2,000 Greater & Lesser Mysteries/2,000 Greater and Lesser Mysteries.iso/geninfo/mys00597.txt/text0056.txt

Text File | 1994-06-10 | 1.9 KB | 38 lines

In article <[e-mail]>, Tony Heatwole <[e-mail]> writes:
> Daryl Morse brought up the question of jet engine mounting
> angle in a previous article. This is something I've often
> wondered about myself.  Most tail-mounted jet engines on
> commercial airliners *appear* to slant downward toward the
> rear. This is particularly noticable on the tail-mounted
> engines of DC-10s and MD-11s. Here, presumably, downwash
> from the wings isn't a mounting factor.
>
> What are the design tradeoffs in setting the mounting angle
> for tail-mounted engines?  How do the effects of engine loss
> and changes in engine thrust figure in this tradeoff?
>
The main reason for setting the engine nacelles at an angle is to align
the engine inlet with the local flow angle so as to achieve the maximum
inlet efficiency.  An improvement of just a few tenths of a percent in
cruise flight can mean the savings of thousands of gallons of jet fuel
over the lifetime of the aircraft.  Rear mounted engines tend to be canted
upward, as already observed.  Engines mounted on swept wings tend to
be toed-in a degree or two to align with the spanwise flow component.

If you resolve the components of the thrust vector into components along and
normal to the direction of flight, you find

               FX = Fcos(A)cos(B)
               FY = Fsin(A)cos(B)
               FZ = Fcos(A)sin(B)

where A is the toe-out angle and B is the pitch angle.  Since the angles are
only a degree or two, the X component is only slightly below the total thrust.
The cosine of two degrees 0.9994.  Since the magnitude increases as inlet
efficiency increases, the X component may even be larger than if A and B
were zero.  The Y and Z components of thrust can affect engine-out handling.
For rear mounted engines, the nose down pitching moment due to thrust will
increase as the upward cant angle is increased.