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Figs. 4a-5-6-7-8-9-10-11-12. The Values of the Resistance Co-efficient K for Different Forms and Positions of Solid Objects. Arrows Indicate the Direction of the Relative Wind. (Eiffel.)

With the apex forward there is nothing to fill up the vacuous space when the air passes over the large diameter of the base as the curve of the spherical end is too short to accomplish much in this direction.

Skin Friction

Rf = 0.00000778L⁰.⁹³V¹.⁸⁶b.

It will be noted that the resistance increases at a lower rate than the velocity squared, and at a less rate than the area. That is to say, that doubling the area will not double the resistance, but will be less than twice the amount. Giving the formula in terms of area and miles per hour units, we have: Rf = 0.0000167A⁰.⁹³V¹.⁸⁶. Where A = area in square feet and V = miles per hour. The area is for one side of the surface only. A rough approximation to Zahm's equation has been proposed by a writer in "Flight," the intention being to avoid the complicated formula and yet come close enough to the original for practical purposes. The latter formula reads: Rf = 0.000009V² where Rf and V are as above. Up to 40 miles per hour the results are very close to Zahm's formula, and are fairly close from 60 to 90 miles per hour. This approximation is only justified when the length in the direction of the wind is nearly equal to the length. If the length is much greater, there is a serious error introduced.