Morphing rotor blades for more sustainable helicopters
SABRE Shape Adaptive Blades for Rotorcraft Efficiency Project Objectives
Dr Benjamin Woods, Principal Investigator of the SABRE (Shape Adaptive Blades for Rotorcraft Efficiency) project explains the complexities and significant benefits of developing a new approach to helicopter design, which uses blade morphing technologies to optimise performance and reduce fuel burn, CO2 and NOx emissions by 5-10%. Helicopter rotor blade
design has progressed significantly over the last 50 years, with major advancements having been made in materials, manufacturing methods, and aerodynamic sophistication, leading to improved performance. Engineers are approaching the limits of what can be achieved with traditional designs however, as even the most modern designs are stuck with a single, fixed aerodynamic shaped which is expected to perform well in the widely varying operating conditions the blades will see. This is where the teams working on SABRE come in. Their goal is to create a rotor blade that can continuously adapt its shape to the changing operating environment of the helicopter. Considering that rotor blades spin at hundreds of rotations per minute, generating huge forces and strains on the parts in the process, this is no mean feat. But by adapting to these very quickly changing aerodynamic conditions allows the rotor to always be at peak efficiency. The same morphing geometries can be used to adapt to slower changes in conditions, such as changes in air temperature or the amount of payload carried. As Woods puts it: “The industry has spent decades trying to ‘eke’ out performance with rigid blades and we are way into the diminishing returns portion of that curve, so now if you have a blade design that is one per cent better in terms of fuel burn, through airfoil shape or some fancy new material, that is considered quite an achievement with current technology. What we are proposing is to have a step of between a five to ten percent reduction in fuel burn, which would be fantastic for the industry. Once you introduce the option to have the blade shape change continuously at the same rate that the operating conditions change at, you have a lot more potential for efficiency and the main focus for us is to reduce the fuel burn required for the helicopter to fly.”
When airflow is a drag Current blade geometries are prone to an inevitable flaw in efficiency. The problem with the helicopter, particularly when flying forward, is that the part of a blade sweeping into the wind, known as the ‘advancing’ side,
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Reducing energy consumption and environmental impact of aviation: Reducing helicopter fuel burn and emissions by 5 – 10% Improving European industrial competitiveness and reinforcing employment: The results of this project will generate substantial recurring savings in direct operating costs, offsetting the initial cost of the added technology over the life or the airframe thus keeping Europe competitive and securing employment within the industry Knowledge transfer of morphing technologies: The methods used in SABRE could have positive implications for fixed wing aircraft and through active blade control it is also potentially adaptable to large scale wind turbines, making them more cost efficient Delivering international collaboration for innovation: Fostering new partnerships which will help integrate new knowledge into industry after the project end
FishBAC morphing airfoil.
will see very high velocities since the forward flight speed and rotation velocity add together. However, on the other side of the rotor (the so called ‘retreating’ side) the blade is moving away from the oncoming wind, and so the rotation velocity subtracts from the forward flight speed – leading to much lower air velocities. These differences in velocity greatly affect the amount of thrust the rotor generates, and significantly increase the amount of drag
Six ways to change a blade SABRE’s research efforts derive from several European teams working in collaboration including University of Bristol, Centro Italiano Riecerche Aerospazaili, the German Aerospace Center, Delft University of Technology, Technical University of Munich and Swansea University. Between them, six promising morphing blade concepts are being investigated. These are Fish Bone Active Camber (FishBAC), Translation
If we can handle the physics of making morphing work in a tiny helicopter blade spinning around at hundreds of rpm with huge centrifugal forces trying to rip them apart, there is a good chance we can make it work for wind turbines and commercial airliners. that must be overcome by burning more fuel to make more power. Current designs of helicopters are able to change the overall pitch of the blade as it spins around the helicopter in order to partially balance out the impact of these changing velocities, but since this moves the entire blade, it is not able to account for desired changes along the length of the blade – leading to increased fuel burn. If instead the blade geometry could be continuously changed and optimised – both along its length and as it spins around, then the need to compromise on the aerodynamic performance would be removed, with the rotor always having the best possible shape for the exact combination of conditions it is experiencing at that moment in time. Furthermore, these morphing blades would be automatically controlled by computers and they would respond to measurements of the exact current conditions using pressure and strain sensors, to ensure that the reductions in fuel burn don’t make the pilot’s job any more difficult.
