The diagram shows an aircraft in level flight with an airspeed, V. Four forces are shown acting on the center of mass of the plane: the weight, W, the lift, L, the propeller thrust, T, and the drag, D. In steady state, that is no change in altitude or plane airspeed, the lift force is equal to the weight, L = W, and the drag force is equal to the thrust, D = T. The diagram also shows the resultant forces from the addition of lift and drag, and the addition of thrust and weight. These resultant forces are equal in magnitude and opposite in direction.

Power is the product of a force and a speed. For this steady state situation, the propulsion power required is:      P = TV = DV. Typically, drag increases as V², so that the power required to maintain steady state increases as V³. 

The specific energy consumption in this level flight condition is: E = P/(WV) = T/W = D/L. For the lowest possible specific energy consumption, drag should be as low as possible for a given lift.