Failure - Interactions
These factors would be cited in other cases of inflight upset in later decades.
Within the Boeing Company, Eddie Allen and George Schairer worked in the wind tunnel to discern the design variables affecting the human's control of yaw-rate, while operating at combinations of high alpha and beta. Testing involved various iterations of sizing-shapes of the Fin-Rudder, Hinge placement, and resulting rudder Hinge Moment (thus avoiding non-linear Pedal-force curves).
Schairer's paper was presented at the Aerodynamics Session of the IAeS- gathering in NY, January 31st, 1941. Skip ahead two pages to read excerpts from George Schairer's work, describing lesson learned and design modifications [the revolution in Fin-Rudder design after 1939].
During the preflight meeting to confirm the Test Plan, Von Baumhaure (a visiting Dutch engineer) said he wanted to see what took place at 110 to 115 mph when one outboard motor was cut and Pedals were held-in-position, and when completely stalled.