Look at the rotor head of many conventional helicopters and you will find a complex collection of hinges, bearings and mechanical joints that allow each rotor blade to move as it rotates.

Look at an Airbus H135 and the arrangement is strikingly different.

The H135 uses a four-bladed, bearingless and hingeless main rotor system. Instead of relying on conventional hinges and bearings to accommodate the movement of the rotor blades, the design makes use of the flexibility of modern composite materials.

It is one of the most interesting pieces of engineering on the aircraft – and its origins go back considerably further than the H135 itself.

From the Bo 105 to the H135

The technology behind the H135 rotor is part of a much longer development programme.

Bölkow began experimenting with glass-fibre rotor blades during the 1950s. A new hingeless rotor system was successfully tested on an Alouette II during 1962–63, and this development work subsequently contributed to the Bo 105, which appeared in 1964.

The Bo 105 became particularly associated with its rigid rotor system and demonstrated what could be achieved by replacing conventional articulated rotor-head arrangements with structures capable of accommodating blade movement through flexibility.

When the Bo 108 was subsequently developed as a replacement for the Bo 105, that philosophy continued to evolve. The Bo 108 eventually developed into the EC135 and, later, today’s H135.

Why Do Helicopter Rotor Blades Need to Move?

A rotor blade cannot simply be bolted rigidly to a rotating shaft.

During flight, each blade experiences continuously changing aerodynamic and inertial forces. The rotor system therefore has to accommodate three fundamental blade movements: pitch change, flapping and lead-lag.

Pitch change alters the blade’s aerodynamic angle and therefore its lift. Flapping allows the blade to move above and below the nominal rotor plane, while lead-lag – sometimes called hunting – allows movement forwards and backwards within the plane of rotation.

Traditional articulated rotor systems accommodate these movements using mechanical hinges and bearings.

The H135 takes a different approach.

How the H135 Bearingless Rotor Works

The H135 main rotor uses composite construction to provide the flexibility necessary for blade movement without installing conventional flapping, lead-lag and pitch-change bearings. Airbus describes the resulting arrangement as beneficial in terms of maintenance, weight and cost.

The complete system consists principally of four main rotor blades, the main rotor hub-shaft, swashplate, four rotating control rods and the scissors assembly.

At its centre is the main rotor hub-shaft. This transfers driving torque from the main transmission to the four blades while also transmitting rotor forces and moments back into the transmission and aircraft structure.

Each rotor blade is attached to the hub-shaft by two bolts.

The result is an unusually clean rotor-head arrangement compared with a conventional fully articulated rotor.

From the Pilot’s Controls to the Rotor Blade

The swashplate provides the connection between the stationary flight-control system and the rotating rotor.

Four rotating control rods transmit control movement from the swashplate to the individual main rotor blades. The rotating portion of the swashplate is driven synchronously with the rotor mast through two scissors assemblies.

Movement of the swashplate therefore changes blade pitch as the rotor turns, allowing collective and cyclic control inputs to alter the lift generated by the blades.

The important distinction is that the required movement of the blades themselves does not depend upon the collection of conventional hinges found on a traditional articulated rotor head.

The Rotor Blades Are Complex Structures

Although their external appearance is relatively simple, the H135’s composite rotor blades incorporate considerably more engineering than their shape suggests.

The blades include provisions for dynamic balancing and tracking. Trim tabs on the trailing edge can be adjusted during maintenance to achieve correct blade tracking and rotor performance. The blades also incorporate leading-edge erosion protection, protection against ultraviolet exposure, static discharge provisions and lightning protection.

Even seemingly minor features have a purpose. Drain holes, for example, allow moisture entering the pitch-control cuff area to escape rather than accumulate inside the structure.

Later P3/T3 rotor blades also incorporate lead-lag dampers and a bearing support arrangement. The geometry provides coupling between blade lead-lag movement and pitch angle, contributing to lead-lag damping during flight.

Maintenance of a Bearingless Rotor

“Bearingless” should not be confused with “maintenance-free”.

Removing conventional rotor-head bearings eliminates certain components, but the rotor remains a highly loaded dynamic system requiring careful inspection, adjustment and monitoring.

Preflight inspection includes the rotor blades and hub as well as the swashplate, rotating control rods and associated components. Blade attachment bolts, control cuffs, dampers, trim tabs and static dischargers are among the items checked for condition.

Blade tracking and balancing are also important. The rotating control rods are length-adjustable for track-and-balance purposes, while provisions within the blades allow their dynamic behaviour to be adjusted.

This illustrates an important point about modern helicopter engineering: simplifying the number of mechanical joints does not make rotor dynamics simple.

Monitoring Loads Through the Rotor Mast

Some EC135/H135 configurations also incorporate a mast moment indicating system.

Rather than measuring rotor speed or blade pitch, this system provides information about the bending moment acting on the main rotor mast. The indication allows the crew to monitor structural loading, particularly during operations where high mast moments may occur. Recorded overlimits can subsequently be accessed for maintenance purposes.

It provides another example of how the mechanical rotor system and the aircraft’s monitoring systems work together.

A Different Approach to Rotor Design

The H135’s rotor demonstrates how composite materials can change the way a mechanical system is designed.

Instead of adding mechanical components to permit movement, the rotor uses carefully controlled structural flexibility to accommodate much of that movement within the blades themselves.

The result is a rotor system whose ancestry can be traced through decades of Bölkow and MBB development, yet which remains one of the defining engineering features of the modern H135.

The rotor, however, is only useful if sufficient power can be delivered to it. That brings us to the next article in this series: How the Airbus H135 Main Transmission Works.

Explore More About the Airbus H135

If you haven’t already read them, see Airbus H135: From the Bo 108 to a Modern Light Twin Helicopter] and [How the Airbus H135 Fenestron Works.

For maintenance personnel and engineers requiring a broader understanding of the aircraft, our Airbus H135 General Familiarisation Course covers the aircraft’s major systems, their operation and component locations.