FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0232.PDF
232 WIND TUNNELS, MISCELLANEOUS... In the spacious and well-laid-out observation room, steps haveagain been taken towards a high degree of automatic instrumenta- tion. Pressure-distribution readings from a bank of 50 auto-matic manometer dials are photographically recorded, sting- mounted electric strain-gauges and recording equipment areused to measure and record the aerodynamic forces and Schlieren optical apparatus enables full observation of the shock-wavepattern to be made. A test run at M = 1.6 with a straight stub-wing model wastaking place as we entered. The Schlieren screen here was in Technicolor; blue compression shock-waves and yellow expan-sion waves produced a complex pattern around the model silhouette. The strange device illustrated on page 231, and known as theVertical Spinning Tunnel, was next on our list. Its main pur- pose is to enable the behaviour of free models in spinning flightto be observed, and particularly to investigate methods of recovery from a spin. The installation is almost complete and will beginoperation in a few weeks' time. The tunnel consists of a vertical steel pressure-shell, some 80fthigh and 45ft in diameter, with a fan and internal ducting to provide an airflow up the central working section and down theouter annular return circuit. The working section is 15ft in diameter and can accommodate models of up to about 3ft wingspan (exterior photograph, p. 192 in last week's issue). The tunnel can be pressurized to 45 lb/sq in (equivalent to 4atmospheres), and arrangements are provided whereby the model can be launched into the airstream and brought back to thelaunching chamber by remote control. The model can be viewed through a periscope, and television equipment is also to be fitted. The airflow is produced by a fan driven by a 1,500 h.p. electricmotor, and sensitive speed control is provided by hydraulic adjustment of the pitch of the fan blades. The operator adjuststhe upward airspeed until it equals the rate of descent of the spinning model, so that the model remains at approximately thesame height and can be held under continuous observation. Maximum airspeed is 60 m.p.h. at full pressure and 95 m.p.h. atatmospheric pressure. Apart from the free-spinning tests the tunnel can also be usedfor aerodynamic force measurements on a model mounted on a rotating support. Measurements thus made are complementaryto the qualitative results obtained in the free-flight tests. The 4,000 h.p. compressor used to pressurize the spinningtunnel also provides a common air supply for the four Small Supersonic Tunnels forming N.A.E.'s high-speed laboratory.This supply enables tunnels with working sections up to about 9in square to be run at Mach numbers up to 3.5. As Mr.M. B. Morgan (Deputy Director of the R.A.E.) put it, "These are really tools for the long-haired boys"—and they are usedmainly for pure research along advanced lines. An auxiliary pump is used to evacuate the tunnel circuit or topressurize up to four atmospheres, and a conventional system of air driers and coolers is provided. Three tunnels have so farbeen in operation, a 4in-square and a 5yin-square working section for tests on tunnel designs and an 8in X 9in tunnel for general use. The Low-speed Tunnel at Bedford is a massive concretestructure having a working section of 13ft X 9ft and being capable of speeds up to 300 ft/sec. Although the main function of N.A.E.is to obtain design information for high-speed aircraft, knowledge of the performance of those aircraft at low speeds (and, in par-ticular, knowledge of control effectiveness during landing and take-off) is also essential. The low-speed tunnel is used, too, formare fundamental investigations into aerodynamic theory. The air is circulated around the closed circuit by a six-bladed30ft-diameter fan driven by an electric motor of 15 h.p. To ensure a low turbulence-level, a large settling chamber measuring46ft across is included and is fitted with fine-mesh wire screens. The mammoth scale of engineering involved in the 8ft-square supersonic tunnel at N.A.E.; token during construction, this photograph shows part of the main tunnel circuit, including corner, cooler and settling chamber. FLIGHT, 25 February 1955 The wooden working section is suitable for tests on modelswith wing spans up to 10ft and the usual measuring equipment is provided, including an automatic six-component overheadbalance for measuring aerodynamic forces, and manometers tor obtaining detailed pressure-distributions. The prototype of anautomatic recording manometer, in which the levels of 20 tubes are registered by an electric typewriter, was in use on the occa-sion of our visit. A further four of these multi-tube installations are to be fitted.Auxiliary plant for the tunnel includes compressors and evacua- tors