To: helmet fire
There were actually two different problems described and both problems had a common thread. That thread was the movement of the ship during the breaking of ice. Under the icebreaking condition the ship was made to heel left and right by shifting water ballast and at the same time water was pumped forward and aft. This imparted a rolling motion. If you can imagine a disc that is spinning and then loses momentum and it starts to roll on its’ edges until it finally stops. It is this final rolling motion that that best describes the action of the ship. This motion is further exacerbated by the vertical movement of the aft end of the ship (where the flight deck is located) caused by the screws and the major vibration imparted during the ice breaking process.
In the first instance the helicopter is mounted on tricycle landing gear and it is disposed over the lateral axis of the flight deck. The movement of the ship causes the helicopter to pitch and roll on the oleo struts and some sideward movement on the tires (tyres). When the rotor is brought up to speed the blades will due to inertia hang back on the dampers. With the blades not being in a 120-degree relationship with each other the system will be out of balance. The design of this helicopter S-51 (HO3-S) placed the cg very high. This imbalance would further exacerbate the rolling motion caused by the ship and the helicopter could possibly go into ground resonance. That is why I stated that the pilots had to be prepared to lift off as soon as rotor rpm had built up.
In the case of the Bell HTL-1 the helicopter could be aligned with either axis of the ship. Under icebreaking conditions the helicopter would bounce on the floats and would also shift position on the deck caused by the constantly changing side loads on the floats. It did not move from one position to another, it just moved from side to side as the floats rolled in relation to the flight deck.
Ground resonance although not a major problem was still possible. The first time we fired the helicopter up under icebreaking conditions the blade was free to teeter. With the springs removed from the centrifugal clutch as soon as the engine started the rotor would start rotating. The first time under these conditions the blades were at their maximum teeter angle and when they started to rotate they tried to position themselves at the neutral teeter position and the forces generated almost tipped the helicopter over. After that, we held the blades in the mid position during start up. Mast bumping was not a problem with this rotor system as they had Sprague cables that limited the rotor movement in relation to the teeter axis.