14th Nov 2006, 18:46
In my simplistic world view I always considered a 4 engine aircraft to be the optimal for long /extreme range ie. 5000-8000nm legs. This was due to my understanding of the twin having to be so overpowered at T/O to get the weight off the runway in the event of an engine failure ,that it was overpowered for those 10/12/14 hour legs up at altitude.A quad would jump off the ground in a less sprightly manner but could sit up there more happily with its engines less throttled and therefore more efficient.
However with the latest 777ER/LRs beating the socks off the A345/6s it seems I may be missing something .
Anyone care to explain? Is it advances in engine technology or something else?
Also how much do structural considerations affect this debate? Surely 2 engines require bigger fin/rudder assemblies for the assymetric case thereby increasing weight and what about the wing bending relief of 4 vs. 2 engines affecting the weight of the wing structure?
14th Nov 2006, 20:21
An optimally-designed commercial engine will probably NOT have its lowest Specific Fuel Consumption at TO, but instead somewhere down the power curve (an SFC "bucket", if you will.) Thus most of the trip fuel gets burned in the region of greatest engine efficiency.
Additionally, the "overpowered" twin will get to cruise altitude quicker, and thus (additionally) have fewer traffic conficts during climb.
Given these real-world issues, I'm not surprised the LR twin can beat a quad.
17th Nov 2006, 05:10
A generic answer because no two aircraft are the same - Caveat over!
It would be very surprising to find a 2 engined jet aircraft that did not better the performance of a 3 or 4 engined equivalent.
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.
CLIMB - A very fuel-expensive phase of flight, a poorly executed climb can destroy the fuel advantages gained during an efficient cruise, even a very long cruise. Typically, Climb Thrust is at an engine speed in excess of optimum TSFC, and the shorter the time of climb the better. Climb Rate depends upon excess POWER (Thrust X TAS), thus the aircraft with the greatest excess thrust will have the shortest climb time. The 2 engined aircraft wins 'hands down'.
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.
DESCENT - The 3 and 4 engined aircraft win slightly here, and it's their only win. As the 2 engined aircraft has much more thrust, the total idle fuel flow, at terrible TSFCs, will be more. It's only a slight win for the 3 and 4 engined aircraft, as the benefits to the 2 engined aircraft during climb far outweigh the disadvantages during descent.
That's the thumb-nail sketch for a generic aircraft.
I do not completly disagree with the above, but me thinks that in the most stated argument 340 vs 777 it's not only about no. of engines but also airframe/foil generally. I would conquer that a 777 with 4 engines, all about half the size of a GE90 or similar and half the FF (which should be achievable) would be as efficient as the present one, and thus more efficient than a 340.
The problem with Airbus in this instance lies in my view that they are generally heavier. Same comes to my mind when you compare 380 to 847-8F. But also very apparant 320 vs 737. Airbus puts more emphazise on solidity, technical perfection (sometimes perverted) and passenger comfort. 787 might be the first out of this series.
17th Nov 2006, 12:07
One area in which the 3 or 4 donks have a potential, albeit unrealized advantage, is descent. It should be perfectly feasable to shut down 1 or 2 for 15 or 20 minutes for a real fuel savings.
So far no one has gotten that desperate about fuel, but considering modern engine reliability, it may someday make sense. :hmm: