REQUIREMENTS OF FLIGHT CONTROL

What is true of general control systems (Appendix C) is also applicable to automatic flight-control system (AFCS) for an aircraft. The need of aircraft to fly in clouds and fog and to reduce the pilot workload during flight operation paved the way for the devel­opment of devices to stabilize the aircraft by artificial means [3]. In the case of an FBW (fly-by-wire) aircraft, the artificial stabilization is required because the aircraft is inher­ently designed to have static instability for performance gains. The automatic maneuver envelope protection takes place due to the limits provided (in the computer) on the responses. Any reasonably desired limit on altitudes, speed, AOA, and “g” force can be provided. The relaxed static stability (RSS) allows to plan smaller tail surfaces on such FBW aircraft and hence weight saving could be up to 10%.

Confluence of Aero-Control-Formative Years

Period	Aircraft Dynamics Theory	Feedback Control of Aircraft	Feedback Control Theory
1980	—	Gyroscope stabilization (Maxim); torpedo course control (Obry)	—
1990	Study of phugoids (Lanchester); small perturbation theory (Bryan and Williams)	—	—
1910	Measurements of derivatives and calculation of motions (Bairstow and Jones); the methods introduced in the United States (Hunskar)	Demo of two-axis aircraft stabilizer/aerial torpedo (Evan and Lawrence Sperry)	—
1920	Measurements/calculations of derivatives (Glauert, Bryant, Irving, and Cowley) Full-scale flight tests confirm the theory	—	Study of aircraft under continuous control (Gates, Gamer)
1930	The status of theory surveyed (B. M. Jones)	RAE-Mark IV-Siemens automatic pilot/development of pneumatic-hydraulic A2-Wiley postflight	Stability of feedback amplifier studied (Nyquist) Logarithmic plots, sensitivity (Bode)
1940	For a variety of aircraft and conditions, the advances in calculations made Study of response under control	Rudder control, missile development (Germany), all-electric and maneuvering automatic pilots, flight of “Robert R. Lee”	The techniques applied to servomechanisms (Harris), thesis by Hall The new texts
	Frequency methods (Greenberg, Seacord, and others)	—	Root locus method (Evans)
1950		_

TABLE 8.1

Lecture by Bollay Volumes by the BuAer-Northrop Improvements in understanding of flight control

Source: McRuer, D. and Graham, D., J. Guid., Control, Dynam., 27, 161, 2004.

FCS aids a pilot in controlling the aircraft safely and effectively throughout the entire flight envelope (Figure 7.1). Besides providing the basic stability, if needed, AFCS in particular provides good handling qualities (Chapter 10). However, funda­mentally AFCS provides the required stability and improved damping to the basic airframe, if these are inherently poor or insufficient. This aspect could be realized either by an autopilot or a limited authority command and stability augmentation system (SAS).

Since the response of AFCS would be much more rapid than that of a human pilot, the effect of the disturbance would not reach sizeable magnitude. The pilot can detect a change in the pitch altitude of 1° in 0.3 s and there would be a further delay of 0.5 s by the pilot in his decision making on the amount of the correction required to be applied [4]. However, the autopilot can detect the disturbance of 0.1° in 0.05 s or even less and then apply an input to overcome the disturbance in 0.1 s, thus gaining considerable advantage in the control response. This also means that less disturbance is seen by the aircraft.

In some cases AFCS could be a full authority FBW/FCS. Modern high- performance aircraft are designed to have RSS or are inherently unstable in longi­tudinal axis. The RSS concept means that the CG of the airframe is more aft than the conventional designs. If the CG is aft of the neutral point (Appendix A), then the basic aircraft would be statically unstable. This allows reduction in the area of the horizontal tail surface. Weight and trim drag will be reduced. Certain advantages can be gained by this type of design. The major reason for this approach is that a more favorable aerodynamic force balance is achieved than the conventional stable airframe configuration. The wing lift is aided by the control surface lift contribution and for a given AOA, there is higher lift and reduced drag (for a given lift) for the inherently unstable/RSS airframe configuration. In conventional aircraft, the balance of momentum is achieved by downward lift of the elevator. This has a negative effect on the total lift. Whereas in the case of the unstable configuration the moment balance is achieved with the upward lift of the elevator and hence the total lift of the aircraft is increased. Other merits for such a design are as follows [5]: (1) about 10% to 15% gain (depending on the configuration) in the maneuver margin (Appen­dix A), (2) reduction in fuel consumption, and (3) increased climb rate. Since the inherently unstable aircraft cannot fly, it needs artificial stabilization. It could have FBW/FCS with either limited or full authority command. Rafale, Gripen, F-22, and Eurofighter Typhoon aircraft are designed to be unstable.

