Aircraft-Performance Calculations

In previous chapters, we introduce other ways to describe the aerodynamic efficiency. One that is useful is the “lift-to-drag ratio,” or L/D. For the Bf-109G, the L/D ratio for the high-speed cruise condition we analyze is:

CL 0.21

CD 0.036

which is based on the lift coefficient for cruise and the total drag coefficient (i. e., the total drag area divided by the wing area). This is a fairly good value, indicating that the Bf-109 is a relatively clean aircraft; however, it would not make a good glider (at least, not at 610 km/hr) because from a 1-km altitude, it would be on the ground in less than 6 km with the engine off. Why is this so? Is this L/D ratio a good indication of the overall aerodynamic performance of the aircraft? Is there a speed at which the L/D ratio is higher? These questions deserve careful consideration, and we do this in the remainder of this chapter. We seek the means to represent the overall performance over the entire speed range, along with useful information such as the maximum rate of climb, ceiling (i. e., maximum altitude), stalling speed, power required as a function of flight speed, maximum range, maximum endur­ance, speeds to fly for maximum climb rate, best range maximum endurance, and so on. These matters usually are considered as part of the subject of airplane per­formance. However, it is useful to complete this book with a short introduction, because it summarizes what has been accomplished to this point and emphasizes key results.