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Aviation History
1910
1910 - 0078.PDF
[/yGHT] the normal flying angle and speed. If these conditions are not fulfilled some other force must be introduced to balance the machine. It follows from the above that if the angle of incidence is decreased the centre of pressure moves forward from the centre of gravity, thus producing a couple which tends to restore the plane to its normal angle. A similar, but reverse action takes place when the angle of incidence is increased, as the centre of pressure then moves in the opposite direction. The authors are at present experimenting with propellers, but not having yet completed their investigations, they propose to confine themselves to a few general remarks on the subject. It has taken many years of experience to enable the marine engineer of the present day to design a ship's propeller with com parative success, and it is even now not an uncommon thing to read of a ship being fitted with a new design of propeller in the hope that it may be more efficient than the old one. It seems to be quite evident that the same doubt applies to air propellers, and it is probably on this account that so little information is at present obtainable. The construction of propellers for aerial purposes cannot be too carefully considered. It is admitted that the high-speed propellers of French design have an efficiency of only 40 per cent., and this is not to be wondered at, considering the method of construction adopted ; the large pieces of metal running up the back of the blade are as detrimental to the efficiency of the propeller as a hump along the back of the main plane would be to the efficient lifting power of the plane. On the other hand, we are told that an American-made propeller has an efficiency of 70 per cent. This is built up of wood, the surfaces made smooth and highly polished, thus reducing skin friction and air dis turbance to a minimum. Although there are marked differences between the two propellers mentioned, that of American make being of large diameter and run at a comparatively slow speed, while the French design is of small diameter and runs at a high speed, for machines of practically similar size and weight, it may be said that the American propeller is much more suitable for the work. Considerable weight may be saved by using a small propeller running at a high speed, but it is questionable whether this is really a saving on the efficient running of the machine. In a certain English machine, the method of propulsion is of interest, in that two propellers are fitted, one directly in front of the other, running on the same shaft, and so arranged that they revolve in opposite directions. Air propellers are usually made with two blades, as it has been found that they are more efficient than if fitted with three or more blades, the reason for this being that the blades follow each other too closely, and their pitch being of necessity small, the disturbance set up by one blade overlaps that due to the following blade, thus reducing the efficient thrust appreciably. There has been considerable controversy as to the merits or demerits of placing the propeller in front or at the back of the machine, and it remains to be seen which is really the better system. It is fairly evident that the propeller at the back of the machine has a certain advantage, in that it works in the " wake" or following current. Power is spent in producing this forward current, and if the propeller is able to work in this, some of the power is recovered, because the propeller may be said to be working in air more favourable to its efficiency. When the propeller is fitted at the fiont of the machine, the power expended in producing the wake is all lost, and also that part of the machine directly behind the propeller destroys the effective thrust to" a considerable extent. The authors would like to draw attention to the design of the root of the blades. The blades should be amply strong at the root, as so many serious accidents have occurred through the blades breaking WV2 off. The pull at the root = , gr where W = weight of blade, V = velocity in ft. per sec., r = radius in ft. It may be of interest to know that in a steel propeller with which the authors are at present experimenting, the stress is 26,000 lbs. per sq. in., the propeller being run at a tip velocity of 550 ft. per sec. ; and also in the case of a wood propeller, the factor of safety is only 3. In the list of aeroplanes given above will b? found particulars oi a number of propellers fitted to actual machines. Control,—The problem of obtaining sufficient effective com bination of the necessary surface and power is now overcome to a JANUARY 29, 1910. great extent, owing to the excellence of design and construction, bat the greatest problem which has to be faced in nearly all types of aeroplanes is the system of control, for obtaining lateral and longitudinal stability. For lateral stability, the two principal methods of balancing are by total flexing or partial flexing of the main planes, thereby altering the angle of incidence, or by supple mentary planes or ailerons. Of the former, the R.E. P. monoplane and the Wright biplane are the most notable examples ; of the latter the Curtiss biplane, which is fitted with two supplementary planes, placed at the extreme ends of and between the main planes, is a good example. The use of hinged edges or tips is common to a number of successful machines. The elevator has also been used to effect this object, but, apparently, not with much success. Longitudinal or phugoid stability is in nearly all cases obtained by an elevator, either in front or at the back of the machine. The actual steering of the machine is in all cases effected by means of one or more vertical planes placed fore or aft. All the operations for the control of the machine in the air bring up some rather difficult points in mechanical construction ; and also it will be evident that, in the control of an aeroplane, there is a third movement, or sense, which does not exist in any other form of locomotion. There is not only the side balance and steering required as, say, in a bicycle, but it is necessary to cultivate a third sense of fore and aft balance. Thus it will be seen that the system of control The "CR." biplane, designed and built by the authors. of an aeroplane must be made as simple as possible, as the sudden gusts and variations of air currents which have to be contended with leave little time for thought on the part of the aviator. The Wright biplane has only two main levers, one at each side of the aviator's seat. On the right is placed the lever, which moves in a fore and aft direction, to operate the elevator, and moves athwartships to move the steering rudder. The other lever is placed on the left hand, and moves in a fore and aft direction to flex the main planes. Wright found that the flexing of main planes was inefficient, as a swerving movement took place due to the drag caused by the end of the main plane which had the greater angle of incidence. To check this and also the action of centripedal force, he added supplementary vertical rudders, which he operated by a lever placed in such a position that it could be readily moved with, and at the same time as, the lever for flexing the main planes. This has been one of the most effectual and simple methods of control so far used. To obtain automatic stability is naturally the desire of every designer, and will be the only method of control to make the aero plane a safe and pleasant means of transit. This may be more or less obtained by two methods, viz., (a) machines so constructed that they possess automatic stability without movement of main or supple mentary planes ; &) by mechanical means, either by moving weights, pendulum, vanes, or action due to gyroscopes. The first method is practised by the Voisin or the cellular class of machine, which rely on a box-like form of construction, but it remains to be proved that this class of machine will be stable under all conditions, because, if certain oscillations are once set up, these oscillations will ultimately capsize the aeroplane. This has un doubtedly been proved by Lanchester's phugoid theory. 74
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