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Aviation History
1910
1910 - 0040.PDF
i/llCHT JANUARY IS, I9IC. AERIAL PROPELLERS AND SOME POINTS WHICH MAKE THEM INTERESTING. Cot/tinued Thrust and Slip. THE primary condition which it must obviously fulfil is that of creating a thrust (P) sufficient to overcome the resistance of the aeroplane in its line of flight. This thrust, as has already been shown, can only be maintained by imparting a rearward velocity or slip (v ft./secs.) to a mass per second (m) of still air; the value of the thrust in absolute units being given by the very simple equation P = mv. Expressed in words, this mathematical state ment shows that the pressure or thrust of the propeller depends on the mass of air which it sets in motion per second, and on the real slip which it imparts to that mass. It will be observed, however, that increasing the speed (r) also automatically increases the mass per second (m) in the same proportion, for the mass (m) is itself an expression of pA.v where p is the density and A is the area of the column in motion at velocity v. It follows, therefore, that the expression P = mv can be written P = phv1, from which the important deduction is made that whereas the thrust is only directly propor tional to the area, it varies as the square of the speed of the slip. * Disc Area/' For the above, the area may be assumed as equal to that of an imaginary disc attached to the boss of the propeller and having a diameter extending to the tips of the blades. As such a disc would have an area equal to (TTD2 -r 4) it therefore follows that the propeller- thrust is proportional to the square of its diameter. In order to increase the thrust when it is no longer feasible to increase the diameter of the propeller, it is necessary to increase the speed of the rearwardly moving column of air either by increasing the revolutions of the propeller itself or by increasing the pitch of its blades. Pitch of a Propeller. The pitch of a propeller is the distance which it will advance, due to the angle ©f the blades, through a solid nut in one revolution, and for any given diameter it is therefore an indirect measure of the angle which the tips of the blades would make to the plane of a disc fastened to the propeller boss. Such being the case, it does not require a very technical mind to come to the conclusion that the ratio between the pitch and the diameter is limited for practical utility, because of the churning action which would be set up by the blades if they were given -too great an angle. If, for instance, they were set wholly at right angles to the imaginary disc, the device would not be a propeller at all, but a churn pure and simple. At the other extreme, if the blades had no angle at all, they would merely be equivalent to a disc revolving in a plane at right angles to the shaft. It can be shown that the intermediate value of 450 is the theoretical angle at which a blade should be set if working in frictionless fluid, but the values adopted in practice are commonly far below this, and the exact value is a factor which depends upon the skill and experience of the designer. There is another point to be borne in mind, too, and that is that the angle of the blade is neither constant from root to tip nor constant from leading edge to trailing edge. A constant blade angle from root to tip would result in a variable pitch, and a constant blade angle . from leading edge to trailing edge would produce a flat from page 25.) blade, which, from the analogy of the aeroplane, might well be expected to be less efficient than one suitably cambered. For approximate reckoning it is sufficiently near the mark to regard a propeller as being a portion of a helix having such variations in blade angle as will produce a constant pitch at all diameters from the boss to the periphery. As a matter of fact there are practi cal considerations which cause a departure from this principle, especially in propellers like those constructed of wood, where the blade itself actually does spring from the boss, and is not mounted on a separate arm. Wood v. Metal. In any propeller based on the helix principle, the blade angle essentially increases from tip to root, and there comes a point where the blade itself ceases to be any longer of value, owing to its relative inefficiency. From this point to the boss it is the designer's chief object to reduce loss rather than to gain thrust, and thus it is that in metal propellers the blades are commonly cut away entirely where they cease to be efficient, and are mounted on steel arms of circular section which offer "Flight" Copyright Photo. The Short propeller is constructed entirely of wood, and consists of six separate layers which are joined together to form a solid piece. relatively small resistance to their passage through the air. In wooden propellers, where the blade is an integral part of the whole construction, considerations of strength prevent any such cutting away as this, and consequently it is usual in well designed propellers to find the blade gradually converging into a smooth stream-line form, as it approaches the boss. Weight and Strength. When considering a propeller, it is not alone sufficient to have regard to its theoretical qualities, for like all else in practical mechanics, considerations of weight and strength put a limit to the achievement of ideas. Excessive weight, for instance, is the principal factor which checks the use of propellers of very large diameter, and always where there is a question of weight concerned with a moving part, the question of strength has enhanced importance. On the whole, of course, the designer's object is to use as large a propeller as possible without running up against any of the difficulties which beset his path. " Cavitation." There is one other consideration which is involved in an analysis of the equation (P = mv) for the thrust of a 36
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