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Helicopter Electric Regenerative Rotor Brake

Final Report Summary - HERRB (Helicopter Electric Regenerative Rotor Brake)

Executive Summary:
This report highlights the technical developments made in the field of an integrated, optimised electric regenerative braking system for the main rotor of a medium-sized helicopter.

The majority of rotorcraft use mechanical heat dissipating braking systems similar to those used on modern cars in order to decelerate the main rotor which stores considerable energy. The result is that the entire main rotor kinetic energy is lost as heat to the surrounding environment every time that the helicopter lands. In a medium sized aircraft, the energy stored in the rotor when it is at full speed is equivalent to roughly 2MJ of energy. Electrical machines can be used to decelerate the main rotor of the aircraft regeneratively reducing the proportion of energy wasted upon every landing. At the time of writing, no electrical system of significant power exists on medium sized aircraft and so this provides an opportunity to explore not only the specific technologies that could be used but also understand the reliability and certification issues as well as opportunities for closed integration possible with these new, modern systems.

The work contained in this project has provided a first step in providing design frameworks and understanding and demonstrating solutions for the use of significant electric power based systems in a rotorcraft.

A design process in which an optimised electric machine and power electronic converter is required that integrates tightly with the surrounding platform is developed and assessed. Tools for predicting performance of the target electric drive are developed and validated. The benefits and shortcomings of using such tools are presented and discussed. Technical highlights in terms of the electric drive design are presented and their performance analysed.

A static braking system for rotor holding, not dissimilar in nature to the existing hydraulic/mechanical systems used currently on aircraft is conceptualised considering the existing system's shortcomings. One of the principal factors to be addressed is that of the brake coming on in flight. This is tackled from a conceptual and reliability perspective. The concepts are benchmarked and the most promising taken forward for prototyping and characterisation. The final design is presented and its performance evaluated. New data for materials used in the braking process are characterised and a novel framework for the design of such a platform presented.

Tools for assessing the impact of integration of the electric drive system into the existing drivetrain have been developed and used to rationalise the design where appropriate or highlight areas for further investigation. Dynamic analyses have demonstrated that removing the need to rigidly constraint/mount the electric machine part of the electric braking system on both sides would mitigate excessive vibration in the machine from the main drivetrain. System level simulations have been developed that allow some scenarios to be played out without the need for the entire toolchain developed as part of the design process of this project. This allows scenarios such electric start of the main rotor to be investigated and will improve the case for switch from a hydraulic system to an electric equivalent as part of a wider transition to electric systems.

Whilst the technology being developed is not considered safety critical in terms of its availability, it does lie on the primary mechanical driveline for the rotorcraft. Hence in order to demonstrate its suitability in this respect a full reliability and fault analysis has been undertaken. The tools developed are new in terms of electric power systems on rotorcraft and have provided a capability not only to assess this but other electric platforms being considered for rotorcraft.

Finally, the prototype systems for each of the developed technologies are presented and characterised. The electric machine meets its primary requirement of providing a flexible non-contact braking capability and is as designed. The machine fits into the target volume, however the system’s ability to generate power during flight is seen to be less than expected. This is thought to be as a result of a mechanical interface in the machine that has provided extra resistance in the thermal path that is between the primary heat source and the main heat sink. Whilst this does not compromise the ability of the machine to generate torque and power it does reduce the ability of the machine to dissipate that heat via the integrated gearbox mounting and would need to be addressed prior to adoption on an aircraft.
Project Context and Objectives:
The civil fixed wing aircraft sector has, for the last 10-20 years, sought to replace pneumatic, hydraulic and mechanical systems with electric equivalents wherever possible. Fuel efficiency has traditionally been the primary driver especially as removal of the bleed air system and replacement with electric engine start and cabin conditioning systems has the potential to save significant mass. As a result of this drive a significant pool of experience, research and available equipment has been amassed in the area of flight critical electric systems.

