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Aviation History
1929
1929 - 0410.PDF
12 SOTPLEMENT TO FLIGHT FEBRUARY 28, 1929 THE AIRCRAFT ENGINEER is sufficiently supported in order to •withstand the severe toraional effects should the outer floats become operative to an unusual extent. The normal system of biplane bracing would be insufficient to provide the necessary degree of torsional rigidity in combination with a lower plane of such small dimensions. The upper plane lias, therefore, been stabilised by the inclusion of bracing wires over the top surface, and the usual anti-lift wires between the two planes have been excluded as redundant. In order to relieve the lower plane and interplane struts of hydrostatic drag loads, bracing wires are attached to abutments on the main float, and terminate at the lower wing extremity. These wires are fitted fore and aft and in the same horizontal plane, and not easily seen in the front elevation. The tail unit calls for no more than general comment. The tail plane and elevators are carried by the fin which in turn forms an integral part of the boat structure. The aft part of the boat is sufficiently substantial to accommodate stresses induced bv directional control and stability. The fuel tanks are suitably disposed immediately under the C.C4. and on either side of the main strut root extensions. The assumed characteristics of this machine have been included in the diagram shown by Fig. 1, in order that an approximate comparison may be made of the respective merits of both types of seaplane. LECTURES AND PAPERS MONOPLANE OR BIPLANE?* By W. S. FARREN, M.B.E., M.A., F.R.Ae.S., M.I.Ae.E. This paper is an attempt to deal with the problem '' Mono- plane or Biplane ? " with reference to the large commercial aeroplane of the future. I hope to be able to show that, if certain premises which appear to me to be reasonable are conceded, it is probable that for commercial aeroplanes of between 15,000 and 40,000 lbs. in total weight, the biplane arrangement which has been developed mainly in this country has on the whole the advantage. By a " commercial" aeroplane I mean one which may reasonably be expected to pay its way—to "fly by itself" financia.ly. From a general survey of such real civil aviation as exists, I can see no early prospect of making such an aeroplane with a cruising speed exceeding 100 m.p.h., and I propose to consider the problem of " Monoplane or Biplane " ? on this basis at first. In one respect the solution of a purely commercial problem is in principle simple. We seek for the combination of characteristics which will produce the best return on the capital involved. Here we have a common factor in terms of which every proposal can be expressed—money. The items of expenditure with which our problem is concerned may be divided into two classes. First, general commercial expenses :— (1) Interest on first cost. (2) Depreciation. (3) Maintenance and repair. (4) Insurance. (5) Housing (as affected by size). As to the first three I propose to say nothing, leaving it to those who are in a better position to judge to give their views. Insurance rates on certain British commercial aero- planes are, I believe, lower than on any other aircraft— presumably because they are judged to be safer. I hope to show that they are safer to some extent because they are biplanes. As for housing, it will appear that a biplane is appreciably more compact than the equivalent monoplane. The above items are difficult to assess mainly because a detailed analysis of the expenditure of Imperial Airways is not available. Sscondly, we have items of a more strictly technical nature:— (6) Fuel consumption. (7) Non-paying load. • Summary ot Paper read before the Royal Aeronautical Society onJanuary 31, 1929. The problem under discussion is essentially one of engineer- ing (i.e., commercial and technical) compromise. No points of great scientific interest are in question and the most useful treatment is probably to consider in outline alternative machines (monoplane and biplane) to comply with some suit- able specification. Let us take as defined by reference to some agreed standards such as the I.C.A.N. (1) the structural strength, (2) the characteristics of stability and control, (3) the take-off, (4) the stalling speed, and let us further agree to fix : (5) the total weight of the machine, (6) the engines, and their maximum and cruising rating, (7) the range in air miles at cruising rating. In virtue of having fixed (5) and (6), we may take the rate of climb at ground level as a measure of the take-off. Hence the total resistance under take-off conditions must not exceed a certain amount. Supposing that the resistance of the biplane under these conditions is such that the required rate of climb is attained, the lower parasite resistance of the mono- plane makes it permissible to have a higher wing resistance (both profile and induced). It will appear that the structural problems associated with the monoplane are such that it is desirable to reduce its span, and therefore to increase its induced resistance, as much as the requirements permit. In what follows the adjustment of the proportions of the wings has been made on this basis. It results in the span of monoplane and biplane being practically the same. Under cruising conditions induced resistance is relatively less important, so that in virtue of (6) the monoplane's cruising speed is higher by some 4 or 5 per cent, than the biplane's, and the fuel consumed for a given range is correspondingly less. The alternative—to reduce the cruising speed of the monoplane to the same level as that of the biplane by throttling the engine further—involves the difficulty of assessing the benefit to the engine's life and maintenance charges. This is avoided by the course adopted. The mono- plane saves a definite amount of time and fuel, compared with the biplane, but in the other respects enumerated above (1) to (7) is essentially identical with it. To arrive at equivalent dimensions for other essential parts, it is necessary to consider the problems involved in stability and control. Although there is some scope for difference of opinion on what particular combination of tail length, tail area, and rudder and fin area represents the best compromise the appropriate general characteristics are not in serious doubt, and the nett effect on the structure weight can be estimated closely enough for my purpose. The final, and most difficult step in the technical com- parison is to assess the structure weight of the monoplane and hence its paying load. When this is done there remain the commercial considerations enumerated in the third paragraph of this paper (1) to (5), but for reasons there stated I do not propose to deal with them. It may well be that they will prove to have more weight in the final decision between the alternatives than the technical points here con- sidered. But to attempt to discuss them in the absence of a thorough exploration of the technical problems cannot lead to a sound conclusion. As I happen to be familiar with its details, and as it is a type which apparently meets the requirements of Imperial Airways for certain services, I propose to take as an example the Argosy and to lay out the general proportions of equi- valent monoplanes. According to my estimate, based on the usual data, the resistance of the wing bracing of the Argosy may be expressed approximately in any one of the following ways :— 12 per cent, of the total resistance at cruising speed (90 m.p.h.); 21 per cent, of the parasite resistance of the whole machine; 57 per cent, of the profile resistance of the wings; 1 -8 per cent, of the weight at 100 m.p.h. \6?d
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