Originally Posted by
galaxy flyer
tdracer,
Would care to expand on your last paragraph, please. I'm interested in learning, not doubting your analysis.
GF
I'll give it a shot
If you look at a plot of TSFC for a typical turbofan engine (specific fuel consumption on the vertical scale, thrust on the horizontal) it looks rather like a stretched out 'U'. At very low power and very high power, the efficiency is fairly poor, but it's really good in the middle. This is 'OK' because you're still not burning much fuel at idle, and you don't spend more than a few minutes per flight at takeoff. The efficiency at idle is so poor that for some engine types, as you accelerate from minimum ground idle to flight or approach idle, the EGT actually drops. Now this 'U' curve moves around with altitude, airspeed, and total temp (it's mainly a function of inlet total pressure) but the basic shape remains.
Now this is a broad generalization, but because twins are overpowered relative to quads they tend to cruise closer to the bottom of that 'U' shaped TSFC curve while a quad tends to move more up the increasing TSFC slope due to the higher relative thrust demand.
As an extreme example, think of the case of a 747 with an engine out. Now, a 747-400 or -8 will happily cruise on 3 engines at 35k or above provided it's not really heavy. But the fuel consumption skyrockets relative to 4 engine cruise because now you're way up on the high power side of that U curve.
Does that all make sense (or you at least confused at a higher level
). This is very top level and general, but hopefully you get the concept.