Advanced Flight Control Systems

Over the last decade or so new approaches to aerodynamics have brought with them different control philosophies with new acronyms to identify them. These approaches are referred to generally by the title control configured vehicles (CCV), in which means are employed, such as fuel redistribution, to set up a state of relaxed or even negative stability in the aircraft’s pitch axis. It results for example in the case of combat aircraft, in a decrease in manoeuvring inertia and enhanced control response.

To achieve this a computer is interposed between the pilot and the surface controlled to ensure the basically unstable aircraft obeys the control inputs. The control commands now take the form of digital electrical signals and such systems are known as fly-by-wire (FBW). Not surprisingly the most recent manifestation of this type of system, which employs fibre optics in place of electrical conductors, is called fly-by light (FBL).

The next step towards that design concept, which has been on aircraft manufacturers’ drawing boards for some time, the all-electric aircraft (AEA), involves the replacement of the existing hydromechanical units with electromechanical actuators (EMA). In the past the high weight of available motors and an inadequate reliability level have precluded their use.

New types of motor becoming available for power by wire (PBW), as it is called, have power-to-weight ratios and reliability levels high enough to permit their use on both military and civil aircraft. The great advantage of the electrical motor, compared with the hydraulic actuator, is that its rotational output enables it to transmit torque directly about the hinge line of the control surface.

An additional advantage of the new DC motors, which use magnets of rare earth alloys such as samarium cobalt and neodymium iron, is that they can tolerate high peak overloads for short periods. Their use in a PBW control system is said to save hundreds of kilograms in the overall weight of a transport aircraft compared with a hydraulic system. It should be noted, however, that for optimum motor design a voltage higher than the standard 28V aircraft supply is necessary.

At the forefront of actuator development is Lucas Aerospace, a member of the 65,000-strong group, Lucas Industries and based in the English midlands. The company in fact first introduced FBW in the early 1960s for the TSR2 aircraft and development has progressed apace since then. The development and manufacturing divisions of Lucas Aerospace encompass a wide range of aircraft products including flight and engine control systems; electrical power, nacelles and thrust reverser systems; missile and auxiliary power systems; metal and composite fabrication; fuel system equipment and cryogenics; aircraft transparencies; electroluminescent lighting (EL) and Spraymat ice protection; starting and electrical distribution; mechanical equipment; Hi-Rel hybrid microcircuits; software and combustion technology.

Since 1946 Lucas Aerospace has designed flap and slat operating systems for 14 different aircraft types. Of these the progression in the development of actuation systems may best be illustrated by consideration of the actuation system for Airbus Industrie aircraft produced by Lucas Aerospace in conjunction with Liebherr Aero Technik.

The Airbus A300 and A310 have the most modern design of ball-screw actuators for both flap and slat layouts. The control unit in each system has differentially coupled hydraulic motors with pressure-off brakes and transmission shafting of stainless steel tubing. Both flap and slat systems on the A300 have six actuators per wing, the flap system having torque limiters between control unit and the inboard actuators as well as jackhead torque limiters.

On the Airbus A310, the control of flap (four per wing) and slat actuators is FBW using microprocessors. As well as transmitting the input command signals, this ensures continuous monitoring. Any malfunction such as asymmetry or uncommanded movement is detected and made safe. In the Airbus A320 system under development, rotary actuators incorporating the latest in gear design form the final drive elements and take the design progression one stage further. This type of rotary actuator is compact and may be driven via transmission shafting or individual ‘rare earth’ electrical motors.

Apart from developments for Airbus Industrie, which also includes aileron, elevator and spoiler actuators, Lucas Aerospace is engaged on the design and supply of a complete leading-edge flap control system for the SAAB JAS-39 fighter and is participating in the design and supply of the complete thrust reverser actuation systems for General Electric CFM56-5 and International Aero Engine V2500 engines.