The decision to raise the main rotor had nothing to do with vibrations. It had everything to do wit a 20-25 kt deficiency at the high speed level flight end of things, compared with predicted performance. An aero performance expert performed a detailed analysis which indicated the culprit was the upflow of air over the relatively flat sloped cockpit front, which was stalling the entire inner front part of the rotor disc. He produced some dramatically impressive, wall sized computer visual flow mosaics “ proving “ his theory. Big meeting and a decision to raise the rotor to gain back the predicted speed. A monumental effort, including a full scale fatigue test to the modified main shaft, resulted in flying the extended shaft 30 days after the meeting.
The first flight test ( done at design GW ) forward flight Vh ( max speed at 100%Q-2800 SHP then ) revealed an increase of exactly zero knots. BUT, the 5P vibes ( not a typo ) were reduced dramatically and the raised rotor also eliminated an “ elephant in the room “ issue with regard to main blade to cockpit/engine nacelle/tail cone clearance* in operational/maneuvering flight conditions and victory was declared. Said engineer departed the company some months later.
*there is a picture in the SA archives showing the cockpit view from behind with yours truly and Dick Wright, then Chief Pilot. On the top of the instrument panel is a horizontally oriented instrumentation box showing real time blade clearance over the cockpit, engine nacelles and tail cone. There were four modified Chicago Aerial electronic blade tracker sensors feeding blade clearance data to that box and thru the telemetry system to the flight test engineers. I’ll add one early data point: just doing low speed flight in the Stratford flight field, and slowing down normally back to a hover from a paced forward flight at 35 kts, the clearance to the cockpit roof structure was down to 5 inches. There is a great story about how that box got there, but you won’t find it in the book.