The initial B707 Rudder-design was for a manual Tab, a "balance" Tab: no hydraulic-boost.
The Rudder Pedals were connected to the Tab, the Tab then flew the Rudder.
Initial Fin-Rudder design: a shorter-height VertStab, Rudder -- manual, spring- Tab [no hydraulic boost].
The early B-707 Rudder was operated only by aerodynamic forces on the TAB and by differential pressures acting on the BALANCE PANELS ahead of the rudder Hinge Line on the fin.
Per _Flight_, 25July58, p145:
"Flying Controls Internal balance plates, aerodynamic tabs . . . are the basis of the Boeing 707's mainly manual control system, which also provides proportional feel, but rudder boost is used on the 707-220 and Intercontinental models for maximum rudder deflection in the low-speed, engine-out case...."
. . . the beginning of rudder "blowback," where the rudder hinge moment is overcome by slipstream forces and causes the rudder to streamline . . .
The various Fin-Rudder modifications continued through the next decade, with the Rudder-TAB rotation redesigned to an Anti-Balance function, with various "boost" iterations [from the initial manual-Tab to a "power augmented rudder" then later a "full range boosted Rudder"] , and various iterations of Rudder-Tab ratios. The artificial stability was added: first effort was the "parallel" Yaw Damper, then after AA Flt-One, a redesigned "series" Yaw Damper.
But what was the real cause of all the loss of control training accidents?
Investigators of the 1960's, '70's, early '80's, would have you believe that it was simply a case of pilot error.
As we now know it [training mishap fatalities] were a result of poorly designed and improperly certificated airplanes ... attempting to fly with thrust-asymmetry [engine failure] at too low speeds.
This hindsight conclusion is not meant to cast any aspersions on anyone . . . but only to put into perspective the evolution of the design of jet transports. . . . To understand this, it is necessary to review just how the aircraft were certificated for minimum control speed.
Marthinsen, H.F. "Engine-Out Flight Training Revisited". Presented at ISASI's 13th Int'l Seminar, Tel Aviv, Oct' 11-15, 1982; ISASI Proceedings 1982, p46-63.
Probable Cause The Board determines the probable cause of this accident was the crew's failure to recognize and correct the development of excessive yaw which caused an unintentional rolling maneuver at an altitude too low to permit complete recovery.
[C.A.B.'s AAR, p11.]
By February 1960, more than a year before the C.A.B.'s accident report was adopted, the manufacturer had initiated Fin-Rudder modifications:
Failure - Interactions
Details of the C.A.B.'s "Peconic" investigation . . . [in-progress, standby ... ]
A public hearing was ordered by the Board and held in two phases. The operational phase of the investigation was held at the Henry Perkins Hotel, Riverhead, Long Island, New York, on August 27, 1959. The technical phase of the investigation was held at the Forest Hills Inn, Forest Hills, Long Island, New York, October 7, 1959.
Press story reported on first day of Public Hearing, Thursday, August 27th 1959:
Thrust asymmetry, yaw, roll.