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Aviation History
1948
1948 - 0183.PDF
FEBRUARY 5TH, 1948 FLIGHT 159 Flight Testing of Helicopters matically in Fig. i. The rapid contraction of the slipstream close to the rotor shows the inapplicability of measure- ments close to the disc. The velocities measured at the fully developed section (X—X in Fig. i) and corrected to give their appro- priate positions on the disc rotor are given in Fig. 2. The theoreti- c a 1 distribution calculated by pro- peller strip theory is included in this figure for com- parison. As the helicop- V#3f descends from hovering flight, it enters the vortex- ring state, a con- dition in which little is known of the physical con- struction and where no theory has, as yet, been devised which can be applied satisfactorily. The air associated with the rotor does not form a regi^ar slipstream develop- ment, but there exists a circulation of air in a very turbu- lent state about the rotor, the flow being directed down- wards through the rotor and then moving radially outwards and upwards out- side the rotor disc. Some of this air, as it passes upwards above the rotor, is again drawn O O-2 O-4 O-6 O-8. . 1-0 FRACTION OF BLADE RADIUS (-t-j Fig. I. Diagrammatic presentation of smoke-filament development in hovering. O-2 O-4 O-6 O-8, FRACTION OF BLADE RADIUS fe) Fig. 2. Comparison of experimental and theoretical axial induced velocity in hovering. inwards and thus some of the air may circulate through the rotor many times, in the manner from which the state derives its title. It is obvious that under such conditions, the accurate measurements of the velocities in the vortex circulation I are virtually impossible. The fluctuations caused by the turbulence of the stream raake it difficult even to identify the order of the velocity at any point in the circulation and, even if the measurement could be made, there is no relationship by which the conditions at the rotor disc could be obtained. However, some extremely valuable informa- tion on the nature of vortex circulation has been obtained by the use of the smoke-filament technique. One of the first photographs of a vortex ring obtained in flight is shown in Fig. 3. The photograph leaves no doubt that the air flowing through the rotor is largely maintained in the circulation and also shows clearly that the form is elliDtical with the vertical direction forming Fig. 3. One of the first photographs obtained in flight of a vortex ring. The true position of the blade tip is about I fu inboard of the end of the smoke-generator pole. Fig. 4. Smoke filaments obtained in approach to auto-rotation state. There is very iittle flow through the disc, and rate of descent is about 2,200 ft/minute. . the major axis. Another important point to note is that the centre of the vortex ring in this condition lies below the rotor disc. The condition when the helicopter is just able to main- tain auto-rotation is shown in Fig. 4. This may be taken roughly as representing the ideal auto-rotation state. The rotor, in absorbing the energy from the air, is in a state of virtually no-flow through the disc. There is, probably, still a vortex circulation above the rotor but the velocities would be so small that for all general purposes the air above the rotor may be considered quasi-stagnant. Beyond the boundary-condition ideal auto-rotation state lies the windmill-brake state of the ordinary propeller, but on the helicopter the conditions do not form a parallel. If the blade pitch is further reduced, the rate of descent increases and more work is absorbed from the air. For a helicopter there is no means of absorbing power, as the rotor is free to over-run the engine, and also a constant thrust must be maintained to sustain the helicopter. The result is that the rotor speed builds up to some new value where the energy is used in overcoming the profiile drag of the blades at their higher rotational speed. Thus, with progress further into the windmill-brake state, the rota- tional speed of the blades increases. In considering the boundary between hovering and the vortex-ring, and between the latter and the windmill- brake state, there is one important distinction to be noted. In hovering there is a definite slipstream pattern of a laminar nature. In the case of the ideal auto-rotating rotor, the air is being brought to rest, there is still a vortex circulation in existence and a certain amount of turbu- lence or mixing will still be taking place. (To be continued.)
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