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Aviation History
1953
1953 - 0124.PDF
122 FLIGHT, 23 January 1953 THE HELICOPTER IN PRACTICE . . . In forward flight the flying technique resembles more closely that of the fixed-wing aircraft, although fundamental difference* are still present. Perhaps the feature in which the helicopter differs most is that changes in the flight-path from climb to descent do not necessarily involve a change in the fore-and-aft attitude of the fuselage. Climbing or descending is carried out by raising or lowering the collective-pitch lever, and the position of the nose relative to the horizon does not change unless fore-and-aft stick movement is used to adjust the air-speed. Once the helicopter has been settled into a steady condition of straight-and-level flight the collective-pitch lever and throttle can be largely ignored until a change in flight-path is desired. Each has a separate friction device (or sometimes, in the case of the collective-pitch lever, an irreversible mechanism) which ensures that it will not move from the position in which it was last set. Control movements are confined mainly to the stick and rudder pedals, leaving one hand free to operate the radio, etc. It is not proposed here to discuss all the helicopter flight evo lutions, but mention should be made in passing of the phenomenon known as "auto-rotation." In terms of fixed-wing flying, this means nothing more nor less than "gliding." In the event of power failure the pilot must lower the collective pitch lever to its bottom stop, which will leave a small positive angle of incidence on the blades. At this blade angle, a little over 2 deg, the rotor will continue to rotate due to the aerodynamic forces acting on it, and will provide sufficient lift to allow the machine to glide in and make a flare-out or short run-on landing in any convenient space. Engine failure does not affect the cyclic pitch or tail-rotor controls and the helicopter is manoeuvred in the glide in exactly the same way as when under power. Rate of descent varies with forward speed and all-up weight, and is normally about 1,000 ft/min. Of the secondary controls the trimmers vary from one machine to another, but are usually in a position convenient to the pilot's throttle hand. The mechanism in most types is a mechanical one, comprising two sets of opposing springs fitted to the control- column linkage, one set in the fore-and-aft and one in the lateral sense. They hold the stick in tension at a central datum which may be shifted at will by increasing or decreasing the spring tension. In forward flight the stick must be held forward of the central hovering position to impart a continuous forward tilt to the rotor, and it can be trimmed in this position. Handling-notes on Selected Types On the Westland-Sikorsky S-51 tximming is effected by two small electric motors controlled by switches below the instrument panel. An upward or downward movement of one switch adjusts the fore-and-aft trim, while the other adjusts the lateral. This type of trimming device also provides the control column with artificial feel, a desirable feature when the cyclic-pitch control is operated through an irreversible mechanism to prevent unde sirable loads being transmitted back from the rotor to the stick. Without this artificial feel the stick would seem "dead." Earlier versions of the S-51 employed irreversible screw jacks to operate the cyclic-pitch control, but they resulted in rather high frictional loads in the control column. The latest version, the Mk3 now in service, overcomes this drawback by the use of metal blades, which are much smoother in operation, and by hydraulic jacks which eliminate friction and make stick movements light and easy. Of all the aerodynamic problems confronting the helicopter designer, stability is probably one of the most acute. The single- rotor system is at best only neutrally stable, and is easily upset by outside air disturbances. Directional stability is not a problem, as it is provided in ample measure by the tail rotor, but in the fore-and-aft and lateral senses there is no restoring moment at all. An efficient mechanical trimming device can do much to alleviate this problem and allow short periods of hands-off flight in calm air conditions; but it is not a cure in itself and is of no assistance in bumpy weather conditions. Two machines in service today—the Hiller and the Bell—incorporate a device to tackle this problem at source. The Hiller 360 uses two aerodynamic paddles on an arm at right angles to the blades, while the Bell 47 has a similar arm, in this case fitted with weights at the extremities so that the gyroscopic forces supply the restoring moment. The stable handling characteristics of the Hiller 360 come as a welcome change to the pilot accustomed to the average single rotor machine. It is light on the controls and reasonably easy to fly. Hands-off flight may be accomplished for long periods in weather conditions in which one might expect a fixed-wing air craft to do the same thing. The control system departs from the orthodox layout in having a hanging stick direct from the rotor head, reminiscent of the old Autogiro days, and also substitutes a small throttle lever on the side of the collective-pitch lever in place of the twist-grip. These two features have little effect on general handling qualities, and are largely a matter of individual preference. The