FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1910
1910 - 0920.PDF
(/JJGHT] NOVEMBER 5, 1910. CORRESPONDENCE,. The name and address of the writer (not necessarily for publication) MUST in all cases accompany*letters intended for insertion, or containing queries. Correspondents communicating with regard to letters which they have read in FLIGHT, would much facilitate ready reference by quoting- the number of each such letter. NOTE.—Owing to the great mass of valuable and interesting corre spondence which we receive, immediate publication is impossible, out each letter will appear practically in sequence and at the earliest possible moment. EFFECT OF ALTITUDE IN AIR AND WATER. [864! I listened recently to an interesting discussion on deep- sea diving. Ic was pointed out that all descents and ascents had to be made very slowly, since a too sudden increase, or decrease, of pressure on the human body was attended with fatal results. The idea immediately presented itself that this state of things would also obtain in the air Assuming that this is so, and in view of the altitudes recently attained by aviators, it is a point which would seem to call for some attention. Not so much in rising, which is done comparatively slowly, but in the quick glides where hundreds, and even thousands, of leet are dropped in the space of a few minutes. Except in exceptional cases (such as M. Morane's magnificent flight at Deauville), it should be possible to descend in more or less gradual circles, and obviate another, but unnecessary, danger to aviators. I write as one who has no practical knowledge of the subject, and who therefore asks the opinion of those who have. Stratford-on-Avon. E. L. W. [The difference of pressure per foot difference in altitude depends on the density of the medium, and is much greater in water than air. Hence, much quicker changes of altitude may be made in air than in water without serious effect.—ED.] FLYING IN THE WIND. [865] I have followed the correspondence of FLIGHT since its inception, but strangely enough do not remember anyone querying or explaining why an aeroplane does not lift so well with the wind as against it. I have given the matter some thought, and ask your permission to set out my views as follows :— Supposing an aeroplane's flying speed is 40 m.p.h., and that it is started from rest into a 25 m.p.h. wind. When the machine reaches a land velocity of 15 m.p.h. it will rise and soar at that speed so long as the initial conditions are maintained. Now if the flight is curtailed to the limits of an aerodrome, and a return journey with the wind is essential, to do so, and to sustain flight, the land speed of the machine must be accelerated from 15 to 65 m.p.h. Naturally the difference in the momentum of the machine travelling at 15 m.p.h. and when travelling at 65 m.p.h. is some what considerable, and the obvious result is the dropping of the aeroplane until the required momentum is obtained. Then we have another question for thought. The power required to accelerate a mass to a speed of 65 tji. p, h, is greatly in excess of that required to accelerate the same mass to a speed of 15 m.p.h., so that any surp'us power which might previously have been used (up wind) for regulating the altitude, &c, must be used entirely or for the most part in overcoming inertia, and maintaining the necessary flight speed. Chiswick. ERNEST WARDE FOX. PADDLE V. PROPELLER. [866] From the time I began to follow the progress of the science ot flight, a little more than a year ago, 1 always wondered why I never read about any attempts to use a paddle-wheel instead of a screw to propel the machines. In February of this year I saw an article on the Lesier-Best airship model, which has paddle-wheels with adjustable paddle-boxes. Then in FLIGHT of January 22nd, 1910, a letter (305) appeared, with photo, describing a paddle-wheel on which the " floats ' seemed to feather laterally. The following is how my thoughts ran on the subject. I imagine a paddle-wheel placed just behind the main planes of a biplane, tne paddle to be as broad as the main planes are long. Imagine, again, the paddle-wheel divided in the middle to permit the passage of the body of the aeroplane, thus making two wheels of equal breadth, the diameter to be any suitable size—perhaps a little more than the depth between the two main planes. Suppose also, that by means of suitable gearing, the "floats" of the paddle-wheel are made to feather in such a way that, for the greatest part of the revolution possible, they shall press the air downwards and bickwards, so assisting actively ; and during the remainder of the revolution they shall place themselves at a certain angle of incidence that will help the " lift," so assisting passively. To me, the advantages of