. Possible Configurations

The possible solutions for a suitable configuration are readily discussed in terms of the requirements on Cmo and dCJda. We state here without proof (this is given in Sec. 2.3) that dCJda can be made negative for virtually any combination of lifting surfaces and bodies by placing the center of gravity far enough forward. Thus it is not the stiffness requirement, taken by itself, that restricts the possible configurations, but rather the requirement that the airplane must be simultaneously balanced and have positive pitch stiffness. Since a proper choice of the CG location can ensure a nega­tive dCJda, then any configuration with a positive Cmo can satisfy the (limited) con­ditions for balanced and stable flight.

Figure 2.5 shows the Cmo of conventional airfoil sections. If an airplane were to consist of a straight wing alone (flying wing), then the wing camber would determine the airplane characteristics as follows:

Negative camber—flight possible at a > 0; i. e., CL> 0 (Fig. 2.3a).

Zero camber—flight possible only at a — 0, or CL = 0.

Positive camber—flight not possible at any positive a or CL.

For straight-winged tailless airplanes, only the negative camber satisfies the con­ditions for stable, balanced flight. Effectively the same result is attained if a flap, de­flected upward, is incorporated at the trailing edge of a symmetrical airfoil. A con­ventional low-speed airplane, with essentially straight wings and positive camber, could fly upside down without a tail, provided the CG were far enough forward (ahead of the wing mean aerodynamic center). Flying wing airplanes based on a straight wing with negative camber are not in general use for three main reasons:

1. The dynamic characteristics tend to be unsatisfactory.

2. The permissible CG range is too small.

3. The drag and CLmm characteristics are not good.

The positively cambered straight wing can be used only in conjunction with an auxiliary device that provides the positive Cmo. The solution adopted by experi­menters as far back as Samuel Henson (1842) and John Stringfellow (1848) was to add a tail behind the wing. The Wright brothers (1903) used a tail ahead of the wing (canard configuration). Either of these alternatives can supply a positive Cmu, as illus­trated in Fig. 2.6. When the wing is at zero lift, the auxiliary surface must provide a nose-up moment. The conventional tail must therefore be at a negative angle of at­tack, and the canard tail at a positive angle.

An alternative to the wing-tail combination is the swept-back wing with twisted tips (Fig. 2.7). When the net lift is zero, the forward part of the wing has positive lift, and the rear part negative. The result is a positive couple, as desired.

A variant of the swept-back wing is the delta wing. The positive Cmo can be achieved with such planforms by twisting the tips, by employing negative camber, or by incorporating an upturned tailing edge flap.