Drag is a function of the square of airspeed, so the same changes in speed cause the drag to vary by a factor of four. To reduce the net force as much as possible, helicopter blades are designed to be as thin as possible, reducing their drag, although this makes them inefficient for lift. In the 1950s, helicopter blades were made in much the same fashion as fixed-wing aircraft wings; a spar ran the length of the rotor blade and provided most of the structural strength, while a series of stringers give it the proper aerodynamic shape. This method of construction, given the materials of the era, placed enormous stresses on the spar.