Re: Rear Engined Aircraft

From:         shevell@leland.stanford.edu (Richard Shevell)
Organization: Stanford University, Dept. of Aero/Astro
Date:         21 Dec 94 02:17:41 
References:   1 2 3 4
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In article <airliners.1994.1784@ohare.Chicago.COM>,
drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard) wrote:

> In article <airliners.1994.1751@ohare.chicago.com>,
> Richard Shevell <shevell@leland.stanford.edu> wrote:
> >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 of 3 degrees is
> >.99863, and lift is increasd by 5% of the thrust since sine of 3 degrees is
> >.05234.  

> Actually, it isn't the thrust line change that gives the increase in lift.
> It is the fact that the body is flying at a 3 degree angle of attack (about
> the most you can get without incurring an offsetting drag penalty).  The
> thrust line is then correctly positioned for an efficient cruise attitude.
> A small difference, I know, but I think it is important.

Maybe it is just a way of looking at it, but a positive incidence of the
thrust line with respect to the flight direction does in fact provide lift
equal to the thrust times the sine of the thrust incidence angle referred
to the flight direction. This discussion  depends on the definition of
thrustline incidence.  The thrustline incidence is generally defined with
respect to the fuselage reference line which is an arbitrary line in the
fuselage defined, in a transport, to be parallel to the floor.  Normally
one selects the wing incidence such that the fuselage floor is level in the
usual cruise configuration.  Therefore the thrust does provide a lift if
there is a positive incidence angle.  Flight attendants do not appreciate
puishing 100 lb. carts uphill so a substantial airplane incidence in
cruise, defined by the fuselage reference line, is a negative, although it
has happened sometimes.  In any case if the thrust has a positive incidence
with respect to the reference line, a positive contribution to lift occurs
compared to a zero incidence thrust line.
-- 
Richard Shevell
Email: shevell@leland.stanford.edu