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

wrote:
>
> 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?

Thrust line.

Most likely to line it up with the local flow at a design point. Such as:
high altitude, heavy wt, cruise or single engine climb at max gross or
.... or .... or a compromise between several design points.

I remember an access door on the outboard side of a rear mounted nacelle
that came off in flight. It hit the rudder.

" > wrote in message
ps.com...
> 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
>

Picture the airflow from the trailing edge of the wing, you want this well
aligned with the intake.

Todd

"B A R R Y" > wrote in message
...
> wrote:
>>
>> 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?
>
> Thrust line.

wrote:

> 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?

A wild-assed guess would be to unload
the tail in cruise.

I'm no aeronautical engineer, but .. I would think the angle may be
chosen so that, at some chose speed (like final approach speed), there
is minimal trim change over a wide range of power (thrust) settings. I
wouldn't want the airplane to pitch up, or down, on short final when I
found it necessary to add power to make a missed approach.

I would think a similar reason would explain the off-axis alignment of
the motor on single-engine airplanes. I'll ask my mechanic.

george