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Originally posted by DeltaFlyingProf
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No it does not have twice as much maintenance cost. But it is higher, you have to examine 4 smaller engines (smaller, thus spare parts are cheaper, hence it is not twice as expensive) instead of 2 large ones. Reliability is slightly lover since now you have one of 4 engines that can have a glitch.
As for why a 2 engine aircraft is not more efficient than a 4 engine one, I have to go into a few technical details:
For a given weight and aircraft, the thrust to be provided is independent of the number of engine. It is actually the weight divided by the lift to drag ratio. The lift to drag ratio is a function almost exclusively of the shape of the aircraft, nothing else. In general, the larger the aircraft, the better it is because the Reynolds number increases.
Now for a given speed, both jet engines, turbo-fans, turbo props or any kind of engine consumption is proportional to the thrust it provides. So if you have 4 engines, each engine has to provide only 1/2 the thrust the engine of the 2 engine aircraft have to produce. So now you have 4 engines consuming each 1/2 of the fuel of the 2 engine aircraft and I will leave it to you as a homework to show that the total fuel consumption is the same.
I expect your copy by tomorrow noon on my desk and will have no mercy in my grading. You get either 100 or 0, no in between!
Where do difference come in in term of fuel burn? Larger engine of the same generation tend to be slightly more efficient (but we are talking at most a 1 or 2% difference in today's engines) and the smaller engine have an area exposed to the air which is more than 1/2 of the large engine. This adds some drag, but again, it is rather small compared to the total drag and may affect the lift to drag ration by at most 1 or 2 percents.
So difference in efficiency total? Maybe 2%. That can be outdone by differences in design. And in fact, because of the weight distribution, the structural weight of the wing of the 4 engine aircraft may be lower because of the stress relief due to having a better weight distribution along the wing... So at the end, maybe 1% difference.
Any more questions?
As for why a 2 engine aircraft is not more efficient than a 4 engine one, I have to go into a few technical details:
For a given weight and aircraft, the thrust to be provided is independent of the number of engine. It is actually the weight divided by the lift to drag ratio. The lift to drag ratio is a function almost exclusively of the shape of the aircraft, nothing else. In general, the larger the aircraft, the better it is because the Reynolds number increases.
Now for a given speed, both jet engines, turbo-fans, turbo props or any kind of engine consumption is proportional to the thrust it provides. So if you have 4 engines, each engine has to provide only 1/2 the thrust the engine of the 2 engine aircraft have to produce. So now you have 4 engines consuming each 1/2 of the fuel of the 2 engine aircraft and I will leave it to you as a homework to show that the total fuel consumption is the same.
I expect your copy by tomorrow noon on my desk and will have no mercy in my grading. You get either 100 or 0, no in between!
Where do difference come in in term of fuel burn? Larger engine of the same generation tend to be slightly more efficient (but we are talking at most a 1 or 2% difference in today's engines) and the smaller engine have an area exposed to the air which is more than 1/2 of the large engine. This adds some drag, but again, it is rather small compared to the total drag and may affect the lift to drag ration by at most 1 or 2 percents.
So difference in efficiency total? Maybe 2%. That can be outdone by differences in design. And in fact, because of the weight distribution, the structural weight of the wing of the 4 engine aircraft may be lower because of the stress relief due to having a better weight distribution along the wing... So at the end, maybe 1% difference.
Any more questions?
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