Control Friction and Apparent Spiral Instability
A spirally stable airplane, when disturbed in bank angle from wings-level flight, will return on its own to wings-level flight, although on a different heading than it had before the disturbance. On the other hand, the bank angle of a spirally unstable airplane will increase without limit after an initial disturbance. A spirally unstable airplane’s bank angle must be corrected continuously to maintain wings-level flight. However, for the usual case
Figure 15.4 Time history of a precision instrument approach in light turbulence, using ILS, for an unspecified general-aviation airplane. This model has low Dutch roll damping and other stability and control deficiencies. The pilot uses large stick and rudderpedal forces, but airspeed, heading and path angle variations are excessive. (From Barber, Jones, Sisk, and Haise, NASA TN D-3726, 1966)
of moderate spiral instability, with times to double amplitude of the order of 20 seconds, corrections are made instinctively. Pilots are generally unaware of the instability.
Therefore, it is not surprising that Ralph Upson’s 1942 set of objectives for a safe personal airplane do not include spiral stability. That is, if one were only to make airplanes “as easy to fly as cars are to drive,” spiral stability would not necessarily be an objective. Yet, positive spiral stability has over the years been of interest to personal-airplane designers.
One reason for this is that Federal Aviation Regulations for airplanes operating under Visual Flight Rules (91.205) do not require gyroscopic rate of turn indicators. Without one of these instruments the pilot of an airplane that blunders into clouds has no way of maintaining wings-level flight, unless the airplane happens to be spirally stable. In that case, freeing the controls prevents a “graveyard spiral.” Another reason for spirally stable personal airplanes is to enable a solo pilot to be able to read a map without finding the airplane in a bank upon looking up.
Unfortunately, an inherently spirally stable airplane can appear to be spirally unstable with rudders and ailerons free, as a result of control friction (Campbell, Hunter, Hewes, and Whitten, 1952). To correct this, NACA researchers designed a rather complicated controlcentering device to overcome friction without interfering with normal control activity.
A cylindrical barrel encloses two preloaded compression springs and a shaft passing through the barrel. A shoulder on the shaft and corresponding shoulders on the inside of the barrel are at its midlength. A flat circular pickup ring under the end of each spring is forced against both shoulders with a force equal to the spring preload. The shaft cannot move relative to the barrel without moving one of the pickup rings and consequently compressing one of the springs. Campbell’s group installed the preload barrels in both the rudder and the aileron control systems of a Cessna 190. An electric motor provides rudder trim, a jack screw provides aileron trim, and solenoids engage or disengage the preload devices (Figure 15.5).
Without the centering devices, the airplane diverged in the direction of a rudder kick, after a kick and release of all controls. However, the centering devices allowed the Cessna’s inherent spiral stability to take effect. After a rudder kick and release of all controls the airplane returned to wings-level flight, and would continue so indefinitely.