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Aviation History
1926
1926 - 0206.PDF
SUPPLEMENT TO FLIGHT 32 THE AIRCRAFT ENGINEER MARCH 25, 1926 Silver soldering has much the same disadvantages as brazing, but it is carried out at a somewhat lower temperature, and it makes a very strong job. It is scarcely ever used, partly on account of the cost of the silver solder, but chiefly because ordinary brazing is generally good enough, and the temperature is still not low enough to get away from ill-effects on high tensile steels. Soldering with low temperature tinman's solder need not affect high tensile tubing, but a good deal of care and super- vision is necessary, because it is easily possible to raise the temperature higher than needed. So long as joints made with soft solder are in pure shear, and there is little direct tension or tearing action, soldered joints will stand up to a shear of 2 tons per square inch. It is generally considered advisable to reinforce the joint with taper pins or rivets up to at least two-thirds of the strength of the joint. Sockets built xip of sheet metal and dip brazed can be soft soldered on to tubes without difficulty, and the writer has experience of a number of frames built up in this way that have been quite successful. The chief objection to the method is that it is expensive. A great deal of care has to be taken in making each joint, particularly to ensure that it is properly cleaned of flux. If this is not done corrosion is almost certain to start sooner or later. 3. Mechanical Joints. It was on account of these manufacturing difficulties that the attempt was made to make up a frame which was fastened together entirely without welding, brazing or soldering. The problem is fairly simple if channel or angle sections are used for the frame, but for various reasons it was decided to keep to round steel tube. The difficulty in making a frame that is bolted together is in keeping the joints tight and preventing them from working and fretting the holes. Very accurately fitting bolts would put up the cost of manufacture, while rivets are not convenient for use with tubes. The system which was eventually adopted is illustrated in Fig. 1. The tube itself is reinforced bv a sleeve, generally of the same size tube, split lengthways and sprung into place, and the actual fastening is made by using collar nuts fastened together with a screwed tie-rod across the diameter of the tube. Direct tension tests were made of samples joined in this manner, when if was found that the apparent bearing pressure at which the joint failed was about 150 tons to the square inch on the surface of the collar nuts if friction were neglected. As this is a much higher bearing pressure than the metal will take it was obvious that friction could not be neglected, and that it was in fact largely by means of friction that the joint held together. This encouraged us to use the joint on a large scale, and it has been found to be completely successful in a number of steel fuselages in aeroplanes that have been supplied to the Air Force and also in aeroplanes used by the Armstrong Whitworth Reserve Training School. The joints are quite rigid and show no signs of movement after hundreds of hours of flying and thousands of landings. The advantages of using a purely mechanical joint are many. In the first place, frames can be built up on jigs and the saving of time in erecting and trueing a frame in this way is con- siderable. The frame is very much more easily repaired, because a member can be easily replaced, and finally there is the big advantage that the tube can be used in its best heat- treated state and there is no fear of spoiling it or rendering it liable to corrosion. Hitherto the writer has only experienced this method as applied to steel tube, but there seems no reason why it should not be used with equal success to fasten light alloy tubes. Besides the wings and fuselage there are a number of other places in an aeroplane where riveted-up units can be used. It has been common practice to make the levers that work the ailerons, elevator and rudder of a stream-line shape by welding two steel pressings together. An alternative way of making up the same pieces is illustrated in Fig. 2. It will be noted that no welding is used and that the whole construction is riveted together and can be fixed to a tube by means of a ferrule, nuts and tie-rods. A similar lever can be made in aluminium alloy. There is still another way by which components can be joined together. This has been used extensively in the metal wings made by Sir W. G. Armstrong, Whitworth Aircraft Ltd., and is best understood by referring to the illustrations, Figs. 3 and 4, where the attachment of a rib to the leading edge of the plane is shown. The rib itself is made up complete with an aluminium fitting in each end (Fig. 3). The leading and trailing edge sections are pushed over this, as shown in fig. 4. In order to illustrate the method the leading edge on the right- hand side of the picture has not yet been sprung into place. The leading edge is locked into place by means of two small clips, one on each side of the rib. These clips embrace the lips of the section and are slid up into place close alongside the rim. It has been found in practice that no further locking is necessary. There are many other types of joints which might be described, but the few illustrations given are intended only to show that mechanical jointing is not necessarily a difficult or costly process. The examples chosen have all been tested out on Service aeroplanes with results that have been per- fectly satisfactory, while the economy in manufacturing has been at least up to expectations. NOTES ON THE GROUND OPERATION OF FLYING- BOATS. By R. J. MITCHELL, Assoc.M.Inst.C.E., A.F.R.Ae.S. Sometime in the future the design of flying-boats will have reached a state of development at which they can be moored out for indefinite periods. Even at the present time, modern flying-boats are often moored out for quite long periods, especially when carrying out extended cruises away from their base. It is the usual practice, however, for them to be brought ashore and housed in hangars when not in operation, and no matter what state of development flying- boats reach, it will always be desirable to bring them ashore periodically for inspection, maintenance and repair, just as an ocean liner is dry-docked for its periodic overhaul. It may be argued that large flying-boats may be dry-docked in a similar manner, but it is considered that for the sizes at present being built, and contemplated building, it will be found much more efficient and economical to bring them ashore. The methods employed in launching, bringing ashore, and handling flying-boats on the ground are of very great import- ance, and have a very considerable bearing on the length of their useful lives. The Beach Cradle has generally been used and is still used to a large extent for this purpose. It consists of a cradle, mounted on swivelling wheels, which is made to fit the bottom of the hull amidships for a considerable portion of its length. The beach cradle has been found to have many disadvantages, the chief of which is its liability to damage the hull. It is often stated on very good authority that more damage occurs to flying-boats during the use of beach cradles than during all the remainder of their normal operations. Some early efforts were made to eliminate the beach cradle by fitting an axle through the hull fitted with external detachable wheels. This was not very successful, however, as the intense localisation of the loads at the axle was found to be very little better for the hulls than the beach cradle, and was liable to cause leakage unless the hulls were streng- thened very considerably. It was not until the development of the boat amphibian that the great advantages of the amphibian type of chassis for launching and beaching were realised, and that by design- ing a chassis which could be very easily fitted or removed, the advantages could be utilised for flying-boats. A chassis of this type has been designed and has been ex- tensively used for the operation of the " Southampton" type flying-boats. The accompanying photographs serve to illustrate its general features and method of operation. The chassis is made in two separate parts, one to fit on each side of the hull. Each part consists of a light tubular frame built on to a bent axle carrying an aero wheel. An adjust- able strut is pinned to the axle near to the wheel in such a way that it is free to swivel in the plane of the axle through 178/*
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