A rim

Dec 29, 2023

Wrexham Glyndwr University Dr ROB BOLAM CEng FRAeS and Doctoral Research Assistant JHON PAUL ROQUE provide an insight into the Faculty of Art, Science and Technology’s patented prototype rim-driven ‘FAST-Fan’ electrical jet engine.

In the Faculty of Art, Science and Technology (FAST) at in Wrexham Glyndwr University (WGU), engineering students and staff are concentrating their efforts on delivering Net Zero Wales.

As part of the Welsh Government’s SMARTExpertise programme, the FAST-Fan is one of the last projects to be funded via the European Regional Development Fund. WGU has collaborated with six UK based SMEs (Ad-Manum UAS Technologies Ltd, Invertek Drives Ltd, Motor Design Ltd., Drone-Flight Ltd, Geola Technologies Ltd and Tirius Ltd) to provide the analyses, design and manufacture of a patented prototype rim-driven, ‘FAST-Fan’ electrical jet aircraft engine. The resulting propulsion unit is capable of delivering more thrust per frontal area and higher efflux velocities than current fan configurations.

Rim Driven Fans (RDFs), are a type of ducted fan that have a continuous rotor-rim attached to the tips of the fan blades and are driven by means of electro-magnetic motor circuitry arranged within the duct. (WGU)Although the onboard generation and storage of electrical energy is considered the primary obstacle to aircraft electrical propulsion, the distribution of electrical power and its conversion to propulsive thrust still present significant challenges. One of which is how to efficiently achieve propulsion for high-speed flight.

Conventional civil aircraft use propellors for regional operations to attain speeds of up to Mach 0.6 and altitudes ranging up to 30,000ft but for longer journeys that require higher speeds and altitudes (up to Mach 0.9 and 45,000ft) ducted by-pass fans are used. Both technologies are hub driven with the main propulsive airflow passing over a central energising unit, namely the gas turbine engine, and the propulsive drives being transmitted via rotating shafts.

It is, therefore, quite logical, that hub-driven technology is automatically considered for aircraft electrical propulsion solutions: It presents a familiar technology and appears to provide lower development risk. However, it remains questionable whether hub-drives always provide the ideal configuration to meet the needs of aircraft electrical propulsion.

The aim of the FAST-Fan project is to explore the potential of rim driven technology in this context. The objectives were to research, analyse and develop the technology and to utilise the latest available rapid prototyping techniques to realise design concepts through the construction of a scaled prototype device suitable to power a small unmanned test aircraft.

Bladed rotor (or "blotor") finite element software analysis. (WGU)Rim Driven Fans (RDFs), are a type of ducted fan that have a continuous rotor-rim attached to the tips of the fan blades and are driven by means of electro-magnetic motor circuitry arranged within the duct. RDFs offer some key advantages when compared with conventional hub-driven fans. Such as an increase in thrust per frontal area owing to the removal of the flow restriction caused by the hub mounted motor; shorter overall length of the fan assembly; a reduction in the motor tangential forces required to generate torque owing to an increase in the radial moment arm; improved fan aerodynamics from the omission of fan-tip losses; provision of air cooling for the motor windings and the ability to easily install two contra rotating RDFs in tandem. This latter advantage also gives efficiency gains owing to the removal of flow swirl and facilitates the generation of increased Fan Pressure Ratios thus enabling faster efflux velocities and flying speeds. A drawback to an RDF configuration can be its weight if the electromagnetic circuit is not optimised.

RDF technology traces its history back to the early 1960s and the Ryan Vertifan. (NASA)One of the earliest recorded accounts of rim driven fan technology for aerospace applications took place in 1961 when the Ryan Aircraft Corporation developed the XV-5A Vertifan aircraft. The XV-5A fans were pneumatically driven by jet engine exhaust gasses acting on turbine blades located around the periphery of the aircraft’s lift producing fans. Although the fan technology was deemed a success the rim drive element was halted, and the fan design went on to be developed for the General Electric CF6 high by-pass engine designs.

