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Aviation History
1920
1920 - 1217.PDF
NOVEMBER 25, 1920 THE PROBLEM OF THE HELICOPTER" BY LOUIS DAMBLANC IT is a very great honour to me to have to speak this evening before this learned assembly, and I have to thank you very sincerely for the occasion which you have offered me of discussing before you a very important question, one which is of interest to the whole of the aeronautical world, namely, the problem of the helicopter. My aim will be but modest, for I shall simply attempt to develop those essential arguments which have given me such entire confidence in the practicability of this type of heavier- than-air machine, and I shall be content if at the end of this paper I have been able, not to convince you, but at least to interest you in the cause which I have at heart. The helicopter is not a competitor of the aeroplane. It is an entirely different type of aircraft, but the one is the complement of the other, and if their respective uses are not the same, they nevertheless will be of equal importance in that immense future which we all believe to be reserved for aircraft. For military purposes the helicopter will be an incomparable observation machine, and, when its horizontal speed becomes equal to that of an aeroplane, a formidable bombing machine. For work at sea its advantages are evident. Aeroplanes cannot land on the decks of warships -except in the face of great difficulties and with the aid of special and encumbering landing decks, which involve a waste of space which can scarcely be tolerated. The helicopter alone is capable of getting over these difficul- ties in a satisfactory manner. By its use it will be possible to rise vertically from the deck of a ship and to land in the same way. Merely to mention these several applications is to justify the interest which has been taken in the attempts to develop this type of machine. I will not waste time in discussing the history of the heli- copter.. All the world knows that for more than a century many inventors in England as in France have taken intense interest in this problem. The greater number of them have stopped short at the production of children's toys, using for motive power, either bent whale-bone, skeins of rubber or steel springs. One has only to read the numerous patent specifications which have been taken out concerning helicopters to realise that most of the authors have had either the most elemen- tary or the most fantastic ideas. The majority of the patentees have considered only vertical ascent of the machine, and have concerned themselves neither with the maintenance in the air nor the gliding descent of the machine. Such conceptions will not stand examination. In order to be practicable, any kind of flying machine must give complete guarantees of stability under all circumstances of flight. It was in 1903 that the late Col. Renard, in a very striking communication to the Academie des Sciences, brought the first gleam of light to bear upon the conditions of vertical flight. This scientist with sure intuition, with enlightening and fertile reasoning, has established the fundamental laws which still rule in all practical enquiries into the behaviour of • Paper read before the Royal Aeronautical Society on November 18. lifting airscrews. I propose to speak' of his works, and Ishall divide my study of them into three chapters :— (1) Lifting force and lifting airscrews ; (2) Construction of helicopters ; and (3) The gliding descent of a helicopter with stoppedengine. (1) Lifting Force and Lifting Screws Col. Charles Renard, in three communications to theAcademie des Sciences, has dealt completely with the question of lifting airscrews. I will recall to you the title of hiscelebrated notes. The first (November 23, 1903) was entitled " Upon the Possibility of Sustaining in the Air a Machine of the Heli- copter Type, using Internal-Combustion Engines' in their present state of Lightness." The second (December 7, 1903) bore the title " Upon theQualities of Lifting Airscrews," and finally the third, presented several months before his death at Chalais-Meudon (whereduring twenty-seven years he had unceasingly pursued his scientific researches) was entitled "A New Method of Con-struction for Airscrews." With the aid of aerodynamic balances, Col. Renard hadspent eighteen years in studying completely the functioning of lifting airscrews, working with no translational motion, andhe had determined the characteristic equations of their operation. The fundamental formulas arrived at by Col.Renard are the following :—• Thrust in Kg. = F = «n2DJ Power expended in H.P. = T = ,8w»Ds Where n = revolutions per second, D = diameter of thescrew in metres, and o and £ are constants for any member of a family of similar airscrews. The expression for the useful thrust per horseabsorbed is given by F T' It can be seen at once that for the same airscrew the thrustper unit of power is greater and greater as the speed of revolution becomes smaller. Sustantative Value (Qualite sustentatrice).—Col. Renardhas given this name to a characteristic figure used in the study of an airscrew; which is determined in the following manner :— For an airscrew of diameter D the area of the circle swept TTD*out by the blades will be :—S = If we call S' the area of an imaginary plane which dropping vertically at a speed, V' will produce the same thrust F as that given by the airscrew for a power expended = T, then the " qualite " or sustentative value denned by Renard is q = i'.S It can be seen at once that S increases with the " qualite "of the airscrew. In order to obtain given thrust F, the greater the equivalent plane is the less is the work expended in orderto obtain this given thrust. This simple statement proves that augmentation of the " qualite " is equivalent to increas-ing the aerodynamic efficiency of the airscrew in question. power a I iSD n THE DAMBLANCJiHELICOPTER : Photograph of a scale model of the two direct-lift screws employed on the Damblanc "Alerion."
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