Important factors are the thrust to weight ratio of the engine, fuel consumption and the aerodynamic characteristics of the aircraft (Dole and Lewis, 2000). Overall engine efficiency is cycle efficiency x propulsion efficiency (Hunecke, 2010). Propulsion efficiency (np) = 2/(1+c/v) where c is exhaust speed and v is aircraft speed. Thus, maximum efficiency is achieved in a balanced equation. Looking at the equation, at takeoff, c exceeds v and thus np is low and as V increases np will increase however, to extend endurance, c is reduced (power reduction), once again affecting efficiency. In considering overall efficiency, propulsion efficiency must also be considered in relation the aircraft itself. One such method is the Oswald efficiency factor (FAA, 2001) whereby: CD = CD0 + (CL)2/ e AR where: CD is the overall drag coefficient, CD0 is the zero-lift drag coefficient, CL is the aircraft lift coefficient, is the aircraft circumference-to-diameter ratio e is the Oswald efficiency number, and AR is the aspect ratio. (FAA, 2001). From this equation in conjunction with the propulsion equation can be seen the multiplicity of factors that affect efficiency. The dilemma in performance versus efficiency arises from both design and operating environment. For example, a fighter jet may have low efficiency (a high SFC) however, the aircraft performance is high.