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Aviation History
1955
1955 - 1110.PDF
222 FLIGHT, 12 August 1955 DESIGN OF SMALL HELICOPTERS . . . £17,000. Since even the most expensive civil Auster costs less than £3,000 it would seem that the cost/weight correlation in question was not general enough for arbitrary use. In the writer's opinion, curves of this type are of use only within a particular firm, where the inefficiency of the design, planning and production departments is statistically determinate because of the inherent tendency to tackle problems in a set manner. But in comparing products which differ as widely as the Hiller Hornet and the S-56 it is obvious that a much more fundamental approach must be used. It seems safe to say that with small helicopters the design emphasis must always be on cheapness, an exercise for which the average aircraft designer is quite unsuited. With a competent aerodynamic and stressing team, performance and strength will be in accordance with specification almost automatically, but low cost can only be won by great effort on the part of the entire team, particularly the draughtsmen. A stimulus large enough to engender this effort cannot be discussed as a technical problem, since it is chiefly political in nature, although many technicians probably have ideas on the subject. Part of the task of cheapening helicopter design involves a complete re-appraisal of the fundamental components which are essential to any design. In many cases there is no reason why a flat sheet or strip of steel should not be substituted for the conven- tional flapping hinge. Drag hinges and dampers can nearly always be eliminated by correct blade design, as various American designs have shown. These two simplifications, coupled with a logical development of the Seibel blade-root arms and the Hafner torque bar principle, can result in hub/root arm assemblies which cost only a fraction of present-day assemblies. In the case of tip-driven helicopters it seems foolish to construct the fuselage and direc- tional control fin in metal or plastic for small quantities; the well-tried form of spruce and plywood construction still used on sailplanes is not only far cheaper but generally lighter as well. These comments could be extended almost indefinitely, to cover such items as removing the necessity for adjustment of the pilot's seat to the use of friction pads in an undercarriage shock absorber. Quite a long period could be devoted to "electrics" and how to avoid them at any cost, with a discussion as to why the magic stamp of the A.I.D. can transform a fifty-shilling boost coil into a fifty-pound electrical component. Unfortunately space considerations preclude any further examples. Dr. Focke could be right in his assertion; whether he will be right depends not on an inscrutable fate, nor on the technicians, but on whether somebody with a good supply of money gets down to the job. And despite the fulminations of Mr. "A. Heretic" in Flight some months ago (March 4, 1955) in these enterprising times "money" means "The Ministry." c-.„:-....-•... References(1) Cmdt. H. Boris: "Problems associated with small helicopter development." Journal of the Helicopter Association, October 1953.(2) A. V. J. Vernieuwe: "Helicopter operating experiences." Journal of the Helicopter Association, October 1952. (3) P. G. Masefield: "Some thoughts on the operational future ofthe helicopter." Journal of the Helicopter Association, March 1953. (4) P. R. Payne : "Booster ramjets for helicopters." Flight, December31st, 1954. ROTTERDAM'S HELICOPTER CONGRESS ROTTERDAM was, from July 27-29th, the scene of animportant meeting between leading figures in the heli-copter movement on both sides of the Atlantic. Organized by the Rotterdam Helicopter Syndicate, the congress consisted of a series of half-hour lectures on commercial and technical aspects of both helicopters and heliports, each lecture being followed by a discussion. Four of the papers are reported briefly below. We hope to give space to the remaining lectures in a future issue. The place of the helicopter in scheduled air transport today was discussed by Mr. Robert L. Cummings, Jr., president of New York Airways, who operate a fleet of five S-55s on ultra-short- haul work in the New York area. Mr. Cumrnings quoted some recent statistics illustrating the type of operations performed. In May 1955 the five helicopters carried 2,553 passengers, 90 tons of cargo and express and 60 tons of mail, representing an overall load factor of over 51 per cent. Route mileage flown daily was now 1,738 miles, representing a scheduled utilization of 5 hr 13 min per aircraft per day. Passengers of New York Airways paid an average of a little over 34 pence per seat-mile, but this did not offset the cost of operation, the difference being made up by Government service pay; this support would be essential for some years to come, but New York Airways recognized it as temporary and were making every effort to enlarge their commercial revenues. In the year ended March 31st, 1955, overall costs per mile amounted to 25 shillings. The indirect