2 ms·

If you must allow for the failure of one engine, adding more engines reduces weight. 2 engines: You need to install and carry 200% of the engine power needed t

by burfog 7y ago

If you must allow for the failure of one engine, adding more engines reduces weight. 2 engines: You need to install and carry 200% of the engine power needed to fly the plane. 3 engines: You need to install and carry 150% of the engine power needed to fly the plane. 4 engines: You need to install and carry 133% of the engine power needed to fly the plane. It is really quite interesting that twin-engine planes are cheaper to operate than planes with more engines. The engines in twin-engine planes should be larger and thus more expensive in every way. It seems that larger engines must be discounted somehow, either by the manufacturer or by the maintenance crews, or that there is a scaling factor that hugely effects efficiency of large passenger jet engines. For example, are maintenance costs dominated by paperwork? That could be enough to overcome the extra expense of needing powerful equipment to lift and otherwise manipulate the parts of a huge engine.

nemetroid 7y ago
Flight with inoperative engine(s) is altitude constrained. The aircraft is going to be able to fly with one engine operative, but not at its usual altitude, so the 200/150/133 idea doesn’t really hold. Further, larger engines are more efficient than smaller ones. You pretty much want to install the largest engines you can fit. It’s the underlying reason for the 737 Max debacle.
- burfog 7y ago
  Engines are sized for failure at take-off. At that time they are at maximum power, the aircraft is heavy with fuel, and the ground is mighty close. I see no evidence that larger engines are more efficient than smaller ones. Larger total frontal area (counting all engines together) is more efficient, as is a higher bypass ratio. Both of those tend to lead toward larger engines, but you could leave those unchanged while modifying the number of engines. For example, switching from 2 engines to 8 engines, you could maintain the same frontal area by changing the diameter from 10 feet to 5 feet. The bypass ratio, which is the portion of air that bypasses the combustion, could be 12-to-1 or 10-to-1 or whatever, and need not change with engine diameter.

by burfog 7y ago

nemetroid 7y ago

Flight with inoperative engine(s) is altitude constrained. The aircraft is going to be able to fly with one engine operative, but not at its usual altitude, so the 200/150/133 idea doesn’t really hold. Further, larger engines are more efficient than smaller ones. You pretty much want to install the largest engines you can fit. It’s the underlying reason for the 737 Max debacle.

burfog 7y ago
Engines are sized for failure at take-off. At that time they are at maximum power, the aircraft is heavy with fuel, and the ground is mighty close. I see no evidence that larger engines are more efficient than smaller ones. Larger total frontal area (counting all engines together) is more efficient, as is a higher bypass ratio. Both of those tend to lead toward larger engines, but you could leave those unchanged while modifying the number of engines. For example, switching from 2 engines to 8 engines, you could maintain the same frontal area by changing the diameter from 10 feet to 5 feet. The bypass ratio, which is the portion of air that bypasses the combustion, could be 12-to-1 or 10-to-1 or whatever, and need not change with engine diameter.