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Aviation History
1948
1948 - 1091.PDF
JULY 15TH, 1948 FLIGHT 79 The Gyrodyne words, the normal operating condition of the gyrodyne is with both the fuselage and the tip-path plane substantially level. Periodic Blade-tip Stall At one time it was thought that the better propulsive effi-ciency of a driven sustaining rotor compared with that of an airscrew would result in a higher top speed for the helicopterthan for the gyroplane. In this respect the helicopter has not come up to expectations. A bugbear of the helicopter, affect-,ing its propulsive efficiency in forward flight, has been the periodic blade-tip stall. This shortcoming is minimized inthe gyrodyne. Perhaps this point is best explained by considering the axialcomponent of flow through the rotor disc, i.e., the flow of air in a direction perpendicular to the tip-path plane. The auto-rotative rotor of the gyroplane is inclined at a positive angle of incidence with respect to the flight path, and the rotor iskept rotating by the axial component of flow which, in this case, is directed upward relatively to the tip-path plane. Achange in axial flow does not alter the blade angle of attack equally from root to tip, but affects the root portion of theblade most. Hence, an increase in axial flow merely extends the periodic stall at the blade root, and the operation of the rotorreiitams fairly smooth* though of course, if too great a propor- tion of the blade becojhes stalled, the vibration from the Totormay be excessive. Basic Limitation of the Forwardly Inclined Rotor The periodic blade-root stall of the autorotative rotor, how-ever, is relatively innocuous in comparison with the blade-tip stall occurring when the rotor thrust of the helicopter isinclined forwardly for propulsion. The rotor disc then makes a negative angle of incidence with respect to the flight path,as a result of which the axial flow through the rotor increases with forward speed. To compensate for the reduced angle ofattack caused by the increased axial flow, the blade angle has to be increased, and at a high forward speed it is generallygreater than the maximum value at which the rotor would autorotare in power-off flight. Hence, in the event of poweifailure, the blade angle must be quickly decreased to prevent the blades from losing their kinetic energy and stalling, though,on the other hand,"a sudden large reduction in pitch may be extremely dangerous. The effect of the increased axial flow on smoothness of opera-tion of the.rotor is more marked than in the gyroplane because, in the .helicopter with the rotor forwardly inclined, the axialflow is downward, and an increase in axial flow affects the tip portion of the blade least. Consequently, when the meancollective pitch is increased to compensate for the increased axial flow, the biade angle at the tip is excessive, and theblade tip approaches the stall cyclically at high translational speeds, especially as the angle of attack on the retreating bladeis already high due to blade flapping or cyclic feathering resulting from the aerodynamic dissymmetry of forward flight.The helicopter rotor, therefore, becomes rough in operation at high pitch in forward flight, though it may be quite smoothat the same forward speed in auto-rotative flight. Advantages of Low Pitch It may be argued that there is no need for the helicopter totravel fast; there are plenty of uses awaiting slow-speed machines for some time to come. But even if one neglectsthe possibilities of increased speed offered by the gyrodyne principle, the smoothness of low-pitch operation is in itself adesirable attribute. It is unnecessary to exceed the maximum autorotative blade angle to compensate for an increased axialflow resulting from a forward inclination of the rotor. In other words, the propulsive powered-rotor departs too far from thesatisfactory condition of operation of the autorotative rotor, and the relatively non-propulsive power-rotor of the gyrodyneavoids the limitations of the two extremes. (To be concluded next week) "- '" MODEL AIRCRAFT RALLY AT RADLETT THE third annual All-Herts Model Aircraft Rally will takeplace at Radlett airfield, by kind permission of Sir Frederick Handley Page, on August 22nd. -J$T. F. Guest, who last year took first prize for a speed of 98 m.p.h. with a "Dynajet" model, hopes this year to fly a twin " Dynajet" model aircraft which may exceed 150 m.p.h. If conditions permit and the model has completed its tests an attempt may be rffade on the world's speed record, now held by the U.S.A. with a speed of 162.3 m.p.h. The programme for August 22nd begins at 10.30 with rubber and glider