FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1910
1910 - 0041.PDF
JANUARY 15, i910- propeller, and that is the effect of the revolutions per minute. With a given pitch, increasing the revolutions •will increase the speed of the rearward column of air in •direct proportion, and will in consequence have a similar -effect upon the resultant thrust. It must be remembered, "Flight" Copyright Photo. In the Beedle propeller the aluminium blades are made self' supporting by being bent at right angles, so that they over' lap in an extension of the boss. however, that this thrust has to be transmitted to the aeroplane through the blades of the propeller, so that once again the question of strength has to be taken into consideration. In marine propulsion there is another difficulty, for it is there found that if the pressure per •sq. in. on the blades exceeds a certain value, the water gets thrown off from the surface, so that the propeller •commences to revolve in a cavity. This phenomenon— known as " cavitation "—does not have to be taken into consideration in the design of the propellers for aerial work. High and Low Speed. Besides the question of strength, high speed involves other difficulties which are well known to all associated •with mechanical apparatus. The moving parts have to be particularly well balanced, and on an aeroplane the gyroscopic force has also to be taken into account lest it interfere with the control or possibly fracture the propeller-shaft, due to a sudden swerving of the axis. . Practical experience, so far as it has gone at the present •day, however, seems to indicate that propellers can be satisfactorily operated at the same speed as the engine, and for the time being that is as fast as anyone is likely to wish to use them. Efficiency. So far, we have considered only one aspect of pro pulsion, that of obtaining the greatest possible thrust from the screw. It has been convenient to take this side of the question by itself, because by fixing the idea on ® ® Ae.C.F, Doings. THE Aero Club of France have decided to send three challenges to the Aero Club of America for the tiordon-Bennett Flight Cup, and they have also entered •a full team for the Gordon-Bennett Balloon Cup Race which will also be held in America this year. It has been decided to present gold medals to Mr. Henry Farman for his duration and distance records and to Mr. Hubert Latham for the very sporting flights •he has carried out on' his monoplane,, while silver-gilt one point at a time it shows up more clearly the natural relations which exist between what may be termed the fundamental factors in propeller design. There is the all-important question of efficiency to be taken into account, however, before final conclusions can be drawn. It has been shown that the thrust of a propeller depends on setting a column of air in rearward motion, but what of the energy which that impressed velocity represents ? In a boat it is quite obviously lost, and for the moment such may also be assumed to be the case in a flying machine; how, therefore, can it best be reduced to a minimum ? From the simple fundamental equation Energy = 0-5 »nr, it is obvious that whereas the waste is only directly proportional to the mass (m) of air in motion, it is proportional to the square of the speed (v2), so that it is this latter factor which should be kept as low as possible. The Best Propeller. In Rankine's words: "That propeller is best, other things being equal, which drives astern the largest body of water at the lowest velocity." " Flight " Copyright Photo. The Helice integral is a wooden propeller of French con struction ; its blades are built up from separate layers. The theoretical efficiency, it is interesting to note, is given by the very simple equation Efficiency i V -=- (V - o'5 v) where V is the speed of the aeroplane, and v is the real rearward speed or slip of the column of air from the propeller. Some of the considerations which limit the diameter of a propeller have already been discussed, as also have some of those which affect the questions of revolution, speed, and pitch. This latter factor, however, has also to be considered on its own efficiency basis, and here again a limitation of the blade angle on the lines already pointed out is indicated. It would appear to be advisable there fore to adopt an efficient angle in any case, which means that if the diameter is also made as large as possible, the revolution speed will be the sole remaining factor by means of which the other two items can be adjusted to give the thrust required. To be concluded.) ® ® medals are to be given to Mr. Maurice Farman and M. Jacques de Lesseps for their performances across country. Pilote-aviateurs' certificates have recently been granted to the Hon. C. S. Rolls, Mr. Mortimer Singer, MM. Jacques de Lesseps, Molon, Bregi, Mament, Metrot, Aubrun, Balsan, and Prince Bibesco; while pilote-aeronauts' certificates have been given to MM. Pierre Clerget and Louis Godard. N. Clerget also has qualified for a balloonist's certificate.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
