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Aviation History
1954
1954 - 0526.PDF
242 FLIGHT 26 February 1954 HELICOPTER RESEARCH UNDER the above title, and bearing the sub-title A Review of Outstanding Problems together with an Account of Some Recent Work at A. and A.E.E., Boscombe Down, a lecture was given to members of the Royal Aeronautical Society and the Helicopter Association of Great Britain at a joint meeting on February 18th. The speaker was Mr. F. O'Hara, M.A., and the chairman was Mr. N. E. Rowe, C.B.E., D.I.C., B.Ss., F.R.Ae.S. In the first part of Mr. O'Hara's paper, the major items for research in the helicopter field were reviewed, the subjects including rotor aerodynamics, stability and control, evaluation of configurations, vibrations and fatigue, and operational aspects. An outline was given of work being done at Boscombe Down in connection with the assessment of the longitudinal handling characteristics of helicopters. The second part of the paper contained an account of recent A. and A.E.E. work on the low-speed and take-off performance of a helicopter. An empirical method of low speed performance estimation was described, and the variation with wind speed of the ground effect on a rotor was discussed theoretically and on the basis of experimental results. A theoretical analysis of the forward take-off motion of a helicopter was briefly presented, and the final section was concerned with the performance of a multi engined helicopter in the event of failure of one engine, with particular reference to die possibility of safe operation from the type of site proposed for civil use. Some extracts of general interest from the first part of Mr. O'Hara's paper follow. Stability and Control.—The stability characteristics of heli copters were not yet satisfactory, and consequently their opera tional use had been restricted, particularly in blind flying conditions. Efforts had been made to improve the stability in such devices as the Bell stabilizer bar and the Hiller servo rotor, but recently the trend on larger machines appeared to be towards auto-stabilization. Much basic work had been done to analyze the stability and control characteristics of helicopters, and it appeared that more attention should now be given to develop ing methods to assist in designing helicopters with selected handling properties. More information was first required on desirable handling qualities, and an effort was being made at present at Boscombe Down to correlate pilots' impressions and quantitative measures of the simpler aspects of handling on a single-rotor machine. Evaluation of Helicopter Configurations.—In the earlier days of helicopter development, attempts were made to develop types of many and varied configurations, but the field had now been narrowed down considerably. The relative merits of single- and multi-rotor systems, however, still required investigation; for example, the interference effects with tandem rotor arrange ments were of particular interest. Estimates showed that the tandem arrangement without overlap appeared more efficient than the single rotor in hovering, but less efficient at fairly low speeds; overlapping reduced interference losses at higher speeds, but was less efficient in hovering. For an overall assessment of the configurations, however, other factors had to be taken into account. Rotor design itself required continual study, and the effectiveness of schemes designed to permit higher speeds should be assessed. A study of the various types of rotor propulsive SYCAMORES FLY AGAIN T^HE Ministry of Supply agreed on February 17th to •*• "unground" the Bristol Sycamore helicopter. As announced in our issue of February 5th, these machines were grounded on the advice of the manufacturers, pending the results of an inquiry into "a potential source of weakness in the shaft driving the tail rotor." The necessary modifications—comprising the inter position of a torque-limiting clutch in the tail rotor drive, a stiffer shaft and a small change in the setting of the tail rotor itself—have been proof-tested, and modification sets despatched. NEW LIGHT-AIRCRAFT RADIO NEW versions of the Plessey PTR 61 V.H.F. transmitter-receiver, designed principally for use in light aircraft, are now in quantity production. The set provides R/T. communica tion facilities on any one of six crystal-controlled channels in the 116-132 Mc/s band, and one of its advantages is that crystals can be removed and exchanged in a simple manner during flight, thus extending the possible range of frequencies. Typical per formance of the set is a range of 50 miles at 3,000ft. Small size and low weight are other advantages; the front face of the set measures only 8in x 6in, and the depth is llin. The standard version, in which the main controls are on the systems would also be of great value and would enable power plant developments for helicopters to go ahead along the most promising lines. Vibrations and Fatigue.—Much vibration trouble arose from the rotor systems in which freely-articulated blades subjected to fluctuating loads were the source of vibration excitations which might be communicated directly through the control system or might result in resonance disturbances in the fuselage. Aero dynamic interference between rotors, or from rotor down-wash on the fuselage, and mechanical units such as the engine, or com plex and lengthy transmission systems, were also possible sources. A low vibration level was important for crew and passenger com fort, but the vibratory characteristics were of greater significance in their effect on fatigue stresses. A general study of rotor dynamics, including aeroelastic effects, was required. Another form of vibratory disturbance was the phenomenon of ground resonance. This usually occurred with the aircraft on the ground and partly supported by the rotor, and appeared to arise from resonance between the motion of the aircraft on its undercarriage and oscillatory in-plane motions of the blades. Operational Aspects.—The wider use of helicopters in both military and civil roles required operation in blind flying con ditions, and it now appeared that instrument flying in a limited flight envelope was at least a near-possibility on current types, provided that the question of limited panel flying in the event of instrument failure was satisfactorily answered. Investigation into this question and into the possible application of recently developed flying aids should be made. For instrument flight at low air speeds, instrument improvements alone would hardly be enough, and auto-stabilization of the helicopter appeared the only immediate hope. Other outstanding operational problems still to be solved included the possible accuracy of dead reckon ing navigation, de-icing, blade-motion control for starting and stopping the rotor in high, gusty winds, and the suppression of noise for civil operation. Discussion.—The following are some points from the discus sion which followed the lecture:— Mr. Raoul Hafner (Bristol Aeroplane Co.) doubted whether Mangler's mathematical analysis of down-wash was the most realistic approach. The variation of down-wash with time was possibly more significant than the variation in space, he submitted. In reply to Dr. G. S. Hislop (Fairey Aviation Co.), the lecturer amplified his reference to the possible use of combined controls to improve stability and control characteristics. The addition of a tail, it had been found in the U.S.A., did increase manoeuvrability, but a combined rotor and elevator control might be difficult. Professor A. R. Collar (University of Bristol) asked how important was the development for helicopters of a flight instru ment able to measure very low air speeds? At Bristol University, work had been performed on such a project involving pitot tubes fitted to the ends of the rotor blades. Mr. O'Hara replied that low-speed measurement was particularly required for testing purposes. Research into methods for increasing helicopter speeds, sug gested Mr. J. Shapiro (consultant), could include with advantage a study of blade profiles. Mr. O'Hara agreed that the merits of all methods of increasing speeds should be assessed. front panel of the transmitter-receiver chassis, has a weight of under 15 lb, while for aircraft with tandem seating, dual remote- control boxes are available with a modified set, at a small weight penalty. This latter installation was in fact developed for use in the D.H. Chipmunk. The power consumption of the PTR 61 is approximately 60 watts. MATTERS OF STANDARDIZATION RECENT publications by the British Standards Institution include standards for Laboratory Tests for Conductive and Antistatic Rubbers (B.S. 2044 : 1953) and for Extrarlow and Low- voltage Spring-return Non-latch Switches for Aircraft (G.145 : 1954). Copies of these standards are obtainable, price 2s 6d and 2s respectively, from the B.S.I, at 2 Park Street, London, W.l. In another sphere of standardization, the F.E.18 Ultrasonics Panel of the British Welding Research Association is considering the need for a standard, single-hole, steel reference block for use in ultrasonic testing. Such blocks would need to be commercially available and should bear a mark showing that they are of approved design and quality. Firms and individuals interested are asked to communicate with the Secretary, the F.E.18 Committee, British Welding Research Association, Abington Hall, Abington, Cam bridge, from whom further information may be obtained.
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