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Aviation History
1949
1949 - 0629.PDF
FLIGHT April 14th, 1949 427 A Further Examination of the Largest and Heaviest Helicopter in the World : Control Features Horse Part Two BEFORE proceeding to anexamination of the mediabetween cockpit controls and the rotors themselves, it is desir- able that a general statement be given on the fundamentals of con- trol involved. Regarding a heli- copter rotor simply as an airscrew- rotating in a substantially horizon- tal plane, the ability of the aircraft to hover, ascend and descend ver- tically is readily understandable. If the lift exceeds the all-up weight, the aircraft ascends; if the lift exactly balances the weight, the aircraft hovers ; and if the lift is not as great as the weight, the aircraft descends. The lift (or thrust) developed in the rotor is a function of rotational speed and the angle of attack of the blades, the power input being, in turn, determined by the power required to keep the rotor turning at a given speed with its blades at a given angle of attack. Broadly, it is desirable that the rotor should rotate at its optimum r.p.m., the lift being adjusted by variation of the blade pitch and the resultant power demand met by variation of engine supercharge. Unfortunately, when a mass is rotated on a shaft, an equal and opposite turning reaction is produced, and in a helicopter the effect is to tend to spin the body of the machine around beneath the rotor. It is for this reason that, in single-rotor aircraft, compensation for torque reaction is provided in the form of a lateral rotor at the tail, or, as in the Fairey Gyrodyne, an offset airscrew. With rotors driven by jet reaction at the blade tips, torque reaction is non-existent, and there- fore does not have to be compensated. Equally, by em- IN the preceding section of thisanalysis ("Flight," April 7th) the general features of the Cierva Air Horse • were reviewed and brief reference was made to the control system employed in this very interesting aircraft. In the present article the picture is completed by a survey of the mechanical elements of control, the process of pitch-change application to the rotor blades, and the distribution of thrust as between the three rotors resulting in the variety of Bight manoeuvres. ploying two rotors turning in oppo- site directions, the reaction of one can be made to compensate that of the other, but this configuration can bring with it undesirable prob- lems of pendulum instability. As explained in the first instal- ment of this review, the original Weir helicopters conceived by Mr. Pullin were both twin side-by-side rotor aircraft, and, as a result of the flying experience with these machines, the soundness of the con- trol characteristics was proven, and the necessity for a third rotor indicated. In addition, as also mentioned previously, there is a (present) limit of practicability on rotor diameter, and this means that, in order to lift greater loads, more rotors are required. These were the contributory causes in pro- ducing the Air Horse in its present form to fulfil the per- formance requirements. As regards torque reaction, the three Air Horse rotors all turn anti-clockwise as seen from above, and thus tend to rotate the aircraft as a whole in the opposite direction. However, by attaching the rotors to the outrigger booms in such a manner as to provide an inclination of the natural thrust axis, compensation for torque reaction is provided. Naturally, this compensation will afford an accurate balance only at one operating condition (in this case, cruis- ing), but the degree of unbalance for all other flight con- ditions is quite small, and is sufficiently met by the pro- vision of adjustable vertical surfaces at the tail of the fuse- lage so that the unbalance can be trimmed out. Having examined the basic lift action of a rotor, we can Detail of rotor head showing swashplate, drop arms, flapping and drag articulation linkage and hydraulic snubber units. The two-seat cockpit Is fitted with fully duplicated primary controls, whilst the secondary controls are all carried on a central pedestal. ELEVATOR & AILE CONTROL COL ENGINE COOLING FAN-PITCH CONTROL CC*. - COLLI PITCH
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