Tail Location, Size, and Shape

Horizontal and vertical tails are commonly located about a wing semispan behind the center of gravity. While horizontal tail sizes normally range from 15 to 30 percent of the wing area, the actual size is a complex function of desired center of gravity range, ground effect, and other factors. There is a minimum tail size that will trim a neutrally stable airplane at maximum lift in ground effect. Horizontal tails that are larger than this absolute minimum permit a useful operational center of gravity range.

Optimization theory has been proposed to size horizontal tails for particular center of gravity ranges, considering actuator rate and amplitude and flying qualities constraints. A particular application (Kaminer, Howard, and Buttrill, 1997) starts with a particular horizontal tail volume. Then, the most aft center of gravity location and feedback gains are found that (1) put longitudinal short-period eigenvalues into a region of MIL-STD 1797 Level 1 or 2 flying qualities and (2) do not exceed actuator rate or amplitude limits in response to a severe vertical gust. The problem as stated has reasonable solutions. The method, although involved, may be useful in preliminary design.

There seems to be no upper limit to desirable vertical tail size from a stability and control standpoint, but vertical tails that are too small lead to a variety of undesirable characteristics. For example, airplanes with low weathervane stability require heavy coordinated rudder – aileron inputs when beginning and stopping turns, especially at low airspeeds. When Walter Brewer, Professor Otto Koppen’s former student, brought the Curtiss XSB2C-1 wind-tunnel model to the Wright Brothers Wind Tunnel at MIT in 1939, Koppen said, “If they build more than one of those things, they’re crazy,” and further, “You don’t need wind-tunnel balances for data, all you need is a record player under the tunnel saying, ‘put on a bigger vertical tail!’”

The necessity of a powerful rudder for recovery from erect and inverted spins led to notched elevators to allow full rudder deflection in either direction. The much neater and lower drag solution of the P-51 Mustang, in which the rudder hinge line lies behind the eleva­tor trailing edge, seems to have occurred independently to designers at Focke-Wulf and was rapidly adopted by other designers. Aerodynamic damping in pitch and yaw is proportional to the square of the tail arm. Since the damping of the Dutch roll oscillation is inherently poorer than the short-period pitch oscillation, it is better to have a longer vertical tail arm.

Before the constraints on vertical tail function were well understood, airplane manufac­turers built vertical tails in distinctive shapes. L. Eugene Root, then at Douglas El Segundo, changed all that with a U. S. patent that describes straight-tapered tail surfaces with leading edges and hinge lines all at a constant percentage chord.