That's a double barreled question Stallie. In the title you say jet engine and in the text jet aircraft. The jet engine bit appears to have been covered pretty well so I'll put in my two bobs worth, specifically in relation to the B744.
Optimum altitude for cruise is that altitude at which the engine and airframe are operating at maximum efficiency. Airframe efficiency is maximized by selecting a cruise speed which corresponds to the most efficient angle of attack and engine efficiency is maximized by operating in the design rpm range around 90% to 95%. If you climb at the best angle of attack with the engines operating at their design optimum and you reach a point where you are climbing at 300fpm or less then you have reached, for practical purposes, the optimum altitude. To operate either faster or slower than the optimum speed will be less fuel efficient as will operating higher or lower than the optimum altitude. So for any given aircraft weight and set of environmental conditions there is one optimum altitude at which you have maximumefficiency. For a RR powered B744 taking off at close to maximum all up weight (MAUW) of around 400 tonnes, generally, the initial optimum altitude is around FL290.
There are three cruise speed schedules used and they are ECONomy or minimum cost cruise, long range cruise (LRC) and maximum range cruise (MRC).
ECON is a variable speed schedule starting off at about M.85 and decreasing as fuel burns off and is a function of aircraft weight, cruise altitude and headwind component. Faster in a headwind and slightly slower in a tailwind.
MRC is also a variable speed schedule starting at about M.83, with speed decreasing, which gives maximum range though not necessarily maximum efficiency.
LRC is 99% of MRC (not speed, fuel efficiency) and starts at around M.86 and decreases. No wind corrections are applied to either MRC or LRC.
For the B744, ECON is midway between LRC and MRC! That’s right, LRC is faster than the normal cruise speed. The optimum altitude increases with fuel burnoff at around 100' every eight minutes.
It's confused me, I hope it helps you. Now I need a cold Crownie after that lot and what do you know there's one right here!
