TC
"My observation is: why does an a/c designer complicate matters by driving the TR from the engine?"
My own observation is that far from complicating matters, the AS350 and B206 actually simplify things. For a single-engined heli with the engine aft of the M/R transmission, in order to drive the T/R as you consider best, it would mean doubling up on drive shafts and complicating the MRGB to generate a T/R drive coupling. In the case of the Gazelle, as well as the extra drive shaft and the more complex MRGB, it also has an additional intermediate inclined g'box to route the T/R drive under the engine. All this adds extra components, building in failure points, but most importantly, extra weight. Furthermore, an independant T/R driveshaft passing under the engine means the engine has to be mounted higher reducing clearance from the main rotor, making the C og G higher and increasing panel sizes to cover the engine.
In the AS350 and B206, the design is simple, light-weight, compact and reliable. All the talk of Short shaft / High speed shaft failures is mainly theory as on the whole, as VP asserts, these failures are extremely rare in any case.
JJ