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Fig. 2 is a diagram of an aeroplane that shows the connection between the kite and aeroplane principles. In this figure, the wing surface of the aeroplane, X-X corresponds to the kite surface X-X. The relative air W-WW-W striking the wing from the left is deflected down along the arrows R-R-R-R and results in an equivalent lift force L, and a drag force D as in the case of the kite. The resultant force required to maintain the relative velocity between the air and wings is indicated by D¹, opposite and equal to the drag force D. The resultant required for overcoming the drag is provided by the screw propeller P instead of the string S shown in Fig. 1. The propeller thrust (D¹) is parallel to the air stream instead of being inclined as in the case of the string, but the total effect is the same since both are "Resultants of the lift and drag." To sustain the aeroplane, the lift (L) must be equal and opposite to the weight shown by M. The fact that M and L are opposite and equal makes it only necessary for the propeller to overcome the horizontal drag, and hence the thrust can be made parallel to the air flow—or nearly so. The aeroplane is provided with a small tail surface (T) that corresponds to the kite tail (T). It maintains the lifting surfaces X-X at a given angle with the air stream. The tail may, or may not aid in supporting the machine, but in modern machines it is common to employ a tail surface that is non-lifting under ordinary conditions of normal flight. The body (B) contains the pilot, motive power, fuel, and such useful load as it may be necessary to carry.