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Aviation History
1958
1958 - 0623.PDF
FLIGHT, 9 May 1958 639 Tailplane and e/eyotor control linkages, showing layout of tailplane follow-up and trimming systems. HUNTER F.6 ... him, for aerodynamic loads are in any case too high for him toachieve full deflection. The horizontal tail-control is rather more complex, because itinvolves a combination of hydraulic fully-powered elevator with electrical follow-up and trimming of the tailplane. Follow-up isachieved by a pantograph linkage from the elevator making con- tact with micro-switches which control the Rotax motor changingtailplane incidence. Consequent movement of the tailplane adjusts the position of the micro-switch datum to relieve die switches.These are also separated, so that the elevator will move one or two degrees before the linkage contacts a switch. The result is thatfine control-adjustments, e.g., during formation flying, are made with elevator alone, while tailplane follow-up occurs with thelarger movements needed during manoeuvring at transonic or approach speeds. The "neutral" zone also avoids tailplanehunting. Feel is provided by a standard telescopic spring unit connectingthe pilot's control rods and the leading edge of the tailplane. The neutral or trimmed position is therefore reached when the ele-vator is substantially in line with the tailplane; and stick force remains directly related to stick movement regardless of speed.Overstressing is avoided by jack stalling. A centring effect is pro- vided by a steepening of the rate curve near the neutral position. Trimming is effected quite simply by an electric actuator which,moving simultaneously with the tailplane actuator, alters the datum of the micro-switches controlling tailplane follow-up. Thepilot's control for this purpose is a sliding switch on the top of the control column. As the Hunter control system is a mixture of hydraulic and elec-tric components, and has many electro-hydraulic elements, the consequences of failure or malfunction of either electric orhydraulic supplies or components can be complex. Careful study of the very clear Hawker technical publication on the subject makesclear the various combinations with which a pilot might have to deal. For the purpose of this description some of the remoterintricacies may be omitted. The basic stand-by is, of course, manual reversion, which occursautomatically; and hydraulic accumulators in the tail and aileron circuits (respectively at 1,575 and 900 lb/sq in) can provide severalfull control cycles to allow establishment of trimmed conditions for manual flying. Control forces are then very heavy, particu-larly on the ailerons, but a landing in this condition is perfectly feasible and is a standard training exercise. The tailplane foHow-up will in any case continue to operate, though it may sometimes be better to switch it off. For practice in manual flying, power isswitched off by closing the electrically actuated cocks governing oil supply to the servos. In the event of electrical failure and a need to revert to manualcontrol, a stand-by battery can be used to close these cocks. An engine windmilling after a flame-out will supply enough hydraulicpressure to provide adequate powered control, but in most such cases the pilot is recommended to revert to manual before landing.Prior to reversion it is also highly desirable to adjust the tailplane to an angle suitable for trimmed flight at a moderate airspeed andMach number in order to avoid sudden and heavy out-of-trim stick loads at the moment of disengagement. In the event of tail-plane actuator main-motor failure, a second motor integrally mounted with the jack, can be brought into operation. Openingof the safety cover which protects the switch provided for this purpose automatically cuts the main motor out of circuit. The horizontal tail control is now such that accurate gun-sighttracking is possible at all speeds. At trans- and supersonic Mach numbers the tailplane is the most effective element and its rate offollow-up to the elevator will allow application of g at a rate of about 0.5 g/sec. Stick forces per g are satisfactory, although forhigh I.A.S. they may feel a little light. Stick forces which vary primarily with stick movement rather than with speed are anoticeable feature of the control system, but in this case the effect is not disconcerting. Hydraulics. In the Hunter, hydraulic power is used to operateundercarriage, wheel brakes, flaps, airbrake and the aileron and elevator Hydroboosters. The main supply is drawn from areservoir, located in the centre fuselage, through a Micronic filter and into either the main engine-driven pump or a hand-pumplocated in the engine bay. The latter pump can operate any of the services except the airbrake but is not available to the pilot inflight. The engine-driven pump is a Dowty Vardel two-stage unit pro-ducing a working pressure of 3,000 lb/sq in, measured by a gauge in the cockpit. If pressure falls below 600 lb/sq in a warning lightglows and an