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Aviation History
1962
1962 - 0280.PDF
282 FLIGHT International, 22 February 1962 GHT SYSTEMS Af.l \£l in wing-Dome flight with gear and flaps down normal aerodynamic controls, also control jet reaction valves at the nose, tail and wing-tips. When the nozzles are away from the horizontal position, engine-bleed air is automatically supplied to these valves to provide attitude control in non-conventional flight. A simple form of single-channel autostabilization is available but, in the event of unserviceability, hovering and transitions can readily and comfortably be carried out without it. Control of the thrust vector is by a single lever alongside a conventional, vertically mounted engine throttle. The only cockpit control extra to the conventional ones, this lever is moved backwards to rotate the engine nozzles downwards. The engine is started up with the nozzles rearwards, to minimize any ground erosion or exhaust gas recirculation effects, and they are only selected down immediately before take-off. The aircraft is best lifted off by increasing power fairly rapidly until unstick occurs. Height is then maintained by natural manipulation of the throttle, a suitable hand-rest being provided to facilitate precise movements when required. Compared with a helicopter, very slightly more attention is required for height control, because throttle move ment gives an acceleration rather than a velocity change, ground- effects tend to give a small unstable cushion rather than a stable one, and the available thrust/weight ratios tend to be less. Neverthe less, the crisp response of the Pegasus engine combined with a suit able throttle-to-engine gearing makes the pilot's task straight forward. As with a helicopter, judgment of height and rates of ascent or descent are made visually in the normal manner. The view from the conventional fighter-type cockpit of the P. 1127 is adequate in this respect, special attention having been given to the provision of good visibility over the nose and sideways. Positioning of the aircraft over the ground is maintained by suit able adjustment of its attitude, using the stick, and heading by use of the rudder pedals. It should be made clear that the engine nozzles remain fixed while hovering, as quite high speeds can be attained in any direction merely by tilting the aircraft, and thus the thrust vector, appropriately. During hovering the aircraft can be likened to a brick riding on a jet of air, having neutral stability about all three axes and no natural damping. Control deflection therefore causes an angular acceleration of the aircraft, and it has been argued that this provides a situation which the pilot cannot cope with unless artificial damping is provided. But the P. 1127 has proved that hovering without autostabilization is not only feasible, but easier than in many helicopters—even in conditions of extreme turbulence. This has been achieved by providing suitable stick sensitivities (i.e., aircraft angular acceleration per unit stick- deflection) and feel characteristics. Unlike a helicopter, the jet VTOL aircraft has no significant cross-coupling effects. Transition from the hover to conventional flight is simply achieved by leaving the throttle open and moving the engine nozzles pro gressively backwards. As this is done the P. 1127 accelerates smoothly and quickly, and automatically picks up aerodynamic lift to compensate for the loss of jet lift. There is no marked change of stability, trim or control behaviour. Although climbing transitions may be made, the simplest technique is to hold the air craft at low incidence in a flight-path roughly parallel with the ground, thus doing away with the need for monitoring incidence separately. The transition to well above normal stalling speed is achieved with the nozzles only about 45° from the vertical position, and the only evidence of a completed transition comes from the a.s.i. Transition from conventional flight to the hover is made from normal approach conditions, the permissible tolerance of speed and height being generous by any standards. When required, the nozzles are moved down to the braking position, which is forward of the hovering position, and the resultant combination of braking- thrust component and intake momentum drag produces a powerful and smooth deceleration of about £g initially. As the speed falls off the throttle is progressively opened to provide jet lift to supplement the declining aerodynamic lift. This is a natural process and can be almost directly compared to checking sink with power during a conventional precautionary landing. As in the case of the accelera ting transition, the simplest technique is to fly roughly parallel to the ground at low incidence. Just before the aircraft comes to a standstill the nozzles are moved back to the hovering position and it can then, if necessary, be manoeuvred by tilt to position it above the intended touch-down point and a vertical landing carried out by setting up a slow rate of descent. Where vertical take-offs are impossible, due either to overload or to utterly unsuitable conditions from a ground-erosion aspect, the vectored thrust system of the P. 1127 really comes into its own by providing an extremely simple short-take-off facility. By accelera ting with the engine nozzles rearwards to a predetermined take-off speed, usually quite low, and then rotating them down to an inter mediate position, incredibly short ground-rolls can be achieved. The piloting problem is quite straightforward although the time from brakes-off to full throttle to selecting nozzles down is very short. In a similar way, landings can be made with forward speed. The encouraging thing about V/STOL operation using the vectored thrust concept is that it requires no more than an ex tension of normal flying techniques: the pilot largely does what comes naturally. It is no exaggeration to say that in many ways flying a V/STOL is easier than flying a conventional aircraft, because all the problems associated with operation from runways of fixed length and direction are eliminated. What about the problem of pilot conversion ? Experience with the P. 1127 suggests that this will be straightforward provided the pilot has a jet background and is given a few hours' dual hovering and translation experience in a helicopter. Ideally, of course, the use of a two-seater aircraft is the correct approach. A programme of conversion recently carried out by an RAF pilot from Aero Flight, RAE Bedford, was completed in 43 minutes' total flying time, only 16min of which was hovering and transition time and demonstrates the ease of conversion. The conversion included one session of taxying and one 25min conventional flight; then, without autostabilization, one tethered hover, one free hover, one double transition (vertical lift-off, accelerating transition, con ventional flight, decelerating transition, vertical landing), one double transition using nozzles at braking position for decelerating, two short take-offs and landings, and one lift-off from solid ground. Finally, what of bad-weather operation of VTOL aircraft? It has been argued that with VTOL one now has the potential to operate under zero-zero weather conditions. This will doubtless be achieved in the future, but only at the cost of increased aircraft weight, complexity and unserviceability. It is important, therefore, to ensure that the first generation of military V/STOL aircraft is not embarrassed by such a sophisticated requirement, in view of the need for simplicity and early introduction into service. Experience with the P.l 127 to date suggests use of a conventional ILS or GCA approach to below-cloud, followed by a visual transition. The latter should be less exacting than a conventional landing as the per missible tolerances on height, airspeed and heading are greater. This is because the pilot is free and able to make any required adjustments in the air throughout the decelerating transition. Over and above this the aircraft can subsequently be translated to the desired landing spot if there is still need to do this. Such freedom of operation means that the normal fuel reserve for the overshoot case can be greatly reduced, and the worst that can happen is that the pilot may have to make a vertical landing a short distance from the desired spot. So much for flying the P.l 127. The future will undoubtedly see larger, heavier and more complex V/STOL aircraft using either vectored thrust or a battery of lift engines, or a combination of both. For the immediate future, however, the keynote must be simplicity and this, in the authors' opinion, strongly favours the vectored thrust concept. In any event it is vital to ensure that the logistic-support problems of future V/STOL aircraft do not nullify the striking advantages of flexible operation away from prepared bases.
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