Originally Posted by
Old Smokey
As you say, the twin needs more total thrust for the same weight at Takeoff. Following engine failure, similar (but not the same) performance must be obtained with 1 engine instead of 2 or 3, thus, when all engines are operating (the 99.9999% NORMAL situation), the twin has a 100% thrust excess above minimum requirements, whereas the 3 engined aircraft has a 50% excess, and the 4 engined aircraft a mere 33% excess. Thus, the 2 engined aircraft is significantly over-powered in normal flight situations.
Including normal cruise... right?
Originally Posted by
Old Smokey
CRUISE - The optimum fuel consumption occurs when the aircraft is flown at the optimum airframe speed, and the engine speed is at optimum TSFC, SIMULTANEOUSLY! Optimum airframe speed, and optimum TSFC engine speed cannot be looked at separately, they must be considered together.
Consider a 4 engined heavy aircraft early in the flight. The optimum altitude for the wing may be, for example, F/L 350, but this would require engine speed in excess of optimum, so, although the aircraft may have sufficient thrust to climb to and maintain F/L 350, engine speed will be above optimum, and the aircraft limited to a level lower than the wing 'wants'. If the same aircraft was powered by 2 engines, there will be significantly more excess thrust, thus it is much more feasible to climb straight to F/L 350 where engines are at optimum TSFC. It should be noted that in most cases, it is more fuel expensive to cruise above optimum level, than to be the same altitude below optimum level (about twice the penalty in fact). So, the 2 engined aircraft wins in the cruise phase by being able to cruise at significantly higher levels for the same phase of flight.
But why should a 4 engined heavy operate below the optimum altitude for the wing?
Why wouldn´t a twin be forced to choose between flying above the optimum altitude for the wing or running engines below optimum speed?
Also: the total installed thrust is limited by takeoff requirements - but is it optimized for cruise fuel burn or is it not?
Also, comparing 777 vs 340 may be difficult - is it twin/quad issue or general weight of construction? But compare 340 vs. 330.
Is it a general rule of high-bypass engine construction that 2 engines giving a certain amount of thrust always weigh less, produce less drag and burn less fuel than 3 or 4 engines would for the same total thrust (and engine development level)? Does it mean that a 2-engine B747 or A380 or L-500 or An-124 or An-225 would be sure to have longer range than the 4-engined or 6-engined originals? And that a single-engine jet would have the best range of all - like Virgin Atlantic Global Flyer? Virgin motto seems to be "1 engine 4 long haul!"... any data for cruise and more importantly, climb fuel burn of Global Flyer?