MAXIMUM LIFT

Airfoil theory based on potential flow methods predicts the lift of an airfoil in its linear range but does not provide any information concerning maximum lift capability. As discussed previously, Cimiu, is determined by flow separation, which is a “real fluid” effect. Separation is difficult to predict analytically, so the following material on Qmax is mainly empirical.

Typically, conventional airfoils without any special high-lift devices will deliver C|mai values of approximately 1.3 to 1.7, depending on Reynolds number, camber, and thickness distribution. The appreciable dependence of Cimiu on Я shown in Figure 3.12 for the GA(W)-1 airfoil is typical of other airfoils. Figure 3.23 presents Qmia as a function of R and thickness ratio for NACA four-digit airfoils having a maximum camber of 2%, located 40% of the chord back from the leading edge. At intermediate thickness ratios of around 0.12, the variation of C(m„ with R parallels that of the 17% thick GA(W)-1 airfoil. Note, at least for this camber function, that a thickness ratio of 12% is about optimum. This figure is taken from Reference 3.14. This same reference presents the following empirical formula for C/max for NACA four – digit airfoils at an R of 8 x 106.

„ „ (0.123 + 0.022P – 0.5z – tf

-2.6 рз

t, z, and p are thickness, maximum camber, and position of maximum camber, respectively, expressed as a fraction of the chord. For example, for a 2415 airfoil,

t = 0.15 z = 0.02 p =0.40

so that according to Equation 3.42,

For a plain wing (unflapped), there is little effect of aspect ratio or taper ratio on Cw Even the presence of a fuselage does not seem to have much effect. As the angle of attack of. a wing increases, is reached and any further increase in a will result in a loss of lift. Beyond the wing is said to be stalled. Although taper ratio does not significantly affect the overall wing Cw it (and wing twist) significantly affect what portion of the wing stalls first. As the taper ratio is decreased, the spanwise position of initial stall moves progressively outboard. This tendency is undesirable and can be compensated for by “washing out” (negative twist) the tips. One usually wants a wing to stall inboard initially for two reasons. First, with inboard stall, the turbulence shed from the stalled region can shake the tail, providing a built-in stall warning device. Second, the outboard region being unstalled will still provide

aileron roll control even though the wing has begun to stall. The lift charac­teristics of three-dimensional wings will be treated in more detail later.