5.6 Power Curves

For propeller-driven aircraft, engine performance is specified in terms of power. A chart that is analogous to Fig. 5.13 is easily developed, but with drag expressed as power required P _R using the relationship

Figure 5.15 illustrates a power-required curve for a typical propeller-driven aircraft. The power-available model for an appropriately sized reciprocating engine/propeller combination, obtained by multiplying Eq. (5.9) by V _?, is also plotted on the figure. The airspeed where power available equals power required is the aircraft's V _max for that altitude and aircraft weight. For a propeller-driven aircraft, the airspeed where power required is a minimum is, among other things, the speed at which the aircraft can maintain level flight at that altitude and weight with the minimum engine throttle setting.

Figure 5.15: Power available and power required for a propeller-driven aircraft.

Power curves are also useful in predicting the performance of turbojet-and turbofan-driven aircraft. Figure 5.16 illustrates power-available and power-required curves for the same aircraft whose thrust-available and thrust-required curves are shown in Figure 5.13. The curves are obtained by multiplying thrust and drag at each point by the freestream velocity. Note that minimum power required occurs at a lower velocity than minimum thrust required. For this flight condition, induced drag is three times as great as parasite drag, that is, 3 C _{D 0} = kC ² _L.

Figure 5.16: Power available and power required for the jet aircraft of Fig. 5.13.