The rotary wing aircraft sector has traditionally had lower levels of installed electric power on board, primarily serving avionics and wing ice protection systems. However, there exists the potential to exploit the developments made in the fixed wing sector and use these to further advantage, provided a aircraft-wide approach is taken. Electric systems can be used to replace a far greater proportion of the installed equipment, even extending to the tail for medium sized aircraft and even full propulsion for small aircraft. Over and above the efficiency savings, a significant benefit in terms of through life costs can be made if large mechanical, single function systems can be replaced by compact, easily replaceable electric systems that serve multiple purposes and provide a versatile, and potentially reconfigurable power system. This is reflected in the Clean Sky, Green Rotorcraft (GRC) ITD challenge statement, which states:

“The eventual elimination of noxious hydraulic fluids and the reduction of CO2 emission through efficiency and weight optimisation of on-board energy systems constitute the two crucial objectives pursued in this subproject. The emerging high performance electrical systems and equipment enable a range of new solutions but need to be adapted to the specific helicopter constraints.”

The Helicopter Electric Regenerative Rotor Brake (HERRB) project aimed to investigate, design and quantitatively assess a system of electric technologies that could be used to replace the existing mechanical brake used for the deceleration of the main rotor of a helicopter once the aircraft is on the ground and the engines have been stopped. Rather than wasting all of the stored energy in the rotating rotor blades, as the current mechanical system does, the electric system will recover a significant (~50%) proportion of this energy and store it for auxiliary purposes, such as fast engine start.

In order for the proposed system to be considered viable as a replacement for the existing system, as thorough an understanding of its direct (rotor deceleration) and indirect benefits (in-flight generation and rotor control capabilities) has to be gained in terms of its primary functions. Furthermore, an assessment of the impact of including signify new technologies such as this must be undertaken in order to ensure that the direct benefits are neither undermined nor the designed system deemed unflightworthy due to either integration or certification issues.

Although the main benefits come from being part of a wider electrical system, in order to be tested on existing platforms, the weight of the full system needs to be below existing commercially available machines, exploiting the very low duty cycle required for the application whilst maintaining the levels of safety and reliability needed for this type of aircraft system. In order to achieve this, the main components of the work were:

1. Full, concept system design followed by down-selection and then integration and reliability studies for the chosen design case.
2. Development and experimental validation of integrated design tools encompassing the dynamic thermal and electro-magnetic behaviours of both the electro-mechanical (machine) and electrical power conversion stages.
3. Design of a fully optimised prototype capable of being installed into an existing rotorcraft. The relevant aircraft safety, reliability and certification standards were incorporated into the design.
4. Full-scale test rig capable of both system characterisation over the full working envelope and of reproducing the dynamic inertial energies stored (and hence torques exerted) in the main and tail rotors during the full deceleration operations being considered.
5. Building of a fully characterised, aircraft worthy machine and power electronics-based energy conversion system allowing assessment of the system capability.

As a research project HERRB developed a series of tools to allow the early stage selection and detailed design of these technologies on a medium to large scale for use on rotary wing based equipment. The objectives of the proposed project were:

1. To demonstrate the suitability of the finally selected electrical topology as the ideal candidate for the dynamic braking application being studied.
2. To deliver the capability of predicting accurately the thermal behaviour of a full regenerative rotor braking system for the selected topology under a range of rotor braking scenarios.
3. To evaluate the thermal and dynamic performance of the supplied prototype regenerative rotor braking system.
4. To demonstrate the capability and suitability of the final solution to be integrated to an existing rotorcraft’s transmission with minimal design modifications.

Each of these objectives has been achieved.
Project Results:
1 Introduction
The work in the HERRB project can be broadly categorised into four main areas being covered by seven technology focused work packages:

WP0. Design Specification
WP1. Rotor Deceleration System
WP2. Electrical Power Conditioning
WP3. Rotor Stop and Hold Mechanism and Control
WP4. Aircraft Installation Configuration
WP5. Safety and Certification
WP6. Prototyping and Test

and a management work package:

WP7. Management

The inter-dependence of the technology focused work packages can be seen in Figure 1.

1.1 Specification (WP0)

1.1.1 Overview
The call for proposal outlined the aim of the project and gave an outline of aircraft level requirements specific for a braking application. In order to allow for a meaningful comparison and to agree target requirements that meet the Technology Readiness Level (TRL) of the Call for Proposal (CfP) a detailed specification was carried out based upon research of the state of the art in the area, current standards, focus groups with project stakeholders and the relevant Green Rotorcraft team (GRC3) lead. The Preliminary Design Specification (PDS) that resulted drove the design of each of the various systems in each of the project packages (WPs).
The preliminary design specification encapsulated requirements for both the mechanical and electrical systems of a potential solution to the problem of electrically actuated main rotor braking.

