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338 AIRCRAFT ENGINEERING December, 1936 Th e Type of Equipment Used in a Modern Electrical Aero-Engin e Test Laboratory N Figure 1 is shown an example of a purely electrical dynamometer, which uses two electrical carcases in tandem; its range of power and speed being suitable for dealing with medium-sized aero-engines and the more powerful automobile and lorry engines. Its speed range is from 500 t o 3000 r.p.m. and its maximum continuous absorption 240 b.h.p. The two carcases arc yoked together by steel members, and th e two shafts are also coupled together by a flexible coupling. Both carcases are mounted on anti-friction trunnion bearings, so as to permit the utilisation of th e principle of torque reaction for the measurement of load. An interesting feature is that by a simple arrangement of levers the steelyard which measures the load caters for forces in either sense, so that it indicates torques arising either from the absorption of the engine power output, or from the use of electrical power to motor the engine. Official Requirements Engine testing regulations in this country necessitate the employment on official tests of a form of dynamometer in which the re actionary torque must be measured, and it is th e general practice to instal for such purposes a form of torque weighing mechanism of which th e accuracy can from time to time be easily checked. For this reason simple and direct weighing gear such as is shown in Fig. 1 is nearly always employed, but the development of the practice of enclosing the operators in a sound-proofed cabin has led t o th e adoption in some cases, whethe r with electric dynamometers, control panel. This is laid out with the In our issue for November we gave or th e more generally employed hydraulic dyna minimum of electrical instruments, the system some details of typical modern Electric mometers, of system s permittin g the load indica employing variously coloured lamps to signal Dynamometers . This month we are to r to be placed remote from the dynamometer essential information in a way which is quickly giving a further review, dealing particu in the cabin. Here again, however, a mechan understood by th e ordinary test-shop operative. larly with a combination form of testing ical system of links and shafts, as simple Engine control levers arc mounted on th e same machine , partly electric and partly and frictionless as commercially possible, is panel, and side-by-side with it is usually placed hydraulic, which has some important th e general practice, as it readily permits a further panel carrying the normal instruments, advantages as regard s economy of first of checks on its accuracy, needs no calibra gauges, flowmeters, etc., pertaining to the cost and convenience of operation tion, and is thus more readily acceptable by engine itself. Air Ministry inspection staff. I n Fig. 6 we illustrate a close-up view of a in b.h p., the sum of the two readings being, further electro-hydraulic unit, which also of course, the effective engine output. Electric B.H.P. Indicators gives truly accurate motoring and absorbing The power absorbed electrically is converted Where routine tests do not demand such power readings by torque reaction. Interlocks into current which feeds the separate motor arc employed to ensure safe and convenient high standards of accuracy, electrical means of driving the fan, an d renders th e latte r independ operation of the plant, and the control panel indicating b.h.p. are sometimes employed, ent of the town's electricity mains, so tha t the in all cases carries means for controlling the such as by the wattmeters (calibrated in b.h.p . main plant with its auxiliaries becomes a dynamometer (both electric and hydraulic from motor efficiency curves) incorporated self-contained unit generating sufficient elec in the electric dynamometers shown in Fig. 2 . portions) from a distance. For example, tricity for its own needs. These machines are now installed in the tost a small geared motor is used to vary the Fig. 4 shows a further form of electro- shop of a large motor manufacturer in this power-regulating devices of the hydraulic hydraulic dynamometer under construction country, using a battery of several score of dynamometer, and apart from built-in auto in the makers' works ; in this machine also matic regulation this motor is also controllable machines of a similar nature ; and the same th e electric dynamometer bye-passes to the from the panel by push-buttons. principle of power measurement is employed hydraulic dynamometer powers in excess of for part of the load in the large aero-engine its own capacity, but in this case both carcases test plant shown in Fig. 3 . Accurate Torque Readings are mounted on trunnions and yoked together so tha t truly accurate torque-reaction readings Whatever the proportion of load absorption Electro-Hydraulic of motoring and absorbing powers are given. between electric and hydraulic portions of the This is an electro-hydraulic dynamometer ; By means of a clutch the shaft of th e hydraulic dynamometer, the torque readings remain portion can be disconnected from that of the th e large wind tunnel and fan supplying the accurate. These electro-hydraulic units for electric portion, while remaining coupled to cooling blast over the engine are also shown, aircraft engine testing are of a particularly th e engine, and in this way the superior range flexible nature, as the very wide scope of the small electric motor at the right-hand side of powers and speeds of th e hydraulic portion powers and speeds inherent in the hydraulic being only for th e purpose of turnin g th e engine can be fully exploited for testing specially portion can be used to the full. They are slowly. The engine power is partly absorbed powerful or specially high-speed engines, with naturally less expensive than the entirely- by an electrical machine, the remainder being out being handicapped by the more limited electrical type; in addition their inertia is automatically transferred to a hydraulic dyna scope of th e electrical portion. usually somewhat less, a point which usually mometer. In th e latter, powers are measured by torque reaction, while the electrical absorp needs to be taken into account in considering A view is given in Fig. 5 of another unit tion is indicated by a wattmeter calibrated airscrew shaft stresses. of this type completed, together with its
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Dec 1, 1936
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