Electrical rim driven fan technology for aircraft applications has also been the subject of various theoretical publications since the 1960s, one of the more recent (2006) being conducted at the NASA Glenn Research Centre during which a 32-inch (813mm) diameter rim driven fan was the subject of an active magnetic (levitated) rim bearing study. However, this study did not consider the performance characteristics of the hub-less fan over a range of operational speeds.

A more recent account of electrical RDF technology, conducted at WGU in 2016 involved the manufacture and concept demonstration of a low-cost, plastic, 3D printed, electrical RDF intended for small UAV applications. The study successfully tested a 115mm diameter brushless DC RDF at various speeds but the fan blades and electromagnetic circuit were not optimal, and the input power and thrust values obtained from the test were considered un-representative of the true potential. Even so, the underpinning concept was considered viable and the seed was sewn for the FAST-Fan project.

CFD was used to numerically model the flow through the energiser unit. (WGU)At the heart of the FAST-Fan lies the energiser unit which transfers torque from the motors to the air via the fan blades. There is a finite rate at which energy can be transferred in this way, as governed by Euler’s Work Equation. The pressure rise across the fan is also important because it determines the achievable efflux velocity, which depends on the amount of energy (torque) that can be transferred per swept volume. The energiser unit is therefore optimised to deliver the required maximum amount of energy to the airflow, and the air is entrained in a duct of gradually decreasing volume to achieve the desired pressure rise over the dual fan stages. The closed fan-tips minimise pressure leakages and contra-rotation of the fans ensures that there is no energy lost in efflux swirl as the air flows through the nozzle to generate high-speed thrust. The relative rotor speeds can be independently modulated to control fan performance and in common with turbo-jet engines the FAST-Fan should benefit from ram-effect, especially whilst operating at high speeds and altitudes. Cooling-air enters from vents on the outer surface of the intake and is drawn over the motor windings by the motive flow of the exiting air.

Achieving a low-weight, high-strength and an efficient design was the greatest challenge. From the project outset it was critical to get an accurate prediction of the FAST-Fan’s torque versus speed characteristic, so that the motor circuit design would not be too powerful and heavy for its intended purpose. CFD was used to numerically model the flow through the energiser unit and Motor-CAD and FE (finite element) software was used to model the electromagnetic circuits and analyse the rotor stresses.

As the intention was to manufacture a prototype device. The dimensions of the FAST-Fan were determined by the available budget and prototyping machine capabilities. A 200mm fan diameter was selected based on initial calculations which also allowed a generous margin of up to 300mm diameter for designing and manufacturing the intake and nozzle. A novel manufacturing technique and bespoke tooling were developed to manufacture the bladed rotor assemblies.

The aerodynamic fan blade profiles for modern aircraft are proprietary in nature and information is difficult to access. But such fan blades are often optimised to operate at super-sonic speeds such as 450 m/s, which is much higher than those anticipated for the FAST-Fan with sub-sonic operational air speeds closer to 200 m/s.

Compressor blade profiles were considered more suitable for this application and fortunately the profile and performance data was openly available. A standard C-Series blade profile was finally selected and, based on this, a low aspect ratio (<1.5) fan blade was developed to ensure maximum energy transfer to the airflow, whilst maintaining a minimal frontal intake area.

The Concept Demonstrator FAST-Fan at Wrexham Glyndwr University. (WGU)The analysis, design, and manufacture of the bladed rotor presented unique challenges and provided some interesting and unexpected gains. The rim structure gives support to the blade tips which alleviates the bending, torsional and centrifugal stresses that normally concentrate at the root end and hub region of a fan. The dispersion of these stresses, in the bladed rotor, meant that the blade roots could be structurally and aerodynamically optimised and the adverse effects of operational blade deflections avoided. The blade roots were thinned down and there was no need for complicated dovetailing to the hub structure. Earlier attempts at dovetailing both ends of individual blades into the hub and rim structure had been abandoned on the grounds of unnecessary complication. Instead a homogenous bladed-rotor “blotor!” design was created.

