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Aviation History
1961
1961 - 0392.PDF
400 FLIGB 30 Ma\ 196. Cockpit layout is simple; the right-hand stick controls fore-and-aft propulsion and on top of it is a twist-grip to govern hover-height. Pitch and lateral trim are adjusted by the left-hand stick. Instruments are kept to the minimum; the larger four grouped dials show selected cylinderhead temperatures in the Alvis Leonides lift engine Off the Ground . . . These gate valves are positioned each side of the T-junction inthe trunkmg. They work differentially fore-and-aft, and in unison side-to-side, in response to movement of the stick. Their action canbe observed from the cockpit, but the control is a fairly coarse one. Stick-load is high, particularly forwards, but in a cruise case afriction nut at the pivot can be tightened to relieve the load. Lateral and pitch control are influenced—hardly more—by astick on the left. It has two planes of freedom. Control surfaces are spring-loaded spoiler flaps over the inner and outer curtainslots, which are deployed to destroy a segment of the air curtains, thus inclining the platform and adding a thrust component to lift.Fore-and-aft spoilers are about 2ft long—the lateral ones rather longer, as the side curtains are weakened by air ducted-off forpropulsion. Maximum inclination that can be achieved with the pitch-and-roll stick is about 1 J°, though that control would be moreeffective were the spoilers larger. It is sufficient to hold the Hover- craft beam-on to a light cross-wind; for anything stronger thesolution is simple—turn into wind. Elevons in the propulsive ducts were found ineffective and were abandoned early on. A pair of adjustable aircraft-type rudder pedals complete thedirectional controls. They operate on rudders at each end of the longitudinal ducts, those at the forward end being for control inreverse. Pedal loads are light, even at maximum speed. Hover-height is controlled by r.p.m. of the Alvis engine, variedby a throttle twist-grip on the propulsion-control stick. There is no "altitude" control other than a tachometer; and, as pitch of thefan is fixed, there is no pilot's control over boost, although a mani- fold pressure gauge appears on the panel. The only other variablecontrol, for the Mabore, consists of a simple throttle lever and friction nut on the port side of cockpit. Initial travel opens thehigh-pressure fuel cock—it is as simple as that. On the hardstanding at Cowes, Peter Lamb ran through thestarting procedure. First, a quick visual check on the plastic pipes carrying kerosine and petrol upwards from the deck tanks; then,petrol pump on; ground-flight switch on; magneto switches on; a prod on the starter button; and the Leonides burst into life. Aturn on the twist grip and the lift engine settled down to a steady rumble at 1,000 r.p.m. The next action was to turn on the kerosinepump, crack open the H.P. cock and hold in the two-stage starter button until the Mabore lit up. A mirror on the cockpit keeps theengine in view. Careful throttle-control is necessary to keep j.p.t. under 600°C until the Mabore is properly burning and turning at11,000 r.p.m. We were ready to go. At 1,600 r.p.m. on the lift engine Nl began to lighten, and as theengine accelerated to its governed maximum of 2,900 r.p.m. the craft rose to hover lOin over the hardstanding. We were at maxi-mum weight of 13,3771b and slightly nose down. Manoeuvring outside the hangar required judicious use of directional ruddercontrol and some busy pump-handling on the propulsion control stick. The Mabore was left to idle. Lamb switched on the pilot'shigh-speed rotary wiper and a second traverse type on the passenger's windscreen. We turned slowly towards the slipway, accelerated,and rushed forwards and downwards towards the sea. The expected splash never came. The Hovercraft dipped itsnose, lifted and rode steadily lOin above the water, hobbling over the waves like a rowing boat. A great cloud of spray seemed toengulf it completely. Lamb pushed the propulsion stick forward, opened up the Mabore and we gathered speed towards the openSolent. Progress at first seemed disappointing. The Hovercraft seemed to make heavy weather of acceleration, and visibility throughthe spray was virtually nil. But suddenly the water mist began to diminish and Nl pressed forward with most satisfactory urge.The wave which, as we were hovering, had formed in front of depressed water under the hull had been over-ridden. We wereover the hump and on the downhill side of the drag curve. With both throttles wide open the Hovercraft slammed ahead. Noise wasconsiderable. The trim stick was brought back to lift the bows, but the effect was small. Nl covered the water in a series of short fore-and-aft oscillations—like a well-damped car on bumpy road, or the chop of a speedboat (but without the smack of hull againstwater). Occasionally the front skirt slapped against a wave, breaking-off the crest, and as we crossed the wake of a steamerpitch amplitude increased. To stop quickly a considerable momentum must be destroyed.At 30kt Lamb closed down the Mabore, kicked on full right rudder and the Hovercraft rotated, hardly deviating at all from its previoustrack. It pirouetted almost on its vertical axis. Half-way through the turn he snatched back the propulsion stick to give a full 5001b ofreverse thrust and then, as the nose came round, slammed it for- ward again. Still following its original line, the Hovercraft stopped;the pilot snapped shut the twist grip and the Hovercraft came to rest only 50yd from where the spin was commenced. Peter Lambclimbed out to lower the water-speed probe, and we changed places. I wound open the twist grip and the Hovercraft lifted off. Evenwith zero forward speed, directional control was good—drifting slightly downwind I could, without difficulty, point the nose in anydirection. I selected an into-wind heading and pushed the propul- sion control fully forward. Spray mist surged up over the bows anddrenched the screen beyond the capability of the wipers to clear it. Then I opened the Mabore up to full 21,000 r.p.m., with j.p.t. at560°. At lOkt on the sensitive water-speed indicator acceleration was sluggish. Then the Hovercraft cleared the hump, and at 15kt themist dispersed and we skimmed away over the water. Distances shrank and the sensation of speed was exhilarating. A slight beam wind made it necessary to compensate for drift inorder to hold a required track; to make track corrections I used rudder to swing the nose through double the change in head-ing. When the required track-change had been achieved I brought the nose back on to track. Every turn is a skid, and a high-speedturn through 180° consists of one long drifting slide; the analogy with a fast car out of control on an arena of ice is inescapable. Athird of the way round a fast turn the Hovercraft may be riding sideways over the water at a full 30kt clip and with up to 1 \g lateralacceleration. Momentarily it feels out of control—the sensation is unlike that of any other air or water-borne vehicle; in car terms onecan only say that the Hovercraft is in a state of vast oversteer. I reduced hover-height a few inches by throttling the revs by 300r.p.m. and speed increased by 2kt. I couldn't understand why. until Lamb explained that reduced momentum drag through the intakeoffset decreased propulsive thrust. Under ideal conditions Nl has exceeded 58kt water speed, and with the Viper should reach65kt. Future hovercraft may use Doppler speed-measurement technique; a number of trials have already been carried out,and problems of accurate measurement over running water are under investigation. SR-N1 has also been used to investigatenavigational methods. Decca is preferred, and there is talk of a moving-target system being developed, in which the trace movesover a stationary map. There have been demonstrations to the Army and water-borne forces; commercial operators are interestedand there are applications to be explored over land and coastal and inland waters. Nl has carved its own niche in history. Back at the harbour the Hovercraft demonstrated its tractabilityby taxying in the water up to the approach to the slipway. There was a surge of power as it lifted off, rushed up the slipway gradient,checked with reverse thrust, hovered, and dropped quietly back on to the hardstanding.
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