relation to its width at each point, the mass dealt with per unit length will be everywhere as the width. But for a given height of the arched section the values of and will be inversely as the width, and consequently the load sustained for any particular element of the length will be constant in respect of width; that is to say, the sustaining power of the aerofoil for a given midsection and a given grading is constant, no matter what the plan-form, both as to total and as to each element of length.
Fig. 121.
This is a very convenient rule to remember, but one which, from the nature of the assumptions made, is more or less approximate; it can be applied legitimately to all ordinary modifications of plan-form.
Fig. 122.
When an aerofoil is designed according to the foregoing specification, whether as a solid as in the case in point, or as a lamina of the same mean section, the equivalent area for uniform values of and will evidently be that of a plane whose plan ordinates are those of a segment, that is, proportional to the thickness ordinates. Such a form may be taken as having two-thirds the area of the circumscribed rectangle; that is, if be the length of the aerofoil the equivalent area will be:—
By adopting and adhering to some standard such as that above defined, the experimental information obtainable becomes of greater value than when a variety of forms are employed. It
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