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Aviation History
1927
1927 - 0052.PDF
SUPPLEMENT TO FLIGHT THE AIRCRAFT ENGINEER JANUARY 27, 1927 of the price paid for installation of engine and equipment, provision of access from one part of the machine to another, freedom for gun fire, view, &c. This, of course, is necessary, as an aeroplane which cannot perform its military functions, is of little use. But there is an important point to remember. Once an aeroplane exists, the loss of performance or high cost of manufacture occasioned by military installation require- ments cannot be recovered, change of policy means complete redesign to return once more to best economic considerations and to get best performance consistent with military require- ments. So also do considerations of policy affect optimum size. The object of the bombing aeroplane is to drop bombs. In order to function, it requires certain signalling and navigation apparatus, certain defensive armament and certain crew, plus its load of b^mbs The former represent a nx?d mini- mum and may represent a greater weight than the bombs load. In such case a small increase in size may mean a large increase in effectiveness (i.e., bombs load). As a set-off there is an increase of true structure weight with increase of size. For example, suppose the fixed load of a certain aeroplane to be 25 per cent, of its gross weight. By fixed load, 1 mean military load and the weight of that part of the aeroplane which is unaffected by change of size. Suppose the true structure to be 30 per cent., and the power plant (including fuel, &c.) 55 per cent. The aeroplane, let us say, weighs 10,000 lb. The fixed load is 2.500, of which the military load might be 2.000 and the bombs load 500 lb. of that. Increase the size of the aeroplane to 12,000. Suppose true structure to increase to 31 per cent,, power load remains unaltered. The remainder available for "fixed'" load is now 2,880 and the bombs load will be increased to 880 lb., an increase very large compared with that of the aeroplane. The actual figures used are not real, but illustrate the principle. If. as may be the case, bomb load is small com- pared with fixed load, change in policy which would modify this ratio would have considerable effect on the optimum size of aeroplane. It is true that owing to engine powers available being discontinuous that the " best " size or best performance cannot always be chosen, but this has no effect on the argument since the border line cases are passed from one step to another with more serious results if of less probable occurrence. We may usefully ask ourselves in what direction weight saving may be expected in the structure. In previous articles 1 have already suggested maximum simplification of military requirements, careful investigations into forces arising in flight. It seems unlikely that we can afford to consider acceleration factor of less than 2-5 for aeroplanes meant for straight flying. With this factor we shall require a load factor of 1 -5 at least or a stress factor of 2 if alternating loads are to be dealt with or materials of low proof/maximum stress are used. There is however, the possibility of new materials or new material technique to be considered. Strip steel for the manufacture of aeroplane spars, etc., has already reached a point of development where improvement in mechanical properties is unlikely to be reflected in much weight saving unless the specific value of Young's modulus could be raised— an extreme improbability. A substantial raising of the specific proof stress for light alloys might have a very marked effect on structural economy, but the large amount of work on aluminium alloys in the last decade has not held out much promise of this. Magnesium is an interesting material and a solution of the corrosion problem would encourage a vigorous attack on the development of magnesium alloys. These developments are perhaps not for the moment. Just now technique is perhaps of more importance. Solid drawn tubing, and streamline wires both lag behind steel strip members and offer profitable opportunities for improvement as structural members, and this is a case not of materials and their manipulation. It will be as well to explain here the importance of tech- nique to the engineer. In his note to my article on the Paris Show the e.litor raises the query, a propos of a Duralumin stamping illustrated and commented upon, "Is it really necessary ? " I suggest that this point is negligible to the engineer. Dr. Aitchison in hie interesting articles on Duralu- min has pointed out that Duralumin, like other light alloys, has to be forged in a state of less plasticity than steel owing to limitation of safe working temperature. If I look at the example I am surprised that it can be made, more surprised that it can be made cheaply. The acceptance of these two facts modifies my knowledge of the technique of Duralumin drop stamping, that is to say of what I can reasonably ask the drop stamper to make as a commercial article. The actual use to which M. Breguet puts this part is to me of quite secon- dary importance, except in so far as I can judge from that use the accuracy of stamping necessary and a reasonable com- mercial value for the part, knowing the complete aeroplane to have certain value. I think a picture of one section of the technique of the engineer may be given in this way. Imagine a rectangular block of pigeon holes thus : MATERIALS 46/ The vertical rows are labelled with different materials, e.g.. various kinds of steel, light alloys, yellow metals and so on. The horizontal rows are labelled with methods of manipu- lation, e.g., plastic working (such as rolling and drawing, forging, etc.). machining, joining processes and so on. In each of these mental pigeon holes is a specification, partly definite, partly nebulous of the properties and limitations, mechanical and economic, associated with the conjunction of a material and a method of manipulation. This specification is con- tinuously undergoing modification sub-consciously as the result of experience or information and represents the stock- in-trade drawn upon for detail design, etc. Anything which may modify the3e specifications has consequently an im- portance which quite transcends its intrinsic utility. For the purpose of considering the influence of structural and material technique on performance we require a set of pigeon holes as fully filled up as possible and steady improve- ment is always going on, not only in completing the set exemplified, but the vast number comprising engineering technique as a whole, and each section only exists at anv time in its most complete state in many minds. The development of technique is a slow laborious task which makes appeal only to the professional mind but it is of vast importance in its slow attack on obstructions to development and in consolida- tion of advances made. In aeronautical engineering as in bridge work there may be noticed a difference in applied technique as scale varies. This is due to the entry of the financial economic factor (or in times of war the man power factor which comes to the same thing). In certain classes of engineering the cost rate is nearly independent of scale (up to the point where handling problems become serious) and in this class raw material is the important factor, cost may be considered as so much per ton. Such an application to an aeroplane, particularly a metal aeroplane, is inadmissible. Geometrically similar wings on a different scale show a decreasing cost per pound with increase of scale. This has the effect of raising the true economic size, since more elaboration can be afforded on the larger aeroplane. Here again modification of technique will affect economic size. Numerous as all these factors are, they are not likely to have more than a secondary influence on design and performance. It is to the engine and propeller that we must turn for the most marked results. (To be continued.)
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