In addition to developing the lift equation, the brothers also developed the equation for drag. It is of the same form as the lift equation, except the coefficient of drag replaces the coefficient of lift, computing drag instead of lift. They used this equation to answer the question, "Is there enough power in the engine to produce a thrust adequate to overcome the drag of the total frame...," in the words of Combs. The Wrights then "...measured the pull in pounds on various parts of their aircraft, including the pull on each of the wings of the biplane in level position in known wind velocities...They also devised a formula for power-to-weight ratio and propeller efficiency that would answer whether or not they could supply to the propellers the power necessary to deliver the thrust to maintain flight...they even computed the thrust of their propellers to within 1 percent of the thrust actually delivered..."