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Principles of the Aeroplane

A few more words in regard to the "relative velocity" between the surface and wind. In the figure, the kite is assumed as being stationary, while the wind moves from left to right. With a thirty mile per hour wind, the relative air velocity in regard to the surface would be 30 M. P. H. If the air particles are now considered stationary, and if the kite is towed toward the left (opposite to figure) at 30 miles per hour, the relative velocity between the surface and air would still be 30 M. P. H. In other words, the kite may be stationary, or may move in regard to the earth, but its lift is unaffected as long as the relative motion between the surface and air remains constant. The motion between an aeroplane and the earth depends upon the difference of the aeroplane and wind velocities. For example, a aeroplane with a relative speed of 60 miles per hour, flying against a headwind of 30 miles per hour, moves 60-30 = 30 miles per hour in regard to the earth. The same aeroplane flying with the above wind would have a velocity of 60+30=90 miles per hour past a fixed point on the earth's surface, yet in both cases, the relative velocity of the surface in regard to the air would be the same.