Partial Power Control

Another control system compromise made during the jet’s awkward age was to try to get by with direct manual control for one or more surfaces. The Douglas F4D Skyray’s rudder was a good example. The F4D was a small, single-engine jet whose demands for rud­der controllability seemed minimal. Of course, there were no asymmetric power conditions to consider. Rudder control in cross-wind landings and takeoffs and to make coordinated turn entries and recoveries was shown to require only modest amounts of rudder deflection and pedal force.

The F4D could be spun, and was required to have good spin recovery characteristics. Ordinarily, this would require full rudder in opposition to the spin, and the corresponding pedal force for a manually controlled rudder would be high. However, the F4D’s inertia distribution made the elevons the primary spin recovery control. The rudder, in any case fully shielded from the airflow by the wing at spin attitudes, could be in any position without affecting spin recovery

All was fine until F4D test pilot Robert O. Rahn inadvertently entered an inverted spin. The rudder was now unshielded. The air flow direction in the spin drove the rudder in the pro-spin direction. Not only that, but the unshielded rudder’s effectiveness in the inverted spin was high enough to require that it be deflected in the opposite, or anti-spin, direction for a satisfactory recovery. With no hydraulic power assistance, the best Rahn could do with an estimated 300 pounds of pedal force was to neutralize the rudder (and then use the emergency spin chute for recovery). This unanticipated demand for rudder deflection meant that the original decision to save the cost and complexity of hydraulic power for the rudder was not justified.