17.2 The Problem

An automated aircraft controller is useful in hazardous flight situations in which it is desirable to manipulate the aircraft without having a pilot present. Since automatic control systems for fixed-wing aircraft are quite prominent, they seem to be a viable solution. Occasionally, however, the aircraft must be maneuvered in tight areas or maintained at a specific position for extended periods of time. In such instances, an unmanned helicopter is the best solution (Sugeno, Griffin, and Bastian, 1993).

Control of a helicopter is much more difficult to achieve than control of a fixed-wing aircraft. This is true mainly because when a pilot releases the controls of a typical fixed-wing aircraft, the aircraft eventually reaches some steady state flight condition — the aircraft is generally stable. The same is not true for a helicopter. Without constant corrective control inputs, a helicopter tends to diverge from steady-state conditions — the aircraft is generally unstable. In addition, the flight dynamics of a helicopter are highly coupled and vary from aircraft to aircraft as well as from one flight regime to another. The coupling is due, in part, to the large gyroscopic moment created by the main rotor. Any forward pitch motion, i.e. “nose-down”, in a helicopter results in a corresponding roll to the right. Control-induced aerodynamic effects introduce additional coupling. These coupled effects make helicopters one of the most difficult aircraft to control.

A typical helicopter configuration is shown in the schematic of Figure 17.1. Here, notice that the body of the helicopter is suspended from a main rotor that provides lift. A tail rotor is employed to counteract the torque produced by the reaction of the air against the main rotor. The pilot uses three primary controls to fly the helicopter: (1) the collective is used to adjust the pitch of the main rotor blades in order to increase or decrease lift, (2) the pedals are used to adjust the pitch of the tail rotor blades, thus producing more or less torque on the aircraft body as needed, and (3) the cyclic, for all practical purposes, determines the orientation of the main rotor with respect to the aircraft body. These three main control mechanisms are shown in Figure 17.2.

The main rotor can be thought of as a disk that produces lift. When the cyclic is pushed forward, the disk tilts forward with respect to the aircraft body causing a forward acceleration. When the cyclic is pushed to the right, the disk tilts to the right causing an acceleration to the right. Unfortunately, each control action excites motion on multiple axes. In other words, the controls are highly coupled and, as a result, flying a helicopter is an extremely difficult task.

Figure 17.1  The helicopter produces forces on the surrounding air. These forces are channeled to accomplish flight maneuvers.

Figure 17.2  The primary controls in the cockpit of a helicopter include: (1) the collective that is used to adjust the pitch of the main rotor blades to increase or decrease lift, (2) the pedals that are used to adjust the pitch of the tail rotor blades, thus producing more or less torque on the aircraft body as needed, and (3) the cyclic, which for all practical purposes, determines the orientation of the main rotor with respect to the aircraft body.

As a specific example of the coupling effects present in a helicopter, consider a maneuver in which the pilot wants to descend from a hover to some predetermined altitude. To decrease the lift from the main rotor, and thus begin the descent, the collective is pulled downward. This causes the pitch on the main rotor blades to decrease, producing less force on the air. As a result, the counter torque provided by the tail rotor is no longer sufficient to maintain a constant heading. Therefore, the pedals must be adjusted to change the pitch on the tail rotor blades, thereby providing the correct counter torque for the new collective position. Unfortunately, the torque provided by the tail rotor is not generally limited to the vertical axis of the helicopter. In many instances, the tail rotor affects torque about the longitudinal axis resulting in roll. The roll causes a pitch due to gyroscopic precession from the main rotor. Each control produces multiple effects in the motion of the aircraft. As a result, all control actions must be meticulously coordinated in order to perform even the simplest maneuver.