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Aviation History
1910
1910 - 0921.PDF
NOVEMBER 5, 1910. not the only loss, and the problem of propeller design resolves itself into obtaining the maximum thrust at a desired speed for the least total power.—ED.] AEROPLANES V. HELICOPTERS. [868] In reply to letter No. 763, by " Octavius," I should like to make a few remarks. Your correspondent says that " an aeroplane rising at an angle of 1 in 30 experiences 900 times the head resistance per unit area of body of a helicopter rising at the same speed," and he gives as the reason for this that the aeroplane has to travel a much further distance at a much greater speed than a helicopter in order to rise to the same height. But he appears to overlook the fact that in order to rise the blades of a helicopter must revolve, and consequently must experience resistance apart from that produced by its upward motion. The peripheral speed of the blades of a helicopter must be considerably greater than the speed of an aeroplane at the present day. For instance, a helicopter with blades of 14 ft. diameter, revolving at 240 r.p.m., has a peripheral speed of 176 ft. per sec, or 120 miles an hour. Since resistance increases with the square of the velocity, the resistance per sq. ft. at the end of the blades (the most effective pari) of such a helicopter must be at least four times that of an aeroplane, assuming that both are moving through undisturbed air. Your correspondent assumes the toul head resistance of a helicopter to be equivalent to that which it would experience by virtue of its upward motion also. Apart from this, the aeroplane is not. and was never intended to be, a direct-lift machine, and therefore it cannot be compared with the helicopter when used as weight-lifter only. In order to com pare the aeroplane with the helicopter on an equal basis, it would be necessary to propel the latter at the same speed and the same angle as the aeroplane. This would mean that more power would be consumed by the helicopter, thus placing it at a still greater disadvantage compared with the aeroplane. It is well known that the aerial screw is inefficient when used as a propeller, and it is probably still less efficient when used as a heli copter. Even supposing it to be equally efficient, it gives the heli copter system a lower efficiency than the aeroplane system. Your correspondent gives the aeroplane system a total efficiency of 54 per cent. If a helicopter, having an efficiency of 60 per cent., were propelled by a screw-propeller, also having an efficiency of 60 per cent., its total efficiency would be only 36 per cent., or 18 per cent. less than that of the aeroplane. The low efficiency of the aeroplane system is almost entirely due to the screw propeller. In order to increase its efficiency some new form of propeller is required. The aeroplane system being the most efficient, its chief dis advantage is that it can only rise from the ground at a very small angle. In order to adapt the aeroplane system to a direct-lift machine it would be necessary for the planes to be made so that they could move rapidly while the machine is stationary. In connection with this principle I will quote Mr. Edison's views on the subj-ct. He says: " If I were to build a flying machine I would plan to sustain it by means of a number of rapidly-revolving inclined planes, the effect of which would be to raise the machine by compressing the air between the planes and the earth." If the planes are to move in a direction at right angles to their span while the machine itself is stationary, they must either move in a circle or oscillate backwards and forwards. A machine built on this principle would, I believe, contain all the advantages claimed for the helicopter, and in addition would carry more weight for less power. Leamington Spa. G. B. BENNETT. ASPECT RATIO. [869] In reference to letter 762, you mention that aeroplane engines are at present started by turning the propellers. In this case how are the engines restarted after a vol plane ? What is meant by the aspect ratio of an aeroplane ? Your excellent paper is invaluable to all interested in aviation. Crewe. F. B. W. A. [Pilots do not allow their engines to stop turning in mid air, although they may momentarily switch off or throttle down. Aspect ratio is the ratio of the span to the chord.