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Aviation History
1949
1949 - 1869.PDF
FLIGHT, 17 November 1949 635 The size and method of operation of the structural test frame areindicated by this view of the Canberra wing (full span) assembled for bending tests. . traversed by leadscrews driven by reversible electric motors. Deflection of the free end of the beam transmits a signal to the motor which moves the iockey weight until the beam is in balance, and a Selsyn records the number of turns of the screw on a counter on the control desk. The weighbeams are fitted with single contacts operating a spring-loaded relay so that the motor runs one way when contact is made and reverses immediately the contact opens. The motor, therefore, is never at rest and hunts about the median position of balance. When the weigh beam is appre- ciably out of balance, a second contact shorts an eddy- current brake, so allowing the motor to run faster until the balanced condition is approached. When the possibility of having a high-speeu tunnel was under discussion up at Warton, the suggestion that a turbo- jet engine might be used as an ejector pump to induce the requisite high-velocity mass flow was propounded only half- seriously. The light-hearted suggestion was then given further thought and it was realized that there was much more to it than the initial flippancy implied. The result was a patent in the names of Messrs. D. L. Ellis, R. G. Fowler, H. C. Harrison, and R. F. Creasey, and the 44m by I2in high-speed tunnel on the edge of the Warton airfield. (The main features of the tunnel were reviewed in the September 29th issue of Flight.) At the time of writing, the high-speed tunnel has had about 54 hours' On this test rig the complete hydraulic system designed for the Canberra is operable through the correct lengths of pipe lines and at the appropriate jack loads. running, and although by no stretch of imagination couldit be called quiet in operation, the fact that the outlet is directed across the Ribble Estuary and the nearest housesin the '' line of fire '' are at Stockport, about eight miles away, has perhaps been the reason why no complaintsfrom residents have been received. The Rolls-Royce Nene turbojet, as may be seen in the illustration, is carried ina naceEe housed coaxially within a swelling in the tunnel trunk and, of the mass flow through the tunnel, approxi-mately one-third is breathed by the Nene, the remaining two-thirds passing through the annular throat outside thepower-nacelle. At the maximum turbine r.p.m. 12,500) the tunnel- quite fortuitously—chokes when empty at the start of the first diffuser section immediately downstream of the work- ing sect^>n, the choking speed being approximately 0.97/0.98 Mach number. With a semi-span model in- stalled, the choking speed is reduced to about M 0.88, whilst, with a larger full-span model for pressure plotting, the speed is further reduced to M 0.84, the tunnel choking in these instances at the model. With a 15m chord model, the corresponding Reynolds number is 5 x io" and the flow is steady. Air is drawn from the settling chamber into the con- tracting intake flare through a gauze filter-box and, after passing through the 3ft 8in by ift working section, is then diffused down to the pumping (engine) section. The, jet efflux, together with the induced and by passed flow, is then ejected to atmosphere down a lengthy trumpet "exhaust pipe." The local humidity at Warton averages on most days round about 80 to 90 per cent and a straight- through tunnel of this type would therefore suffer from condensation troubles were not means taken to lower the humidity at the intake. This is achieved by raising the temperature of the inlet air by approximately 20 deg C by abstracting and returning into the circuit approximately one-tenth of the hot gases delivered from the exhaust. Although the combustion of 1 1b of paraffin liberates 1.37 1b of water and, therefore, the actual humidity of the inlet air is increased, the fact that the inlet temperature is increased makes the inlet air capable of holding a greater quantity of water and, as a result, the overall relative humidity is lower. Mounting and Visualization The normal type of balance is not suitable for use with models subjected to very high speed flows, because the struts cause premature choking and so limit the maximum speed. The most satisfactory method is to carry the model on a sting extending downstream from the tail, the forces being measured by strain gauges built into the model itself. Models of jet-propelled aircraft lend themselves particu- larly well to this method of mounting and, in many instances, the mounting sting can be arranged so as to simulate the effects of jet interference. The higher the designed speed of an aircraft, the more important become the problems of low-speed stability and control. Visualization of flow is often of value, particu- larly in connection with swept-back wings. This can be achieved in a variety of ways: by attaching threads of silk or wool to the model, or by introducing filaments of smoke into the airstream. The latter method is, however, of little practical use because the smoke is visible only at speeds so low that the Reynolds number is much too low for accuracy. A far better method of obtaining flow visualiza- tion as well as useful Reynolds number is to use a water tunnel since, for the same size model and the same flow speed, the Reynolds number obtained in water is 13 times that in air, owing to the ratio of kinematic viscosity of the two fluids. The English Electric water tunnel is similar in principle to a closed-circuit wind tunnel but, of course, is much smaller. The working section is i8in square, and the maximum speed of flow is about 25 ft/sec. Normally, semi-span models of 7 to 8in chord are used, and the flow is made visible by means of a strong light projected through the glass walls of the working section to illuminate finely (Concluded on page 650)
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