What It Looks Like

Designing an airplane is a long, technically arduous task driven by myriad goals: The airplane must perform as advertised, conform to minimum FAA certification standards, and be cost-effective for the manufacturer to produce. The designing process doesn't end once certification is achieved and the airplane is in production, either. Continued testing by the manufacturer, combined with customers' practical experience, often results in required modifications to the original design to correct existing problems or eliminate potential problems. The mass balance device depicted in the accompanying photo is one such modification.

The large white antenna-like part protruding from the top of the single-engine Comanche rudder (aft of an actual "cat's whisker" antenna) has an identical twin on the opposite side of the rudder. The matching horns are a set of weights perched out at the end of metal supports. Look closely and you can see that the weights are positioned ahead of an imaginary plane running through the hinge line where the rudder attaches to the vertical stabilizer.

This weight, or mass, serves to dampen any tendency of the rudder to "flutter," or vibrate rapidly from the effects of aerodynamic forces on the control surface. According to Aerodynamics for Naval Aviators, a highly regarded text that looks at aerodynamics and aircraft engineering from a pilot's perspective, the distribution of mass and stiffness in a structure, such as a rudder, determines certain natural frequencies and modes of vibration. Aerodynamic forces can interact with the structure to excite or negatively damp the natural modes of the structure and allow flutter. Flutter is a bad thing - the high-frequency vibrations can rapidly lead to failure of the structure.

A simple way to change the natural frequencies and modes of vibration of a control surface by altering the distribution of mass is to place additional mass ahead of the hinge line. That's exactly what Piper did in the early 1970s with its single-engine Comanche models. In response to a perceived problem with vibration in the rudder, Piper designed a mass balance device that could be attached to the top of the rudder. Though inelegant in appearance, it's an elegantly simple engineering solution to a potentially serious problem.