duration competitions, followed by power duration, concours d'elegance, and control- line flying events. Admission to the airfield is free and visitors may obtain copies of the programme in advance by writing to the Hon. Secretary, Mr. P. Brown, 6, Abbeymill Lane, St. Albans, Herts, and enclosing 7|d which includes postage. PRESENTATION DAY AT CRANFIELD ON July 7th, the first presentation day at the College ofAeronautics, Cranfield, 36 diplomas were awarded—10 with distinction—to students going down at the end of the f first two-year course. Of this number 17 had specialized in aircraft design, 12 in aerodynamics and seven in propulsion. All the students, apart from those who are permanent com- missioned officers in the Royal Navy and Royal Air Force and who will return to their Service duties, have obtained good positions in industry, research establishments or in the airline corporations. Mr. E. F. Relf, principal of the College, said that this presentation day represented the end of the first stage of this new experiment in education, the results of which would be shown by the students when they went out into the world. It also marked the beginning of the traditions of the College. It was generally agreed, he said, that the best way in which the staff of such an establishment could preserve a fresh out- look was by taking part themselves in original research and although, because of the difficulties with which all concerned with the inception of the College had had to contend, it had not yet been possible to do much yet in this direction, eighteen original research reports had been produced at the College during the first course, to which three students had contributed as co-authors. Diplomas were presented to students by Mr. George Tom- linson, Minister of Education, who was entertaining and effec- tive in a speech in which he mentioned the importance of beginnings in the founding of a tradition and paid a tribute to the work of the Principal who, he said, could make or mar the College at the outset. While congratulating the successfulstudents on the results of their work, he stressed the need for good fellowship and for being proficient in the business ofliving as well as in earning a living. In addition to the diplomas, there were two special awards—the Governors' prize of £20, to be spent on books, for the best all-round student of the course, and the Principal's prize of^10, to be spent on books and instruments, for the best piece of original work done during the course. The first was wonby A. Lightbody and the second by J. H. Hunter-Tod, partly for his thesis on Sandwich Construction and partly for hiswork as co-author in the preparation of two of the research reports produced during the course. Air Chief Marshal Sir Edgar Ludlow-Hewitt, chairman ofthe Board of Governors, Lord Brabazon of Tara and Sir A. H. Roy Fedden also spoke. The fifth Sea Lord, Vice-Admiral Sir Philip Vian, Rear Admiral (E.) Sir John Bedale. Air Marshal Sir Ralph Sorley, Air Marshal Sir R. VictorGoddard and Sir Melvill Jones were among those on the plat- form. D. R. POBJOY AMONG the passengers who lost their lives in the accidentnear Northolt on July 4th was Mr. D. R. Pobjoy, whose name will be remembered by many from the early daysof the light plane movement, although during the last two or three years his activities were outside aviation. Douglas Rudolf Pobjoy was a Bristolian and a Captain inthe R.A. (Territorial Army Reserve). He came into the aviation fraternity in the middle 'twenties, when he designed the littlelight plane engine that came to be used in several aircraft types in later years. His first engine was intended for the Lympnetrials in 1926, but as it did not pass its type tests in time, one of the two Cranwell light planes designed by F/L. Com per wasunable to take part in the competition. Pobjoy's collaboration with Comper continued after Pobjoy Airmotors was formed,the Pobjoy engine being fitted in the Comper Swift. In the course of time, others obtained a controlling interestin Pobjoy's firm, and he next appeared at Rotols, where he designed the sleeve-valve engine for driving auxiliaries in largeaircraft. When the policy of that firm reverted to purely air- screw work, Pobjoy's fertile brain sought other avenues. Herecently designed a tractor of which he expected great things, and he had actually been on a visit to Finland to obtainfinancial backing for it when he met his death. In this he was successful, and it seems a cruel stroke of fate that his lifeshould Jbe taken just at the moment when there was every prospect of making a good start in this new sphere.
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