1.2 Rotor Deceleration System (WP1) and Electrical Power Conditioning (WP2)

1.2.1 Overview

WP1 & 2 focused on the design and optimisation of the dynamic (or regenerative) aspect of the rotor brake. Detailed and coupled modelling of the thermal and electromagnetic and electric machine, power electronics based converter and drove the final design towards a minimum installed mass of the entire system rather than on a component wise basis.

1.2.2 Results Generated
During the course of the project the following scientific/technological results were generated:
• Integrated machine design for used in a medium sized rotorcraft without supplementary cooling
• Development and calibration of a full thermal model for the finally selected topology of electric machine
• Validated optimisation process for balancing of DC and AC copper losses in the design process of a fixed speed electric machine.
• Process for early stage (pre-prototype) calibration of complex stator assemblies in thermal models
• Design and test of a magnetically active mechanical wedge for secure, vibration resistant coil retention in concentrated wound open slot machines.
• Temperature dependent system level models for fast simulation of converter/heatsink thermal behaviour including machine and controller behaviours.
• Validated switching period based calculation of losses with potential for real-time estimation of losses

1.3 Rotor Stop and Hold Mechanism and Control (WP3)

1.3.1 Overview

WP3 considered the design of the electric static (or holding) brake from the concept stage right through to embodiment and final prototype.

Unlike the dynamic brake, topological constraints existed on this aspect of the design resulting in a large solution space that needed be methodically and objectively searched resulting in an integrated and optimum final outcome.

A morphological approach was used to devise concepts that could potentially satisfy the requirements based its individual sub-functions. These were then rated and ranked objectively yielded the final design. A new modelling framework and generation of new data was created/measured to support the design optimisation to ensure a low-mass final design.

A final optimisation phase was conducted that demonstrated that without functionally altering the method of operation and hence final performance, an aircraft system could be realised at a mass much lower than that of the initial prototype.

1.3.2 Results Generated

During the course of the project the following scientific/technological results were generated:
• Morphological approach to a system design for a static electric helicopter brake
• Validated generalised design framework for the finally selected static, electrical braking system
• Static and quasi-static co-efficient of friction data for key materials used in friction-based braking systems
• Validate and optimised solution to the static braking requirement for a medium size helicopter

1.4 Aircraft Installation Configuration (WP4), Safety and Certification (WP5)

1.4.1 Overview

WP4 considered the designs proposed in WP 1, 2 & 3 and based jupon analysis or optimisation of the designs, ensured that they met as closely as possible at the prototype stage requirements for a modern rotorcraft in terms of packaging, integration, reliability and safety.

A full analysis of the machine integrity (static and dynamic) was undertaken and demonstrated that the assembly was both functional and designed to safety factors appropriate to the function of the brake and the TRL being pursued. The principle exciting frequencies were identified and the implications of tight integration of the rotor brake highlighted.

A system-level model was constructed that took low- (or component-) level data as its input and abstracted it to the detail required to allow simulations of long time durations to be undertaken allowing loss analysis under various scenarios to be reviewed.
Various case studies were compared in terms of time to achieve a complete braking cycle, energy recovered on land, total predicted conversion efficiency. The toolchain also permitted the ability to conduct an initial study into the potential for the electric machine and converter (electric dynamic brake) to act as a rotor acceleration device prior to the rotorcraft engines being engaged for a simultaneous engine and main rotor start.

As well an an integration study, a reliability and full failure modes and effects analysis was undertaken for the prototype solutions proposed within the HERRB project based upon their state of development at the critical design review (CDR).
The study demonstrated several cases for all severity ratings in which the reliability of the developed systems fell outside of the target ranges for that severity rating highlighting those system that would require further attention prior to being considered to be sufficiently fault tolerant to enter service.

1.4.2 Results Generated

During the course of the project the following scientific/technological results were generated:
• Design modelling framework for assessing the compatibility of the rotor brake prototype with an existing airframe
• Full system non-linear level coupled model of the chosen electric drivetrain derived directly from component level analysis
• Application of system model for optimisation of prototype subsystems based upon system-wide constraints.
• Generation of reliability data and associated fault assessments for full permanent magnet electric drive systems of significant power.
• Fault tree analysis of a braking system for a helicopter considering all possible interactions between various subsystems and harsh environment experienced during rotorcraft flight.

1.5 Prototyping and Test (WP6)

1.5.1 Overview

In order to prove, characterise and validate both the electric machine and the power electronic converter, a test rig was constructed that could operate over the operating ranges of the prototyped equipment.

