Originally Posted by
tuna hp
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.
True, but nobody making modern jets puts the other two on the tail if they can help it.
Some regional jets are the only exception, because with a short wing span they don't need longer (heavier) landing gear to provide wingtip clearance during a xwind landing. Larger airplanes with longer wings need tall gear for tip clearance, so that weight is already a done deal.
But even the newest RJ's have gone to wing mounts.
Originally Posted by
tuna hp
-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
Simpler structure, yes but I doubt it saves a great deal of weight.
Originally Posted by
tuna hp
-can build more flex into the wings, absorb more turbulence in the wings
Whatever flex is required, gets designed in. The exact design doesn't matter. Again I suspect not much weight savings by carrying the spars all the way through.
Originally Posted by
tuna hp
-possibly better for unpaved runways
Modern commercial transports don't operate on unpaved runways. Besides I disagree...the gear would kick up rocks which would get sucked into a tail-mounted engine.
That's why military transports have their engines mounted on the wings, with the wings mounted high on the fuselage.