In the previous discussion, the one speed or angle of attack concept was used to explain how longitudinal stability was attained. It is important for the pilot to know that the airplane is stable at various speeds or angles of attack, not just one. The controls, which allow the pilot to depart from the one speed or angle of attack concept or the controls used to give the pilot longitudinal control around the lateral axis, are the elevators and the elevator trim tab. [Figures 1-25 and 1-26]

Figure 1-25.—Effect of elevators.

The function of the elevator control is to provide a means by which the wing’s angle of attack may be changed.
On most airplanes the elevators are movable control surfaces hinged to the horizontal stabilizer, and attached to the control column in the cockpit by mechanical linkage. This allows the pilot to change the angle of attack of the entire horizontal stabilizer. The horizontal stabilizer normally has a negative angle of attack to provide a downward force rather than a lifting force. If the pilot applies back elevator pressure, the elevator is raised, increasing the horizontal stabilizer’s negative angle of attack and consequently increasing the downward tail force.

This forces the tail down, increasing the angle of attack of the wings. Conversely, if forward pressure is applied to the elevator control, the elevators are lowered, decreasing the horizontal stabilizer’s negative angle of attack and consequently decreasing the downward force on the tail. This decreases the angle of attack of the wings. [Figure 1-25] The elevator trim tab is a small auxiliary control surface hinged at the trailing edge of the elevators. The elevator trim tab acts on the elevators, which in turn acts upon the entire airplane. This trim tab is a part of the elevator but may be moved upward or downward independently of the elevator itself. It is controlled from the cockpit by a control which is separate from the elevator control. The elevator trim tab allows the pilot to adjust the angle of attack for a constant setting and therefore eliminates the need to exert continuous pressure on the elevator control to maintain a constant angle of attack.

Figure 1-26.—Effect of trim tabs.

An upward deflection of the trim tab will force the elevator downward with the same result as moving the elevator downward with the elevator control, and conversely a downward deflection of the trim tab will force the elevator upward. The direction the trim tab is deflected will always cause the entire elevator to be deflected in the opposite direction. [Figure 1-26]