The research programme capitalised on the most recent advances in electrical machine technologies, thermal modelling, design optimisation and manufacturing processes to realise and evaluate a full-scale helicopter electric tail rotor (ETR) motor that is engineered for flight-critical operation and is representative of an aircraft installation.
A direct drive ETR solution was optimised against the objectives of a peak output power of 3kW per kg of installed active mass making it weight-competitive with current mechanical technology, and was a fault-tolerant design specified to an integrity target of 10 to the -9 failures per flight hour, whilst offering demonstrable performance and whole-life cost benefits.
The Electrical Tail Drive-Modelling, Simulation and Rig Prototype Development (ELETAD) project investigated the feasibility of powering the tail rotor of a helicopter with an electric drive, replacing the current mechanical system comprising high speed shafts and gearboxes. The electric tail rotor system comprised a lightweight and compact electric motor connected via cables to power electronic converters which act to condition and control the power feed to the electrical machine. The potential advantages include:
- Reduced future reliance on hydraulic and oil-based systems;
- Low maintenance, high service life, with reduced repair and overhaul cycle impact;
- Inherent torque control and power limiting directly inferred measurement of operating load;
- Higher system integrity, and inbuilt fault tolerance;
- Improved overall aircraft fuel efficiency when part of a ‘more electric’ power management system.
For the electrical drive alternative to be viable it needed to deliver the performance requirements, whilst satisfying the 10E-9 failures per flight hour integrity of a safety critical system. The integrity requirement dictated fault-tolerance and redundancy within the system design. Further, the tail drive motor should be sufficiently compact to be mounted at the end of the tail boom together with the blade pitch mechanism and should therefore be weight-competitive with the existing mechanical transmission. Additionally, the location and reliability requirement dictates air-cooling. All these challenges had been addressed within the ELETAD project, where the viability of a compact, fault-tolerant air-cooled electric tail rotor motor has been demonstrated.
The ELETAD project had centred on the use of high efficiency and performance permanent magnet brushless machines, requiring innovative use of modern materials and advanced design methods. Designs and prototypes of full-size axial-flux and radial-flux electric tail rotor motors had been delivered, accompanied by comprehensive test data and supporting analyses to enable a full performance assessment to be made. The characteristics of the double airgap axial-flux and the modular wound radial-flux topologies had been compared. It was found the axial-flux and radial-flux designs had near identical active weights, and the prototypes exhibited similar continuous output capabilities. However, the radial-flux design proved to be overall more efficient and better suited to the tail rotor drive configuration. Importantly the work had identified areas where improvements in manufacture and materials would enhance performance and where future design and analysis effort should be focussed.
An innovative test capability for multi-phase fault-tolerant aircraft propulsion drives had been established, capable of dynamically characterising electrical drive systems with peak ratings up to 340 kW, 2000 Nm and 4500 rpm. A major element of this test rig was the complete functional representation of a redundant/fault-tolerant electrical supply and control system that would be needed for a safety-critical aircraft application. In the near term this ‘copper bird’ was to be used to support further developments development of electric tail rotor drive and additional research into fuel efficient more-electric helicopters.
Additionally, the project resulted in a new suite of validated software tools and techniques for predicting the loss, thermal behaviour and in-service life of high performance fault-tolerant electrical machines, over dynamic operating duties. These design tools and methods have widespread applicability, for example in developing electrical machine designs for other aircraft propulsion drive applications or for high torque wheel motors in electric vehicles. The new capabilities for modelling electrical machines offered by the software tools have been incorporated within commercially available design software marketed by one of the project partners.
The project had been undertaken by a pan-European consortium of two SMEs and two research institutes; The University of Bristol (Lead partner), Lucchi Rimini Elettromecanica, Motor Design Limited and the University Politehnica of Bucharest.