Future Trends in Commercial Aviation Engines’ Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 133

Fig. 21 Ignition and lean stability limits versus normalized reference velocity in addition to key
engine operating points; reproduced from Bruce et al. ( 1977 )

article, we will use a rule of thumb (viz. ʴ isen = 0.33) with attendant altitude relight

capability of 35,000 ft. 0.85 cruise Mach number.
Another design requirement introduced with the twin-engine aircraft operation is
in regard to restarting the engine at the cruising altitude after an extended exposure
to the local

flying at
35,000 ft. or 40,000 ft. unlike the older requirement for 3- or 4-engine airplanes
where one could relight the hot engine with attendant higher T 3 levels. Clearly the
cruise altitude relight for the latter case is much easier to accomplish than the
former with a cold-soak engine.
When a twin-engine airplane is operating with one engine only and the pilot
turns off the good engine by mistake, the engine must restart immediately while
essentially running turbomachinery at the cruising speed. In other words, the relight
capability at the cruising altitude must be met on both the low- and high-corrected
fl

thermal condition, namely ʸ = 0.7958 or 0.7519 when fl

flow limits with attendant big variation in the corrected reference velocity as shown
in Fig. 21 .
The range of the resulting full-scale annular rig data is very wide as listed in
Fig. 21 . Some of the key operating points (viz. sea-level ground idle, max cruise
fl

flying 0.8 Mach number at 40,000 ft. and sea-level takeoff, engine fuel schedule for
cold day and altitude operation) are shown in order to identify the minimum value
of V R ʴ 3 ʸ 3 below which ignition would not occur and the corresponding engine
windmilling points, viz. pairs of altitude and Mach numbers along the left-side of