Originally Posted by
megan
The inlet has a downward cant but I think the engine and exhaust are parallel to the waterline. Perhaps on the ground all that engine weight hanging from a beam ahead of the wing leading edge gives it a bit of a droop, the pods do dance about in turbulence, the wing too is a flexible structure.
Megan basically has it - the engine centerline is generally parallel to the aircraft 'waterline' (although some aircraft installations do have a slight angle - it's considerably less than what you see at the inlet). The front of the inlet is 'drooped' to better align with the airflow at a typical cruise AOA. This gives the inlet a slightly better pressure recovery at cruise (and hence slightly better fuel burn). And inlet pressure recovery is a huge design consideration - especially at cruise - since it has an outsized effect on fuel burn (reduced ram drag
and increased thrust).
While the droop does help a bit with preventing inlet separation at high AOA (takeoff) - that's not the primary reason why it's there. Inlet lip separation at very high AOA (actually prevention of the separation) is a major design criteria for the inlet and is addressed at the overall inlet design stage, since the absolutely last thing you want to have happen if there is an over-rotation at TO is for the engines to surge due to inlet separation. The original JT9D installation on the 747 was particularly prone to inlet separation at high AOA, and there were a couple close calls when outboard engine's surged due to a takeoff over-rotation (outboard engines were more prone due to the wing flex).
Years ago, Pratt came in with a proposal to do a 'cambered' inlet instead of a drooped inlet (basically incorporating the inlet droop over the entire length of the inlet, rather than the typical droop design which incorporates it over a short length of the inlet). Claimed they could get something like a half a percent to one percent improved fan efficiency. Our inlet people looked at it and said 'no measurable improvement', but Pratt persisted with a bunch of ground testing they claimed validated the concept. Then they did some back-to-back flight testing (A310 IIRC) and couldn't measure any change. They quietly dropped the concept after that
.