Classical Versus Canard Configurations

A classical configuration consists of the main wing in the front and the tail in the back. In a “canard” configuration, the two lifting elements are reversed (see for example the Wright Brothers’ flyer). The common wisdom is that in a classical configuration, the tail has a negative lift, whereas with a canard design, the forward control surfaces have positive lift. This makes the canard a desirable configuration for a heavy lift airplane. Many teams have used this design configuration, but only

few have succeeded in having a stable airplane. See Fig. 11.13 for illustration of the difference in cruise. In Fig. 11.13a the moment of the main wing about the center of gravity is a nose down moment that needs to be balanced by a nose up moment from the tail. With the canard, Fig. 11.13b, the same situation requires a nose up moment from the forward lifting elements, which provide a positive lift.

However, it is possible to design a classical configuration in such a way that the tail will be lifting at take-off. The key point is to oversize the tail design, which is not a significant empty weight penalty, since it can be made out of light materials and it will lift more than its own weight. In this option, one requires that the center of gravity of the airplane be located aft of the main wing quarter-chord (main wing aerodynamic center). To do so, and still insure that the necessary static margin (SM) is preserved, the tail needs to be quite large, in order to move the aerodynamic center of the complete configuration sufficiently downstream. At take-off, when the airplane is fully loaded, the incidence is quite large, say 15° or so, and as we know from thin airfoil theory, the main wing lift force will move close to the main wing quarter-chord. As the airplane rotates to take-off, the lift force moves towards the quarter-chord and passes in front of the center of gravity which creates a nose up moment that must be balanced by the positive lift force of the tail, see Fig. 11.14a. But in cruise, the main wing lift force moves back past the center of gravity as the airplane speed has increased and the incidence decreased, requiring a negative lift force on the tail, Fig. 11.14b.

The equilibrium code is used to size the tail in order to achieve the above result. From the pilot point of view, flying such a configuration did not make any difference in handling qualities.