Thanks to SAS for a PM- could not recall the name of what was, in the CH-47A I flew from 1963-65 called the Differential Collective Pitch system, whose function was similar to the Pittch Bias Actuator in the S-76. The early UH-60’s had them as well, until the Army decided to take them out because: 1) they were a maintenance headache, and )2 the pilots couldn’t tell whether it was on or off anyway ( something we had told them a long time before that ).
The speed trim really wasn’t put in to provide a level fuselage attitude-that was only partly true-the other part was that it was there to reduce the main rotor shaft bending loads at speed and thereby allow for a higher shaft component replacement time. You might recall some difficulties the USMC had with pilots using the CH-46 speed trim manually to control attitude during approach to an LZ in Vietnam. Think they had at least one shaft failure as a result.
( Background: after flight school I had wangled an assignment to the Test Board at Ft Rucker. The prototype 47A’s they had ( 3 of them ) had been landing at various spots around Ft Rucker, and maybe it was the landing on the golf course* that got me, a 2nd LT into getting a Chinook checkout, and by two of the Boeing Test Pilots to boot, as they were there to assist the program. One could ask them questions and get very straight and detailed answers. It was ironic that after a short tour there and a visit to SE Asia, I wound up at Sikorsky, and while the CH-53A could fly rings around the CH-47A I’d get into some “interesting” conversations over the years with SA folks who failed to pay attention to the gradual and effective improvement programs the Army and Boeing effected into the Chinook product line.
*there were two explosive failures of the nose gearbox, traced to a gear resonance and which caused nose gear box failure and parts etc being ingested into that side’s engine with very noisy further results. Another ship had what was referred to as floating SAS links, resulting in a ship rolling on its side. Lastly, there was an aft shaft bearing failure, in which the crew was able to get it on the ground on the north part of Cairns AAF, but th blades hit the tunnel and the engine controls were cut, so they were shut down via a fire truck directing their turret into the inlets, one at a time. I happened to be landing at the north helipad and folks were running across the path to my tie down in front of me, so I turned the D model around and watched all of this one. No injuries. Anyway, the field grade types started finding other things to do. Except one: we had a civilian Joe Givens who was an O-6 in a USMC fighter sad at NAS New Orleans and he still flew them. Turned out later that this Joe Givens was in the same F4U-4 squadron in the S. Pacific with Byron Graham, my future boss as Ch Exp pilot at Sikorsky. Sorry, had to keep the record straight re field grades.