Figure 10.44. Uninstalled maximum cruise rating (turboprop)

is held constant. Fuel flow at the initial climb is obtained from Figure 10.43b. The example gives 0.522 × 914 = 477 lb/hr. With varying values of altitude, climb calculations are performed in small increments of altitude, within which the variation is taken as the mean and is kept constant for the increment.

Maximum Cruise Rating

Figure 10.44 shows the maximum cruise SHP and fuel flow in nondimensional form for the standard day from a 5,000to 30,000-ft altitude for true air speed from 50 to 300 kts. Intermediate values may be linearly interpolated. The graph takes into account the factor k1 (varies with speed and altitude) as indicated in Section 11.3.3, Equation 11.19.

In the example, the design initial maximum cruise speed is given as 300 kts at a 25,000-ft altitude. From Figure 10.44a, the uninstalled power available is 0.525 × 1,075 = 564 SHP. In Figure 10.44b, the corresponding fuel flow is 0.436 × 564 = 246 lb/hr. The integrated propeller performance after deducting the installation losses gives the available installed propeller performance.

10.11.3 Turbofan Engine: Civil Aircraft

All thrusts discussed in this section are uninstalled thrust. There is loss of power when an engine is installed in an aircraft, as discussed in Section 10.10, from 7 to 10% at the takeoff rating depending on how the ECS is managed. At cruise, the loss discreases to 3 to 5%. For simplicity, both military and civil aircraft installation losses are kept at a similar percentage, although the off-take demands are significantly different.

Figures 10.45 through 10.51 show the turbofan power at the three ratings in a nondimensional form for civil aircraft engines with low and high BPRs. Civilaircraft turbofan performance is also divided into two categories: one for a lower BPR on the order of 4 and the other at 5 and above. The most recent engines (i.e., engines for the newer Boeing787, Airbus350, and Bombardier Cseries) have

reached a BPR of 8 to 12; however, the author could not obtain realistic data for this class of turbofans.

The higher the BPR, the less is the specific thrust (TSLS/m˙ a, lb/lb/s). There is a similar trend for the specific dry-engine weight (TSLS/dry-engine weight, nondimensional). Table 10.7 may be used for the computations.

Turbofans with a BPR Around 4 (Smaller Engines; e.g., Bizjets)

Turbofan performance. An engine-matching and aircraft-sizing exercise that gives the TSLS is conducted in Chapter 11. Chapters 11 and 13 work out the installed thrust and fuel flow for the matched engines of the sized aircraft under study.

Takeoff Rating. Figure 10.45 shows the takeoff thrust in nondimensional form for the standard day for turbofans with a BPR of 4 or less. The fuel-flow rate remains nearly invariant for the envelope shown in the graph. Therefore, the sfc at the takeoff rating is the value at the TSLS of 0.498 lb/lb/hr per engine.

Maximum Climb Rating. Figure 10.46 gives the maximum climb thrust and fuel flow in nondimensional form for the standard day up to a 50,000-ft altitude for three Mach numbers. Intermediate values may be linearly interpolated. There is a break in thrust generation at an approximate 6,000to 10,000-ft altitude, depending on the Mach number, due to fuel control to keep the EGT low.

Equation 11.14 (see Chapter 11) requires a factor k2 to be applied to the TSLS to obtain the initial climb thrust. In the example, the initial climb starts at an 800-ft altitude at 250 VEAS (Mach 0.38), which gives T/ TSLS = 0.67 – that is, the factor k2 = TSLS/ T = 1.5. At a constant EAS, the Mach number increases with altitude; in

Figure 10.45. Uninstalled takeoff performance (≈<BPR 4)

Figure 10.46. Uninstalled maximum climb rating (≈<BPR 4)

the example, when it reaches 0.7 (depending on the aircraft type), the Mach number is held constant. Fuel flow at the initial climb is obtained from Figure 10.46b.

With varying values of altitude, climb calculations are performed in small increments of altitude within which the variation is taken as the mean and is kept constant for the increment.

Maximum Cruise Rating. Figure 10.47 shows the maximum cruise thrust and fuel flow in nondimensional form for the standard day from a 5,000to 50,000-ft altitude for Mach numbers varying from 0.5 to 0.8, which is sufficient for this class of engine–aircraft combinations. Intermediate values may be linearly interpolated.

The coursework example of the design initial maximum cruise speed is Mach 0.7 at 41,000 ft. From the graph, that point is T/ TSLS = 0.222, which has TSLS/ T = 4.5 (i.e., k2 in Chapter 11). Chapter 11 verifies whether the thrust is adequate for attaining the maximum cruise speed. Fuel flow per engine can be computed from Figure 10.47b.

Figure 10.47. Uninstalled maximum cruise rating (≈<BPR 4)