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Aviation History
1955
1955 - 0968.PDF
HIGH DRAGEVE SIDE-BY-SIDEROTORS WIT MODEL SPEED 80 THE ANGLO- AMERICAN CONFERENCE... Fig. 3.—The high drag- level curves correspond to a high setting of wing incidence in relation to the angle of the rotor masts on the model, while the low drag curves correspond to a low setting. Ignoring any possible developments in materials or manufac-turing processes, a fixed value for disc loading of approximately 4 lb/sq ft and a tip speed of 700 ft/sec had been arbitrarilyassumed for the purpose of comparing different rotor sizes and configurations. A fixed value for CL had also been assumed.A numerical value for this coefficient was rather arbitrary as it varied for different designs of blade involving twist and taper. The lecturer went on to discuss the factors affecting the prac-tical limitations in rotor size. As rotors grew larger, and if the tip speed, disc loading and lift coefficient were kept at a constantvalue, the torque required to drive the rotor increased as the cube of the diameter. This meant that the weight of transmissions andall torque-sensitive parts would have a tendency to increase in weight at a faster rate than the gross weight of the helicopter(Fig. 1). Such weight increases would reduce the payload and at some point it was obviously more economical to use two smallrotors instead of one large one. An analysis had been made of seven two-bladed semi-rigidrotors, designed by Bell, with diameters ranging from 35ft to 55ft. Using a digital computor, the following exponential equa-tion had been obtained: ("(Disc loading X n)°-257 X (Solidity)04 (Radius)3018 L (Tip speed)1"65 j where Wj = the weight for one of the two blades and n = thedesign ultimate load factor. This equation applied only to the blades themselves and not tothe whole rotor assembly. The figure of 880 was different for different rotor systems and varied with the number of blades.The remarkable thing about this equation was that, for a given family of rotor designs, it showed blade weight increasing withthe cube of the rotor radius if disc loading, tip speed and solidity were kept constant. Blade taper and twist had been varied but stillthe cube law emerged. It was not possible to determine a rotor diameter which shouldnot be exceeded as so much depended on the purpose for which the helicopter was to be designed. From their own experience,however, they had concluded that the single-rotor /tail-rotor con- figuration was the most practical for helicopters up to a grossweight of approximately four tons. For machines exceeding six tons' all-up weight, it was advantageous to have more than onemain lifting rotor. The transition from single-rotor to multi-rotor helicopters wasa unique process having no parallel in fixed-wing history and was one which profoundly affected many aspects of design. In thetandem rotor configuration certain advantages were immediately apparent: (a) Less power was required for hovering; (b) thefuselage, surprisingly enough, became smaller; (c) the machine was capable of relatively high-speed sideways flight, allowing it tocome to a hover in strong cross winds; (d) during autorotational landings, a steeply flared attitude could be maintained during andafter contact with the ground, thereby simplifying considerably the emergency technique. Against these had to be weighed certain disadvantages of thetandem configuration: (1) Longitudinal static instability, mani- fested in the tendency of the machine to pitch upwards whena gust was encountered. (This had been overcome at Bell Aircraft by the use of an accelerometer control which automatically re-duced the collective-pitch of the forward rotor when the fuselage was subjected to an upward acceleration.) (2) The tandem heli-copter required more power than the single rotor machine in the cruising flight condition (Fig. 2). It followed from these con-siderations that the twin rotor side-by-side < configuration possessedthe advantages of the tandem machine without its disadvantages. In the side-by-side rotor helicopter the power required for cruisingflight was even less than in the single rotor machine. This was an important consideration for twin-engined machines since itresulted in a good rate of climb on one engine (Fig. 2). The condition applied even at very low airspeeds, indicating a goodemergency single-engine performance. Simplification of controls FLIGHT was an additional advantage since the lateral cyclic-pitch controlcould be dispensed with and roll stability could easily be obtained by equipping each rotor with the accelerometer control.The most interesting aspect of the side-by-side configuration was the design of the fuselage itself. Earlier helicopters of thistype had had more or less conventional aeroplane fuselages with the rotors carried on outriggers. However, for larger machines itappeared that a better solution would be to design a long, thick fuselage (to include outriggers, engines and space for passengers)which would