Needed Devices Are Not Installed

In spite of an evident need for redundant, irreversible power controls and electronic series-type stability augmenters, these devices were rarely used in the early jet aircraft. Stability and control designers and their chief engineers were quite justifiably reluctant to do so for reliability reasons, but also to avoid high cost and weight penalties. What was done instead? Some stability and control case histories of jets in that awkward age are given in what follows.

The case histories are of interest not only as history but as cautionary tales for the stability and control designers of future advanced general-aviation aircraft. These case histories tell mostly of failure and shortcomings. But since this poor record was made by some of the brightest stability and control designers of the 1950s, future designers of high-performance aircraft should be wary of trying to avoid irreversible power controls and series-type stability augmentation, in the name of simplicity and cost saving.

7.1 F4D, A4D, and A3D Manual Reversions

In the first two of these early Douglas jets, hydraulic power-assisted controls were indeed used in the original layouts, but only in single channels. That is, in the not-infrequent case of a flight failure of some part of the hydraulic system, the pilot could pull an emergency lever that disconnected the hydraulics. Control would revert to ordinary manual connection of the stick and pedals to the control surfaces.

Thiswasfine if the failure wasa jammed hydraulic valve that would interfere with control. However, there were a few regrettable cases in which a pilot incorrectly diagnosed a control difficulty as a hydraulic system failure and made a one-way reversion to manual control. For cases in which this happened at high airspeeds, where manual control was only marginal, manual reversion made a bad situation far worse. Redundant hydraulic power controls, in which several hydraulic actuators in parallel supplied the needed hinge moments, could not come soon enough for the fast jets.

Dual aileron hydraulic boost was used for the A3D Skywarrior (Gunston, 1973), but only to avoid excessively high boost ratios. Two 20:1 boost systems were used instead of one 40:1 boost. One-way manual reversion was used, as in the other two Douglas jets. Because of the A3D’s relative large size, even emergency manual control was not possible without a shift in wheel-to-aileron gearing. For manual reversion the pilot shifted gears by 2:1, requiring twice as much wheel throw for a given aileron deflection.