What It Looks Like

Why is it that the elevator on some airplanes is shaped somewhat like a squared-off C when viewed from above, with a "horn" area on each end that extends into the fixed horizontal stabilizer, while others do not have these extensions?

The reason is aerodynamic balance. The engineers who designed the airplane wanted pilots to be able to pitch the airplane nose up and nose down using the proper amount of control force - not too much, but not too little either. One way aircraft designers tailor control forces is to employ some sort of aerodynamic balance on the control surfaces. The horn-type balance is one fairly common design.

When you pull back on the yoke to initiate a climb, you're causing the elevator to deflect upward. The slipstream pushes against the elevator surface, forcing the tail down and the nose up. It stands to reason that the amount of force it takes to deflect the elevator should increase as airspeed increases. That's desirable because it inhibits abrupt, gross changes in angle of attack at higher airspeeds, which could overstress the airframe.

It is not desirable, however, to have it be too difficult to push or pull on the yoke to pitch the nose down or up. If the control surfaces are too difficult to move, the pilot may be fatigued by the effort, or worse yet, unable to control the airplane. The way to fine tune control forces is to use aerodynamic balance.

Notice that the horn portion of the control surface is ahead of the hinge line where the movable elevator attaches to the fixed horizontal stabilizer. When the elevator de- flects up, the horn deflects down, and vice versa. The force of the slip- stream acting on the horn helps to counteract some of the force of the slipstream acting on the main portion of the elevator, thereby lessening the amount of force the pilot must use to push or pull on the yoke. Aerodynamic balances can help impart pleasing flying qualities to an otherwise truckish airplane.

Aerodynamic balances also serve to increase the hands-off stability of an airplane by inhibiting the tendency of a control surface to "float" with changes in angle of attack. For example, if a gust caused the airplane to pitch nose up, the elevator could streamline in the slipstream, allowing the nose-high attitude to continue. The aerodynamic balance fights this tendency, causing the airplane to naturally return to the attitude for which it was trimmed before it was moved by the gust.