TABLE OF CONTENTS The algebraic form of the aircraft drag polar we have assumed leads to some simple and helpful relationships. These mostly pertain to the parameter ( L/ D) max, which describes the maximum efficiency of the aircraft. Because lift is the aerodynamic force that allows aircraft to fly and carry payloads and passengers, it represents the usefulness or economic value of the aircraft. Drag, on the other hand, must be opposed by thrust, which is typically created by burning fuel. Drag therefore represents the cost of providing the economic value, mostly the cost of fuel. L/ D, therefore, represents the aircraft's benefit-to-cost ratio, and ( L/ D) max describes the maximum achievable value of this ratio.
To find where this ratio is a maximum on thrust and drag curves like Fig. 5.13, we need only remember that these curves are created by assuming lift equals weight and that weight is held constant. Therefore, if lift is constant ( L/ D) max is achieved where drag is minimum. This point is identified on Fig. 5.13 as " V for minimum thrust required."
An interesting phenomenon occurs at the speed for ( L/ D) max, which is once more a consequence of the assumed algebraic form of the drag polar. Figure 5.13 was created by carefully calculating parasite and induced drag values for a fixed assumed aircraft weight and drag polar, then summing the two...
