Originally Posted by
Gipsy Queen
tdracer. This is an interesting point. Beyond considerations of compression and polar moments of inertia, I'm wondering what the principal disadvantages might be?
There are several big ones. First off, tail mounted engines are relatively high off the ground, making servicing and maintenance more difficult and time consuming. Not a big difference, but over a ~25 -30 year service life of an aircraft, it mounts up. Second, during the early design phases of a new aircraft, the engine weight is little more than an educated guess. Worse, the engines seldom get lighter - they are usually heavier, often a lot heavier. Now, if the engine is mounted on the wing - it's located near the aircraft CG, so a heavier engine is generally not a big concern to the overall design. Tail mounted engines - by definition - are way behind the aircraft CG. So when the engine gets heavier, the wing is suddenly in the wrong place. The only solution (short of starting over with a new wing position) is to add ballast to the nose - effectively doubling the weight penalty. According to people I worked with that worked on the MD-90, this hurt the MD-90 massively. After the basic design was frozen, the FAA required much better blade out and uncontained failure protection - resulting in a massive increase in the engine weight. Between that and the associated required nose ballast, the resultant aircraft was so heavy that it lost most of it's fuel burn advantage from the new engines (relative to the MD-80).
Structurally, putting the engines on the wing has significant advantages relative to the tail. I've been told there are some aerodynamic advantages as well although I admit it's not obvious to me what those might be.