An Introduction to System Modeling and Control

1Introduction

In this chapter we simply want to present some examples of physical control systems. The intent here is to show why feedback control is necessary and to give the “big picture” of what must be done to control physical systems.

1.1 Aircraft

Figure 1.1 is a drawing of a simple propeller powered airplane. There are four forces on the airplane: lift which is primarily provided by the wings, drag which is basically wind resistance, thrust which is provided by the propeller, and gravity.

Figure 1.1 The four forces on an aircraft.

Lift is due to the difference in air pressure between the top and bottom of an airfoil. The airfoil is a generic term that here refers to the wings, horizontal tail or the vertical tail of the aircraft. Figure 1.2 on the next page shows streamlines of air moving past an airfoil (wing). The air going over the top travels faster (as it goes a greater distance) than the air on the bottom of the wing. Because of this speed difference, it turns out that the air pressure on the top of the wing is then less than the air pressure on the bottom of the wing. The resulting upward force is what we call lift. We call lift an aerodynamic force. Figure 1.3 on the next page shows the air flow streamlines across an airfoil in a wind tunnel.

Figure 1.2 ...