STRUCTURAL DESIGN - AEROPLANE or AIRPLANE.
The General Problem.—The aeroplane shown in fig. 9, article AERONAUTICS, was designed about 1914; in general aerodynamic and structural design it is not fundamentally different from most modern aeroplanes. Regarded as a problem in structural design, a normal aeroplane wing, such as that illustrated, is a double canti lever, i.e., two similar cantilevers joined at their roots, carrying a load distributed approximately uniformly over its span. The inten sity of the loading begins to diminish appreciably at about one chord from the tip, but is still 3 of its value at the centre, at a dis 'To a passenger in an aeroplane i,000 or 2,000ft. up, there is generally no sensation of speed relative to the ground.
tance of chord from the tip. The wing itself is essentially a lam ina, the wing sections most commonly used (see fig. 2) having a greatest depth of from to -1- chord. The resultant loading is nearly perpendicular to the chord, but its line of action (as seen in side elevation) intersects the chord at a point (the so-called "centre of pressure") whose distance from the leading edge of the wing varies from to i chord in normal flight, but may travel much further to the rear—even beyond the trailing edge—in special conditions. As a whole, therefore, the wing structure must be capable of taking torsion.
From general principles it would appear that the lightest way of bracing such a lamina would be to stiffen it internally by longi tudinal and transverse beams (spars and ribs), and to support these by external bracing. The arrangement of two planes, one above the other, connected by struts and wires to form a braced tubular girder, suggested itself to the earliest experimenters in flight. Such a solution (see fig. 9 of AERONAUTICS) is typical of the modern biplane.
Alternatively, by tapering the wing in plan (fig. 7 b) and thus both reducing the intensity of loading outwards from the centre (though not, it appears, in proportion to the reduction of the chord), and increasing the available depth at the root, the whole of the necessary structure may be contained inside the covering. This is the principle of the internally braced wing characteristic of nearly all modern monoplanes. It has the advantage of avoid ing the resistance of the exposed bracing which is involved in the first solution.
With the aid of the Prandtl theory it is possible to arrive at comparative overall dimensions for a normal biplane and a mono plane of otherwise similar characteristics. Such a comparison is shown in fig. 12. The span of the monoplane (full lines) is about o% larger than that of the biplane (thin lines), and its chord 65% larger. Both the length of the body and the tail area required by the monoplane are greater by about 28%. Each type has other advantages and disadvantages, but the ever increasing rela tive number of monoplanes indicates that, all in all, they are superior. (See also BIPLANE and MONOPLANE.) Refinement in Details.—The details of the structural design depend largely on the materials used (see below). But the main problems are not essentially different from those of the design of ordinary engineering structures, except in one particular. The importance of saving weight in every item of an aeroplane (§ 5) makes it economically possible to carry refinement in design to a point not usually attempted in any other structure. For the same reason elaborate calculations are made during the design, and the effect of every minor strengthening is carefully considered. All aeroplane structures are highly redundant and initial stresses are imposed on the members by tightening the bracing wires, in order to increase the stiffness of the structure. The design of modern aeroplane structure takes account of these features as far as it is possible to do so, in contrast to the gen eral tendency of bridge design, for example, which tends to avoid them as sources of uncertainty (see BRIDGES). Where saving of weight is a prime consideration such a course is not possible.
Most pilots "blank out" or temporarily lose control of certain senses, including sight, at less than 8 times normal gravity. The amount which an individual can stand depends upon physique and training. However, except for fighting aeroplanes the individual can usually stand more than can the aeroplane.
The ratio between the effective and the normal value of gravity, is termed the load factor for the whole machine under the stated conditions. A normal commercial aeroplane need never experience a load factor of as much as two in flight. In practice it is de signed to bear a factor of from four to six, dependent on its size and class. The ratio between the maximum load factor considered in the design, for any stated form of distribution of air pressure, and the factor in any actual condition of flight appropriate to that distribution, is the nearest approach to the factor of safety com monly used in engineering.
For a commercial aeroplane the factor of safety, as thus de fined, seldom falls below three. In normal level flight it is of the order of five. For a war aeroplane it may fall to II-, so that the most highly stressed member is loaded to two-thirds of its break ing load, or even more, but only in extremely rapid manoeuvres.
A commercial aeroplane has a true factor of safety, a margin of strength not called upon under the worst conditions which it nor mally experiences. The load factors corresponding to irregularities in the air are very small. So long as it is in the air, an aeroplane is exposed to much smaller risk of the failure of its structure by stress of weather than is a ship at sea. Any commercial aero plane which conforms to the official criterion of strength, and has been licensed, without which it may not carry passengers, is capable of performing many manoeuvres such as looping and spinning with complete safety.