Originally Posted by
manucordier
Hi BOAC,
I understand that beneath-the-wing engines produce a nose up pitch when you increase thrust (which has a negative effect on static longitudinal stability) and tha aft-fuselage-mounted engines produce a nose down pitch (which has a positive effect on static longitudinal stability).
Thus in both case, increasing thrust has an influence on the pitch and thus on longitudinal stability. However, according to my understanding, none is more influent than the other !
Be very careful to not mix issues of stability and of control; the discipline uses both terms because they are not interchangeable.
In this case, talk of stability is a red herring; the issue is that an underslung engine MUST create a nose-up moment when thrust is increased, because it's practically impossible to arrange for the thrust line to be aligned with the vertical location of the cg - unless you have a BAe146-like configuration, of course, but this discussion is usually in the context of low wing aircraft.
Whereas the aft-mounted engine is actually pretty close to pointing through the aircraft cg in a vertical sense; so the pitching moment is much smaller with variation in thrust. You'd need to mount the engine above the fuselage to get a comparable effect - something like the 727 or Trident type centreline engine WOULD create a perceptible pitch down with thrust application, I expect. But the more-normal side-mounted engines are pretty much neutral in pitch effect, to a first approximation at least.