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Aviation History
1967
1967 - 0089.PDF
FLIGHT International, 19 January 1967 89 The NGTE Rigid Rotor IN THE QUARTER-CENTURY since Mr Igor Sikorsky made thefirst officially timed rotating-wing flight the helicopter hasretained a pre-eminent position in the field of what we now call VTOL aircraft. The reason is the high efficiency of the lift generation mechanism whereby low velocity is imparted to a large mass of air. The advent of the gas turbine engine, with its greater power:weight ratio, has in later years allowed the investigation of other forms of lift system which do not suffer from the helicopter's inherent penalty, lack of speed. At the present time all forms of VTOL aircraft (except helicopters and those with tilt-propellers) use jet thrust direct from a turbine engine, with consequent loss in efficiency in comparison with the rotor. This is the price paid for high cruising speed made possible by the deletion of a rotor system, which, if retained, would present severe mechanical and aero- dynamic problems. The gap between the off-loaded rotor, com- pound helicopter and the STOL aeroplane will, however, be bridged if the circulation-control rigid-rotor technique at present being developed by the National Gas Turbine Estab- lishment at Pyestock, Hants, proves commercially the promise it holds out experimentally. The circulation-controlled rotor is the result of some six years' research by NGTE on a principle which stems from the well-known characteristic of lift generation using rotating cylinders. This work had shown that lift could be generated if an axially rotating cylinder were replaced by a stationary cylinder with span-wise slots through which high pressure air was forced. When this stationary cylinder was immersed in a moving airstream large lift coefficients were obtained, and the application to VTOL aircraft immediately suggested itself. Some of the results of this work were shown at the Establish- ment's open day last May and later at the SB AC exhibition; a brief description appeared in Flight for May 26, 1966. The principle of circulation-control—the basis of the rigid rotor—is shown in Fig 1, which represents a cross-section of a cylinder immersed in a smooth airstream imagined to be moving from left to right. On encountering the cylinder the flow divides symmetrically over the top and bottom. The stagnation point (where the air is brought to rest with respect to the cylinder) is at A. The air passes smoothly over the , Figs 1-3 (left, below). Three stages in circulation control applied to a \ cylinder. The airflow over a plain, unblown cylinder is shown in the upper diagram, while the other two represent the effects of single and double slots with blowing Fig 4 Variation of lift coefficient with momentum coefficient at Mach 0.2 and a Reynolds Number of 360,000. The right-hand 20° curve is theoretical, but the other curves were obtained from model experiments The two rigid-rotor projects initiated at NGTE'. Above, the Mk I model with single rotor and grossing 40,0001b and, below, the Mk 2 model with twin rotors and based on the BAC One-Eleven, at 73,0001b Fig 3 cylinder but separates at a very early stage, because of the adverse pressure gradient, leaving a wake about 120° wide. A force (profile drag) is exerted upon the cylinder; but, because of airflow symmetry, no lift is generated. Fig 2 shows the effect of a slot in the cylinder wall so arranged that high-velocity air may be ejected tangentially over the surface. The effect is to re-energise the boundary layer so that the flow adheres to a much greater area of the surface, thus minimising the area of the turbulent wake behind the cylinder. The stagnation point has moved from A to B, owing to the reduction of pressure round the top surface. Fig 3 shows the effect of two such slots; the extent of the turbulent wake is again reduced, and the stagnation point has moved even further round the cylinder to C. At supercritical Reynolds Number conditions the size of the wake is about 40°. A further effect is to provide, at the trailing edge of the shape, a positive pressure equivalent to a profile drag reduction. For the disbeliever, brought up on a diet of conventional aerofoils, NGTE have a simple demonstration set-up in which a cylindrical model, with a slot at an appropriate position, is immersed in a constant-speed airflow. The model is suspended by a spring balance calibrated in terms of CL from zero to plus and minus 10 units, while a bank of tufts downstream of the model demonstrates the wide area of the turbulent wake. When a high-pressure air supply is switched to the slots the spring balance reading changes instantly from zero (under no- blow conditions) to about five units, while the extent of the disturbed wake is drastically reduced. The lift generated over such a blown cylinder is a function of the momentum of the airstream emerging from the slot, as shown in Fig 4. Momentum coefficient, the important para- meter in circulation control, is denned as C3, the ratio of momentum per unit span of the jet emerging from the slot to the product of the free stream dynamic head and the chord of the cylinder. Jet momentum is the product of jet velocity and mass flow through the slot, while the free-stream dynamic head is that which would be measured by a pitot mounted on the leading edge of the model; the chord of the cylinder is, of course, its diameter. Another quantity which may be obtained from Fig 4 is the
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