From:shevell@leland.stanford.edu (Richard Shevell)Organization:Stanford University, Dept. of Aero/AstroDate:21 Dec 94 02:17:41References:1 2 3 4Followups:1

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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