TOTAL DRAG

The total drag of the aircraft is composed of the total vortex and all other drags at each speed. Where the vortex drag equals all the rest, i. e., where the two lower curves in Figure 2.11 intersect, drag is a minimum for the whole aircraft Since lift is constant for a given mass of aircraft in level flight it is at the minimum drag point on the curve that the best L/D ratio of the aircraft occurs. Another, slightly more elaborate, presentation of this information appears in Figure 4.10, and in Figure 4.4 the shape of the polar curve of a sailplane is directly related to this same curve, although presented in different form in that figure, as sinking speed plotted against velocity rather than total drag against Cl.