Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/302

§ 191 uniform throughout the length of the aerofoil, but of changing scale, i.e., that it should tail off to a point at each extremity. Such a form is not generally found in Nature; the section nearly always becomes flatter and the angles of dip and trail become less as the extremities are approached. It is not known whether this fact is due to the reasons suggested in the preceding section, or whether it is attributable to aerodromic considerations alone, or whether there is some further subtle reason that has hitherto escaped detection. It is certain that the feature in question is valuable from the aerodonetic standpoint, and that is all that can be said with certainty at present.

§ 192. A Standard of Form.—In 1894 the author, with a view to embarking on some experiments in flight, took measurements of the plan-form of several of the soaring birds, including an albatros, a herring gull, and a kittiwake gull, with the result that an elliptical form was adopted as being a simple geometrical form whose ordinates approximate very closely to the average of those adopted by Nature. No attempt was made to imitate the "sinuosity" of the bird wing plan-form, this being regarded as an anatomical accident.

The form of section adopted has been given in Fig. 58; the aerofoil being made of timber, it was necessary to adopt a form easy to produce; on this account the hollow in the underside was very soon abandoned owing to the results not justifying the additional labour.

The "grading" is segmental or parabolic, that is to say, the maximum thickness of the section at different points along the length of the aerofoil is given by the ordinates of the segment of a circle whose cord is constituted by the flat face. This method of grading, independently of the plan-form to which it is applied, ensures the proper tailing off of the load towards the extremities as set forth in § 120. It is evident that if the thickness of the stratum at different points along the aerofoil is in constant