Various motor architectures were analysed including Brushless DC (BLDC), Synchronous AC and Induction Motor technology. A BLDC configuration was soon ruled out (based on previous experience with rim drives) as the concentrated windings, cogging torque, and torque ripple effects were anticipated to cause motor starting and low-speed vibrational problems.

However, permanent magnet technology did seem to offer better torque generation than inductance motor designs. So, a high voltage AC synchronous motor design was developed. The FAST-fan motors are lightweight and incorporate minimal iron stators, aluminium windings and iron-less rotors. Multi-slotted distributed windings are used to minimise torque ripple and vibration. These are only subjected to relatively low currents, because of the higher supply voltage and lower rim force requirements. Which significantly reduces the resistive heating of the air-cooled windings and increases the attainable motor efficiency to above 93%. The resulting compact concept demonstrator design has a maximum power rating of 30 kW and provides an overall Specific Power of 2.5 kW/kg. Which includes the active (circuit) and non-active (structural) components such as the. pylon, intake, nozzle, nacelle, and fans etc.

The FAST-Fan's rim winding. (WGU)Performance predictions were made for FAST-Fan speeds up to the maximum design speed of 15,000rpm, at which the anticipated thrust is 350 N (36kg) and the average efflux velocity 112 m/s (250mph). The initial low speed qualitative testing phase (up to 5,000rpm) has demonstrated that the fan runs quietly and smoothly. This benefit was mainly attributed to two design features: the distance between the rotors provided by the structural stator and the fixed shaft construction which avoids the ‘shaft wobble’ problem inherent with rotating shafts. Initial thrust measurements already indicate that the performance is better than predicted and this is considered to be owing to the effects of thrust induced on the intake lip. The next stage of testing is intended to examine the effects of higher (up to 15,000rpm) and differential rotor speeds on the fan’s performance.

Wrexham's Glyndwr University welcomes collaborations with industry, universities and research organisations and has made its research openly available. (WGU)

The Quick Electric System Test (QuEST) UAV is already in-progress at the university’s UAS laboratory with a team of students, staff and partners constructing this 4m wingspan, 50kg MTOM aircraft intended for flight testing the FAST-Fan.

Looking at wider applications, it became evident that there was a compelling case for the adoption of rim-driven technology for larger aircraft. The results of a comparative study between the theoretical dual-stage RDF and an existing modern small fan-jet engine indicated that, as a thruster device, the RDF offers a compact and lightweight alternative to small fan-jet engines. It will operate at much lower core temperatures than a jet engine and is likely to be lighter in weight, more efficient, easier to monitor and control, quieter and offer much greater values of specific thrust. The next steps involve the design and modelling of an efficient high-thrust (≈10kN) high-speed (up to sonic airspeed), zero-emission rim-driven propulsion device for regional passenger, distributed thrust BWB business aircraft and even high-speed hovercraft applications.

Climate Change has resulted from the use of engineered devices and is likely to be solved in this way too. Seemingly the challenges that lie on route to a Net Zero 2050 are surmountable, provided that the aviation community works in a spirit of collaboration and openness.

WGU is keen to participate in this way and has made all of its published research on the FAST-Fan project freely available via the Wrexham Glyndwr University online research repository https://glyndwr.repository.guildhe.ac.uk/.

Wrexham Glyndwr University also welcomes collaborations with industry, universities, and research organisations. This project was funded by the Welsh Government (WEFO) under the SMARTExpertise initiative (Project Reference 82321) and supported by the European Regional Development Fund.

Dr Rob Bolam CEng FRAeS and Doctoral Research Assistant Jhon Paul Roque 14 March 2023