In particular, FBW/FCS-related control laws have several major functions to perform: (1) SAS for excellent HQs and aircraft maneuverability, (2) automatic speed and flight trajectory control (via autopilot), thereby reducing pilot workload, (3) safe operability in all weather situations, (4) effective gust load alleviation, (5) an extended service life and enhancement of ride qualities, (6) performance optimization, and (7) reconfigurable/restructurable control tasks in the event of sensor/actuator faults or control surface damages.

From the point of view of ‘‘flight path,’’ the resulting control problem is to generate adequate deflection of aerodynamic control surfaces or changes in power or thrust to maintain the shape of the flight path and the velocity along this path. In all transport and most other aircraft, autopilot is used to reduce the pilots’
workload. The autopilot is used to control the motion of the vehicle in pitch, yaw, and roll axes in order to follow the required trajectory. The main aim is to ensure the stability of the vehicle in the presence of disturbances/forces/moments caused by various sources while maintaining structural integrity. While the autopilot strives to achieve the desired instantaneous values, the guidance system aims at achieving the desired end-of-the flight conditions. The autopilot is the inner control loop, whereas the navigation/guidance is the outer loop. Normally, the BW of the vehicle control loop is 0.2 to 0.6 Hz and that for the navigation/guidance is 0.02 to 0.04 Hz. The guidance system employs the navigation system as sensors to detect the instantan­eous velocity/position of the vehicle and generates the guidance commands to reach the desired conditions. Navigation is based on the high-precision measurements of acceleration and high speed/accuracy of the required computations. The autopilot consists of precision angular and angular rate sensors. All the autopilot modes help the pilot in effective maneuvers mostly with hands off. The autopilot could have several types of modes as shown in Table 8.2 [5].

Due to the fact that some additional information/data are required to per – form/conduct certain coupled/integrated modes, the interaction of autopilot with IMU and air data system would be definitely required. The autopilot types are: (1) roll (bank angle position) autopilot (holds the aircraft in level position), (2) rate controller (stabilizes the aircraft), (3) heading select and hold autopilot (keeps the course of the aircraft), (4) vertical speed, (5) airspeed select and hold, (6) Mach number hold, (7) pitch altitude hold, (8) altitude select and hold, (9) glide slope, (10) approach/flare, (11) localizer, (12) runway align, and (14) flight path angle controller (helps in climbs). Accelerations autopilots are generally used in missiles and altitude hold autopilots alleviate pilot workload. A brief description of a few control aspects is given here [4].

Altitude hold: The pitch attitude sensing element detects any change in the aircraft attitude. This change will be compensated by appropriate feedback law. However, changes in vertical displacement cannot be detected by an attitude sensor. Therefore, for automatic leveling off at any desired altitude, an altitude hold function is required. The proper sensor is based on a pressure transducer, and it senses a change in altitude as a change in (static) pressure. Correspondingly, the elevator servo actuator is operated to apply elevator control to restore the aircraft to the selected altitude. The altitude hold signal can be obtained from an inertial reference

system. This signal is augmented with a barometric pressure correlated altitude signal (from the altitude sensor) in an air data computer that interfaces with the FCS.

Airspeed hold: An airspeed sensor measures the difference between static and dynamic pressures. The speed error signal is applied to the pitch sensor control channel.

Mach hold: The airspeed hold mode is normally used for the low-altitude flights. The Mach hold is used during the high-altitude phase.