Power Offtake Research Articles

Design rules to establish a credible More‐Electric Engine baseline power architecture concept

AbstractThe More‐Electric Engine (MEE), with its electrified engine auxiliary systems and increased multi‐shaft power offtake, is likely to become an increasingly central aspect of future More‐Electric Aircraft. Consequently, lightweight but resilient electrical power architectures are needed for these future MEE applications. However, whilst a range of MEE architectures exist in the research literature, no effective baseline architecture or standardised feature identification has been proposed to specifically address their unique design requirements. Accordingly, any underpinning technology‐focused research for critical MEE subsystems may ultimately have a reduced effectiveness without this credible baseline. Based on comprehensive design analyses, preliminary design requirements and anticipated operational modes, this article proposes key design rules for the formation of the first generic baseline MEE electrical power system architecture concept. Guidance is provided on features such as the number of power generation systems, the number and topologies of distribution channels, type of power conversion, essential load redundancy, and the location of emergency power supply. This article also provides full transparency of the design process so that key decision points can be revisited to capture application‐specific requirements and updates to certification requirements.

Electric Power Transfer Concept for Enhanced Performance of the More Electric Engine

Abstract In future electrified aircraft, multispool more electric engines (MEEs) are expected to be equipped with electric generators connected to each shaft for power offtake and supplying onboard electrical loads. These can be interfaced to a common high-voltage DC bus architecture via power electronic converters. Such system architecture enables the establishment of an “electrical bridge” to circulate the desired amount of power between the engine shafts, and decouple their speeds. This paper introduces the possible benefits from the electric power transfer (EPT) for engine performance and scrutinizes a novel EPT-adopted design (EPTAD) for future MEEs. For this purpose, a zero-dimensional (0D) engine model has been developed by using the intercomponent-volume (ICV) method. By using the engine model, the CFM56-3 engine is redesigned to realize the EPTAD. Comparing the simulation results for the EPTAD and baseline CFM56-3 engines shows significant improvement for engine performance in terms of specific fuel consumption (SFC) and surge margin (SM), mainly at cruise (CR) condition. Results show that almost 3.2% and 2.2% of fuel burn reduction is achieved for the short- and medium-haul flights, respectively, with a 1150 kW EPT system. It is also shown that variable bleed valves (VBVs) can be eliminated in the EPTAD engine with a 1150 kW EPT system.

Performance analysis of turbo-electric propulsion system with fuselage boundary layer ingestion

A propulsion system analysis of an aircraft concept featuring a fuselage tail cone integrated turbo-electrically powered fan is presented. The aircraft has two underwing podded geared turbofan engines and an aft-fuselage mounted boundary layer ingesting fan. The fuselage fan is driven by an electric motor that is powered by power offtake from the main power plants under the wing. A long-range tube-and-wing aircraft with a year 2050 entry into service is used as a reference. A coupled multidisciplinary method for system level assessment of the turbo-electric boundary-layer ingesting propulsion system is presented. A correlation-based method is used to predict fuselage drag and a series of optimizations are carried out for a range of fuselage fan diameters. The optimal level of ingested drag is 30%-57% of the total fuselage drag, resulting in a net reduction in mission fuel burn of 0.6%-3.6% depending on technology assumptions. Further analysis reveals that installation effects, mainly increased mass, offset some of the gains of boundary layer ingestion for the smallest fuselage fan sizes. For larger fan sizes, it is instead the losses in the electric machinery together with the lower efficiency of the fuselage fan, compared to the main engine fan, that compensate for the gains from boundary layer ingestion. However, the by far strongest effect for determining the optimal level of ingested drag is that it is very difficult to obtain a net benefit from ingesting the half of the total fuselage drag contained in the outer part of the boundary layer. The benefit is outweighed by losses in the electric transmission system, installation effects and efficiency deficits of having an aft-mounted fan instead of larger size under-wing main engines. This is true also under the assumption of radical technology such as superconducting electric machinery. It is concluded that the studied aircraft architecture, despite having a high theoretical potential, faces a large difficulty in beneficially ingesting the significant amount of the fuselage drag contained in the outer part of the boundary layer. This severely limits its potential to substantially reduce the fuel burn compared to a conventional twin-engine tube-and-wing aircraft in the year 2050 timeframe.

Design, construction, and testing of an aero‐engine starter‐generator for the more‐electric aircraft

This study describes the design, construction, and testing of an aero-engine starter-generator and its associated power electronic converter. A high-speed, permanent magnet machine and a dual-channel machine-facing converter with an electrical power offtake rating of 95 kW have been developed for a small civil turbofan application. The study also describes the more-electric architecture into which the machine and converter are integrated and reviews the multi-faceted performance specification, which is a common feature of machines of this type.