cost per mile, at 12 shillings, represented nearly half this figure. The relatively high in- direct cost was, of course, aggravated by shortness of stage-length (the average being only 14 miles), though it was certain that expansion of traffic volume would bring a substantial reduction. Referring to direct operating costs, Mr. Cummings said that the major problem was that engine overhaul costs were at least three times as high as those experienced when the same engine was used in a fixed-wing aircraft. The engine was not designed or constructed for use in a helicopter, and while it was possible to remedy the primary cause of high overhaul costs—vibration, and operation at hi<?h power settings—what was really required was a power plant designed specifically for helicopters. In this context he regarded turbines of great importance. It was not an over-statement, Mr. Cummings stressed, to say that if the present noise level were not drastically reduced, the helicopter would never realize the useful future that was being so enthusiastically promised by designers, manufacturers and operators—some other type of vehicle with vertical-flight ability would inevitably replace it. Looking ahead, he described both the Piasecki H21 and the Sikorsky S-56 as representing "a long step in the right direction"; with single piston engines thev would probably be certificated by the end of 1956. It was hoped that bv 1959 both aircraft, as well as the Bristol 173, would be powered by multiple turbines; they shou'd then approach "an economic status not even remotely possible" for the S-55. For the type of short-haul operations performed by New York Airwavs it seemed that a fully instru- mented helicopter, carrying 18-20 people at 135 m.p.h., at direct costs not exceeding tenpence per seat-mile, would meet most requirements. But this specification implied a maximum useful range of about 75 miles. Larger, faster vehicles with lower seat- mile costs would be required for inter-city operations below premium fares on longer routes. In Mr. Cummings' view it was not possible to forecast when such equipment would make its appearance. The immense short-haul market lying ahead of the vertical-lift aircraft was no illusion, and whether or not the rotary wing concept became the final answer could make no difference. A paper on the future technical development of the helicopter was delivered by M. Paul Morain, a senior helicopter engineer of the S.N.C.A.S.E. concern. After outlining many problems in- volved in the rotor design, M. Morain concluded that the only really efficient means of widening the functional limits of a rotor during forward flight was to relieve it of part of its lifting and thrusting function by adding fixed wings and auxiliary airscrews— thus producing the compound helicopter. To provide maximum fatigue life, blade structure would be as homogeneous as possible, avoiding stress concentrations caused by such discontinued joints as bolts and rivets; joints by welding, brazing, bonding and clamping would therefore be preferable. Rotor materials would also be chosen with fatigue properties in mind. For light machines, wood would give place to plastic; for medium-sized helicopters light alloys were interesting, and steel would be used for large machines. Discussing engine systems, M. Morain made plain his belief that all major developments will stem from the use of turbines. While single-shaft engines could successfully be employed on light or medium helicopters, free turbines were more suitable for the large multi-engined helicopters of the future. Gas generators now under development opened a new vista for the compound machine. Such units were capable of furnishing mechanical energy on the shaft or adiabatic energy, in the form of air from the compressor, alternately or simultaneously, the only effort required being adjustment of automatic-control devices. In this way it was possible to pass progressively from feeding compressed air to the reaction rotor to driving an airscrew—a solution of ideal simplicity for the fast transport helicopter. M. Morain believed that the safety of the twin-engined heli- copter was acceptable only when the engines were turbines; with two piston engines it was "il1usory," assuming a reasonable t>ower- weieht value. He predicted that the large transport helicopter won'd be one with at least three turbines. Discussing helicopter airframes. the lecturer forecast refinement of fuselage shape for transport helicooters, and widespread use of both vertical and horizontal tail surfaces, which were necessary to eive cruising stability approaching that of the aeroplane. He believed that the best solution for the undercarriage problem might be the use of one or two landing skids placed longitudinally beneath the fuselage and attached to it by shock-absorbing elastic material; these skids would be used in combination with wheels, which need only carrv the weight of the empty aircraft when it was being manoeuvred on the landing site. M. Morian believed that the limiting cruising speed for (Concluded on page 242)
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