Originally Posted by
rickair7777
The main factor actually was the requirement that you have three or more engines for extended over-water ops. When ETOPS relaxed that requirement to two engines, the trijet was done.
I know thats the narrative for the life of the trijet airliner, but couldn't there be applications where the trijet made sense? Maybe the airplane is too small for wing-mounted engines and maybe its mission is to be able to do long ranges out of difficult runways.
Originally Posted by
rickair7777
Two engines are more efficient...
- Less rotating-machinery drag (saves energy).
- Fewer support systems (saves weight).
- Less flow energy-loss (no long s-duct).
- Larger fans are actually more efficient anyway. The core is going to be a little bit heavier, but it can optimized for max fuel efficiency at cruise speeds vice T/O.
Yes I understand all of this, that to produce X thrust is more efficient in every way out of 2 engines than 3. But there has to be some fraction of X where its more efficient to get it out of 3 engines than 2. For example, 3x5,000lb engines have to use less fuel than 2x50,000lb engines powering the same plane.
So I ask, whats that fraction of X? As I said above, if the plane needs .5X to take off then the twinjet has to have 1X total thrust. What total thrust could the trijet have to have the same cruise efficiency as the twinjet? The minimum that it could be is .75X because of the takeoff requirements