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Aviation History
1957
1957 - 1752.PDF
842 FLIGHT WESSEX • . • The tail rotor drive shaft is taken back through the tail cone to an intermediate gearbox beyond the tail hinge-point and thence up to the tail-rotor gearbox. The transmission system is essentially similar to that in the American S-58, with the exception that the absence of clutch allows a marked simplification. Rotor Head and Blades. The main rotor head follows the classic Sikorsky design-pattern. It is of light alloy construction and incorporates offset flapping and drag hinges employing Timken tapered roller bearings. The flapping-hinge offset gives the machine a remarkably good e.g. range, comparable to that on larger fixed-wing aircraft. The limits are from 9 deg aft to 7 deg forward of a vertical datum line below the centre of the rotor head. The head has a built-in 3-deg forward tilt which reduces the characteristic helicopter nose-down attitude in for- ward flight. Fuselage attitude on the ground is approximately 1 deg nose-up and, with the aircraft normally loaded, the hovering attitude is also slightly nose-up. The larger size of the rotor head has enabled a much neater design to be effected than in earlier models. The drag-hinge dampers are concealed within the head and, in contrast to the arrangement in previous models, a common reservoir for damper fluid is mounted centrally on top of the head. Spring-loaded centrifugal droop and anti-flapping stops are fitted. The latter prevent blade-sailing when the rotor is being started or stopped in high winds. As a result it is possible to operate the Wessex in surface winds of up to 50 kt without danger of excessive blade- coning. The blades are folded by a method similar to that in the Whirlwind. Rotor-head control is also of conventional Sikorsky design, incorporating a tilting swashplate actuated by two lateral and one fore-and-aft hydraulic jacks, with the azimuth star above carry- ing the connecting rods to the blade-pitch arms. Both main and tail rotors are four-bladed and both have all- metal blades. The main rotor is 56ft in diameter. Blade construction embodies a D-shaped extruded alloy leading-edge spar, with the trailing edge attached in sheet-alloy "pockets." The tail rotor is of 9ft 4in diameter and its blades are fashioned from sheet alloy. A conventional tail-rotor spider, operated by rudder pedal movements, controls the collective pitch of the tail-rotor blades. Range of movement is greater than in previous heli- copters, being from plus 30 deg to minus 20 deg. Hydraulic damping is incorporated in the rudder-pedal control run to pre- vent excessive loads being applied suddenly to the tail-rotor by the pilot. Cockpit and Control Systems. The basic cockpit layout of the Wessex is essentially similar to that of the American S-58, but there are naturally differences in the engine instruments and controls. Access is normally gained to the cockpit through the two hatchways at the forward end of the cabin, but external steps are also provided to enable the pilots to climb up and enter through the sliding windows on each side. This method will be useful when the heli- copter is engaged on freighting duties and the cabin is loaded with bulky equipment. Full dual control is fitted, with the right hand for cyclic-pitch control and the left for collective-pitch and throttle lever. The captain's position is on the starboard side. The rudder pedals carry differential toe brakes operating on the main wheels, and a parking brake lever is located centrally between the two pilots. Fuel cocks and fuel transfer and pressure controls are on the central console, while the rotor brake lever is mounted centrally overhead. The cockpit is, in fact, a slightly larger version of that in the Whirlwind, and visibility is substantially the same. The side panels are slightly bulbous, making it possible to see directly aft or below by leaning over in the seat. As in the Whirlwind, the rudder pedals are adjustable for leg length. Two separate hydraulic servo-systems, primary and auxiliary, are provided to guard against control-system failure. All controls have duplicate power operation with the exception that the rudder pedals are connected only to the auxiliary system. The hydraulic pump supplying power to the primary system is driven by the main gearbox (and, therefore, remains in operation during auto- rotation) and the auxiliary-system pump is engine driven. The two systems come together to actuate the main control jacks at the swashplate. System pressure is relatively low, at a normal operating value of 1,400 lb/sq in. Both systems are normally in operation at the same time during flight. Thus, in the event of failure of one system the other is already coupled. A three-position switch on a Arrangement of bag-type fuel tanks in the Wessex: Forward (right) and rear groups, with filler and header tank on left. control box on the