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Aviation History
1914
1914 - 1085.PDF
OCTOBER 30, 1914. 9ne of the earliest types that the writer remembers were the twin-screw models of Mr. Burge Webb, having one propeller in front (tractor) and one beh.nd (pusher). These were very light one-plane models, and although somewhat tricky to launch cave remarkably good results. The design is not, however, a neat one, and becomes more and more difficult as the size of the model increases. During the present year the writer has been making a number of experiments with model submarines: (I) rubber-driven, (2) fitted with electrical motors and automatic controls worked by the same agency. In the case of the earlier rubber-driven models, in order to obtain longer runs and overcome the reaction to the motor torque, a model was fitted with twin propellers and rubber motors a U Fig. 1.—Part view of rubber-driven submarine model with tandem propellers and concentric gear. ordinary single-stick model aeroplane. The ugliness and incon gruity of such a monstrosity in design (which worked quite success fully) quickly led to the adoption of the tandem propeller, as shown in Fig. 1. The results obtained were very good, and it is with a viewlto experiments along similar lines in aeronautical models that these lines are penned. Some experiments have, of course, been made in this direction already ; probably the best-known model of this type was Mr. Aston's i-oz. model, built for the famous i-oz. competition, held some years ago, in which quite a number of the leading aeromodellists of those days took part. The results obtained with this model were extremely good. It has rubber motors must be wound nf in tit same direction. In Fig. 2 a central idle or jockey cogwheel is inserted, which allows an ordinary model twin winder to be used. In the former case the strands of robber must be kept well apart or they will interfere with one another. The idle cogwheel used need not be of so large a diameter as the two others, although it is shown so in the figures. In Fig. 1 the rear propeller has tor an abutment the outer wire ring bracket; in Fig. 2 the small extra piece A serves that purpose, and in Fig. 3 if is a ball-bearing abutment. This is necessary if the rear propeller shaft has an abutment against the axle of the leading propeller ; on account of the friction caused by the pull of the rubber motor driving the rear propeller, the propellers do not run evenly. In Fig. 1 the smaller and leading propeller has a diameter of 1 5 ins. and the rear one of 2 ins. In the case of aerial propellers, as shown in Figs. 2 and 3, the dimensions could be 8 ins. and 12 ins. Since the rear propeller is working in the slip stream of the leading one, it must be given either a coarser pitch or n greater diameter, or possibly both. There is also the question of their proximity to be considered. In Figs. 2 and 3 the steel spindle, C, Fig. a, and D, Fig. 3, is supposed to be several inches in length, and the bearing of the rear propeller to be such that it can be placed in any position alone this spindle ; if the spindle were threaded and nuts used on both sides of the propeller, this could easily be done, the idea being to find the position of maximum efficiency. Obviously the whole question is one in which a considerable amount of interesting experimenting could be done. The system is efficient in the case of marine propellers under certain circum stances. Possibly it would be less so in the case of propellers working in air; but so far as we know a sufficient number of experiments have not yet been carried out to enable a satisfactory answer to be given. We shall be glad to hear from any reader who may have experimented in this direction with respect to either aerial or marine propellers. Mr. F. Mayer (of Messrs. J. Bonn and Co.), with whom we briefly discussed the matter a few days ago, writes as follows:— " I enclose herewith two drawings (Figs. 4 and 5) of the gear devices mentioned at our interview. These gear arrangements are technically known as co-centric or concentric spindles. "The one shown in Fig. 4 is what is termed a differential gear, and that in Fig. 5 a lazy shaft gear. As you know, I have had considerable experience in gearing in all its branches, and from time to time have made use of most of the known arrangements and have used co-centric gears in a variety of forms and for a FIG ^TP*®^ FIG 5 Figs. 2 and 3.—Tandem propellers on concentric shafts. Figs. 4 and 5.—Two types of concentric gearing by Mr. F. Mayer. also been used (and mav for anything we know to the con trary still be employed) in the case of modern torpedoes, which have now a range of some 7,000 yards and a speed at any rate of 35 knots, some accounts say 45 knots, but this we doubt. The employment of twin propellers in this manner leads to the building of a model of the best streamline form. At the very first Royal Aero Show at Olympia, a full-sized machine, fitted with tandem propellers, was on exhibition. So far as we remember, the machine had no distinguished career, but in those early days, it might well be due to many causes, other than this particular feature in design. In the designs shown in Figs.I, 2 and 3, the rear propeller is mounted on a steel wire spindle running, right through a piece of brass tubing in which it re volves, this brass tubing itself revolving in suitable bearings by means of cogwheels suitably arranged. The two axles turn, of course, in opposite directions. One rubber motor, therefore, drives the rear propeller directly,yt\\\\st the other drives the leading propeller mounted on the brass tubing by means of suitable cogwheels. In Figs. I and 3 •it will be noticed two cogwheels only are used. In this case both variety of purposes. The commonest type of co-centric gear is the spindle and sleeve on which the handles of clocks and watches are mounted ; in the case of clocks and watches the spindle and sleeve have a ratio of 12 to 1. This is merely a matter of diameter and number of teeth in the various wheels. This is the only instance that I know of common use for this type of gearing ; but I have seen it used on aero-models, on torpedo propellers, on an aero engine, and some cinematograph projectors have the shutter and flicker blades driven by this means. " In Fig. 4, a train of three bevel gears are used. Bevel gear A is fixed to spindle D ; bevel gear B, mounted on its own sleeve, E, which runs loose on the spindle, D ; B is driven by the idle bevel, C. The propellers are mounted at E and F. In place of bevel gears crown wheels and pinions can be used. [This is a very neat arrangement, possibly the neatest; in this method, for rubber-driven model aeroplanes, only one rubber motor would be used, the reaction to motor torque would be got rid of, but the duration would be rather lessened than increased, owing to increased friction, more strands of rubber, &c. This type, there- I08S
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