move sideways through the air—i.e., a wing. Mr. Kelley concluded his remarks by describing certain modelexperiments which had recently been carried out at Bell Aircraft to determine the feasibility of such a design. The technique forthe tests had been developed independently by Arthur M. Young, whose original researches in the 1930s had led to the present rotorsystem in use on Bell helicopters. Mr. Young had devised a whirling-arm test stand, in which the operator sat in a chairwhich, mounted on ball bearings, turned with the arm. A model of the side-by-side twin rotor flying wing was mounted at theend of the arm and described a circle 50ft in diameter. The wing itself was of symmetrical section, having a 40 percent thickness ratio. Diameter of the two rotors was 51in and power was supplied by two Black and Decker electric drill motorswith an interconnecting shaft. Since the whirling arm was suit- ably counterbalanced, the model lifted only its own weight. Simi-larly, the drag of the arm was counteracted by a separate drive motor so that the model only had its own drag to overcome andwas, virtually, in free flight. Centrally mounted levers enabled the operator to make control movements and the wing andswashplates could be pre-set at different fixed angles in relation to the rotor masts. An additional feature was that, by using anexternal source of power to drive the arm, even measurements of the model's characteristics in auto-rotation could be made. At the present stage it was too early to draw any final con-clusions, but certain trends were apparent. It had been con- firmed that the "wing" used less power in the cruising flightcondition than did the conventional helicopter (Fig. 3). Preliminary tests indicated the possibility that no tail would be required as therotors were a powerful agent for supplying control and stability. However, should it prove desirable to be able to introduce wing-moment changes in flight, a small control surface could extend forward of the centrally located pilot's cabin. Discussion Mr. D. L. Hollis-Williams (technical director, Westland Aircraft)conceded that "if Bell had found that one of their best regimes of flight was sideways there was no reason why they should not exploitit." Seriously, however, he expected stability and control problems to arise with the side-by-side-rotor design, and foresaw the necessityfor a rudder. He spoke of the almost insuperable difficulty of making a reliable helicopter and suggested that, in the future, only the simplesttypes would survive. Mr. Young (Douglas Aircraft): It was noted that Mr._ Kelley hadnot mentioned operating economy. It was essential to improve the lift/drag ratios of lifting systems, since cost-per-ton-mile was in-separable from this parameter. Mr. Kelley, replying, pointed out that the heaviest rotary-wingmachine now flying was of the tandem-rotor type. He cited mail opera- tions in Chicago, in which, blade for blade, maintenance of tail rotorswas more expensive than that of main rotors. The need for good L/D ratio, he agreed, might require further reductions in disc loading. MR. GREATREX'S PAPER DISCUSSED THE discussion summarized below followed the Conferencelecture given on June 22nd by Mr. F. B. Greatrex (chief development engineer, installation division) of Rolls-Royce, Ltd.,under the title Jet Noise. A digest appeared in Flight of July 8th. Mr. Miller (N.A.C.A.) stated that the result for take-off and landingnoise agreed satisfactorily with American experience. Conway data, however, seemed to disagree with work by Tyler; it had been foundthat noise from shearing between the by-pass air and the central jet could exceed that due to action between the former and the atmo-sphere. It was American practice to integrate noise intensity over an assumed sphere from readings taken in the plane of the enginewhile the latter was rotated about its longitudinal axis. The Rolls-Royce results had squashed the argument that the corrugated nozzle waseffective only above the choking condition. Mr. Greatrex's equalizing of airliner all-up weights was questionable;any U.S. jet transport would be very large and would require engines of 12,000 or 13,000 lb thrust, and, as the noise of present piston-engined equipment was considered already marginal, such transport turbojets would have to be some 18 to 20 db quieter than comparableengines of today. Attention should also be paid to the noise due to turbulence in the boundary layer around fast transports, particularlytowards the rear of the cabin. Mr. Greatrex replied that, over the whole range of time measured,Comet noise agreed within li db with calculated values based on ground measurements. There was growing evidence that noise froma close pair of turbojets was not twice that produced by a single turbojet. Regarding nozzles, comparisons had been made betweenconvergent-divergent and convergent types and any "screech" expected had failed to materialize; measurements had been taken on both types
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