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Aviation History
1917
1917 - 1022.PDF
OCTOBER 4, 1917. WING BRACING AND HEAD RESISTANCE. By MARCO POLO. (Continued from page 996.) AERODYNAMICALLY the monoplane is more efficient than the biplane or multiplane, due to the fact that there is always, where two or more planes are placed one above the other, a certain amount of interference which renders the superimposed planes, especially the lower one, individually less efficient than if it had been removed from the disturbing influence of the other. It, therefore, follows that the greater the percentage of the total area formed by the upper plane, the better the efficiency of the biplane combination, other things being equal. There are, however, various considerations to be taken into account, which are found to limit, to a certain extent, the reduction advisable of the lower wing. Thus, for instance, if the resistance of the wing bracing of such an arrange- ment compares with that of the more usual designs.. As before, the total area is 265 sq. ft., of which the upper wing accounts for 177 sq. ft., and the bottom plane for 88 sq. ft. These are approximately the proportions of the planes in the Nieuport, although the areas are slightly different. In Fig. 4 is shown diagrammatically the wing bracing of the Nieuport. Apart from the difference in area of the two planes,, the chief feature of the Nieuport is the single spar in the lower wing and the Vee formation of the inter- plane struts, the one being, of course, a consequence of the other. The lift wires, of which there are two- on each side, run from points on the body to the the top wing be retained in its usual biplane position upper ends of the two struts forming the Vee, while above the fuselage, while area of the lower wing is reduced to zero, a monoplane of what has come to be known as the "parasol" type results. In such a machine, although the view in a downward direction is particularly unrestricted, the centre of gravity is very considerably below the centre of lift, while the centre of thrust will probably be a good distance below the centre of resistance. This is undesirable for var- ious reasons, and hence the parasol type of monoplane, in spite of its excellent qualities as regards a good the landing wires run from the top of the centre plane struts to one point at the apex of the inter-plane strut Vee. The total length of wires on a machine of this area and with the Nieuport type bracing is, it will be seen from the table on the right in Fig. 4, 92 ft., while the total length of struts is 40 ft. 8ins. Assum- ing, as in previous cases, that the struts offer twice as great resistance per foot run as the wires, we arrive at a resistance factor of 173.3, compared with a factor of 184 for the standard scout of the same area SPAN CMOM A*0A ££? JUST 7XTTAL A*M*264St TOTAL LENOTH OF 31 Ml/73 •&0*—&** WrM£3 : QQ'—0 " Fig. 4.—Diagram of Nieuport type wing bracing. view downwards, did not remain long in vogue as a military machine. It has thus been demonstrated that although aero- dynamically the monoplane is the more efficient, practical requirements tend to nullify this advantage, and "we return once more to the biplane. It still remains a truth that the greater is the percentage of the total area formed by the upper wing, the better the efficiency—within limits. This proviso has pre- cisely to do with the reasons that ultimately condemned the parasol monoplane, and to the French firm of Nieuport belongs the credit of being the first to introduce, what might be termed a successful compromise between the theoretical efficiency of the monoplane and the practical advantages of the biplane. It should here be pointed out that up till now reference to efficiency has only .been in regard to aerofoil efficiency, not taking into account ihe question of wing bracing. In the Nieuport the monoplane aerofoil efficiency has been closely approximated while still retaining the biplane construction, and the success which this machine has had, and is still having daily, is sufficient proof of its merits. Sufficient has been indicated in the foregoing to show that, apart from questions of wing bracing, the large top plane and small bottom plane, or one-and-a-half- plane arrangement, is to be considered good both aerodynamically and practically, the small lower plane obstructing the downward view to a very small extent only. There now remains to examine how shown in Fig. 1 (page 995). It will thus be seen that this arrangement appears to be slightly better than that of a standard scout, although the overhang of the top plane is about 1 foot longer than that of the former. Probably the advantage of the Nieuport bracing would be greater in a machine of somewhat smaller area than that chosen for this article for purposes of comparison. In actual fact the standard Nieuport is of considerably smaller area. One of the first questions that occur to one when in "pursuit of reduced bracing resistance is naturally whether any wire might not be omitted. In the case of the Nieuport, as already pointed out, there are two lift wires on each side, and it might be thought possible to omit one of these. Let us examine what would be the result. When flying at maximum speed,, that is to say at a very small angle of incidence, the centre of pressure moves back towards the rear spar^ The top plane being of comparatively large chord, this travel of the centre of pressure is probably con- siderable, since it is to be presumed that for the sake of efficiency a stable wing section is not employed. This means that there is a tendency for the plane to assume a still smaller angle of incidence. This tendency can be, and probably is in the Nieuport,. counteracted by choosing a suitable angle for the two lift wires. If only a single lift wire were employed, attached to the point between the upper end of the Vee struts at which the centre of pressure is normally located, and running transversely to the flight path 1022
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