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Aviation History
1951
1951 - 2205.PDF
572 :p£i<,GHT, 2 November it qj POWER-OPERATED FLYING CONTROLS HYDRAULIC POWER SUPPLY PUMP DELIVERY IS REGULATED AND DIRECTION OF FLOW CONTROLLED BYDISPLACEMENT OF SLIDE PILOTS CONTROLLINKAGE , VARIABLE-DELIVERY PUMP PILOTS CONTROLLINKAGE valve-resetting action takes place as the latter moves in response to the initial valve opening. For electrically driven systems the valve may be discarded and control obtained by means of a variable-stroke pump, as shown in Fig. 3. Here the pump delivery is regulated from zero to a maximum in either direction by varying the eccen- tricity of the cam track on either side of a central neutral position. The lever operating the eccentric is linked to the jack and to the input in the same manner as the valve of Fig. 1. In order to ensure that the jack is completely filled with fluid when the pump delivery is nil, and to place this fluid under a positive pressure at all times, a further backing pump is required in order to feed fluid at a pressure of about 500 Ib/sq in into the jack cylinder through non-return valves. The pressurization of the fluid reduces the volume of any air bubbles which may be present and substantially increases the rigidity of the jack. A variable-stroke-pump type of powered control is3 of course, suitable only for use in systems where the pump is directly coupled to the jack, i.e.5 electrically driven systems. Its use on small aircraft undoubtedly imposes some weight penalty, since the conventional engine-driven hydraulic pump with pipe-line distribution of power is lighter. Screw Jacks.—It has already been mentioned that rigidity requirements have led to the development of screw-jacks driven by hydraulic motors. An irreversible screw-jack is comparable with a structural member from the rigidity view- point, and the natural frequency of a control surface operated by such means will be very high. Fig. 4 shows a screw-jack powered control. It will be noticed that the arrangement of control valve and follow-up linkage is identical with that used for the equivalent valve- controlled type of hydraulic jack. As with the hydraulic jack, electrically driven pumps may be close coupled to the hydraulic motor, or the variable-stroke pump may be applied as shown in Fig. 3. In a further interesting variation, the hydraulic pump and motor form a compact unit which may be mounted in the fuselage, where temperatures are comparatively high and servicing in flight is possible, whilst rotary shafting transmits power from the hydraulic motor through the aircraft to screw jacks mounted adjacent to the control surfaces. If advantage is to be taken of the high rigidity of the screw jacks it is, of course, essential that the latter should be placed as close as possible to the control surface in order to avoid elastic effects in the intermediate linkage. Screw-jack Efficiency.—The efficiency of a screw jack is readily controllable during the design stage by adjustment of the helix angle or the type of thrust bearing. If the efficiency from input to output is less than 50 per cent, then the effi- ciency of the screw operating in the reverse direction—i.e., feeding-back forces from what is normally the output to the input—will be zero, and the screw will be irreversible. For normal efficiencies in excess of 50 per cent the screw will become reversible with a reverse efficiency rising from zero to 100 per cent as the normal efficiency rises from 50 per cent to 100 per cent. Generally speaking, efficiencies in excess of 70 per cent are obtainable only by the use of the well-known Fig. 3. (Left) Electrically driven system: method of control by variable-stroke pump. Fig. 4. (above) Screw-jack powered control. recirculating-ball types of screw, with their low friction. When a screw jack is used in a powered flying control its efficiency is of importance in connection with two apparently mutually opposed design requirements, (i) the necessity of maintaining irreversibility and a high degree of rigidity, and (ii) the need to economise in installed power. If engine-driven pumps are used as a source of power, the latter consideration is of less importance than in the electrically driven pump, where the penalty of inefficiency is not only an increase in electric-motor weight, but the need for larger pumps and hydraulic motors; in all cases, however, the efficiency of the screw influences the total weight of the installation. Fortunately, a sufficient degree of rigidity (many times greater than in the equivalent jack designs) can be obtained when using a reversible high-efficiency screw in conjunction with a suitably geared hydraulic motor. The improvement in the case of the screw jack is due to the reduced volume of fluid subjected to compressibility. In a hydraulic jack the entire quantity of fluid required to complete a full stroke is contained within the jack and, when a load is applied, is compressed. A hydraulic motor geared to a screw jack, however, completes many revolutions during the jack stroke, but the quantity of fluid in the motor at any instant corres- ponds only to one revolution and the effect of fluid compressi- bility is proportionally reduced. It will thus be seen that on electrically driven systems a good compromise is possible, the use of high-efficiency screw jacks resulting in a negligible loss of rigidity, although in certain instances irreversible screw jacks may with advan- tage be employed in conjunction with engine driven pumps. (To be concluded) IRAQ SURVEY CONTRACT THE Survey Department of the Iraq Government has awardedan aerial survey contract to Hunting Aero Surveys, Ltd., for aerial photography and contour-mapping of a large part of Iraq. The contract is the first step in the economic development of the country as drawn up by the Iraq Government Development Board and from the survey will be prepared accurate maps which are to be used for many development projects in connection with road construction, planning of water supplies, land settlement and power schemes. The photographs will also be used for preparing a geological map of the area, as part of a proposed mineral survey of the country. The aircraft—a Percival Survey Prince—and crew have already started on the photographic operations, which will take two years to complete. The expedition is in the charge of Capt. H. F. Warwick, D.F.C., and the photographic manager is Mr. J. D. L. Symington, who was a navigator in Coastal Command during the war. An aircraft engineer and two ground photographers are also accompanying the expedition, which is a self-contained unit with its own processing equipment. The company's senior ground surveyor has also arrived in Iraq to supervise the ground control for mapping. The film will be developed at the company's expedition base in Iraq, one set of photographs handed to the ground-survey staff, who will at once brief the ground control for the air mapping. Hunting Aero Surveys, which is the parent company of a world- wide group of aerial survey companies, has been engaged on air survey in the Middle East for some 15 years.
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