Induced Camber (TRIC), Inertially Driven Twist, Shape Memory Alloy Driven Twist, Chord Extension, and Active Tendons. There are many key design parameters that should be considered when creating rotor blades. For example, you should ask ‘how much curvature is there in the airfoil?’, ‘how does the length of the airfoils vary over the blade?’, ‘how much twist is there in the blade?’. These are the types of features that traditionally engineers play with to optimise rotorcraft performance,
with compromises being required due to the wide range of operating conditions. “Those are the exact same things that we would love to be able to change – but in real time – to allow us to fully respond to the different operating conditions without the need to compromise. We are looking at exactly those factors,” explained Woods. “We have two different concepts that can actively change how much twist the blade has. We are also investigating two different ways of actively changing the airfoil curvature (known as camber). We have a concept which can increase the length of the airfoil in a smoothly varying way, and we have an active tendon concept which uses tensioned cables, not to change the shape of the blade, but to alter its dynamic response of the blade. A harsh reality of helicopters is that the huge variations in force during rotation excite, or vibrate, the long skinny blade structures. When you are thinking about changing the blades as much as we are with these morphing devices, it is important that we have a way to mitigate any negative impacts on the dynamic response of the blades, and our active tendon concept gives us that ability. ”
A revolution in efficiency The technologies being developed in SABRE have the attractive mix of making economic sense whilst in tandem helping achieve a step in the right direction for sustainability aims. “Efficiency matters for the operators,” said Woods. “My personal motivation is sustainability, but even if the operator of the helicopter doesn’t care at all about sustainability (which thankfully is becoming less common), they would definitely still benefit from the fuel that they don’t have to
The SABRE project vision
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buy because we made the helicopters more efficient. So, increasing efficiencies is the best way forward for everyone using helicopters.” The implications for the environment are major if the technology is rolled out. Reducing emissions from aviation is a goal of the EU in line with its 2030 climate objectives, so finding technologies that slash fuel use is a vital part of the solution. During the global Covid19 pandemic, aviation as a sector has suffered greatly and when the dust settles, surviving operators will be keen to innovate their aircraft toward higher efficiencies. This brings us to a major point around knowledge transfer of SABRE’s research. Whilst the focus of the project has very much been on helicopters, the concepts that have been developed could be adapted for fixed wing aircraft and wind turbines. Considering these are two sector giants, the real savings and implications for reducing fuel use, pollution and saving money are very exciting indeed. “The physics of helicopters makes it super challenging to find efficiencies. While wind turbines and fixed wing aircraft are by no means ‘easy’ either, if we can handle the physics of making morphing work in a tiny helicopter blade spinning around at hundreds of rpm with huge centrifugal forces trying to rip them apart, there is a good chance we can make it work for wind turbines and commercial airliners.” The partners involved are now preparing to test demonstrators, turning the analysis into hardware and lining up these flexible and adaptive structures for wind tunnel and whirl tower testing. Helicopters may soon benefit from these research efforts in terms of performance, economic savings and reduced environmental impact of aviation by burning less fuel in flight.
Project Funding
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 723491
Project Partners
https://sabreproject.eu/#partners
Contact Details
Benjamin King Sutton Woods Department of Aerospace Engineering Queen’s Building University of Bristol University Walk BS8 1TR United Kingdom T: +44 (0) 117 33 15366 E: ben.k.s.woods@bristol.ac.uk W: https://sabreproject.eu/#header W: https://youtu.be/NqJtZiqPIzY
Dr Benjamin King Sutton Woods
Dr Benjamin K.S. Woods started in the Department of Aerospace Engineering at Bristol in 2015 and is now a Senior Lecturer in Aerospace Structures. He is currently an EPSRC Early Career Fellow and is leading the Horizon 2020 project SABRE. He holds 7 US patents, has authored 28 journal publications, two book chapters, and 58 conference papers.
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