Hi Tailer Jet Airplane Aerodynamics

I've asked a lot of experts this question and have
yet to get a cogent answer. Surprising.

When the engines are mounted by the tail in
everything from MD-80s to CJ Citations, why
do they always tilt up at the front?

What aerodynamic problem does that solve?

We know about similar things... why do they
make the engine on more than a few singles
point to the right? -- To deal with P-factor.
But what's the answer on the jets?

Bill Hale