Thrust Required for Level Flight
If Eq. 9.4 is divided by Eq. 9.5, we find that the ratio of thrust required to total airplane weight is equal to the ratio of the drag coefficient to the lift coefficient, or:
T = TR = thrust required = – jjD ’
which illustrates again the importance of the L/D ratio in performance characteristics. That is, to minimize the amount of thrust that must be used in level flight, the L/D ratio must be as large as possible. It is useful to determine how the thrust required varies with flight speed; Eq. 9.3 provides the needed information. First, we calculate the required lift coefficient to provide the lift to balance the weight as a function of flight speed:
C = _W = 2W
Cl qS pv2s •
Figure 9.4. Thrust required for Bf-109G.
Substituting this result into Eq. 9.3 gives the drag coefficient as a function of flight speed; namely:
Notice that the induced-drag coefficient decreases rapidly (as the inverse fourth power of the speed) as V increases. This indicates that induced drag is most important at lower speeds, whereas the parasite drag dominates at high speed. The thrust required as a function of velocity is found readily by means of Eq. 9.4. We find:
This is plotted in Fig. 9.4 for the Messerschmitt Bf-109G. All values shown are for standard sea-level atmospheric conditions. (The changes that result from higher altitudes are checked in an exercise at the end of the chapter.) Both the induced-and parasite-drag contributions are plotted separately and compared to the total thrust required.