Why do the engines on some airliners (those with the engines mounted at the tail) appear to be pitched up relative to the fuselage? To be clear as mud, what I mean is that instead of being parallel with the main body of the aircraft, the engines appear canted (intake up/exhaust down) a bit. Am I imagining this?
Are you referring to the DC-9 and MD-80 series planes? That’s becasue the engine the use has has a slight turn upward at the end of the exhaust end. You can see it in this picture some. The exhaust comes out parallel to the long axis of the planes, but the engine doesn’t look straight. You can really see it on the 707RE.
The usually well-informed folks at airliners.net beg to differ:
Another factor is that a plane may not cruise through the air at the exact same angle as it sits on its landing gear, and the engines will be oriented for best flight performance.
It’s done primarily to give the plane a better chance to take off from the treadmill.
Having the thrust vector parallel to the roll axis of the aircraft is one important detail. The other important detail is where, elevation-wise, the thrust vector is relative to the center of mass and/or center of drag of the aircraft. As Fish Cheer points out, the front of the engine is up high so it can catch clean air, but if the thrust vector didn’t need to be at any particular elevation, then the entire engine would have simply been level with that high intake.
So: intake high to catch clean air, exhaust at some lower height (and orientation) to get the thrust vector where it needs to be, and tilt/duct the engine so that it connects nicely with both the intake and exhaust.