The force to change the direction of this angular momentum on a helicopter rotor system dictated the need to design and develop the swashplate. I’m going to include Wikipedia’s definition and
description here because I don’t think that I can describe it any better myself: A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to translate three of the pilot’s commands from the non-rotating fuselage to the rotating hub and blades. It consists of two main parts: a stationary swashplate and a rotating swashplate. The stationary (outer) swashplate is mounted on the main rotor mast and is connected to the cyclic and collective controls by a series of pushrods. It is able to tilt in all directions and move vertically. The rotating swashplate (inner) is mounted to the stationary swashplate by means of a bearing and is allowed to rotate with the main rotor mast. It is connected to the mast via a drag link. Both swashplates tilt up and down as one unit. The rotating swashplate is connected to the blades via pitch change rods. The controlled tilting of the swashplates creates the forces on the blades to change the direction of the rotor disc. Ok, we have this spinning rotor disc, attached to and
driven by the mast pole of the main transmission totally supporting the entire weight of the helicopter fuselage. Now we want to move the helicopter in some direction. The pilot moves the cyclic control stick in the desired direction, which through the flight control linkages, tilts the swashplate and changes the pitch of the main rotor blades. To move the helicopter forward, more pitch is put into the rear of the rotor disc and less in the front. You may notice that I didn’t say more pitch was put into the rear blade, I said the disc, because as you may remember, the force to change the pitch must be put in at a right angle. Anyway, overcoming the gyroscopic effects, the disc now wants to change position, relative to its axis (the mast) and that change must be dealt with by the type of main rotor head utilized. The rigid rotor (sometimes called a hinge-less)
that is rigidly attached to the mast pole is capable of transmitting high bending forces to the mast. When a pilot makes a cyclic movement causing the main rotor disc to tilt, the fuselage wants to follow. In flight, the mast bending moment is low. However, when the fuselage is in contact with the ground and cannot follow the main rotor disc, the bending moment can be very high. These high bending forces can damage a mast, so manufacturers have set limits that must be monitored and recorded. Calibrated strain gages are attached to the inside of the hollow mast and a Mast Moment Indicator
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HelicopterMaintenanceMagazine.com December 2019 | January 2020
is mounted on the instrument panel that shows the pilot the total moment being applied to the mast. Exceeding certain limits are cause for special maintenance inspections or even replacement of the mast. The hinged (or semi-rigid) type rotor head can teeter on the mast pole, like a see-saw, which allows the disc to dip forward and rise in the rear. This teetering action decreases the bending forces on the mast pole. There are mechanical stops to limit the teetering action of this head in relation to the mast pole. One negative feature of this teetering action is called mast bumping. It is generally a result of a pilot induced over-controlling of the cyclic leading to a negative G situation. This negative G can also be caused by other factors such as severe turbulence, landing on an excessive slope or a rapid lowering of the collective. Mast bumping is a result of the helicopter main rotor head making contact with the main rotor mast. The head literally “bumps” the mast and depending on the severity of the bump, can damage or snap it off. This rarely occurs when helicopter is flown within its limitations. I know that I have only “scratched the surface” so to
speak, when I describe the hostile environment that the rotor system is exposed to and the extreme demands that are put on the materials that the rotor system is constructed from. You need to respect the designers and engineers for their technology. That is why I have to look back every once in a while and remember the awe I felt when I first started working on helicopters. I guess we all get a little complacent after a few years, but as mechanics, we all need to remember what this system goes through when we are doing inspections or performing maintenance. We must look closely for surface scratches that can propagate into cracks. We must look deep for that corrosion that is indicating deterioration of the materials. We must recognize unusual wear patterns on components and replace when beyond their limits.
It is our duty to see that all of the components stay in
formation. May the forces be with us.
Terry L. Peed has been working in the aviation industry for more than 49 years. He is a Vietnam era Navy veteran and is a graduate of Embry-Riddle Aeronautical University with an Associate of Science degree in Aviation Maintenance. Peed
holds an Airframe & Power plant certificate with Inspection Authorization. He is employed by Metro Aviation Inc. as a field base mechanic at the University of Wisconsin in Madison, Wisconsin. Email
terrypeed@yahoo.com.
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