this paddle-wheel method over the screw, as at present used, appear to be :— First, the action in the air is more direct; it gives the beating motion of nature with the rotating motion which is so efficient in mechanics. Second, with planes having adjustable angle of incidence, and placed so that they gave great resistance to forward movement, the paddle would give a more direct or quick lift when rising from the ground, and so make rising possible where a run could hardly be had. The pilot would level the planes as required while rising. Third, the paddle-wheel would assist lateral stability by tending to eat up side gusts, as a bird helps to steady itself, when caught by contrary winds, by the beat of its wings. Fourth, the wheels might be run independent of each other, and if necessary at different speeds, when turning corners Fifth, the aeroplane would pick up momentum quicker when turning to fly with the wind, because the paddle would exert a more direct thrust on the wind then acting as a " wake." Sixth, the screw has to be supported by the aeroplane ; the paddle- wheel would support itself when in motion. I have not the opportunity of experimenting to find out if this is practical. It looks as if there was something in it but, if so, why is it not in use somewhere ? This idea must surely have been in other minds beside my own. I am told thousands of pounds have been spent experimenting along this line. The objection taken was that no matter how the " floats " were inclined, in the upward and forward parts of the revolution, the reaction of the air on them would press the machine downward and backward respectively, so counteracting, to a great extent, the work done in lifting and pro pelling by the " floats " when at the other parts of the revolution. If'his is so. why does the same not take place with birds when raising their wings between each downward beat ? Some of your readers who have tried something in this line might give their experience. S'nce writing above I have come acoss a copy of FLIGHT for April 23rd, 1910, which had been mislaid when I was hunting through the others to see if there was anything in them about aerial paHdle-.. In this copy I see another letter (478) from E. Simkin, in which he says he has altered the " blades " of his paddle-propeller so as to " give a little lift as well as propulsion." This means that we are both on the same idea, but the methods of putting ihe idea into practice will, no doubt, be quite different. He might let us know what further success he has, and if the reaction of the. air on the passive blades —or floats—is much in evidence. Dundee. ALEX. W. DOWNIE. THE ACTiON OF PROPELLERS. [867] A friend and I have been arguing about the action ol propellers, and now respectfully invite your assistance. I contend that the action of a propeller is as follows :— A 1 ropeller throws a jet of air backwards, which, reacting upon the still air beyo id, pushes the propellet and the machine forwards. My friend contends that the action is the same as a nut on a screw thread; that is to say, that the propeller literally screws itself through the air as if the air were solid. He therefore contends that a perfect propeller would have no back draught, and considers the present back draught an indication of inefficiency. Tuubridge vV'ells. F. HALSEY. [Our correspondents do not appear to have read an article that we published in FLIGHT on |anuary8th, 15th and 22nd, wherein we endeavoured to explain the action of a propeller, and dealt in particular with the two aspects of the problem brought forward by our correspondent and his friend. We showed the limitations of the screw and nut idea, and also dealt with the simile of the jet. The jet theory is correct, but not very well expressed in the above letter, that is to say, the driving force is not due to the impact of the jet against the inert sut rounding atmosphere, as may be proved from the fact that the propulsive effect of a jet of water is the same whether the nozzle is submerg -d below the surface or has free access to the atmosphere. The propulsive effect of a propeller, or of any device designed to obtain a thrust from fluid reaction, results from the acceleration of the molecules of the fluid. All fluids have mass, that is to say, they weigh something, but however light they may be, setting them in motion calls for the exercise of a force and gives rise, by Newton's laws, to a reaction of equal magnitude in an opposite direction. This is the thrust that drives the aeroplane ; the j«t of air thrown behind fr >m a propeller is the " slip." It is necessary to have some slip in order to obtain a thrust at all, but the slip does represent wasted energy. On the other hand, it is 918
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
