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Aviation History
1956
1956 - 0523.PDF
FLIGHT, 4 May 1956 523 Aerodynamic Aspects of Helicopter Design Lecture to the Helicopter Association IN a highly technical and specialized paper, presented to arecent meeting of the Helicopter Association of Great Britain,Dr. H. Roberts, Ph.D., discussed some of the lesser-known aspects of the work of a helicopter design team. He said that untilrecently all the efforts of the aerodynamicists had of necessity been directed towards improving performance characteristics,since these were so marginal in early helicopters. Today, the industrial aerodynamicists' tasks covered a much wider field com-prising: (1) Prediction and analysis (including performance analysis, stability and control characteristics); (2) wind-tunnel andspinning-rig testing; and (3) flight testing and analysis. In addi- tion, the provision of stressing data was an important feature.Much of the work of the aerodynamics office was concerned with establishing techniques, since no existing methods could be appliedas in die case of aeroplanes. Dr. Roberts described at length the various methods for deter-mining performance characteristics, examining some of their merits and demerits, and then continued with a discussion ofstability and control. The problem was more complex than the equivalent fixed-wing problem because in the helicopter (assum-ing the use of articulated blades) the fuselage and rotor interacted one upon the other. The effect could best be explained by meansof a simple diagram. (Fig. 1) Any outside disturbance affected both the rotor and the fuselage together but, in addition, a changein fuselage attitude affected the rotor through a "feed back" and this, in turn, further affected the fuselage. The observed motionas a result of the original disturbance was the total of these effects. The influence of the pitching moments of the fuselage on staticstability was unusual, particularly in respect of short fuselages. Over the normal working range of incidence they tended to beunstable, but at the larger positive and negative angles there was a 150° 240 120' M-0 7 300 60" BLADE ELEMENTS OUTSIDE THIS CONTOUR EXCEED THE CRITICAL MACH N» Fig. 2. Distribution of lift coefficient over the rotor disc where the tip speed is 650 ft/sec and the tip speed ratio is 0.2. The broken-line contours indicate Mach number and blade elements outside the heavy contour exceed the critical Mach number. Direction of rotation is anti-clockwise. DISTURBANCE ROTOR FUSELAGE OBSERVED MOTION Fig. 1. Simple diagram showing the interaction of rotor and fuselage in the disturbance case. region of stability. The addition of a tailplane provided somemeasure of stability in forward flight but the effect was not entirely as might be expected. Over the working incidence range,the combination of fuselage and tailplane was stable. At large positive and negative fuselage angles, however, the question oftailplane stall had to be taken into account and, whereas the fuse- lage by itself might be stable at these angles, the combination wasunstable. Although a helicopter might be statically stable, a divergencemight arise when control movements were applied. Methods of improving stability included the fixed tailplane, the use of offsethinges in the rotor head and variations in e.g. position. The first was very effective in increasing damping in the fore and aftsense although it did introduce other problems. Offset hinges also increased damping to a certain extent; but their efficacy wasquestionable for other than the improvement of static stability, since it had been shown by calculation that the damping effectmight be reduced to quite low values at some speeds as they were moved outward from the rotor axis. The effect of e.g. movementwas similar to aeroplane experience, stability characteristics deteriorating as the e.g. was moved aft. The provision of desir-able handling qualities in roll and yaw could be achieved by having a low rotor speed ir both cases, but this requirement hadto be related to other considerations. The lecturer went on to say that the lack of systematic flightrecords of the normal accelerations to which helicopters were sub- ject made stressing requirements rather a matter for synthesis ofresponse and possible flight loading conditions rather than a matter of experience. Normal accelerations could be inducedin two ways, either by the application of cyclic pitch or collective- pitch. The application of the former gave a small immediateincrease in g, followed by a steady build up to a maximum in a few seconds. The amount of g per degree of cyclic-pitch applieddecreased with speed, and at low speed this was an inefficient method of producing normal accelerations. On the other handcollective-pitch application gave a nearly instantaneous increase in g and was most effective at low speeds. The maximum g in thiscase was determined either by the maximum collective-pitch angle available or by the stalling lift coefficient of the blades. The effect of gusts was not of primary importance on modernhelicopter rotors. As speeds increased, the effects of gusts on winged helicopters were, on the other hand, quite serious becausethe wing lift increment at constant vertical gust velocity increased linearly with forward speed. This characteristic was quitedifferent from the normal rotor one in which g was substantially constant, independent of speed (apart from the effect at very lowspeeds). As was to be expected, the effective low aspect-ratio of a rotor gave low values of lift variation with incidence and thiseffect, apart from the change in form of variation with speed, tended to give low gust load factors. Outstanding problems concerned mainly the need for moredata on a number of subjects, including the following: (1) Blade and fuselage drag; (2) induced velocity—including the inter-ference effects between multiple rotors; (3) blade stall and critical Mach numbers, including the effect of vibration levels on theacceptable degree of permissible stalling; (4) desired handling characteristics and autostabilization; (5) stability derivatives andthe correlation of wind-tunnel and full-scale results on these. Other items which required the devotion of much effort were:(6) The collection and collation of data on flight and gust envelopes; (7) a more accurate analysis of the off-loading wingas an aid to the attainment of higher speeds; (8) specialized wind- tunnel problems, such as scale effects and tunnel constraint effects;(9) the investigation of special devices (e.g., the jet flap). Dr. Roberts concluded by saying that the true function of theaerodynamics office was not entirely that of performance estima- tion, which should be an automatic process dependent uponstandardized text-book procedures. Far greater effort should be concentrated on aerodynamic handling aspects, and the aero-dynamicist had an important part to play in improving the "breed" to a stage where a few successful British types could be produced.This might do much to halt the present trend away from heli- copters and towards other forms of low-speed aeroplanes. SECOND SUPER-CARRIER THE U.S.S. Saratoga, second of the U.S. Navy's super-carriers,was commissioned in the New York Naval Yard recently. There were 6,000 guests at the ceremony. Saratoga cost £74mand is three feet longer than Forrestal, the first of the class, which is already in service. Saratoga's engines, more powerful thanthose of her sister ship, develop 250,000 h.p., giving her an "aver- age speed" of 34 kt. She can accommodate 100 aircraft. Carriersof the same class now building or projected are the CVA-61 Ranger and the still-unnamed CVA-62.
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