Dual Drive Booster for a Two-Spool Turbofan: High Shaft Power Offtake Capability for More Electric Aircraft and Hybrid Aircraft Concepts

The effects of high shaft power offtake (POT) in a direct drive, a geared drive, and a novel turbofan configuration are investigated. A design and off-design performance analysis shows the configuration specific limitations and advantages. The more electric aircraft (MEA) concept promises to offer advantages with respect to aircraft performance, maintenance, and operating costs. The engines for the MEA concept are based on conventional turbofan architectures. These engines are designed for significantly increased shaft POT that is required by the airframe, and the shaft power is usually taken off the high-pressure (HP) spool. This can impair the off-design performance of the engine and lead to compromises during engine design and to operability limitations. Taking the power off the low-pressure (LP) spool mitigates some of the problems but has other limitations. In this work, an alternative novel turbofan architecture is investigated for its potential to avoid the problems related to high shaft POTs. This architecture is called the dual drive booster because it uses a summation gearbox to drive the booster from both the LP and HP spool. The shaft power, if taken off the booster spool, is effectively provided by both the LP and HP spools, which allows the provision of very high power levels. This new concept is benchmarked against a two-spool direct drive and a geared drive turbofan (GTF). Furthermore, it is described, how the new architecture can incorporate an embedded motor generator. The presented concept mitigates some of the problems, which are encountered during high POT in conventional configurations. In particular, the core compressors are less affected by a change in shaft POT. This allows higher POTs and gives more flexibility during engine design and operation. Additionally, the potential to use the new configuration as a gas turbine-electric hybrid engine is assessed, where electrical power boost is applied during critical flight phases. The ability to convert additional shaft power is compared with conventional configurations. Here, the new configuration also shows superior behavior because the core compressors are significantly less affected by power input than in conventional configurations. The spool speed and its variation are more suitable for electrical machines than in conventional configuration with LP spool power transfer. The dual drive booster concept is particularly suited for applications with high shaft POTs and inputs, and should be considered for propulsion of MEAs.

Analysis of Electromechanical Interaction in Aircraft Generator Systems

The supply of electrical power is usually achieved by a generator driven from a prime mover by some form of mechanical drivetrain. Such an electromechanical system will have natural resonant modes in both the electrical and mechanical subsystems. The electrical generator provides a coupling between the subsystems, transferring not only useful power, but also disturbances between the electrical and mechanical domains. These disturbances may excite resonances resulting in cross-domain (electromechanical) interaction. This can lead to lifetime reduction in the mechanical components and instability in the electrical network, resulting in poor reliability for the wider system and potentially catastrophic component failure. Electromechanical interaction is particularly critical in power generation systems onboard aircraft, because the generator is driven by a gas turbine via an inherently low-stiffness drive train. It is then critical to identify electromechanical interaction at the design stage so that these issues can be avoided. However, predicting the occurrence of interaction through simulation is challenging, requiring multidomain models operating with different time scales. This paper analyzes an aircraft auxiliary power offtake to produce a reduced-order mechanical drivetrain model, allowing the modal frequencies to be predicted and cross-domain interactions to be modeled. A purpose-built electromechanical test platform is used to validate the model and demonstrate how electrical disturbances are passed through the generator to the mechanical system and affect the electrical network. Future research will use the test bed to demonstrate strategies for avoiding or suppressing unwanted interactions.

Effect of Pressure and Oil Mist on Windage Power Loss of a Shrouded Spiral Bevel Gear

In many aeroengines the accessory power offtake is achieved using a spiral bevel gear set running off one of the main shafts. The crown and bevel gears are housed in an internal gearbox and there is significant heat generation within this chamber, some of which is attributed to windage power loss (WPL) generated by the gear. Over the past few years the University of Nottingham Technology Centre (UTC) in Gas Turbine Transmissions has been researching spiral bevel gear windage power loss, both computationally and experimentally, using a purpose-built test rig at the UTC. In this study the test rig has been adapted such that chamber pressures up to 8 bar can be generated. Test data has been obtained that shows the effect on the WPL of chamber pressure, advancing understanding of the relationship between data obtained at ambient and pressurized conditions. Three configurations have been studied: unshrouded gear, a gear with 360 deg shroud and a shrouded crown and pinion meshing pair. Furthermore, the effect of oil mist within the chamber on the WPL has been studied. An oil mist generation system was developed for introducing a fine mist into the chamber and results are presented for varying mist flowrates. A mist measurement system was developed to sample mist mass fraction within the chamber and the data obtained is used to calculate an effective (oil/gas mixture) chamber density. Increasing chamber pressure increases the Reynolds number, moving the system behavior further along the moment coefficient-Reynolds number correlation. The Cm-Re correlation is similar in form to that for a shrouded cone, showing transitional behavior around Re = 2 × 106. Beyond this transition, Cm decreases with increasing Re. Introducing an oil spray has two effects: a reduction in chamber temperature and an increase in the effective density of the chamber fluid. Both effects can be accounted for by calculating Re and Cm based on mixture properties but it seems highly likely that the properties of the fluid under the shroud differ from those of the fluid in the external chamber.

