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Aviation History
1964
1964 - 0080.PDF
63 FLIGHT International, 9 January 1964 Typical aircraft equipment suppressors, all capable of operation up to maximum temperature of I5O°C. Right, a range of lead-through radio interference suppressors em- ployed in aircraft equipment By careful unit construction and internal assembly, improvements can be made, but for higher frequency performance other types of construction and a four-terminal configuration must be considered. The lead-through network in Fig 6 shows that elimination of the internal inductance of connecting leads results in the characteristic in Fig 5c, with greater efficiency over a wide frequency range.1 A development of the lead-through suppressor is the duct or bushing suppressor. Designed for panel mounting, it produces a characteristic as in Fig 5d, clearly showing that, over a wide fre- quency range, the capacitor does not reach a final resonance and is effective at VHF. The importance of efficient mounting is shown in Fig 5e where a bushing suppressor is mounted on a bracket and produces a characteristic similar to that of a two-terminal capacitor. Three forms of suppressor capacitors are therefore available: bushing suppressors, which may be small for mounting within equipment, but may have limitations of current rating; lead- through suppressors, which are not as efficient as the bushing design, but may be more satisfactory to mount within apparatus; and two- terminal general-purpose suppressors, which may be small, but of limited frequency range. Lead through Conductors direct to Capacitor Electrodes Fig 6 Lead-through suppressor network, illustrating elimin- ation of shunt capacitor connecting inductance All the above suppressors may be used in conjunction with in- ductors now widely available in a range of sizes and current ratings. In practice, it has been found that efficient use of suppressor capacitors mainly obviates the need for chokes in suppressor net- works. But certain forms of interference may still require line inductance, although the overriding principle of suppression at source must be observed and the suppressor network mechanically designed to form an integral part of the equipment. Radio interference investigations have been carried out on most civil and military aircraft projects over the past few years to ensure that equipment and aircraft installations are satisfactory and con- form to specification requirements. The projects covered by Standard Telephones & Cables include Viscount, Vanguard, Britannia, Canberra, Argosy, VC10, Hunter, Wessex, Herald, Comet IV and Trident. In these varied aircraft the scope of equip- ment generating interference has increased and ranges from small d.c. motors, actuators and control equipment, through main power- generation equipment, transformer rectifier units, static inverters and rotary inverters to autopilots, flight systems, inertial navigation systems and automatic landing equipment. Individual equipment is measured with standard apparatus to BS 727 over a frequency range from 50kc/s to 600Mc/s, in a double- screened room with the equipment fully operation. For simple equipment, measurement of noise appearing at the terminals is generally sufficient and suppressor components are applied where necessary to the main line-terminals. For complex equipment it may be possible to consider the apparatus as a "black box" and suppress noise appearing on cables issuing from the unit, or alter- % 7 Typical sources of radio interference in an aircraft:— '. fct-pipe temperature servos; 2, fuel filter de-icing actuators; 3, fuel pumps; 4, fuel flow- meters; 5, main alternators; 6, rectifier and a.c. control units; 7, hydraulic pump motors; <•, windscreen wiper motors; 9, flasher unit: 10, oxygen supply regulators; II, fatigue peters; 12, search radar; 13, leading-edge de-icing actuators; 14, leading-edge de-icing inching control; 15, window heating equipment; 16, ice detecting equipment; 17 nurnidistat; 18, temperature control amplifiers; 19, air valve, actuators; 20, motorized ••lector natively to identify the individual interference sources within the unit and apply suppression—generally more satisfactory as it elim- inates interference at source and may save suppressor weight. Based on practical experience, STC have evolved an effective procedure. The equipment is assessed in the unsuppressed con- dition, the noise characteristics studied and suppressor components applied to reduce the noise levels. Radio interference suppression has become more art than science, because it has proved impossible to formulate empirical laws to determine the form of network required. A major step is to assess equipment at an early stage, when mechanical design is not fixed, so that suppressor networks can be more easily integrated with equipment design. Final trials are made on fully representative aircraft, with elec- trical equipment suppressed to specification limits and all aerial installations fitted. The radio interference measurement apparatus is installed in the aircraft and a full flight trial carried out with all sys- tems operating. Noise levels appearing at each aerial termination are measured and any further action prescribed. Assessment of non- representative aircraft has been found to be costly, as the results may be completely nullified by noisy equipment subsequently fitted. All major aircraft manufacturers and government and civil aviation authorities recognize the importance of radio interference suppression, and interest and awareness of the work carried out is increasing. Major military and civil projects in the design or prototype stage are now subject to full-scale investigation. Further study of radio interference and its suppression will relate to interference at very low frequencies, in the light of current navigation developments; suppressor networks for low-frequency systems compatible with modern size and weight restrictions; suppressor components for military environment of operating temperature range and mechanical stresses; assessment of speci- fication requirements for new communications and navigation equipment to ensure that realistic limits are maintained. Manu- facturers should not seek levels of interference on aircraft equip- ment that will impose a severe weight penalty on the complete installation. Techniques and specifications employed in the USA must be kept under review, particularly in respect of the US MIL specifications covering test equipment and limits.
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