—ED.] HYDRO-AEROPLANES. [870] In your issue of September 24th, it is stated that M. Fabry's machine has risen and traversed more than 3 kiloms. Some of us have been trying similar experiments (but without the success) on Lake Windemere, and should be very glad to know whether any of your readers would care to undertake or assist in similar experiments. I could offer facilities on Windermere Like if they cared to communicate with me. Kendal. EDWARD WILLIAM WAKEFIELD. I/GGHT] CAN WE FLY FASTERl WITH LESS POWER ? [871] I should like to be allowed to offer my sincere congratula tions to you re your articles under the above head. In my opinion, you are absolutely right in every respect in your conclusions. And you show a determination to disregard the orthodox expert, and trace difficulties to their souice for yourself that is admirable in these days of follow-my-leader science. Some time last year, at the close of a series of very interesting experiments, I wrote a small pamphlet on aviation, which I sent to the aviation papers and to several of the newspapers. Some of them returned the copy with polite regrets, some without any regrets, some did not return it at all. My points were : (1) Smaller planes ; (2) plants nearly or quite flat ; (3) a boat-shaped or streamline car to contain the engine, pilot, &c. My reasons were that a design of this kind, by reducing the head resistance, would allow much greater speed, which would mean greater stability and indifference to adverse gusts, and make up for the smaller supporting surface and the absence of camber. Also, such a machine could be constructed much more strongly with less weight than any of the machines having to stand the tremendous leverage of widespreading planes. I am pleased to see how closely you corroborate evety one of my points. In the pamphlet alluded to I also advocated two other things besides those mentioned ; a new method of propulsion and a new force to use with it. These last two, however, are still affording as much amusement to the experts as my other three points did, and so I expect I had better say nothing further about them till the author of your article or some other equally enlightened writer states his views, and I find myself, much to my astonishment, suddenly arrived in quite respectable and orthodox company. Manchester Street. W. LEMA!I-RB. PATENTS. [872] We should be the last people to discourage the advance ment of new ideas, but in the interests of inventors themselves, it seems necessary to warn them that the descriptions which some of them have allowed to appear lately in the Press unfortunately ruins their chances of obtaining valid patents, unless the application for the patent has been previously made. After the application is made, however (whether for a full patent, provisional protection, or design registration), it is generally safe to make full disclosure of the invention at once, and even 10 begin selling, if an opportunity presents itself. Inventors who ignore this advice simply give away their ideas to the public, which, if they can afford to do so, is perhaps the most generous thing to do. 38, Chancery Lane. STANLEY, POPFI.EWK.LL, AND CO. NORMAL PRESSURE. [873] Could you please tell me what pressure 5 m.p.h. wind would represent when meeting a stationary I It. sq. plane, whole surface against the wind, and would 10 m.p.h. wind represent twice the pressure of 5, and 15 three times the pressure of 5, and so on ? If not, what would be the rule for determining this in the absence of special instruments? Would 6 ft. by 2 ins. have the same resistance against the wind as 12 ins. by 12 ins. ? New North Road. H. H. [The pressure on a normal surface at 5 miles an hour is "075 lb. per sq. ft. (see " Flight Manual," Table 155). The pressure varies as the square of the speed (see " Flight Manual," Note 1), and is also affeced by the shape of the plane (see " Flight Manual," Note 13), in such a way that the mean pressure per sq. ft. is tsreater on long narrow planes than on square planes. The formula for calculating normal pressure is derived from the theory explained in " Flight Manual," Formula 76. and includes a constant for which various values h*ve been found by different experimenters (see " Flight Manual," Table 154). An average working value for approximation is given by the formula P • 003V (where P = pressure in lbs. per sq. ft. and V = flight speed in m.p.h.).—ED.] THEORY OF THE BALLOON. [874] Could you please inform me what is the correct technical the >ry why a balloon or dirigible lifts ? Glasgow. AMATEUR. [The theory of balloons is based on a property of fluids which is known as " Arc dmed s' Law," which is to the eff-ct that anything immersed in a fluid is acted upon by an upwird force equal to the weight of the fluid displaced. The enunciation of this law and its application to aerostatics will be found in " Flight Manual " Notes 129 to 188.—ED.] 919
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