Fuel-Sensitivity Analyses for Jet and Piston-Propeller Airplanes

Two mathematical relationships have been derived to estimate the impact on fuel consumption during cruise – one for jet airplanes and one for piston-propeller airplanes – arising from design changes that alter the airplane’s weight (W), lift-to-drag ratio (L/D) and Specific Fuel Consumption (SFC). Using cruise fuel burn as the figure of merit, the expressions may be used to rapidly assess proposed airplane design changes (or modifications) where simultaneous changes to these three fundamental design parameters occur. Two case studies are presented. The first considers the incorporation of an active drag reduction system, such as Hybrid Laminar Flow Control (HLFC), which reduces the skin friction drag, but increases the airplane’s weight, and – as such systems require power – increases the power off-take from the engines, leading to an SFC penalty. Using a commercial jet airplane as a reference vehicle, the results of the numerical expression are compared to those obtained using a computer program, which is capable of accurately determining the cruise fuel. The underlying assumption of linear independence of the three parameters is explored – the results indicate that the parameters are only weakly linked, the nature of which depends on the airplane’s performance characteristics. The second case study concerns a proposed design change to piston-propeller airplane, where the incorporation of a retractable landing gear reduces drag, but a weight penalty is incurred. A target for the weight change is rapidly computed for the associated drag change.

Fuel sensitivity analyses for active drag reduction systems

Active drag reduction systems, such as hybrid laminar flow control (HLFC), have the potential for significant fuel savings; however, this is at the expense of an increase in aircraft weight and engine power off-take. A computer program – capable of accurately determining the trip fuel for a given mission profile – has been developed. The program was validated against manufacturer’s payload-range data, and then modified to emulate the installation of an active drag reduction system, by incorporating changes to the drag polars, specific fuel consumption (SFC) data and operating empty weight (OEW). Results of sensitivity studies are presented that enable the reduction in trip fuel to be determined for given changes in CD, SFC and OEW. The underlying assumption of linear independence of the three parameters is explored. The linearised method was observed to underestimate the fuel savings of the HLFC aircraft by approximately 0·6% compared to an analysis which took into account the coupling between the parameters. A mathematical relationship has been established to estimate the impact on cruise fuel burn arising from relative changes to the aircraft’s mass, lift-to-drag ratio and SFC.

Comparative Analysis of the Windmilling Performance of Turbojet and Tbrbofan Engines

The windmilling performance of turbojet and turbofan (TF) engines is investigated numerically by means of a performance synthesis program. Different types of TF engines are taken into consideration, including mixed and unmixed exhaust configurations and configurations with and without a booster compressor on the low-pressure shaft. Furthermore, different design bypass ratios are investigated. The free windmilling performance is calculated for various ram pressure ratios, and, in addition, power input and offtake on the high-pressure shaft are considered. The calculated results indicate a rather strong dependency, not only on the design bypass ratio, but also on the engine type. For example, the internal drag and the maximum power offtake for a given ram pressure ratio differ considerably when comparing mixed and unmixed exhaust configurations. However, for the relight capability, the design bypass ratio is identified as the most important parameter. Nevertheless, the results of this investigation make evident that the rather widespread procedure to present the windmilling performance of engines dependent on ram pressure ratio and design bypass ratio is insufficient.

Avionics cooling-rate trade-off modelling for ultra-high capacity aircraft

A computer aided conceptual aircraft design methodology (CACAD) has been developed to size ultra-high capacity jet transport aircraft. Modules were also developed for predicting maintenance costs of each airframe and avionics system and these were incorporated into CACAD. A methodology was developed to enhance the reliability of avionics systems, based on experimentally-proven engineering design solutions. A number of avionics cooling techniques were investigated, and reliability and maintainability models of thermal management were developed and linked to an avionics maintenance cost module. Further models were produced to investigate the impact of proposed changes on the environmental control systems, engine-provided bleeds and power off-takes. It was found that increased flowrates above the normally recommended values for the avionics bay, and to the flight deck instruments, may increase the reliability of the avionics systems, and also increase aircraft dispatch reliability. They may not, however, greatly improve direct operating costs (DOC), due to significant fuel penalties. A separate refrigeration unit was investigated and found to be a feasible cost-effective measure, even allowing for increased engine fuel consumption caused by the effect of the engine power off-take required to drive the refrigeration unit.

The application of acoustic philosophies and techniques to aero engine development

The paper is a general interest one dealing with the way in which acoustic techniques and knowledge assist in the instrumentation, testing and development of Aero Engines. No attempt is made to deal with “Sonic Analysis”, i.e. the technique whereby an external microphone allegedly enables the condition of each and every part of the engine to be determined, nor to deal with the general external noise produced by engines; both of which are subjects in themselves. The use of varied methods of extracting and processing information from compressor blading, turbines, gears, bearings, canware and carcass of the engine are dealt with in some detail as is the associated Sound Room work. The help which can be given by ear and brain in a number of these cases is dealt with as a valuable complement to instrumental analysis, especially in revealing effects easily missed by instruments alone. Coupled vibrations between engine parts and between engine and structure or power off-take are considered and the latest methods for their measurement discussed. The original paper was provided with tape recorded audio representations of many of the effects as heard by the ear; however, the present state of the art does not permit their reproduction here.