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Aviation History
1926
1926 - 0361.PDF
MAY 27, 1926 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT Careful and systematic work will allow the necessary practical modification to be introduced with very moderate increase in drag. Two points in respect of the above present themselves. Firstly, resistance per square foot is not alone the criterion of a good body if the size of the body has been increased above the minimum rendered necessary by the requirements of general design. The size of the body should therefore be kept as small as possible. Secondly, the construction of the body should allow the fullest possible advantage to be taken of the external dimensions in order to keep down cross- sectional area. In many cases where a large amount of internal space is required to allow free movement to the occupants, monocoque construction, or an approach to it. is some help in keeping down overall dimensions. Where the body is of good shape, of small dimensions in relation to the gap between the wings and appropriately placed in relation to them, body-wind interference on drag and lift may be reduced to nothing, or even be negative. Efforts have frequently been made to reduce body-wing interference by making the body fill the whole gap between the wings. The evidence, for what it is worth, of the effect of this arrangement is very conflicting. I have been assured on good authority that raising the top wing of a certain aeroplane so as to give a small gap of about 2 ft. between the body and the top wing increased the speed of the aeroplane by 10 miles an hour, with a proportionately good effect on climb. In another case, a similar alteration. I am informed, has had no effect, and I believe there are claims that closing the gap has improved the performance of certain aeroplanes. One must not lose sight of the probable influence of slip- stream effects in explaining these results, but all indicate the pitfalls which await the designer who is forced to use large bodies. In cases where the engine masks the body there seems no possibility of getting really low drag figures. The best radial engine arrangements seem to give a drag of about 4 lb. per square foot* of area measured as —— where d is engine diameter. Full-scale performance as well as model tests seem to confirm this; it is, however, easily possible to get higher figures, particularly by the addition of exhaust rings of unsuitable aerodynamic design or by excessive air flow over the crank- case. This disturbed air flow undoubtedly has a bad effect on body-wing interference, particularly if the engine is for any reason near the wings. • All these figures are at 100 feet per second. Further resistance figureswill always be so quoted. (To be continued.) DURALUMIN By LESLIE AITCHISON, D.Met., B.Sc, F.I.C., M.I.A.E. (Continued from p. 45) This process of working after quenching is particularly useful in the treatment of rivets. Rivets that have been formed in the ordinary way, and then heat-treated and aged, will not always close without cracking, simply because the metal has become so hard. This difficulty can be overcome by quenching the rivets from the normalising temperature, and then closing them up within an hour or so of the quench- ing operation. In this way, the cracking is avoided, and the metal in the rivet after ageing becomes possessed of its full mechanical properties. This is an alternative process to the use of annealed rivets and results in an article which is quite as trustworthy, from the point of view of freedom from cracks and, at the same time, is decidedly stronger than rivets used in the annealed condition. In the heating of Duralumin prior to quenching, no par- ticular points require special discussion, bar one. The excep- tional point is the very considerable time that has to be allowed for Duralumin to attain to the desired temperature. Those who are accustomed to the heating of steel, either for h cat-treatment or working, will find that Duralumin requires a very much greater time than the ferrous metals. This point requires very careful and special attention, as it may other" wise become a very pregnant source of unsatisfactorily treated material. It is, of course, very usual in the heating of steel to put the metal into a furnace or on to a hearth that is at a temperature much higher than that to which it is desired the metal shall attain. In many instances, it is the intention that the metal going into the furnace shall lower the heat of the furnace sufficiently to avoid any danger. Quite apart from this, it is of course also the intention that the metal and the furnace shall be brought equally to the right heat eventually, when no harm will have been done. The adoption of a similar practice with Duralumin is rather to be deprecated. The heat-treatment of steel is carried out at a temperature very far removed from the melting point of the metal. With Duralumin, the temperature of incipient melting is dangerously near that of heat-treatment, and even though the metal as a whole is not heated to any dangeroui degree, the surface of the metal may become locally over- heated and be blistered, and thereby develop defects which cannot subsequently be rectified. For this reason, it is highly desirable, excepting under very special circumstances and in expert hands, to utilise such methods of heating as do not permit of even temporary and local overheating of the material. That these methods of heating must be precise and readily controllable, is obvious from the fact that the range of treating temperature is so closely circumscribed. It is generally recognised that the most satisfactory method of obtaining a uniform high temperature throughout a con- siderable volume of metal is to immerse it in a bath of liquid. The liquid employed would naturally depend very largely on the temperature that has to be attained, and also upon the nature of the metal that is being treated. For the light alloys, it is generally conceded that the most convenient liquid bath is that produced by a suitable fusible mixture of salts, and a very satisfactory bath for the immersion of Duralumin can be formed by mixing together equal proportions of sodium nitrate and potassium nitrate. This is not the only mixture of salts which could be used, but it is, generally speaking, as convenient as any other. The design of a suitable containing tank is not conspicuously difficult. Mild stee 1 tanks are to be recommended, and it is desirable too that the joints in the tank shall be reinforced by riveting, even if the sealing is achieved by welding. Sharp corners should be avoided in these tanks, as they are prone to crack by expan- sion and contraction when the salt baths are used inter- mittently. During the working of a salt bath there is always a tendency for the collection at the bottom of the bath of a certain quantitv of oxide of iron together with dirt. This accumu- lation of foreign matter has a tendency to insulate the steel plates from the salt. This, in turn, results in an uneven heating of the plates and further scaling on the outside, with the result that sooner or later the tank is burnt through at those places where foreign matter has accumulated. To avoid this contingency it is desirable to remove the oxide and foreign matter from the bottom of the bath, by means of a perforated ladle. This operation, of course, is carried out whilst the bath is molten, and if performed at regular intervals will avoid the burning out of the plates. Duralumin can readily be suspended in the salt baths in baskets of a suitable design. Such baskets are most con- veniently made of steel, and can be made of varying capacity according to the nature of the work that is in hand. Naturally, they will be made as light as possible to avoid the extra expense of heating an idle weight of steel. For the control of the temperature of a salt bath it is probably quite sufficient to use only one pyrometer. The pyrometer that is employed should, however, be a good one and, of course, in consequence of the temperatures employed, must necessarily be of the thermo-electric or electrical resistance type. No form of optical pyrometer can be employed. It is sometimes quite feasible to utilise a thermometer of a suitable design. One of the difficulties encountered in the use of a thermometer on large baths is that of reading it without severe discomfort. A thermo-couple connected to a wall indicator is usually much more convenient. The thermo-couple should be protected by means of a stout iron tube, and should be arranged 308 g
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