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Aviation History
1918
1918 - 0017.PDF
JANUARY 3, igtS. THE PROBABLE TREND OF AEROPLANE DESIGN. By R. F. MANN. DURING the last few months I have been blessed, or the other thing, with considerable leisure owing to i]l-health, and having spent some of the time reviewing the present stage in the development of aeroplane design with the view of determining what changes are likely to occur in the immediate future, it may be of interest to the readers of " FLIGHT " if I set down some of the conclusions arrived at. Aerofoils, Strutting, Bracing and Section. The system of strutting and bracing biplane wings lias undergone fewer large modifications than any other part of an aeroplane, but nevertheless signs are not wanting that considerable changes are likely to occur in the near future with the object of reducing resistance. Since any considerable improvement in streamline sections is very remote, this reduction of resistance can only be obtained by reducing the number of struts and wires hitherto used. Two interesting machines in which attempts had been made to achieve this were the S.E. 4 and the Sopwith triplane. In the former machine use had been made of the I strut, the flying wires being taken from the extremities of the arms, which extended from the front to the rear spar. The chord of the wings was large, and in consequence of the travel of the C.P. the stresses in the strut were high. In the Sopwith, however, these stresses were kept within reasonable limits by employing wings of small chord and obtaining the necessary area by adding a third wing. Un- fortunately, this method is only applicable to small machines, so it cannot be regarded as a complete solution of the problem. Two other forms of built- up strut have been suggested—the X and K. The latter has been employed on a few machines turned out by the Curtiss Company, but they both have disadvantages which are not shared by the I type. (1) Owing to their great side area the range of vision of the crew is greatly impaired. (2), This side area is unnecessary for stability and. liable to make the machine uncomfortable to fly in a gusty wind. Also it is quite possible that the resistance and skin friction of such a strut would be equal to that of two plain struts and the necessary incidence wires. All things being considered, the I strut would appear to offer the best solution, but how are the high stresses in it, caused by the travel of the C.P., to be overcome ? It is possible to design such a strut with sufficient F.S., but a large percentage of the gain due to lessened resistance may be lost through added weight. Failing the advent of an exceptionally light but strong alloy, the only other course is to produce a wing section which, without a greatly impaired value for Ky and L/D, has a considerably smaller travel of the C.P. Naturally, a section with a stationary C.P. would be ideal for this purpose, but I fear that the loss in efficiency would nullify any other advantage. What is required is a satisfactory compromise, and it should be attainable if as much research is expended on it as there has been in getting the present highly efficient sections. Similar large changes in the bracing of the wings are in sight ; in fact, already there is at least one machine in existence—the product of an allied country—in which the usual cables or streamline wires have been dispensed with and the wireless truss system used instead. See Fig. 1. The advantage of this system is that there being no flexible tension members, vibration, and the consequent increase in resistance, is overcome. This vibration, whilst not of much importance in small machines where the length of the bays is small, assumes rather unpleasantly large proportions in the long bays of, say, a bomber. As in the case of the I strut this construction comes out heavier, but as the saving in resistance is considerable it is more than likely that in the end there is a distinct gain—in fact, the performances of the machine mentioned show that this is indeed the case. With a satisfactory I strut on the one hand and a thoroughly tried out wireless truss system on the other, the next step is a combination of the two—i.e., a wireless truss constructed with I struts. Probably there are people who would contend that this is looking too far ahead, and that as really large machines are in existence which use the, so to speak, old-fashioned system of strutting and bracing with satisfactory results there is no need to consider anything else. The obvious retort is that there would not be the convenient and speedy car of to-day if our ancestors had been satisfied witli the pony trap and stage coach. Fuselage, or Body. As the shape and general lay-out of the body are to a certain extent governed by the type of engine employed, it will be as well to devote a few lines to a general survey of engines. When looking back over the past three years, one is immediately struck by three facts, which are : (1) the great and continued increase in h.p., (2) the growing favour of water cooling,' and (3) that although the Allies employ engines of all types—i.e., the vertical, Vee, " broad arrow Vee," rotary, &c.—the German designers continue to employ the 6-cylinder vertical to the almost entire exclusion of any other. Admittedly an 8-cyUnder vertical has been in use recently, but this has now been deleted in favour of a 6-cylinder of the same power approximately, and although using the number of cylinders which are common in Vee type engines the German designers still stuck to the vertical tvpe. What is the reason for this great— almost universal—employment by them of the vertical type engine ? Have considerations of engine or aeroplane design weighed most in their decision ? Personally, I think the latter. A better entry of the body can be ob- tained by using a vertical engine than with any other type, and the almost universal employment of " pro- peller pots" by German designers seems to show that they have realised and make the utmost use of this fact. Although using a narrow engine, their bodies are often of quite generous proportions, but performance does not appear to surfer. Is it not possible that the shape of the nose is of as much importance to resistance as the cross sectional area of a body? Again, can such an excellent entry be conveniently obtained when using an engine of the Vee type ? Isn't the answer to the first question—in the words of the
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