Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/272

If $\gamma _{1}$ be expressed in degrees this will require to be multiplied by ${\frac {180}{\pi }}.$

§ 175. Taking Account of Body Resistance.—The foregoing investigation has included the temporary assumption that the whole direct resistance is constituted by the skin friction of the aerofoil, as in prop. i. of the preceding chapter. We will now take into account the influence of a resistance independent of the surface of the aerofoil, the body resistance or $x_{1}$ of § 165.
Fig. 111.

We know that such a resistance, which may be-represented by an equivalent normal plane, inevitably results in an increase of the gliding angle; also that this increase will be less the lower the velocity, for, according to the equation of the foregoing section (and Prop. V. of § 166), the gliding angle is constant, neglecting body resistance, so long as the aerofoil is properly designed, and does not depend upon the velocity; we may therefore regard the gliding angle as made up of two parts: the part which is constant in respect of velocity, and the part due to body resistance, which varies as the velocity squared,

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