Bell 47, another useful little 2/3-seater trainer and general-purpose helicopter, handles quite well in the air. One British machine which did have a certain amount of stability was the Fairey Gyrodyne. It was built as a private venture by the Fairey Aviation Company and I had the privilege of carry ing out the prototype flight-trials. The configuration was a little different from that of the orthodox single-rotor design in that it used an off-set, forward-facing airscrew for torque correction instead of the tail rotor. This machine derived its stability in the fore-and-aft sense from normal railplane surfaces on the fuselage, and a certain damping effect to unstable rolling motions from the short stub wings carrying the airscrew. In hovering flight the Gyrodyne's rotor was tilted backwards to offset the forward thrust of the airscrew, and as the machine moved forward the rotor came to the level position. At the normal cruising speed it was substantially parallel to the line of flight instead of being tilted forwards, and the airscrew contributed to the forward thrust. By doing so it took about 20 m.p.h. off the average helicopter's speed range in backward flight of 25 m.p.h., and put it on the forward end of the speed range, where it was more useful. Another advantage of this configuration was that since the rotor was substantially parallel to the line of flight the blades were working at a lower angle of incidence and the machine was quite smooth at cruising speed. One of the most spectacular helicopters to be built in this country since the end of the war was the Cierva Air Horse. A three-rotor machine, powered by a single Merlin engine, it had a weight-lifting performance little short of phenomenal at the time. Its handling characteristics—which I experienced while assisting the late S/L. H. A. Marsh with the flight trials— were a most unexpected combination. Its reaction to general air disturbances was slow and ponderous as might be expected in an aircraft of its size and weight; yet response to control, through the hydraulic servo-jacks, was as immediate and light as is found in a small two-seater. It used to remind me of a Great Dane puppy. The second prototype of this machine has been taken over for further development by Saunders-Roe Ltd., who have also under taken development and production of the Skeeter. This is the only two-seater helicopter at present under development in this country and should fill a much needed requirement in this class for flying training and other purposes. There are two machines in the 10/12-seater class. Of these, the Bristol 173—a twin-engine, twin-rotor machine—is a development of the single-rotor Bristol 171, while the Westland-Sikorsky S-55 is the grown-up brother of the S-51, using the same tail-rotor con figuration. The Bristol 173 blades, hubs, and power units are the same as those of the 171 and, if one may judge from the good handling qualities of that machine, combined with experience of the Ameri can Piasecki tandem-rotor configuration, the 173 has all the makings of a good helicopter. Metal blades are being developed by the company but are not yet in service. Control in the 171 is direct from the stick to the blades, there being no irreversible jacks in the system, and this principle is also carried out on the twin-rotor machine, though the controls are naturally more complicated. Fore-and-aft and lateral control are effected by applications of differential cyclic and col lective pitch, and yawing control by lateral differential cyclic controlled by the rudder pedals. The collective-pitch lever and twist-grip throttle operate both rotors and both engines simul taneously, and there is a transmission shaft which, running along the top of the fuselage, is geared directly between the rotors, and allows power to be fed to both rotors from either engine in the event of the other engine failing. Separate throttles are provided on the control box for ground running purposes. The 173 is designed to fly with the fuselage horizontal at the normal cruising speed, and this makes the hovering attitude noticeably tail down; it is, however, quite a comfortable attitude and one quickly becomes accustomed to it. At take-off the nose wheels come off the ground first and on landing the tail wheels are the first to touch. Some improvement in stability may be expected from fixed stabilizing surfaces at the tail of the fuselage. Similar stabilizing surfaces are also incorporated in the West land-Sikorsky S-55's tail boom, though these are rather smaller, and in this case adjustable by an electric control in the cockpit to provide fore-and-aft trimming. Other improvements on the S-55 include a shorter arc of travel for the collective-pitch lever, making it more comfortable for the pilot to operate. Metal blades and power operated controls are similar to those of the S-51 Mk 3. The first S-55 nas recently started its flight-test programme and shows promise of being equally as good as its American counter part. A twin-engined version is planned. In conclusion, if I were asked to venture an opinion as to which helicopter I thought had the most pleasant handling qualities, I would say, quite unequivocally, the American Sikorsky S-52; but I would still like to fly a machine with the same attributes as this helicopter yet affording stability comparable with that of a fixed-wing aircraft.
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