Re: Wing vs. tail-mounted engines?

Date:         31 Mar 2001 16:43:23 
From:         westin*nospam@graphics.cornell.edu (Stephen H. Westin)
Organization: Cornell University
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Wolfgang Keller <w_keller@gmx.de> writes:

<snip>

> What are the actual design tradeoffs today and how did the situation
> evolve over the past decades to favor wing-mounted engines that much? What
> would be the impact of, for example, drastically increased fuel prices
> and/or significantly more strict noise regulations (=> engines with bypass
> ratio >>10)?

The factors I can think of are:

1. Center of gravity. With the heavy engines at the back, but the
   payload forward of that, the CG has to move drastically with different
   loads and different distribution. That would make it more difficult to
   maintain consistent flight characteristics under all situations.
   Aren't there folk tales of unladen VC-10's and IL-62's tipping
   nose-up while parked?

2. Structure. Instead of carrying the weight of the engines
   through the wing roots and fuselage, they are attached to the
   part that holds them up, the wing. Likewise, the wing presumably
   generates most of the drag of the airplane, so you would like
   the engines pulling it directly through the air, rather than
   pushing the fuselage that then drags the wing along.

3. Clean air to the engines. I believe that MD80's and the like
   are somewhat more susceptible to compressor stall, especially at high
   angles of attack or strong crosswinds on takeoff. My wife and
   daughter were aboard an AA MD80 that suffered a compressor stall
   during the takeoff roll at DFW some years back; the pilot aborted
   and went back to have things looked at before trying again.

4. Maintenance issues. Others have pointed out how engines below
   the wing are just easier to get to.

5. Simpler mounting of four engines, though this only matters for the
   747 and A340 these days. In the '60s, there were the VC-10 and IL-62.

I think the DC-9 and 727 got rear-mounted engines in part so they
could sit lower to the tarmac for easy access at primitive
airports. Both have rear integral stairs, at least in some
configurations. Boeing worked around that for the 737 by squishing the
engines in between a low wing and the ground, which created a
challenge in mounting higher-bypass engines. But that all is a moot
point except for regional jets these days, and they have tended to
stick with rear-mounted engines. I suspect that a minimum engine
distance from the ground is also desirable to avoid inhaling foreign
objects from the ground.

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.