The Northrop YB-49 Yaw Damper

The Northrop YB-49 shares with the Boeing B-47 the distinction of being one of the first stability-augmented airplanes in the modern sense (Figure 20.2). Duane T. McRuer (1950) described the YB-49’s yaw damper as follows:

For the sensing part of the system, a Honeywell Autopilot rate gyro was chosen…. An electrical signal is then produced which is proportional to this speed or yaw rate. This signal is fed back through an electrical amplifier and reversible motor. Here the signal is transferred mechanically to a linkage that actuates the rudder cable system. The heavy work, that of opening the clamshell rudder to drag the wing back in line, then falls to the fully-powered rudder hydraulic system.

McRuer since added to this description the information that the reversible motor that put a yaw damping input in series with pilot’s inputs was a turbo-supercharger waste gate servo, as for the B-47. The long cable that runs from the cockpit to the hydraulic servo valves on the clamshell rudders was expected to serve as a backup for the series-installed yaw damper servos. Unfortunately, initial yaw damper actuator motions stretched the cables until the hydraulic servo valve friction was overcome. This created a dead spot until corrected by a reduction in hydraulic valve friction.

McRuer and Richard J. Kulda made the preliminary stability analysis by the method of equivalent stability derivatives, used in the literal approximate factors for the spiral and Dutch roll modes. The detailed design used Bode and Nyquist diagrams, much as in the case of the B-47. The YB-49’s yaw damper had no washout to cancel the yaw rate signal in steady turns. Compared with current practice, the five weeks or so that it took to design, round up parts, install, and check out the YB-49’s yaw damper is of course quite short.