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Aviation History
1981
1981 - 3017.PDF
computer is the software which con verts pilot demands into control- surface movements, based on aircraft motion information from roll, pitch and yaw rate-gyros. The aircraft is flyable on rate sensors alone, although the flying qualities may not be ideal. To im prove handling characteristics, the basic inputs are modified by schedul ing parameters which vary the output from the computers to the control- surface actuators, according to pre vailing conditions. Scheduling allows the flight-control system to be self-limiting, reducing control-surface movement in response to pilot input as the aircraft approaches the limits of its flight en velope. The controls are harmonised to give uniform handling characteris tics throughout most of the flight envelope. Manoeuvre demand The pilot demands manoeuvre g and his inputs are measured by quadru- plex displacement sensors on the stick and rudder. A linear spring-feel system provides uniform stick-force per g, except where the flight-control response is modified by the computer. When landing, for example, pilots prefer larger stick displacements than at other times. Pilot manoeuvre demand is received by the computer and compared with aircraft motion. Each lane of the quadruplex control system has its own set of rate gyros. The sensors are grouped in three boxes, each contain ing four roll, pitch or yaw rate gyros, to prevent local structural distortion inducing differences in the accelera tions sensed by each lane. All 12 gyros are mounted at the apex of the wing, close to the e.g., where vibra tion amplitudes are low. Scheduling inputs include angle-of- attack, sideslip, normal g, lateral g and heading. The integrity of the basic quadruplex control system has allowed BAe to adopt less complex triplex redundancy for these secon dary sensors. The failure of one scheduling sensor will not affect handling quality. A second such failure could jeopardise the flying characteristics, however. After a single sensor failure, there fore, the pilot is advised to respect a more limited flight regime. The loss of a single angle-of-attack sensor, for example, will not disable the stall- prevention system, but the pilot is instructed to limit the angleof-attack in case of a second failure. At all times, the aircraft remains safe to fly on rate sensors only. Angle of attack and sideslip are measured by four skewed probes on the forward fuselage. Redundancy is pseudo-triplex: the upper pair sense angle-of-attack and, differentially, sideslip; while the lower pair measure sideslip and, again differentially, angle-of-attack. Height and Mach number are derived from pitot-static probes—one on the nose and two on the fuselage. Normal and lateral g inputs allow improved handling qualities to be developed. Normal g allows the com puter to sense atmospheric turbu lence and to signal the control sur faces to reduce aircraft gust response, giving a smoother ride. Lateral g may prove a more effective scheduling parameter at high Mach numbers than sideslip. The heading input from the Jaguar's navigation and weapon-aim ing sub-system enables the computer to act as an autopilot, providing such functions as weapon steering com mands. The autopilot software is already in the computers, and BAe hopes that the fly-by-wire test pro gramme will allow the integration and operation to be tested. Quadruplex redundancy is not simply a matter of providing four separate paths for the flight-control signals. The computers are cross linked so that each has access to the sensor inputs of the other three lanes. Provided the signals are within toler ance, they are summed and then averaged to produce a single input which is identical for all four com puters. This voting process takes place throughout the system. If one of the four inputs becomes suspect, its "vote" is reduced so that its effect on the average signal is minimised. An input becomes suspect when it enters a "zone of suspicion" between inner and outer tolerance bands. If the signal wanders out of the wider tolerance band it is then rejected completely. Optically isolated Inputs from all the sensors are analogue, and are converted to digital signals within the computer before being passed to the other three lanes. To prevent electrical interference propagating between computers, each cross-link is optically isolated: a light- emitting diode driven by the trans mitting computer faces a light-sensi tive diode connected to the receiving computer across the "zero-length" fibreoptic channel. Given an identical input value and correctly functioning software, the computer output, in the form of a servo demand to the actuators should be identical for all four lanes. The quadruplex flight-control system must now interface with the dual-redun dant hydraulic system, experience having shown that two hydraulic sup plies guarantee safe operation. Dowty Boulton Paul designed the compact failure-absorbtion actuators. B's F-104 • Vf control-figured vehicle has a >lt»x digital flight-control system. The aircraft, is seel 1o a much greater degree than fh uar, with a combination of aft ba'last, fuci transfer and a canard ioreplane takis =iic centre 35 uer cent of the mean a. i forward of the e.g. The F-'04's rv - > - • -jm is rt-taiiicd, aisd in all but the meat unstable '-on»iguration jtabJiH ("i,i be tvstwd ^.ft-c '• <-3iUire. The General Dynamics Advanced Fighter Technology Integration F-16 (AFTI/F-16) is the first aircraft with a triplex digital fly-by-wire control system. Testing begins later this year. The basic F-16 has quadruplex analogue flight controls with no mechanical backup, and is the first operational unstable lighter. AFTI/F-16 will demonstrate direct lift and sideforce and fuselage- pointing flight modes. The flight-control and ire-control systems will be integrated for automatic target tracking. FLIGHT International, 12 September 198/ 817
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