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Aviation History
1921
1921 - 0691.PDF
OCTOBER 20, 1921 ON MANUFACTURING AIRSCREWS - - * An Industry in which-Craftsmanship is Still Required PROBABLY it is true to say that in no other branch of wood- working, as far as aircraft is concerned, is there still as great a call for skilled craftsmanship as in the making of laminated wood propellers. The reason is not far to seek. The wood components of an aeroplane or seaplane may have to be manufactured to fine limits—many of them do—but they all have this advantage over the propeller, that they are not called upon to have the same aerodynamical accuracy, nor to be in perfect balance when being rapidly rotated. The con- sequence is that propeller making is a fine art, and one in which, for best results, machinery cannot quite replace the skilled craftsman. When one is making an aeroplane spar, or a hollow strut, or any of the various wooden components, use can be made of spindle machines or similar machine tools, which take out rapidly and with sufficient accuracy the superfluous wood which it would take hours to remove with a gouge. It is true that most ingenious machines have been designed—and extensively used during the War—for turning out propellers from the glued-up block. And while it cannot be denied that these machines did excellent service when it was a question of quantity production, the machines cannot compare with a skilled craftsman when it comes to really high-class work. This may appear surprising to many, as it might be thought that a machine, set to certain limits, would work with greater accuracy than could be attained by hand work. While this may be true as regards metals, which are far more homo- geneous, it does not apply to wood. This will be obvious when it is remembered that no two laminations in a propeller are • alike as regards grain and texture. Now the machine, set to follow a template, does not differentiate in its treatment between a piece of fairly soft wood and a harder piece. Its knives cut into the laminations with a fine disregard for the local conditions of the wood. The workman, on the other hand, sees that one lamination is harder than its neighbours. Consequently he adapts his choice and use of tools to the conditions of the wood, with the result that the finished propeller has a " fairer " section than would the majority of those made by machinery. Thns the fact that there is now no call for quantity produc- tion has had one advantage at any rate : it has reinstated, so to speak, the really skilled wood-worker, who during the War was to some extent supplanted by machinery. This was very forcibly brought home to us recently on a visit to the Falcon Airscrew Co., of 113, Cottenham Road, Holloway, where, by the kindness of Mr. D. M. Davies, we were permitted to follow the manufacture of an airscrew from start to finish. Stored in the yards are large quantities of mahogany and walnut of the finest quality. These yards are kept well stocked, and the timber is allowed to remain for many months before being used, so as to ensure that it is well seasoned. The seasoned timber is conveyed from the yards to the saw- mill and planing shops, in which it is sawn into planks and thicknessed ready for use. From templates carefully pre- pared the planks are then marked off and sawn to shape, corresponding to their respective laminations. The next operation is heating preparatory, to the glueing down and clamping of the laminations. In front of a huge fire the planks are warmed to the required^egree, and are then placed in their proper position in the jigs, glued together, and clamped down. Here they are allowed to remain for 24 hours until the glue has thoroughly set. They are then lifted off the jigs and taken to the shaping shop, where the edges of the laminations are removed nearly down to the joint lines. In the shaping and finishing shops these roughly-shaped blocks of laminated wood are turned over to highly-skilled craftsmen, who, making lavish use of gauges and templates, reduce the wood further until the actual sizes called for on the drawings are reached. Here one should interpolate that this is only true with reservations. Before reaching the finished size, the propeller has to be repeatedly balanced to see that one blade is not heavier than another. The efiect of even quite minute differences in weight would seriously upset the smooth running of the airscrew, and would give rise fo vibration. If, then, the craftsman discovers that one blade is heavier than the other, he has to make up his mind, some considerable time before the finished shape is reached, whether the lack of balance is due to the, at that time, more or less rough state of the propeller. If he decides that after finishing one blade will still be heavier than the other, he has to leave the lighter blade just a trifle over size. (We believe the Air Ministry allows 1 mm.) The other blade is then worked to exact size, and the propeller is put on a sensitive machine, which discloses any difference in balance. To finish up, the propeller is sand-papered, covered with fabric which is glued on. Then it is painted, and the brass tips are screwed and riveted to the tips and leading edge. This part of the work also calls for great accuracy; as does indeed every operation in the manufacture of an airscrew. On the occasion of our visit to the Falcon Airscrew Works, we had an opportunity of seeing several examples of fow- bladed airscrews. These call for even greater accuracy in workmanship, as the manner of halving together the lamina- tions in the boss is such that quite minute differences in thickness, or angle of the joints, would throw the blades out of alignment. We also saw several extraordinarily fine examples of what is, perhaps, the most difficult and exacting job which a propeller maker can be called upon to do—three- bladed propellers. The joints in the boss of such a propeller are extremely intricate, and it is a testimony to the skill of the workers employed by the Falcon Airscrew Co. that the firm is receiving quite a number of orders for " three-bladers." AT THE WORKS OF THE FALCON AIRSCREW CO., LTD. : The upper photograph shows a corner of the glhS shoo with several propellers in the clamped-down stage. On the left is seen one end of the shaping shop, wdon'the ri|hta cornerof the finishing shop. Note the stacks of finished propellers on the floor.
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