so as the speed increases, so does
the lift. The extra lift causes the airplane to increase altitude. As it
does, the airspeed falls off, decreasing lift, and thus eventually alti-
tude. In the extreme, at its maximum height the airplane will lose so
much speed that it will stall.
But there is no need to worry about this in a real airplane. The time
it takes to complete one cycle, the period, is on the order of minutes.
Only a sleeping pilot would ignore this. In fact, the period is so long,
most pilots do not even recognize that they are controlling this
motion. Also, the oscillations will damp out eventually by themselves
if they are ignored.

Dutch Roll
The Dutch roll is a motion that couples roll and yaw. The name comes
from the motion of the Dutch speed skaters as they glide across the
ice. It is kind of like the coupling of a small rolling motion with a small

Fig. 4.8. Phugoid motion.