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Aviation History
1964
1964 - 0551.PDF
Air-Cushion Vehicle* WE8TLAND SR.N3 . . . immediately below the input gearbox to the vertical shaft. Westland design and construct their own lift fans. The pattern used in N2 and N3 is a backwards-inclined centri- fugal fan, with 12 three-spar aerofoil blades and overall diameter of 12ft 6in. The assembly is riveted from light-alloy sheet on a cast hub, and requires no spokes to maintain its shape under load. The Dowty-Rotol propellers are derived from those of the Viscount airliner, and have four light-alloy blades with a diameter of 10ft. The blades of each propeller can be hydraulically controlled over a pitch range from +35° to —30°, and extensive strain-gauge running is essential to meet the arduous range of design conditions not met in aircraft (frequent variation of pitch from posi- tive to negative values, operation over a wide range of yaw conditions and opera- tion in the wake of another propeller). Each of the two auxiliary marine propulsion units is a self-contained package consisting of a Rover gas tur- bine, engine-driven accessories and a steerable Schottel-type outboard pro- pulsion unit which can be hydraulically extended into the sea or retracted. Each Schottel unit has a 30in nylon propeller capable of being rotated in azimuth to provide a propulsive thrust in any desired direction, and would be brought into action only when the N3 was operating as a displacement vessel. They may be lowered or raised independently, but are steered together. Control of N3 is effected by means of the propeller pylons, fin and rudder and marine Schottel units. The manner in which these controls operate is as follows:— THRUST AT 6 in ^UNDERSKIRT CLEARANCE 6,000 - ^4,000 - a:a 2,OOO- 2O 4O 6O SPEED (kt) Fig 6 Above, a plot showing the relation- ship between thrust and drag for SR.N3, and the three drag components; below, day- light clearance under the skirt plotted against speed for four weights ranging from 25 to 42.5 long tons 40 SPEED ;)tj products. Each pylon can be rotated through ±30°. Rotation of the rear pylon automatically turns the fin by means of a mechanical linkage; maxi- mum fin deflection is ±20°, and maximum rudder deflection double that of the fin. Operating mode AsACV As displacement craft Command Accelerate forward Decelerate Crash stop Turn left (hovering) Turn left (at speed) Crab to left Turn left (under way) Turn left (hove-to) Driver action Push control wheel forward Pull control wheel to rear Cut engines Select one propeller forward and the other reverse, and depress left pedal Depress left pedal Turn wheel to left Move Schottel switch to left to alter heading As above Control response Propellers go into positive pitch Propellers go into reverse pitch Craft sinks onto surface Pylons ret ate in same direc- tion so that propeller thrusts form couple to rotate craft Pylons rotate in opposition, plus fin and rudder (see Fig 5) Both pylons and fin and rudder rotate in unison Schottels rotate clockwise (seen from above), turning craft to port Turn both Schottels clock- wise through 90° en powers selected Each propeller pylon is turned by a pair of hydraulic jacks in a system energized by the pair of engines which drive that propeller. In the original N2 aircraft-type jacks are used, but those of N3 are standard commercial Unlike the N2, N3 is fitted with a rudder bar. Deflection of the driver's foot pedals causes the craft to yaw about its own Z-axis (Fig 5). In ahead running this control turns the pylons in opposition, so that the lateral compo- FUGHT International supplement, 27 February /96« nents of propeller thrust produce the desired rotation of the craft on to the desired heading. When hovering over a given spot a turn must be accomplished in a different way. The driver must place one propeller in reverse pitch, to counteract the forward thrust of the other propeller, and this automatically reverses the direction in which the reversed propeller's pylon responds to driver demands. This ensures that craft response to all movements of the driver's controls is in the natural and correct sense. Rotation of the driver's control wheel turns both pylons in the same direction (unless one propeller is in reverse pitch). The resulting lateral component of propeller thrust and fin/rudder reaction both rolls the craft and causes it to crab sideways. This control would be em- ployed alone to counteract a cross-wind or a severe asymmetric lateral trim condition. In addition, it is combined with the yawing command controlled by the rudder pedals in order to effect a co-ordinated turn when running ahead at speed. When operating as a displacement craft the two Schottel units have com- plete freedom to rotate together through 360° and thus give thrust in any desired direction. The nylon propeller of each unit has fixed pitch, and can rotate in either direction, and the control of the two thrust-lines, and independent marine throttles, confers complete flexi- bility of craft manoeuvre. Either auxili- ary engine can energize the fin/rudder control system. Traditional DTD. 585 hydraulic fluid is employed, operating at a system pressure of 3,0001b/sq in. Each pair of main engines drives a single hydraulic pump serving the circuit steering the pylon driven by that pair of engines. In addition, each auxiliary engine also drives a pump in a separate hydraulic system serving fin/rudder, skirt raise/ lower and general utilities (operating the cargo doors and the Schottel-unit raise/ lower and steering). If necessary, the hydraulic systems energized by the auxiliary engines can be interconnected. Both auxiliary engines drive a 12kW generator serving a 28V system which includes a pair of 40A-hr batteries. One of the Rover gas turbines runs continu- ously whenever N3 is operating, in order to act as an a.p.u. Security considerations preclude dis- cussions of the extensive military equip- ment for which provision is made in this ACV. But it can be stated that the operating crew will number six dunng most missions, and that basic vehicle equipment includes Decca 969 radar, a Doppler water-speed indicator, and Collins HF, Plessey UHF and stand-by VHF communications radios. 28
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