Active Control Technology: Fly-by-Wire
It is clear that stability considerations are important in aircraft-design configurations. Although the related geometrical parameters are from statistical data of past designs and subsequently sized, this chapter provides a rationale for their role in the conceptual design stage. It also has been shown that to control inherent aircraft motions, feedback-control systems such as a stability augmentation system (SAS) (e. g., a yaw damper) and a control augmentation system (CAS) have been routinely deployed for some time. In this final section, the rationale continues with a discussion on how the feedback-control system has advanced to the latest technologies, such as FBW and fly-by-light (FBL), known collectively as ACT. Today, almost all types of larger aircraft incorporate some form of ACT.
The advantages of FBW are discussed in various sections of this book; the concept is not new. FBW is basically a feedback-control system based on the use of digital data. Figure 12.17 shows the control of one axis, which can be used for all three axes. Earlier SAS and CAS had mechanical linkage from the pilot to the controls; FBW does not have the direct linkage (hence, the name). It permits the transmission of several digital signal sources through one communications system, known as multiplexing. A microprocessor is in the loop that continuously processes air data (i. e., flight parameters) to keep an aircraft in a preferred motion with or without pilot commands. Aircraft-control laws – algorithms relating a pilot’s
command to the control-surface demand and aircraft motion, height, and speed, which involve equations of motion, aircraft coefficients, and stability parameters – are embedded in the computer to keep the aircraft within the permissible flight envelope. Under the command of a human pilot, the computer acts as a subservient flier. The computer continuously monitors aircraft behavior and acts accordingly, ensuring a level of safety that a human pilot cannot match.
Figure 12.17 is a schematic diagram of the FBW feedback arrangement for pitch control. The flight-control computer takes the pilot’s steering commands, which are compared to the commands necessary for aircraft stability to ensure safety and that control surfaces are activated accordingly. Air data are continually fed to the computers (i. e., speed, altitude, and attitude). Built into the computer are an aircraft’s limitations, which enables it to calculate the optimum control-surface movements. Steering commands are no longer linked mechanically from the cockpit to the control surface but rather via electrical wiring. FBW flight-control systems seem to be the ideal technology to ensure safety and reduce a pilot’s workload.
Because analog point-to-point wire bundles are an inefficient and cumbersome means of interconnecting sensors, computers, actuators, indicators, and other equipment onboard a modern military aircraft, a serial digital multiplex data bus was developed. MIL-STD-1553 (in use since 1983) defines all aspects of the bus (i. e., a subsystem of electrical lines for communication, named after electrical bus bars); therefore, many groups working with the military have adopted it. The MIL-STD – 1553 multiplex data bus provides an integrated, centralized system control and a standard interface for all equipment connected to the bus. The bus concept provides a means by which all traffic is available and can be accessed using a single connection for testing and interfacing with the system.
FBW reacts considerably faster than a conventional control system and does not encounter fatigue problems. A strong driver for incorporating FBW in military – aircraft design is the ability to operate at relaxed stability (even extending to a slightly unstable condition) used for rapid maneuver (increased agility) as a result of minimal stiffness in the system. It is difficult for a typical pilot to control an unstable aircraft without assistance; a computer is needed and a regulator supplies the necessary stability. This system does not generate the natural stability of a conventional aircraft but automatically trims the aircraft to the preferred flight conditions. Progress in FBW systems depends to a great extent on the progress of onboard computer power.
An aircraft flying under relaxed stability using FBW does not have the same requirement for geometrical features to provide low stiffness and damping. Hence, stability and control-surface sizing are different than in a conventional design: They are smaller and, hence, lighter with less drag. This is what is meant by a CCV.
Stable designs already have a down-pitching force because of the position of the NP aft of the CG. Any balancing force must be generated by a larger downward lift of the H-tail. Again, this decreases the maximum possible lift and increases the trim drag. In an unstable layout (e. g., the CG moving aft), the elevator’s lift is directed upward to counterbalance the moment. In this way, the aircraft’s total lift is increased; the aircraft wing therefore can be designed to be smaller and lighter and still provide the same performance. There is another benefit from the use of an unstable design: In addition to the aircraft’s increased agility, there is a reduction in drag and weight.
The difference between a conventional and a CCV design is shown in Table 12.1 and Figure 12.18 (see ) for longitudinal stability. The wing area and MTOM of both designs were unchanged; the CCV design yielded a smaller H-tail area with a larger CG range. The directional stability exhibits similar gains with a smaller V-tail area, thereby further reducing the OEM and permitting a bigger payload.
In summary, FBW provides considerable advantages, as follows:
• a simple and flexible system architecture although its design is complex
• consistent handling
• automatic stabilization
• safe maneuvering to the envelope limits
• ability to integrate with a wide range of designs (e. g., slats and swing-wing)
• ability to integrate with engine control through FADEC and the thrust vector
• use of side stick controller – provides free space in the cockpit layout and weight-saving
• incorporates relaxed stability for rapid maneuver, yet uses smaller control surfaces
• permits complex configurations for stealth aircraft, which may not be favorable for aerodynamic considerations leading to unstable aircraft (e. g., the F117 Nighthawk)
• digital data-handling allows multiplexing, which saves weight
• overall weight reduction
• allows standardization
• failure detection
• fault isolation
• built-in tests and monitoring
Figure 12.18. Comparison between a conventional and a CCV design
FBW has been in use for nearly a half-century but the obvious advantages were kept secret for a long time for military reasons. Early development in the pioneering stages progressed slowly with some mishaps. Nearly two decades later, civil aviation took bold steps and Aircraft Radio Inc. (ARINC) standards emerged to control FBW designs. The Airbus was the first aircraft to incorporate full FBW in a major project. The midsized A320 twin-jet aircraft is the first commercial transport aircraft to incorporate full FBW without manual override. The Habsheim (June 26, 1988) and the Bangalore (February 14,1990, near the author’s residence) disasters posed many questions; however, practically all midsized and larger transport aircraft currently incorporate some form of FBW technology.
The FBW system architecture has built-in redundancies. During the 1980s, such systems had quadruple-redundant architecture in which each system works independently. Nowadays, with improved reliability, a triplex system (with voting and consolidation) dominates design. FBW can be applied to one, two, or all three axes of control; modern systems incorporate all three. MIL-STD-1553 specifies that all devices in the system must be connected to a redundant pair of buses, which provides a second path for bus traffic if one bus is damaged. Signals are allowed to appear only on one of the two buses at a time. If a message cannot be completed on one bus, the bus controller may switch to the other bus. In some applications, more than one bus may be implemented on a given aircraft.
To avoid electromagnetic interference, the use of fiber optics for signaling using light was developed recently. Aptly, it is called FBL and is guided by MIL-STD – 1773.
In summary, FBW and FBL designs offer weight reduction with a smaller wing and empennage, fewer control surfaces, less cabling, and the elimination of mechanical linkages. As a consequence, drag is reduced. In addition, FBW and FBL designs provide enhanced safety and reliability as well as improved failure detection.