Wing levelers are single-axis automatic pilots typically used in general-aviation airplanesto prevent spiral divergence. John Campbell’s1952 NACA aileron centering device flown on a Cessna 190 was improved upon a few years later by another NACA group, which converted it to a wing leveler (Phillips, Kuehnel, and Whitten, 1957).
The Phillips device was an aileron trimmer, added to the earlier aileron centering system. The aileron trim point was moved at a slow rate by the output ofa yaw rate gyro (Figure 15.6). The slow 1.5-degree-per-second trim rate left the Dutch roll oscillation unaffected. The Cessna’s flight records showed that the trimming device could maintain a safe bank angle indefinitely, and even could provide a wheel-free recovery from banked attitudes, or effective positive spiral stability Phillips noted (1998) that NACA headquarters failed to file a patent application for the wing leveler, although a patent disclosure had been made. Without coverage by a valid patent, light-plane and autopilot manufacturers would not consider marketing the device.
Modern versions of the Phillips wing leveler device are available. Century Flight Systems, Inc., which started out as Mitchell, became Edo-Aire Mitchell, and finally split off as Century, produces the Century I wing leveler. This device is identical in principle to the Phillips wing leveler flown on the Cessna 190, with yaw rate gyro signals sent to an aileron servo. The ability to command turns and to follow a CDI (Course Direction Indicator) for VOR (VHF Omni-Directional Range), ILS (Instrument Landing System) localizer, or GPS (Global Positioning System) tracking has been added, making the Century I a simple autopilot.