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Aviation History
1910
1910 - 0095.PDF
FEBRUARY 5, 1910. JySiiT] AERIAL PROPELLER*. BY A NAVAL CONSTRUCTOR. CHAPTER I.—The Screw Propeller. THE screw propeller, in some form or other, has been in use for several centuries. It is stated that the Chinese used this method for propelling ships at a very early date. It was not, however, till the beginning of the nineteenth century that the screw propeller became of any practical importance in Europe. In 1836, Smith, an Englishman, and Ericsson, a Swede, applied it successfully to the propulsion of ships. Since then the screw propeller has been used a great deal for the propelling of vessels at sea. The screw propeller, working in water, has been the subject of many theories. Such eminent men as Rankine Greenhill and W. Froude have studied it from the point of view of hydrodynamics, but the results of their labours in this direction have been of little practical value. The complex actions taking place in the water acted upon by a screw propeller, do not, with our present knowledge of hydrodynamics, admit of a mathematical analysis. Recourse, therefore, has to be made to experiments on models. The results of these experiments, carried out under practical conditions, together with quite justifiable mathe matical assumptions, have made the solution of the screw propeller problem a quite definite, although by no means easy, one. Experiments on screw propulsion in water have been made by W. Froude and R. E. Froude in England, and Durand and Taylor in America. This part of the subject, therefore, has been exhaustively treated, and little can be done in this direction as regards the action of the propeller in water. In the absence of data obtained by experimenting with propellers in air, we must look to the water-propeller for a solution of the air-propeller problem. We shall see, in a later chapter, that the two bear a perfectly definite relation to one another. In this way we can utilise the results of experiments carried out in water, for the purpose of designing pro pellers to work in air. The scientific design of an aero plane propeller can, therefore, be carried out. We shall be able to find out the propeller best suited to our requirements. We shall know exactly the horse power required, and the efficiency we are getting if we carefully study the curves given in a later chapter. CHAPTER II.—Definitions. It must be clearly understood that we are dealing with propellers of uniform pitch in the following pages. On many aeroplanes this is not the case, the propeller very often consisting of two inclined flat boards placed radially on the propeller-shaft. Any change from the true screw surface can only result in loss of efficiency. The method of designing the .screw surface will be given in the last chapter. The following definitions are very important, and should be clearly understood. 1. Pitch.—The pitch of a propeller is the distance the propeller would advance for one revolution of the pro peller-shaft, if the whole was working in a solid substance which was incompressible and unyielding. As an instance of this we may take the case of a nut working on a screw thread. For one revolution of the nut it will advance a distance equal to that between two threads. The screw-propeller working in a solid substance can be taken as a nut working on a bolt whose threads have a relatively long pitch. The pitch is usually denoted by /. 2. Slip.—In water and air, however, we have the liquid or gas yielding to the force of the propeller-blade, and in consequence of this the propeller does not advance a distance, p, but a smaller distance. Suppose now it advances a distance, x, per revolution of the shaft. Now the amount x fall short of/ divided by/ is termed the slip of the propeller. This is usually denoted by s. Putting this in symbols we should have 5 = —— Sometimes this is expressed as a percentage, and is then called the " slipper cent," thus j = x 100. The slip varies considerably in any given propeller, according to the conditions under which it is working. When it is working on the shaft of a motor fixed to a bench it obviously does not advance at all, and in this case the slip is 1 or 100 per cent. Obviously this does not represent working conditions. It does not follow, by any means, that a propeller giving a good thrust by a bench test will be an efficient one to fit in a moving body such as an aeroplane. Such tests as these are of little or no value. To calculate the slip in any given instance we may take the following example. Suppose we have a propeller of pitch 4 ft. working on an aeroplane travelling, relative to still air, at the rate of 60 ft. per second (about 41 m.p.h.). Suppose the revolutions of the propeller-shaft are 20 per second (1,200 r.p.m.). Now the actual distance the propeller moves forward in one revolution is £•§ ft. = 3 ft. But pitch is 4 ft. Hence slip is = "25 or 25 4 per cent. A large amount of slip does not necessarily mean loss of power or efficiency, but we should in no case work with a larger slip than 40 per cent. If we go above this the efficiency begins to fall off very rapidly. 3. Pitch Ratio.—This is the ratio of the pitch of the propeller to the diameter of the circle described by the tip p of the blades. It is usually denoted by P. Thus P = g Where D is the diameter of the propeller. That is, a propeller having a pitch of 4 ft. and a diameter of 5 ft. has a pitch ratio of % or -8. In actual practice the pitch ratio ranges between '4 and 1 "2. Between these limits the higher the pitch ratio the better the efficiency. The pitch ratio, however, is largely dependent on the conditions under which we are working in our design, and it may be impossible to fit a propeller having a high pitch ratio. Disc Area Ratio.—The area of the blade of the pro peller is usually expressed as a fraction of the area of the circle described by the tip of the blade of the propeller. The ratio of the developed area of one of the blades of a propeller (whether two, three or four bladed) to the area of the circle described by the tip of the blade we shall call the " disc area ratio." For each pitch ratio there is a certain value of the " disc area ratio " which gives the best efficiency together with 91
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