Optimization of constant altitude-constant airspeed flight of turbojet aircraft

Then we can deduce: 1) y(y) is not negative for all y because of the inequality, Eq. (11), and conditions on y(y)\ that is, -y(O) > 0 and y(l) = 0; 2) the displacement of the trailing edge h(y) is positive, as far as y(y) is positive (Fig. 1); and 3) Eq. (8) implies that c(y) is positive, because both y(y) and h(y) are positive. This deduction leads to the conclusion that Eq. (10) is also the sufficient condition of positive c(y). Hence, we obtain the optimum semispan ratio cropt and the optimum drag ratio