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Aviation History
1947
1947 - 0484.PDF
202 FLIGHT APRIL 3RD, 1947 HELICOPTER DESIGN either by airscrews or by jets. The former suffered from high airscrew losses, gyroscopic couples, and power transmission problems ; the latter from high fuel consumption. One could also visualize blade-tip rotors rotat- • ing epicyclically about the main hub axis, the propulsive thrust being obtained either from the additive torque of the epicyclic rotors or by inclining their tip-path planes in the direction of rotation of the main rotor. Dr. Bennett pointed out that since, for a given value of tip speed, power loading and disc loading, torque in- creased as the cube of linear dimen- sions, the problem became rapidly worse with size and, unless torqueless rotors could be developed to a prac- tical stage, the use of a single rotor was confined to relatively small machines. Limiting Power Loading Turning to the question of power loading, the lecturer explained that YV 38the theoretical limit was — = /—, where the first term is the gross load per h.p. and w the disc loading of the rotor, in lb/sq ft. In practice, due to the fact that sustentation was caused by the downward acceleration of air by the rotor blades over an annulus of the disc and not over the whole disc, the induced power loading was reduced S3to about ../ The ground cushion might, however, enable an overloaded helicopter which was not capable of hovering at altitude to take off and land vertically within the ground cushion. The actual limiting power loading, taking profile drag into con- sideration, might be given approxi- W 25 Vw.mately as fc.H P. " Vw. For ex" ample, a rotor disc loading 2.3 should support between 16 and 17 lb/hp in hovering flight, at sea level but away from the ground cushion. The limit- ing power loading for a take-off with forward speed was nearly double that it zero forward speed. Pitch Limitations Variation in rotor pitch was required to vary the rotor power independently of angular speed ; to compensate for the change of angle of attack of the blades due to variation in axial flow through the disc with forward speed ; and to compensate for change of dens.ty with height. In order to eliminate separate pitch or throttle controls, devices had been developed for governing pitch or i-ngini- speed. In the former there was an automatic pitch reduction in the event of power failure, and the conse- quent loss of lift before autorotation was established might cause the machine to land at excessive sinking A possible helicopter layout with blade-tip rotors rotating epicyclically about the main hub axis. speed. When automatic throttle con- trol was fitted, the pilot retained control of pitch and could utilize the kinetic energy of the rotor for momen- tary hovering, but there was a risk of over-control in flight, the blades then slowing down and eventually stalling. The immediate reduction in pitch was essential in helicopters which oper- ated at a pitch beyond that at which the blades would autorotate, but it was not necessary to/ take this risk at k\ \ ' ——•"•V; \ TCffAL V PARASiTE --•/' /t i i PROFILE INDUCED t FORWARD SPEED Variation of power-required with for-ward speed. These curves show clearly the extra power needed to enable ahelicopter to rise vertically. all. The power could be absorbed equally well at low pitch by a rotor of low blade loading or high tip speed, except at very great altitudes. A further high-pitch limitation was associated with the forward inclina- tion of the rotor for propulsion. The disc then made a negative angle of incidence to the flight path, causing the axial flow through the rotor to in- crease with forward speed and chang- ing the blade angle of attack unequally from root to tip. Least affected was the tip portion of the blade, and conse- quently when the mean collective pitch was increased to compensate for in- creased axial flow, the blade angle at the tip became excessive and might approach the stall cyclically at high translational speeds. The periodic variation aj^ top speed not only impaired the propulsive efficiency but limited operation of the helicopter by the inherent roughness at the higher air speeds. Limiting Translational Speed Of maximum forward speed Dr. Bennett said that in view of the fluc- tuation in relative air speed and lift coefficient at the blade tip, bending and torsional deflections would be serious and would place a definite limit on forward speed. He did not think the speed of the tip of the ad- vancing blade should exceed ^ quarters of the speed of sound. Fo helicopters tip-speed ratios, i.e., the ratio of forward speed to tip speed, would be likely to remain at the Auto- giro value of 2/3 for a long time to come. It had been suggested that this figure might reach a value of i, but that would mean that the tip of the retreating blade would have zero air speed and the rest of the retreating blade would have negative air speed, and the lift of each blade would flue- tuate from zero to its maximum value. This limitation would mean a maxi- mum forward speed of about one- quarter of the speed of sound, or less than 200 m.p.h*. Dynamic Stability Dynamic instability at zero forward speed, at one time thought to be a major limitation, was not now con- sidered very important. The high position of the rotor ensured static stability^ Dynamic stability was con- cerned with the subsequent motion ; it was found that in certain present- day helicopters, where the period of oscillation was about 12 seconds and the time required to double the ampli- tude was 18 seconds, the unstable motion ^as not at all critical, nor diffi- cult to control. Complete dynamic stability could be achieved if more than two-thirds of the oscillation of the tip-path plane were suppressed. Technical Outlook Dr. Bennett wound up by saying that if might be a long time before we had the perfectly stable, jet-driven hingeless, two-bladed, single-rotor helicopter that will operate smoothly and safely in every condition of flighj ranging from zero translational speeu' to a quarter of the speed of sound, but at least the helicopter had emerged from the laboratory stage and, in spite of its present limitations, was already established as a vehicle of practical utility; not a competitor of other forms of transport but with unique uses of its own. For reasons already given, the single-rotor helicopter was at present limited to relatively small sizes. If jet assistance for vertical flight became practical, the large single-rotor heli- copter could operate in forward flight at reduced torque as a gyrodyne, or at zero torque as a gyroplane.
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