Originally Posted by
tuna hp
I read all over the internet that 2 engines is always more efficient than 3. But is that necessarily true?
Yes.
Originally Posted by
tuna hp
It seems that a main factor is the requirement that a plane be able to complete its takeoff run with one engine out. On a twinjet you're losing 50% of your power while on a trijet you are losing 33% of your power.
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.
Originally Posted by
tuna hp
At cruise, which plane is more efficient? The one that is uses the three smaller less efficient engines or the one whose 2 larger engines that are incredibly overpowered for cruise? By how much?
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.
If the weight and fuel savings were not enough, the reduced Mx cost (by 33%) seals the deal.