audio tone is fed in to the pilot's earphones. A push-switch is provided to reduce this note if required. STANDARD SPRING FEEL UNIT MICRO - SWITCH - DATUM ACTUATOR -EXTENDS FOR A/C NOSE-DOWN TRIM -RETRACTS FOR A/C NOSE-UP TRIM TAILPLANE. PIVTOT POINT TELESCOPC SPRING LOADED STRUT SWITCH ARM A-A/C NOSE-DOWN MICRO-SWITCH B-A/C NOSE-UP MICRO-SWITCH Emergency actuation of undercarriage and flaps is by com-pressed air at 2,000 lb/sq in from two spherical bottles mounted immediately behind the ejection seat. Related pressure-gaugesare located on the port cockpit console. Emergency air enters the primary hydraulic system, but does not affect the powered-control circuits if the normal selectors are in the correct posi- tion for the movement required. Oil displaced by the air isdumped overboard. Emergency wheel-brake operation is by oil at 750 lb/sq instored in two Dunlop accumulators mounted in the nosewheel bay. Most of the valves in the system are electrically operated.The majority of the hydraulic,components are supplied by Dowty. A single jack moves each mainwheel leg and the three coyerdoors which are mechanically linked to it. Mechanical sequencing valves then control the operation of the jack which closes theinboard door and engages a mechanical up-lock for the whole unit. An equivalent arrangement obtains for the nosewheel. Alarge spigot in each mainwheel well locates the leg in the retracted position by engaging a hole in the wheel axle. Undercarriage-mounted micros-switches cut out the retractioncircuit when the weight of the aircraft is on the wheels, but this precaution can be overridden by the pilot and retraction- effectedin emergencies where a ground run must be converted into a slide. An electric switch with "up," "down" and six intermediatepositions controls hydraulic flap extension. Synchronization of the two split flaps is achieved by interconnected idling jacks work-ing at 500 lb/sq in, one at each flap. Pressure balance is auto- matically reset after each retraction. The flaps are intended foruse as airbrakes, but stressed for normal descents only, while the airbrake proper can be used without limitation and is controlledby a sliding switch recessed in the head of the engine-control lever. In order to prevent the airbrake scraping on landing, a micro-switch on the undercarriage breaks the circuit on extension and causes the brake to retract automatically. A doll's-eye in thecockpit turns white when the airbrake is extended. The Dunlop wheel-brakes are operated by a hand-grip on thecontrol column, connected with a relay valve on the rudder pedals to allow differential application for steering. A triple-readinggauge shows main supply and individual break pressures. Skid- ding of the wheels is prevented by Dunlop Maxaret units at eachdisc brake. Gauges on the port console indicate the pressure of the two emergency-braking accumulators. Electrical Installations. Electrics intrude into virtually everysystem of the Hunter, for almost all remote control and indication is by electrical means. Though the ramifications of this system aretherefore very considerable, the methods and techniques employed do not differ from the normal. The main system is of the 28v, single-pole, earth-return,voltage-regulated type, the supply coming from two wide-speed- range generators mounted on the accessory gearbox in the enginebay. They are connected in parallel and stabilized by two 24v, 25 A-hr Varley-type batteries which are parallel-connected to give50 A-hr. Voltage regulators for the generators are mounted in the radio bay, where the batteries are also located. For ease oftesting and servicing, the earth leads have been concentrated at a number of points conveniently located throughout the airframe.Flight instruments, engine-temperature control, radio and radar-ranging all require A.C. at 115v, 400 c/s, three phase; andthis is supplied by two inverters mounted on the cabin floor aft of the ejection seat. No. 1 inverter is the main source for flightinstruments and engine temperature control, while No. 2 acts as a stand-by and as the primary supplier for radar-ranging. If No. 1fails, No. 2 automatically takes over and sheds the radar-ranging load. As with most such arrangements, a test switch is providedso that the change-over may be checked by the pilot. A separate low-voltage inverter serves the gun-firing system; and a 1,600 c/ssingle-phase auxiliary inverter is also provided for the radar ranging. Fuel. Internal fuel tankage in the Hunter F.6 is distributed ineach wing, in the forward fuselage and round the exhaust section of the engine, immediately aft of the fuselage break. The inboarduniversal pylons under each wing can accommodate either Bristol 100-gal plastic drop-tanks or Hawker 230-gal metal drop-tanks,while each outer pylon can take either a bomb or a further 100-gal tank. A very respectable fuel load can be accommodated whenlong ferry flights are required. The whole system is divided into port and starboard sections,
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