Originally Posted by
acl65pilot
Trust me if it were more efficient to have three, you would see airlines buying them. They are not. Add it cost to carry, maintenance, added burn etc, and there is no comparison. remember that airlines are taking pillows off of airplanes to save weight. EFB's save weight. Every added pound costs a ton of money over the operational span of a jet.
Airlines are still buying plenty of 4 engine planes. How do the A330 and A340 compare? Again, as I said, I can't see how 3 engines would make sense with the way that larger airliners have evolved to be built. It would have to be something smaller that couldn't fit the engines on the wings.
On the subject of structure and wing root loading, you might also note that 3 engine planes typically carry as much fuel in the center tank as the wings to fuel the number 2 engine, whereas twin engine aircraft typically don't use the center tank until the fuel required exceeds the capacity of the wings. Again, more wing root loading means more structural considerations.
Correct me if I'm wrong, but I believe that the reason larger aircraft mid-mounted wings is for ground clearance for the engines. Airplanes that have fuselage engines can have a 1 piece wing with the fuselage resting on top. Its supposed to be more structurally efficient. Also, I can't imagine why a trijet would necessarily hold more fuel in the fuselage.