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Aviation History
1910
1910 - 0057.PDF
JANUARY 22, 1910. commonly adopted, and in the case of wooden propellers this results in the simplest form of construction. If too many blades are employed they will interfere with one another, but although four blades is probably as many as would be required in most cases, it is not improbable that good results would be obtained in aerial wsrk with propellers having as many as six blades. f/EGfif] is a very convenient and proper way to regard a propeller blade as an aeroplane specially designed to traverse a helical path, and from what is already known of aero planes the immediate deduction will then be that the face of the blade should be curved ; in practice it will be found that this is actually a feature of the propellers commonly in use. It is unnecessary to enter into an " Flight" Copyright. In the Lamplough propeller the blades are each made from a single piece of wood, and are hollowed out from the root towards the tip. They a« mounted in a hollow copper stamping which forms the boss. A Simple Rule, In his very able treatise on Aerodynam'cs, F. W. Lanchester, whose interesting treatment of the problem of the aerial propeller should certainly be studied by those interested in this subject, gives the following rough and ready equation for the number of blades per missible :— Number of blades rt + 2'5 r t - n where f r2 = radius from axis to tip. radius from axis to inner extremity. By increasing the number of blades the thrust is, of course, distributed over a larger area, and the grip of the propeller upon the air column is more evenly distributed ; but, on the other hand, the weight would be considerably increased, and on a machine like an aeroplane it would probably become advisable to con sider if greater ad- B>« vantages might not ensue from using more propellers, with the object of distributing the pro peller streams over the aeroplane sur faces with a view to facilitating starting. Hollowfaced Blades. Although in what has already been said the angle of the blade at any section has, for con venience, been sup posed uniform, such is not really the case in a well- designed propeller, any more than the surface of an aero plane is perfectly flat in a well- <lesigned flyer. It " Flight" Copyright. A curious feature of the propeller on the Pischoff biplane is the lip formed on the periphery of the blade, which thereby somewhat resembles a big wooden spoon. examination of the amount of the curvature at the present moment, although it is very important to point out that it exists. There are various ways of regarding this characteristic of a propeller-blade, apart from that of the aeroplane simile already mentioned, and one of the most convincing is to try and appreciate the desirability of having the air which the propeller blade is acting on at the moment set in motion evenly and without shock ; in other words, to avoid a " fierce clutch " effect. Easy Starting. This condition obviously cannot result if the face of the proDeller-blade represents a flat plate set at a fixed angle, because such a blade as that would cut into the air molecules with a considerable shock, instead of taking hold of them gently and accelerating them by degrees so that they leave the trailing edge of the blade with a velocity which would have been theoretically established by a fiat plate at some equivalent angle. A blade with a curved face, it may be observed, has essentially an increasing pitch from the cutting to the trailing edge considered through any particular section, and it is only the mean effective pitch, equivalent to this range, which has been referred to in the foregoing paragraphs. Wide and Narrow Blades. The necessity of this increasing pitch in a well designed propeller is a very good reason why the width of the blade is limited, and is commonly much narrower than a good many people imagine to be correct. To make a very wide blade on these lines would very soon infringe the limiting angles already discussed, and to make a very wide blade with flat face does not do any good either, because the air molecules have already been accelerated up to their final velocity during their contact with an initial minimum width, so that any Made surface beyond this is wasted except so far as it may be required for increasing the stiffness. The Best Position. Apart from considerations affecting the design of the propeller itself, the efficiency of a proneller is also con siderably influenced by its position, but it is questionable whether the deductions which have been made in marine work on this point will find quite the same parallel in the case of the flying machine. A boat moving through the water sets in motion the water immediately around it, which is caused to travel in the same direciion because of the skin friction between it and the vessel's sides. 53
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