PDR1, I think what the designer wants is not zero lift from the fuselage, but that the lift from the fuselage is what would be delivered by the adjacent bits of wing if they were projected across the fuselage span and there were no fuselage. That is, what you want is a smooth spanwise lift distribution including the fuselage, part of the ideal elliptical lift distribution. I'm not an aerodynamicist but I suspect that this is a lot easier said than done for different angles of attack, and that designers seek a compromise between low drag in cruise and perhaps low drag, say, at liftoff with an engine out.
Also, I think that for long range one would cruise climb to maintain a constant angle of attack, for minimum drag, or in practice a slightly lower angle and higher speed, for speed stability, staying on the right side of the drag curve, I recall reading a graph of Lindberg's IAS against time in the Spirit of St. Louis, showing a continuously decreasing speed at roughly constant altitude, corresponding to a constant angle of attack and a decreasing fuel load.
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Originally Posted by
PDR1
That may or may not be true, but CG position has no effect on flying attitude. The fuselage attitude is set almost entirely by justthe wing incidence - the angle at which the wings are nailed to the fuselage. The CG position would affect how hard the tailplane has to work to hold the wing in that attitude, but not the attitude itself.
FWIW I understand that on transport aircraft (and indeed any aircraft designed for long periods of straight&level cruising) the wing incidence is chosen such that the rear fuselage is aligned to the airflow to reduce drag, and the airflow has significant downwash aft of the wings as a result of generating all that lift. Thus the aeroplane "sits" nose-up. This does mean that the forward fuselage has a significant angle of attack, but the drag produced by that is less than the drag that would be produced by the rear fuselage if flown with the fuselage in a level attitude. Obviously as the fuel is burned off the required wing AoA reduces, and that's why for VERY long ranges the optimum approach is to cruise-climb so that the required AoA increases at the same rate as the fuel-burn decreases it (if you see what I mean).
Someone mentioned using fuselage lift as a contribution to the overall lift - actually you don't want to do this. The reason is simply that the fuselage is very bad at developing lift because it has such a tiny aspect ratio. So each pound of lift (on old money) that you get from the fuselage produces about 30 times the induced drag that you'd get by developing the same amount of lift from the wings. So you really, really want the fuselage to be as close as possible to zero lift coefficient, and that could also be why the wing incidence is chosen to get zero AoA on as much of the fuselage as possible in the cruise.
PDR