Originally Posted by
acl65pilot
Even if you are required to add thrust to a twin jet that cost pales in comparison to the cost to carry the weight of that third engine.
But how much extra weight is it when the 3 engines together only have to put out 75% of the thrust of the twinjet? Each individual engine has to put out 50% of the power of one the twinjet. So even say that it has to weigh 60% as much to make 50% of the power, all together the trijet engines would weigh 180% to the twinjet's 200%. There has to be a breaking point somewhere. In this example it would be that each of the trijets would weigh 67% of the twinjets to make 50% of the power.
Also, there is supposedly some aerodynamic benefit from the s-duct engine where the exhaust out the back of the fuselage effectively lengthens the fuselage more than would ever be practical to build. Even if the trijet has to weigh a little more, it might be evened out by this aerodynamic benefit.
Originally Posted by rickair7777
I see what you're saying. I don't know the break-even point off-hand, but it's generally known in engineering circles that two engines is more efficient than three.
There may also be structural weight savings, since you have to support all that weight on the tail and also transmit the thrust to the CG. Wing-mounted engines are closer to the CG.
I can easily see why large airliners have moved to 2 or 4 engines. If you're already building engines on the wings, then it must take a lot of extra structure to also install an engine at the centerline of the back of the plane. But if you assume an airplane thats already using fuselage mounted engines (say its not tall enough to put the engines under the wing) then I would think that its only marginal extra structure to put a third engine between and behind them.
There are definitely tradeoffs between putting the engines on the wings vs rear of the fuselage. But I'm trying to figure out 2 vs 3 engines so I want to control as many variables as possible. I'm not in aviation and I don't know what you mean about the "CG" (center of gravity?) but some advantages I've read about fuselage-mounted engines are:
-since it doesn't need the ground clearance below the wing, plane can be designed as the fuselage riding on a single wing, which can save weight and structural requirements
-can build more flex into the wings, absorb more turbulence in the wings
-possibly better for unpaved runways