Re: Rear Engined Aircraft

From: (Richard Shevell)
Organization: Stanford University, Dept. of Aero/Astro
Date:         14 Dec 94 02:22:38 
References:   1 2 3 4
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In article <airliners.1994.1764@ohare.Chicago.COM>,
(James Murphy {75881}) wrote:

> In article <airliners.1994.1751@ohare.Chicago.COM>, (Richard Shevell) writes:
> |I think we said this earlier but here it is again.  In addition to the
> |above, an upward inclination of the thrust line provides a little free
> |lift.  The loss of thrust is trivial since the cosine ...

> Aircraft are design nose-heavy to allow for stall recovery, hence the
> tail is already trying to go "up".  The role of the horizontal
> stabilizer is push DOWN on the tail and balance this tendancy, hence
> level flight.  Adding "up thrust" will in fact require more angle-of-attack
> in the horizontal stab to maintain level flight attitude, hence more 
> induced drag.  Now, I have to assume that the added efficiency of canting the
> nacelles up to align with downwash more than overcomes the added drag
> from the stab, howvever, I can't see where vertical vectors that far from the
> CG, on that long a moment arm do much to the CG in terms of vertical
> components.  Sort of like trying to carry a see-saw by lifting one end. :-)

Airplanes are not basically designed to be "nose-heavy".  However, because
airfoils with positive camber are desirable for high maximum lift
coefficient and sometimes for best drag divergence Mach number, and because
cambered airfoils have a nose down (negative) pitching moment about the
aerodynamic center, the wing-body configurations usually have a nose down
tendency that must be balanced by the tail. The nose down moment required
for stall recovery is obtained by designing the wing-tail combination so
that the change in flow when the wing stalls (initially on the inboard
sections) causes a pitch down.  

Ideally the thrust line should go through the center of gravity so that
thrust changes do not pitch the airplane up or down.  Thus the height of
the engines with respect to the c.g. is important.  If I remember
correctly, inclining the engines slightly upward on the DC-9 put the thrust
line very close to passing through the center of gravity. 

Richard Shevell