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Aviation History
1910
1910 - 0029.PDF
JANUARY 8, 1910. [/JJGHT AERIAL PROPELLER*. AND SOME POINTS WHICH MAKE THEM INTERESTING. THE propeller, as we have had occasion to remark before, is a delightfully simple-looking device, except in one or two of its patent forms, but the principles under lying its correct construction are not perhaps quite as generally understood as they might be. They have formed the subject of perennial discussion in engineering circles ever since they were first used on steamships, and it seems by no means unlikely that they will be similarly talked over in their new sphere of flight. Simple as it is in appearance when made, the propeller does, as a matter of fact, call for a considerable amount of skill and experience in its design, and there is a vast difference in the efficiency between one which has been properly proportioned and another that is apparently only slightly different It is probably because of the disproportionate results caused by slight discrepancies that so many inventors have assumed that there is a fundamental merit in a particular shape, and it is doubt less largely due to this point of view that so many " patent" propellers have at one time and another been designed. Whatever virtue there may happen to be in any particular form, however, it is very certain that there must be an underlying principle open to the use of all which causes its success, else it would be impossible to proportion the blades to their work in advance. The Screw and Nut Idea, Much of the confusion which exists in the lay mind as to the action of a propeller is caused by a too ready acceptance of a very popular simile which likens the action of a propeller to that of a nut in engagement with a screw thread. As the screw rotates so does it progress forward through the nut, and as the propeller rotates, so too, does it advance, for its blades are like small pieces of a screw thread which, by their circular motion, make a complete helix. It happens, however, that the medium in which the propeller works is not solid like the steel of which a screw and nut are constructed, and the consequence is that while the propeller advances it cannot help pushing behind it a rearwardly moving column of air or water as the case may be. This mobile nature of the medium is sufficiently obvious to be recognised by everyone, but, curiously enough, the popular conception of the screw and nut analogy leads many people to argue about propellers as if the " solid state " were actually capable of being attained in practice. They overlook the fact that free fluids like air and water in a state of rest can only offer an abutment to a thrust by virtue of having momentum* imparted to them by the thrust, and since momentum means " quantity of motion " it is a very simple deduction to conclude that it is impossible to propel a vessel, aerial or marine, without setting a stream of the fluid flowing in the opposite direction. How a Jet Propels. If a fire-engine were placed on a barge, and the pumps set in action, the whole boat would be propelled by the reaction of the jet, and inasmuch as the picture which this analogy presents to the mind's eye leaves no doubt * Readers who wish to investigate the limitations of the New tonian theory of the continuous communication of momentum, and also the principle of no momentum, will find the subject very clearly treated in Mr. F. W. Lanchester's "Aerodynamics." as to the actual existence of the rearward stream of fluid, it is in many ways a more suitable basis of comparison for screw propulsion than that of likening the propeller to a screw in a nut. Most similes fail at one point or another, however, and the stumbling block in the analogy of the fire engine is the fact that the nozzle velocity of the jet no longer represents its real rearward velocity in respect to the earth once the boat has been set in forward motion by its action. The Jet Under Water. The forward speed of the boat must be deducted from the nozzle velocity of the jet in order to get the effective stem velocity of the water which produces the reactionary thrust; from which point of view it is easy to understand why it makes no difference whether the nozzle be immersed beneath the surface of the river or allowed to play into the air. The boat could never have a forward speed which is quite equal to the nozzle velocity of the jet, because if it did, the column of water in the jet would be stationary to an observer on the river bank; that is to say, each and every particle of it would fall back again into the river immediately above the spot from which it was taken by the pumps, whence it is quite obvious that it can have had no momentum imparted to it and cannot in any way have served as an abutment for a thrust. If the nozzle happened to be immersed on the same level and in a direct line with the pump suction orifice, each water particle would then have been actually replaced in the spot whence it was taken, so that virtually the boat would havi merely run over it, although actually each water molecule will, during the passage of the boat, have paid a visit to the pump. The Propeller as a Jet. By utilising a screw propeller instead of a fire-engine pump, and placing the propeller in the position of the submerged jet orifice, this useless labour of lifting the water up into the pump would be saved, so that on this score alone there is a certain advantage in the propeller even when it is merely considered as a pumping device. It will send a column of water rearward just as a jet would have done, but the column will be of greater area and will move at a lower velocity, which, as will presently be shown, is an advantage. That which holds good in marine propulsion may be assumed to be based on fundamental principles which apply also to aerial propulsion, for the air, like water, is a fluid, although different in kind. It is instructive therefore to pursue the theory further in order to try and establish a few specific conclusions and in this case we may begin at once with the propeller as the accepted device for producing a thrust. To be continued.) ® ® ® ® Flying Handicapped in America. M. SAULNIER, one of M. Bleriot's assistants, who has recently returned from New York, says that it is practi cally impossible to do any flying in that country at present on account of the legal complications arising out of the Wright patents. M. Saulnier,. therefore, sold his Bleriot monoplane and returned to Paris. 25
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