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Aviation History
1909
1909 - 0021.PDF
JANUARY 9, 1909. is not surprising that experimenters should seek to have a propeller on their machine which they can play about with to their heart's content, rather than one which is the last word in refinement and finish. The prrpfller-blade which consists of an aluminium paddie riveted to the flattened end of a steel tube, the other end of which is stuck tangentially into a suitable boss, forms an admirable device for experimental purposes, inasmuch as both the effective diameter and also the pitch can be varied at will. The pitch, it may be remarked for the benefit of those who are interesting themselves in this subject for the first time, is the distance which the pr< peller theo- reticallv advances through the air in making one complete revolution. It is, in effect, a measure of the angle at which the blades are set relatively to their plane of rotation. If they are set wholly in their plane of rotation, that is to say, at right angles to the shaft which drives them, they have no pitch at all, and • consequently exert no thrust upon the machine ; con- versely, if the blades lie parallel with the shaft, they have an infinite pitch, but again exrrt no thrust, although they act very well a~ paddles to disturb the air. It is in this latter form that they make a very useful kind of dynamometer for absorbing the power of an engine for testing purposes. In practice, the blades of aerial propellers are generally set at such an angle that the pitch works out, roughly speaking, at a figure which is between 60 per cent, and 100 per cent, of the diameter. At the present time, however, there is very little of a decided character about the propellers in use, although it is interesting and in- structive to bear in mind that in those at present em- ployed, there is often an approximate equality between the diameter and the pitch. In our remaiks upon the construction and design of aeroplanes, we dealt with the relative positions of pro- pellers, and pointed out certain considerations which have to be taken into account when comparing propellers which are placed behind the machine so as to push, with the screws which are placed in front so as to pull. It is, therefore, unnecessary to deal further with the subject. Flexible Propellers. At high speeds, a propeller, like any other rigid body revolving about an axis, possesses very considerable inertia of a peculiar kind which makes it display a marked resistance to the tilting of its axis of revolution. This resistance is termed gyroscopic force, and is only in evidence when the propeller is in motion ; it increases if the speed increases, and it is more pronounced in heavy propelle s of large diameter than in those which are small and light. This gyroscopic force is so powerful that it may have a serious effect upon the control of a machine as a whole, and has even been known to snap the propeller shaft in two rather than allow the propeller to follow the piiching and tossing of an aeroplane in unsteady flight. This is naturally a serious aspect in propeller design, and the fact that many constructors of aeroplanes favour the use of a tractor-screw mounted direct on the crank-s' aft to the high-speed engine has led the makers of propellers—who in many instances, of course, are the makers of the aeroplanes also—to use flexible blades. These, by springing a little when the shaft is suddenly tilted, absorb some of the gyroscopic force, and thus prevent its effect being quite so pronounced. In some cases this flexibility is carried to an extreme, as for instance in the propeller attached to the Gnome rotary engine, which has long blades made of thin sheet steel. It is particularly interesting to notice, however, that the attribute of flexibility is by no means necessarily confined to metal propellers, for there is a wooden tractor screw on the Vendome aeroplane which can be bent to quite a considerable degree without damage. Wood v. Aluminium. Aluminium, on account of its lightness, is a favourite material for the construction of propeller blades, and in most cases they are, as has been mentioned, riveted to the flattened end of a steel tube. The rib thus formed by the flattened tube on the back of the blade is probably a centre of local air disturbance, and as such may possibly be condemned in future as a source of wasted power. Experiments at all times have ever gone to show that propellers working in a fluid—and air is a fluid—are most efficient when they have smooth, even surfaces undis- turbed by projections and lumps. Wood is finding favour as a material for propellers with many experimenters, and there are some examples of very high-class work to be seen at the Salon. Properly made, a wooden propeller has a beautifully smooth polished surface, and being solid from boss to tip it is free from any joints and projections likely to waste power. In order to avoid warping, the best wooden propellers are built up out of numerous thin layers in a manner which is commonly followed by pattern makers when constructing the wooden cores trom which iron castings are moulded. In the larger wooden propellers, the blade may be made hollow for the sake of lightness and flexibility, and it is thus made in the Vendome propeller. In some cases the wood is bound with fine fabric to give greater strength. A propeller which is of an altogether uncommon description is that employed on the Breguet machine. It has hollow blades built up from strips of aluminium arranged transversely, and overlapping one another like the sections of a lobster shell. Each strip is continuous across the face and the back of the blade, and its extremities meet together to form a sharp edge. It is this edge which trails through the air as the propeller revolves, the advancing edge being quite blunt and round, as it is formed by the bend in the strip of aluminium. Between the folds of the aluminium strips, which are built up about a central steel tube, are light steel springs, and the whole construction is such that the blade is extraordinarily flexible and of a very resilient character. It is stated by the inventor of the machine that the blades have been thus designed to afford an automatically variable pitch. In the table on page 22, details are given of the pro- pellers mounted on some of the leading machines. Aeroplane Engines. IN the next and concluding instalment of our report of the Paris Salon, we shall deal with the aeroplane engines. These form, as everyone interested in flight realises, an all-important section of the industry, for to a great extent it may be said that the records which are achieved in the future by the leading aviators will in a large measure directly depend on the excellence of their motors. Already, as many of our readers know, from various descriptions which have appeared in the columns of The Automotor Journal, some remarkable developments have taken place in the design of motors for aeroplane work. In most cases, in fact, the types which have been developed are quite different from those in use on motor cars.
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