captain's collective-pitch lever enables either system to be switched out if desired. The other switches on the collective-pitch lever control the operation of the landing light. On the cyclic-pitch stick hand-grip are placed another series of switches, controlling the operation of the hydraulic hoist, radio and intercom, and die autostabilizer and stick-trim systems. The last-mentioned can be used irrespective of whether autostabiliza- tion is switched in or not. Stick-trim is brought into operation by a switch on the overhead cockpit panel. The system applies an artificial spring feel to the stick and holds it lightly in a central position. Control movements away from the central position will meet an increasing artificial stick-force. If a new stick position is required for a different flight condition the stick-trim button on the hand grip is depressed whilst the change is being made, and then released at the new stick position. Thus the datum of the artificial spring-feel system is reset and will hold the stick in the new position. Autostabilization in the S-58 is a Sikorsky system with Lear gyro and amplifying components. This equipment will also be fitted in the Wessex. It is primarily a manceuvring-type system, but it does also perform the functions of the conventional automatic pilot. The system control panel is placed centrally on the instrument panel within reach of either pilot. Autostabilized control channels in roll, pitch and yaw are provided, with appropriate trimming facilities, together with barometric control of altitude. The system works through the pilots' standard controls and can be over- ridden by the pilot. The button on the cyclic-stick hand-grip is provided so that by depressing it the pilot can turn the helicopter on to a new heading and release the button, when the aircraft will maintain the new course; otherwise the autopilot would bring it back to the original heading after a control movement. A similar device, in conjunction with which the hand-grip button may also be used, is provided on the rudder pedals. In this case it takes the form of a micro-switch which cuts out the autostabilizer yaw channel whenever the pilots' feet are placed on the pedals The autostabilizer operates through the auxiliary servo system. Further extensions to the autostabilization system to facilitate the handling of the helicopter in its anti-submarine role are also included. The remaining instruments on the instrument panel are con- ventional, including standard blind-flying equipment and a radio altimeter. The turbine installation calls for a number of engine instruments not normally seen in helicopters, such as jet-pipe temperature gauges, and of especial interest is the torquemeter gauge which takes the place of the conventional boost or manifold- pressure gauge. The torquemeter itself is located in the turbine reduction-gear casing and permits exact measurements of the power output of the turbine drive shaft. All instruments on the panel have pillar lighting. General Operating Considerations. In the Westland Wessex the Royal Navy will have a helicopter which is certain to afford considerable operational advantages over existing equipment. Some idea of the improvements may be gained by study of this com- parative table: — Normal maximum speed (kt)Normal cruising speed (kt) Maximum sideways-flight speed (kt)Maximum rearward-flight speed (kt) Maximum wind speed for safe operation (kt)Disposable load including fuel and crew (Ib) The obvious advantage of doubling the lifting capacity with the larger machine opens up new operational prospects in the carriage of anti-submarine detection and strike weapons. Equally important is the ability to continue operations under adverse weather conditions. The figures for disposable load must obviously be reduced to allow for pilot and fuel, but the Wessex has provision for an external load-carrying sling on which bulky freight can be io to the extent of 4,000 lb. The Napier turbines in production aircraft will be adapted to operate on Avcat, the standard Naval turbine fuel, thus ob iaung the need for a carrier to have special fuel for her helicopter: The modification merely entails a recalibration of the fuel sys;em to allow for the different density; the operational efficiency 'X >* turbine is not impaired. B.T.H. cartridge-type starting i-Pn will be used. Alternatives to the anti-submarine role are rescue, communica- tions, troop transport and casualty evacuation. As an ambulance the Wessex can carry eight stretcher cases in two tiers of ^ur °" either side of the cabin. For civil use the helicopter •$& equipped with accommodation for 12 passengers and lugra^ addition to the crew of two—as, indeed, is already being <5ane ^ the S-58s on inter-city operations with Sabena on the Ce ituien •: and New York Airways in the United States. Whirlwind95 80 15 15 35 2,510 Wnn<125 10035 30-35 50 5.000
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