FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0828.PDF
826 FLIGHT, 17 June 1955 Details of the working section and arrangements for interchangeability. MACH 3 WIND TUNNEL . . . BALANCE SECTION WINDOW SMALL SLUICE VALVE STEPS is normally required and it is neces-sary to preserve this dryness during model changes, if delays are to beavoided. Two sluice valves are used to isolate the working section at theend of each run, the valves taking only li minutes to close or open. Theworking section inlet is then released to atmospheric pressure, the windowsswung open, model adjusted, windows closed, wet air sucked out and sluicevalves opened again. The whole sequence from supersonic runningback to supersonic running can be achieved in five to ten minutes. Inthis way a large number of useful readings can be taken per hour onboth short and long runs. The tunnel shell is of orthodoxfabricated welded-steel construction made in sections and bolted togetherwith inset rubber joints. The cooler, elbows, contraction and large end ofdiffuser are rigidly fixed into re- inforced concrete foundations, theremainder being supported on rollers. The shell is 6ft in diameter from the air cooler outlet throughthe first elbow, which contains an eliptical cascade to turn the air through 90 deg. This elbow is then connected to a square-sectionelbow by means of a transition section enclosed in a cylindrical shell. This construction avoids the use of thick fiat plates, andthe cavities so formed are vented to allow the pressure on the inner plates to equalize. The square elbow also contains a rectangularcascade to turn the air again through 90 deg into the screen section in which are fitted four fine-mesh domed screens mounted inwooden frames spaced at 6in intervals. The screen section is attached to the contraction which is made from four cast-ironribbed plates suitably profiled and bolted together. This con- traction in turn supports the large valve which is bolted to itsdownstream end. The diffuser tapers from lft lOjin x 2ft 6in to 6ft 2in diameterwhere it branches into the two compressor inlets. A safety net is provided at the end of the diffuser at an angle of 45 deg to theaxis, and a Jin mesh screen is provided at each compressor inlet to protect the rotors should die model break or become detachedfrom its mounting. Thermal expansion in the diffuser is com- pensated by a specially designed expansion-retraction joint whichcan also be compressed by hand-operated screw-jacks to create a gap at either end of the working section following the removalof the working-section quick-release bolts. Bellows-type copper expansion joints are provided before the small compressor inletand at both compressor oudets, one being attached to the swinging elbow. The changeover from series to parallel operation at M=1.8involves the swinging of the large pipe elbow and the operation of the 3ft valve shown in the main diagram. It is also necessaryto change the small side plates of the balance section to cater for appropriate working section width, which has been done in fourto six hours. Working section changes can, however, be per- formed in less than one hour widiout disturbing die model. In the manufacture of the working section, rigidity and intervalsmoothness were the main objectives. It was decided to avoid the conventional steel box containing replaceable wooden liners.For maximum rigidity, top and bottom of the working section were machined from iron castings, designed as a channel sectionto the approximate contour of the calculated nozzle, with integral cross ribs and end flanges. The contoured surfaces of diese cast-ings, which form the horizontal walls, were profiled from a cam bar manufactured to accurate limits. A pair of castings is requiredfor each Mach number but the finished cost is comparable to that of wooden liners, while subsequent shrinkage is eliminatedand distortion greatly reduced. The nozzle castings were bolted between identical steel side plates, the inside faces of which wereground flat. The nozzles are of such a length that any shock- waves thrown off from the joint between nozzle and liners willpass behind the model. The windows for flow photography must be opposite the modeland hence their leading edge must be ahead of it. This possible discontinuity in the surface leading edge of a removable windowalways presents a difficulty, particularly with circular windows which give rise to a shock wave of complex shape. It was decidedto employ a rectangular window and frame, the vertical leading edges of which might give rise to a pair of plane crossed shock- NOZZLE ADJUSTMENT FOR BOUNDARY LAYER BALANCE BOX WORKING SECTION REMOVAL TROLLEY waves in the working section. Provision has been made to adjustthe model longitudinally in the tunnel to enable these shocks to pass in front of the nose, and the reflected shock from the walls topass behind die tail of the model. The windows are hinged on to die balance section and only the portion comprising nozzleblocks and side plates is wheeled out on the trolley when changing Mach number. A boundary-layer allowance of 0.002in per inchhas been provided on all four walls. The balance section consists of a rectangular shell witii glasswindows in the top to be used for yaw measurements. Into this shell are fitted the hinged liners, the vertical ones engaging widithe small side-plates bolted to the balance-section shell, a tapered support being fitted between shell and liners when set for seriesworking of the compressors. On to the bottom of the balance section is bolted the balance box, in which the balance-carryingand incidence mechanism is mounted. The incidence gear consists of two struts carrying the balanceand model, keyed together and each driven by an electric motor. A third motor traverses the whole assembly horizontally parallelwidi the tunnel axis. By moving both struts together the model may be raised or lowered and by relative movement the angle ofincidence may be changed. Controls are provided whereby any incidence angle in the range from —6 deg to +45 deg (in incre-ments of 0.1 deg) can be selected on a dial, the servo mechanism stopping the gear when die selected incidence has been reached.The incidence-gear control unit ensures mat the model rotates about a predetermined point during change of angle of attack—giving a movement similar to diat obtained widi a "sector" type gear of variable radius and position. Thus the model can be sopositioned as to avoid the worst pair of crossed shock-waves in die tunnel. The front end of the model is attached to die internal strain-gauge sting, and by measuring bending motions at two stations in perpendicular directions the normal force, side force, pitchingmoment and yawing moment can be calculated. These leave rolling moment and drag force to be measured by separate balancesat the rear of the model. The roll balance consists essentially of a heavy spindle mountedon ball-races and restrained from rotating by a strain-gauged cantilever attached at the fixed end to a gearbox. The gearboxand spindle are driven by a small electric motor thus enabling die roll angle to be changed remotely. A combination of pitchand roll will simulate any given combination of incidence and sideslip angles. Rolling moments are measured by the strain-gauged cantilever. The roll-balance body houses a potentiometer on which slides a lightly sprung platinum wiper attached to thespindle. This potentiometer togedier widi die associated roll servo equipment enables die angle of roll to be adjusted remotelyover 200 deg to an accuracy of 0.1 deg. To avoid possible trouble due to mains pick-up, the six-channelself-balancing strain gauge equipment houses a 625 c/s gauge supply derived from a high-stability oscillator. The sensitivityor "balance range" is adjustable in six steps, die least sensitive range giving a full-scale reading with 0.6 per cent strain and themost sensitive range 0.01 per cent strain. Each channel incor- porates a step repeater transmitter, coupled to the motor in the Continued on page 849)
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