for producing flow for boundary-layer control on models, andalso electric equipment for supplying small motors on the model, to drive airscrews or a fan producing jet-flow.Without doubt, the most interesting project seen at N.A.E. was the huge 8ft X 8ft Supersonic Tunnel now approaching com-pletion, in which models up to 6ft span will be tested up to Mach numbers of 2.7. An idea of the scale of engineering involvedin this plant can be obtained from the accompanying photograph, but only by actually seeing the whole complicated web of heavymachinery (the installation includes its own power station) can one appreciate fully the size and extent of this one piece of equipment.The tunnel is of the variable-density type, and the air pressure can be varied from l/10th to 4 atmospheres. Scale effect isminimized, as in other tunnels, by using as high an air pressure as possible, but in practice the working value is limited by con-siderations of model strength (particularly during the unsteady- flow conditions when starting up or stopping the tunnel) and, atthe higher speeds, by the amount of compressor power available. The tunnel structure is a steel shell weighing some 5,000 tonsand with a maximum diameter of 47ft. Flexible joints and sup- ports are provided to permit thermal expansion, and massive con-crete foundations were required to carry the thrusts due to the internal air pressure. The air is circulated by a large ten-stage axial-flow compressorwhich absorbs 80,000 h.p. Four stages are always in circuit, and are adequate for subsonic and low supersonic speeds, whilethe remaining six are brought in for the higher supersonic velocities. Of this 80,000 h.p., 68,000 h.p. is provided by a largeA.C. motor, and the remainder by a pair of D.C. motors for starting and for precise speed control. To attain maximum flexi-bility, an electrical generating station, to be powered by two 20 MW gas-turbine sets, is being built on the site. When com-plete, it will supply the 8ft X 8ft and also other tunnels. The most intriguing feature of this altogether intriguing tunnellies in the method of changing the tunnel-wall contour to obtain the required speed-range. Instead of having changeable linersof various shapes, two of the tunnel walls will themselves be con- tinuously variable in contour. They will consist of flexible steelplates, each 62ft long, 8ft wide and one inch thick. Thirty pairs of hydraulically operated screw-jacks, approxi-mately equally spaced, bend the flexible plates to the aerodynamic shape required. Each jack has its movement controlled by anelectro-mechanical device which converts electrical impulses from. a telegraph tape-reader into small linear movements which openthe hydraulic valve. The position of each jack for the whole Mach-number range of the tunnel is therefore programmed as aseries of holes on a length of paper tape. The accuracy with which the required shape can be maintained is indicated by thescrew-jack movement per hole in the punched tape; it is 0.0025in, or, in regions where the movement is critical, only 0.00125in. Downstream of the working section of the 8ft X 8ft tunnel aretwo diffusers; the first a variable-geometry supersonic diffuser with movable side walls, enabling the shock-wave system to beset up as efficiently as possible (in conjunction with a particular position of the flexible walls); and the second a fixed subsonicdiffuser. At present the supersonic diffuser is mounted on a wheeled base: this is for installation only, however, and the shapeof this section will normally be remotely controlled. The tunnel's auxiliary equipment includes compressor andevacuator plant (totalling over 8,000 h.p.), silica-gel drying plant, and a cooling system which includes a large tubular cooler in thetunnel circuit which reduces the air temperature from 150 deg C a?^ fe^DB "^ compressor to the normal working temperature of 40 deg C. N.A.E.'s 8ft X 8ft installation is an extreme example of whathad become obvious in almost every tunnel seen on our two-day programme—the immense amount of space required by theequipment needed to blow, compress, evacuate, cool and dry the air, compared with the size of the tunnel itself. The workingsections, where most attention normally focuses, and in some cases the whole tunnel circuit, were seen to be dwarfed by thesheer weight and volume of plain heavy bread-and-butter engineer- ing required to provide power and auxiliary processes With these big N.A.E. tunnels, Sir Arnold Hall had pointed °Uw l1^ if3" °^ °!F VisiV came biS constructional problems,which had been solved only by effective co-operation between the Ministry of Supply Ministry of Works and the various contractors.Certainly the resulting facilities, coupled with those at Farn- borough, are unsurpassed at any other establishment.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
