Optimum cruise lift coefficient in initial design of jet aircraft

Abstract

shows the average lift during the ramp-down motion is the dominant factor in the decline of the total average lift at high amax. In the analysis in Ref. 1 a dynamic lift augmentation of AL = 1.5 was assumed, and this resulted in an ALV with a near 20% improvement in straight-and-level range over the conventional baseline vehicle. In Fig. 3c, an augmentation of AL = 1.5 was achieved at the highest pitch rate of K = 0.2 and a0 = 0 deg and for both cases of K = 0.05 and 0.1 for starting angles of aQ = 10 deg. The asssumption of Ref. 1 concerning the existence of augmentation parameters near 1.5 and above would appear then to have some justification. Any benefits that might be gained from augmented lift must be weighed against the penalty levied by the increase in dynamic drag loading. Figure 4 shows the total time-average drag coefficients for each test motion. Figure 4 clearly shows that decreasing amax is advantageous from the standpoint of average drag reduction. From Fig. 3c at the pitch rates of K = 0.05 and 0.1 and starting angles of 10 deg, a value of amax = 35 deg results in nearly optimum lift augmentation with respective AL values of 1.5 and 1.63. The corresponding average drag coefficient is seen in Fig. 4 to be near 0.7, which by some standards is large. Notice, however, that the average drag decreases rapidly with decreasing amax, and for amax = 25 deg and pitch rates of K = 0.05 and 0.1, the average drag coefficient has dropped to 0.3 and 0.24, respectively. At the same time, the lift augmentation remains well above unity and, from Fig. 3c, has values of 1.38 and 1.34, respectively. Thus for the rate of K = 0.1, decreasing amax from 35 to 25 deg results in a drop in the average lift of 18% (though still maintaining significant lift augmentation), whereas the average drag decreases over 60%. In the present study the pitch rate for the ramp-up and ramp-down motions was the same. For a stopping angle of amax = 25 deg the data of Fig. 3a indicate that the average lift during ramp up generally increases with pitch rate, whereas in Fig. 3b the average lift during ramp down decreases with pitch rate. There may then be some advantage in ramping up at a high rate followed by ramping down at a lower rate. In the motions studied in Ref. 1 the rate during pull-up was lower than that for pitch-down. Maintaining acceptable drag loading may be a limiting condition for defining airfoil motions for the purpose of utilizing augmented lift.