Behavioral Animation - Computer Animation: Algorithms and Techniques - page 350

Lift Vertical lift

Lift

Horizontal li f t

Gravity Gravity

FIGURE 11.11
Lifting forces.

In flight, a turn is induced by a roll , in which the up vector of the object rotates to one side. Because
the up vector is rotated to one side, the lift vector is not directly opposite to gravity. Such a lift vector
can be decomposed into a vertical component, the component that is directly opposite to gravity, and
the horizontal component, the component to the side of the object. A flying object turns by being lifted
sideways by the horizontal component of the lift; this is why planes tilt into a turn. For the object to be
directed into the turn, there must be some yaw induced. If a plane is flying level so that lift cancels
gravity, tilting the lift vector to one side will result in a decrease in the vertical component of lift. Thus,
to maintain level flight during a turn, one must maintain the vertical component of lift by increasing the
thrust (see Figure 11.11 ) .
Increasing the pitch increases the angle the wing makes with the direction of travel. This results in
increased lift and drag. However, if thrust is not increased when pitch is increased, the increased drag
will result in a decrease in velocity, which, in turn, results in decreased lift. So, to fly up, both thrust and
pitch need to be increased.
These same principles are applicable to a soaring bird. The major difference between the flight of a
bird and that of an airplane is in the generation of thrust. In a plane, thrust is produced by the propeller.
In a bird, thrust is produced by the flapping of the wings.
Some of the important points to notice in modeling flight are as follows:

Turning is effected by horizontal lift
Turning reduces lift
Increasing pitch increases lift and drag
Increasing speed increases lift and drag

11.1.2 Prey-predator behavior
The previous discussion on flocking discussed modeling the very basics of intelligent behavior (not
much beyond that of a college freshman): stay with your friends and avoid bumping into things. While
flocking behavior can be interesting, especially when interacting with obstacles in the environment, the
objective is to produce a single uniform motion—the emergent behavior of the flock. In order to create
more interesting interaction among the members, two sets of characters can be created with competing
goals and discriminating physical abilities.