Helicopter rotor head

Helicopter main rotor system

Introduction

Lastly I have presented totally new engine concept as a upcoming upgrade for personal helicopter device P01 in One Man Helicopter project for Unreal Engine 4. Based on Rotax engine technology at models 912, 914 and 915 from F to iS type. Today I would like to introduce you to main rotor concept before moving to design transmission system. 

Main rotor system

Rotorhead or main rotor is like a heart of all rotorcrafts. The part of the rotor assembly that joins the blades to the shaft, cyclic and collective mechanisms passed by swashplate. Powered by the engine, through the transmission, to the rotating mast holded by Jesus nut in some helicopters. Blade pitch variations are controlled by tilting, raising, or lowering the swashplate with the flight controls. The vast majority of helicopters maintain a constant rotor speed (RPM) during flight, leaving the angle of attack of the blades as the sole means of adjusting thrust from the rotor.

Swashplate

The swash plate is two concentric disks or plates. One plate rotates with the mast, connected by idle links, while the other does not rotate. This rotating plate is also connected to the individual blades through pitch links and pitch horns. The non-rotating plate is connected to links that are manipulated by pilot controls—specifically, the collective and cyclic controls. The swash plate can shift vertically and tilt. Through shifting and tilting, the non-rotating plate controls the rotating plate, which in turn controls the individual blade pitch.

Swashplate

Types

All helicopters have at least one main rotor to produce the lift that keeps helicopters in the air. Type of main rotor system is defined by how the blades are connected to the rotor hub. The blades may have hinges or bearings to allow them to feather, flap and lead/lag. Depending on what movement the rotor system allows, characterizes it as one of the following:

  • Rigid
  • Semi-Rigid
  • Fully Articulated

Rigid

Developed by Irv Culver and usually refers to a hingeless rotor system with blades flexibly attached to the hub. In a rigid rotor system, each blade flaps and drags about flexible sections of the root. This system is mechanically simpler than a fully articulated rotor system. Loads from flapping and lead/lag forces are accommodated through rotor blades flexing, rather than through hinges. By flexing, the blades themselves compensate for the forces that previously required rugged hinges. The result is a rotor system that has less lag in control response because of the large hub moment typically generated. The rigid rotor system also eliminates the danger of mast bumping inherent in teetering rotors.

Semi-rigid

A semirigid rotor system is usually composed of two blades that are rigidly mounted to the main rotor hub. The main rotor hub is free to tilt with respect to the main rotor shaft on what is known as a teetering hinge. This allows the blades toflap together as a unit. As one blade flaps up, the other flaps down. Since there is no vertical drag hinge, lead/lag for cesare absorbed and mitigated by blade bending. The semirigid rotor is also capable of feathering, which means that the pitch angle of the blade changes. Thanks to feathering hinge.

Fully Articulated

Juan de la Cierva in 1920 developed the articulated rotor, which resulted in the world’s first successful flight of a stable rotary-wing aircraft, with his C.4 autogyro prototype. Fully articulated rotor systems allow each blade to lead/lag (move back and forth in plane), flap (move up and down about an inboard mounted hinge) independent of the other blades to compensate for dissymmetry of lift, and feather (rotate about the pitch axis to change lift). Good control response but high aerodynamic drag and also more complex.

Combination

Modern rotor systems may use the combined principles of the rotor systems mentioned above. Some rotor hubs incorporate a flexible hub, which allows for blade bending without the need for bearings or hinges. 

Flybar

Hiller/Bell Mixed Rotor Head these are very easy to identify because along with the main rotor blades, they also have a flybar with small “paddles” at each end. Usually flybar is orientated at a 90 degree offset angle to the main rotor blades. At any rate, the flybar is allowed to pivot up and down in the Hiller/Bell mixed rotor head so it can stay in a level horizontal plane independent of what the main rotor and helicopter tilt angles are. This is what stabilizes the main rotor disc.

Conclusion

P01 configuration is for single main rotor system, but is good to know how other systems works. Our solution at the moment will be a semi-rigid rotor head with flybars based on two bladed Hiller/Bell mixed rotor head, with different blade type to chose and this process little closer will be described with the next topic. Also available will be simpler version of this rotor head without flybars. In the future we can try other hub and airfoils solutions, but for now this will give us a good template to build upon in the future with various parts and components to change in order to make your racing flying machine unique and more personal.

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