EC145 swashplate and PC rods.


EC145 transmission, mast and swashplate assembly.


mast pole which is attached to the main transmission. Even at flat pitch (no collective input) there is a certain amount of lift designed into the blade to take some of the weight of the rotor blades and main rotor head mass off of the transmission. Once the rotor head and blades are turning, we will refer to them as a rotor disc. Now that we have a rotor disc turning, I will be bouncing back and forth between the rotor disc and the mast pole because there will be numerous forces working on both at the same time:


CENTRIFUGAL FORCE. It is the outward force that is proportional to the mass of the rotating body. Remember the blades that were bending downward toward earth when the rotor was static? Now, they are trying to break free of the head and sling themselves away from the center of the mast. Can you imagine the amount of force working on the main rotor blade bolts and the grip of the head? The blades must be strongly attached to the hub, but they also need the flexibility to change pitch. Back in the 60’s Bell Helicopter designed a Torsion-Tension (TT) strap for use on their main rotor heads. The straps are made of stainless-steel wire windings and covered with a durable elastic material that resembles a pair of elongated rubber bands with reinforced grommets on each end. One end is attached to the hub and the other to the blade. They are strong in tension and at the same time can twist (torsion) to allow the blade pitch change. It takes quite a bit of torque from the transmission,


through the mast to get all of this mass turning but now that it is turning at a predetermined speed, it takes less torque to keep it going. Here we are back to Newton’s First Law of Motion:


INERTIA. The tendency of an object in motion to remain in motion. At flat pitch, with the engine power turning the


main rotor transmission and rotor disc, there is very little resistance to slow down the rotor speed. It takes very little torque to keep the rotor speed up and the air


10 HelicopterMaintenanceMagazine.com December 2019 | January 2020


passing over the airfoil of the blade creates enough lift to ease the weight of the rotor head and blades off of the mast. But now, we want to fly. Let’s just go to a hover


first. The pilot gently pulls up on the collective lever, and through the genius of the invention called a “swashplate” we are able to collectively (equally) increase the pitch of the main rotor blades so that they are taking a deeper cut into the air above them. There are a few forces immediately taking place now. The increased wing incident of the blades causes the air to move faster over them creating lift. It also creates drag on the blades which translates to an increased torque on the mast. The engine must overcome this increased torque to maintain rotor speed so more fuel must be sent to the engine. This increased engine power is driving the transmission to its rated limit and the rotor disc is picking the helicopter up off of the ground. Let’s look at where we are at right now with forces. The rotor blades are now bending upward because of the low pressure on top and the higher pressure below them. They are also being forced outward due to centrifugal force. There are a number of other forces at work on the main rotor disc but let’s look at one important one.


ANGULAR MOMENTUM. The momentum of a disc around an axis. The best example I can think of for this is a toy gyro. If it is set spinning horizontally, it wants to stay horizontal no matter how you moved the support. But if you are determined to move the axis by force, the gyro moves in a strange way — at right angles to the applied moment. Here’s another example of this force. When I went to a motorcycle riders training class, I learned that when riding, if I wanted to turn right I should momentarily move the front wheel to the left and the wheel would immediately lean right and vice-versa. This trick made the motorcycle more responsive in turns. This gyroscopic property is due to the angular momentum of the wheel turning around the axle.


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