Originally Posted by Dani
I'd say the main reason is aerodynamics, i.e. efficiency. There has been generations of engineers calculating it through. I read an article once (sorry, couldn't find the source) that you need less structural strength compared to the size of engine/aircraft with underwing engines, since the propulsion source is right on the center of gravity, the center of lift and (most of) all axis.
Before noise reduction I would also mention the safety factor of having engines as far away from all vital systems and passengers as possible. We all know of some uncontained engine failures that led to complete system failures with fuselage mounted engines while so far its rare to do the same on an underwing design (that's why the engines are mounted in front of the leading edge).
Anyone agree?
Dani
Ooo - disagree.
Taking the second part, about failure effects:
Firstly, what about that 767 (?) at LAX (?) recently which had an uncontained engine failure that took out the 'other' engine; seems to me that would count as an "uncontained engine failures that led to complete system failures ....on an underwing design"
Secondly, the rules on pressurization loss following rotor burst failure for an aft mounted engine constrain the position of the aft bulkhead because you're not allowed to compromise the pressure vessel. There's no equivalent requirement for underwing engines, basically because it would be completely impossible for them to meet it. So the consequences of an uncontained failure of an underwing/podded engine may be far worse if you are unlucky and the disk element heads for the cabin.
And as for engine position effect on design:
"the propulsion source is right on the center of gravity, the center of lift and (most of) all axis."
The underwing engine is much further off-axis relative to the CG compared to, say, the aft-mounted type - there are significant pitching and yawing moments associated with thrust changes, and much more significant VMC concerns. Also, the wing/engine interface isn't without penalties - you;'re disrupting a clean wing design to mount the pod and pylon, and likely giving up some aerodynamic efficiency on the wing. There4 are, of course, benefits too, but it's not a one-way bet in terms of design choice.