Each of the three systems were prototyped based upon the status of the designs at the Critical Design Review.

Each of the systems was tested in order to both characterise that system and provide validation data for the modelling tools developed. The most significant results are summarised here. Test results demonstrated that the final machine prototype had an electromechanical performance very close to that predicted.

Generator ratings were measured based upon needing enough thermal headroom to accommodate the extra measured transient temperature rise required for a single optimised braking cycle. The results agreed well with the predictions made in the thermal model at the Critical Design Review.

Predicted generator continuous ratings were measured at lower values than predicted at the Critical Design Review. Calibration of the thermal modelling highlighted the primary factors responsible for the differences between predicted and measured behaviour.
The power converter measurements demonstrated results commensurate with expectations for a such converter, as predicted by the modelling activity. This confirmed the converter to be an appropriate match for the electric machine.

1.5.2 Results Generated

During the course of the project the following scientific/technological results were generated:

• Manufacture and characterisation of a full-scale integrated electric machine prototype for use on the main gearbox of a medium-sized aircraft
• Validation of the structure of the thermal model and the results output from the finally calibrated thermal model for the electric machine
• Validation of a real-time capable loss predicting mechanism for a IGBT-based power electronic converter which has the potential not only to inform design but allow continuous optimisation of drive operation in real-time.

1.6 Conclusions

The HERRB project delivered a prototype system that allows dynamic regenerative brake of the main rotor of a medium sized helicopter, that can hold the main rotor once power has been isolated from the system, and that can act in a limited capacity as a generator during flight.

Model frameworks have been created for all of the main elements of the design process and where practicable have been simplified to a system level to allow for assessment of performance in case studies. The ability of the modelling processes to predict final system performance has been demonstrated and key areas for future development indicated where the early stage predictions have deviated from final performance.

All research objectives have been met and a range of research areas for further development have been identified.

Potential Impact:
The HERRB project has developed a toolchain for the assessment and detailed understand of the performance of power converters and electric machines for use on helicopter platforms. It is specifically resulted in the development of a prototype electric drive system that serves a specific and well understood function, that of rotor braking post landing on a helicopter. Furthermore, for the first time, a reliability and certification study has address and highlighted the issues that are related to the use of state of the art permanent magnet based electric drive technologies with specific consideration of issues that arise from their use on helicopter platforms (constant frequency high amplitude vibrations, exposure and ingress of contaminants over long periods as part of the normal operating cycle).

In this respect it has provided, for the industrial partner, a first pass at the design and integration of such an electrical drivetrain. On a higher level it has provided the relevant team of engineers with a detailed understanding of the differences and knowledge areas required when using electric systems to perform those functions more traditionally provided by mechanical, pneumatic and hydraulic solutions.

At the airframer, level a team has been formed that has both the motivation and is developing the understanding required to turn the outcomes of this and associated GRC3 projects (REGENESYS, ELETAD) and in partnership with these projects has contributed to follow on projects such as ETR and QUADGEN from individual functional items into a full integrated and hence more efficient primarily electric (propulsion not withstanding) helicopter. This has the potential to be a disruptive technology in helicopter total power management if approached in this holistic manner.

The HERRB project has demonstrated the need for the electric rotor brake system to have a supplementary function whilst in the aim to offset the fuel required to carry it and ensure that it remains an efficiency contributing element in the aircraft. The electric drive system designed has achieved this by operating as a generator in flight. Given existing generate capacity on the aircraft, dynamic rating of the generator based upon operating conditions and scenarios has been proposed and could offer the possibility of providing the extra power required for peak operation were the extra equipment from GRC3 to be installed on an aircraft. This would improve the efficiency of the aircraft further but would necessitate an aircraft wide study of the electric (and possibly other) power management. The possibility of such research will be the subject of future collaborative proposals.

A functional TRL 5 prototype system and associated models is available for further characterisation and drive cycle analysis. Use of the technology for “value-added” functions such as autorotation control, fast electric rotor start, and supplementary generation during the aircraft’s peak periods can be assessed in detail. The prototype is certainly capable, with software modifications, of being integrated into a ground based “copper-bird” style rig.
List of Websites:
http://www.herrbcleanskies.eu/
Dr. David Drury, Tel. +44 117 954 5390, Email: d.drury@bristol.ac.uk