Ever since powered flight was first achieved by Wilbur and Orville Wright in 1903, the aviation community has sought both safer and more efficient methods of aircraft flight control. During the course of this blog, we will trace the development of fly by wire control systems from the basic electrical controls of the 1930s to the enhanced computerized systems of today.
For about thirty years after the Wright Brothers first flight, pilots controlled an aircraft by direct force, by moving control wheels, sticks and rudder pedals linked to cables and pushrods , which pivoted control surfaces on the wings and tails. However, as engine power, speeds and aircraft weight increased, more force was required to effectively direct aircraft control surfaces. Mechanical and hydraulic control systems were introduced to compensate for the increased power needs upon control surfaces. Such systems were relatively heavy and necessitated a careful routing of flight control cables through the plane by systems of cranks, pulleys, hydraulic pipes and tension cables. Although the mechanical and hydraulic systems provided a substantial boost to aircraft controls, they required multiple backup systems in the event of failures, further increasing weight in the design of the aircraft. Another problem of the hydro/mechanical systems was their insensitivity to outside aerodynamic forces such as spinning, stalling and vibrations during flight.
Electrical transmission to a plane’s control surfaces was first accomplished in 1934 on the Soviet ANT-20, the Maxim Gorky. The series of mechanical and hydraulic connections were replaced with electrical ones. This was an extremely large aircraft for its day and the electrical connections worked flawlessly until the collision of the aircraft the following year, proving the potential of electrical flight control. However, a dedicated electronic signal avionics control system was not tested until 1958 on the Avro Canada CF-105 Arrow. Ironically, the first vehicle to utilize an electronic flight control system without mechanical or hydraulic backup was the Lunar Landing Research Vehicle or LLRV, which flew successfully in 1964 as part of the Apollo moon program.
A fly by wire control system is a computer system, which monitors pilot control commands and related factors such as altitude, airspeed and angle-of- attack. The FBW system then relays these pilot inputs to the flight control surfaces in order to keep the aircraft within its designated flight envelope, or safe flight parameters of the aircraft at various speeds, altitudes and other flight conditions. The fly by wire computer employs electrical signal inputs to create electrical signal outputs which affect the flight control surfaces to produce the desired aircraft attitude. FBW computers utilize both analog and digital processing with digital units first appearing in quantity in the late 1970s. The essential difference between digital and analog units lies in how they process information. Analog computers work in a continuous cycle in which data can accept an infinite set of values, resulting in no loss of data. The primary limitation of analog units is the time required to initially configure the hardware to the aircraft, in addition to the difficulty of upgrading existing hardware. Digital systems operate in a designated time environment, in which values are finite. Any loss of data is supplemented by relatively high resolution and sampling rates, which minimize data loss. Upgrading a digital unit is merely a matter of downloading current software, achieving a smooth transition coupled with reduced software and maintenance costs. The flight control systems offer both redundant computer processing and circuitry in case of failure of the primary unit.
Fly by wire technology offers a number of advantages. Aircraft weight is greatly reduced since mechanical and hydraulic linkages are no longer necessary. Safety is enhanced due to both redundancy of electrical circuits as well as a quick response and processing time from the FBW unit, supplanting the skill of the pilot. Fly by wire systems benefit military aviation by allowing engineers the latitude to design an aircraft which may be inherently unstable, but yet be able to attain superior maneuverability under the parameters of the fly by wire computer. FBW systems require fewer parts and less fuel usage while providing more comfort for passengers because of more precise handling characteristics. Fly by wire control systems provide for greater safety by establishing control parameters within the capability of the plane with digital units compatible with the entire range of aircraft sensors.