BinCola – It's the real thing
Wind tunnel tests are an essential phase in the process of aircraft design. But simulating laminar and turbulent airflow at scale, and identifying the transition zone between the two in realistic operating conditions, is a complex undertaking. Clean Sky's BinCola project successfully leveraged temperature sensitive technologies to address the challenge.
Another Clean Sky success story has recently concluded – this time on the outskirts of Cologne at the European Transonic Windtunnel GmbH (ETW), which operates a high Reynolds number advanced wind tunnel test facility. It's one of only two wind tunnels in the world (the other is the National Transonic Facility at NASA, Langley) that is capable of authentically replicating the aerodynamics of real aircraft during take-off, landing and operation at high altitude. ETW's unique pressurised and cryogenically operated testing facilities were a prerequisite for the very specific ambitions of Clean Sky's BinCola (Evaluation of the Benefits of innovative Concepts of laminar nacelle and horizontal tail plane (HTP) installed on a business jet configuration) project.
The BinCola consortium consists of ETW (project coordinator), Deharde GmbH (responsible for manufacturing the wind tunnel models and associated coatings) and Laserline Gesellschaft für Entwicklung und Vertrieb von Diodenlasern GmbH (which developed the array of laser and optics that were used to heat up the surfaces of the scale models in the testing process). With Dassault Aviation as topic manager, they collaboratively investigated the use of extended laminarity in the nacelles (engine housings) and HTP of a business jet, to reduce drag and increase aerodynamic performance. The idea is to improve the efficiency of future aircraft by reducing fuel consumption and lowering CO2 and NOx emissions in line with Horizon 2020 objectives. The total budget for this 16-month project, which ended in February 2020, was €1.5m.
‘We're trying to increase the performance of the aircraft, which in the BinCola project is a rear-engined business jet, and in order to do that we need to reduce drag,’ says Dr. Sonell Shroff, Clean Sky 2 Project Officer. ‘In this project the BinCola consortium tried and succeeded in testing the extended laminarity on the nacelle and horizontal tailplane. It was important to pinpoint where the transition occurs between laminar and turbulent flow. When we use optimally shaped surfaces the flow close to it remains smooth over wide areas so that we have less drag and more lift, contributing to the efficiency of the aircraft and hence lowering fuel consumption and reducing pollutants.’
Relevant wind tunnel tests were successfully performed during September 2019 at flight Mach numbers (aircraft flight speed relative to the speed of sound) and Reynolds numbers (the ratio of inertia force to viscous or friction force). These can be used to determine where flow changes from laminar to turbulent, aka the transition zone at model scale. The tests, carried out at ETW, were effectively a validation exercise to advance computational work carried out by Dassault Aviation, which had performed computational fluid dynamics (CFD) simulations to predict where that transition zone would occur.
‘The difficulty is that when you start reducing the scale of an aircraft to model size you also affect the flow around the aircraft, so you have to adapt the flow gas’ properties accordingly so that the aerodynamics remain the same. This is the speciality of the ETW – they're able to maintain realistic testing conditions with a full-sized flying aircraft,’ says Dr. Sonell Shroff.
Dr.-Ing. Guido Dietz, Managing Director at ETW, explains the details: ‘If you scale down from a real aircraft into a wind tunnel model you need to maintain some fluid mechanics similarity conditions in order to have the flow representative of what happens in real terms. You need to maintain geometric similarity, so the outer shape of the scale model matches that of the real aircraft. The second similarity is that of the Mach number. As the aircraft speed increases, the shorter the time becomes for air particles to evade the flying aircraft, so the outer flow field, far above the aircraft surface, and its topology is strongly governed by Mach numbers together with the geometry.’
‘The next similarity law that must be considered is the Reynolds number, which is the ratio of inertial forces to friction forces,’ says Dr. Dietz. ‘This mainly affects the thin layer immediately above the aircraft surface, also called the boundary layer. On the surface itself, the air is stuck at zero velocity. At a certain distance above the surface, you face hundreds of kilometres per hour, and in between you have the boundary layer, where the flow speed changes from zero to hundreds, and there you have a lot of shear forces. This region is mainly responsible for the drag of a civil aircraft, and it is responsible for the loss of energy in the flow. Thus, it also determines where the flow can no longer follow the curvature of the wing and may even separate such that lift would be lost. The boundary layer on a normal aircraft may have some centimetres of thickness, where all this happens, and if you scale the model down to wind tunnel size you also need to scale the boundary layer down – but this is virtually impossible in conventional wind tunnels.’
ETW’s unique facility made it possible for BinCola to perform their scaled tests by increasing the density of the flow, packing the gas particles closer together and by decreasing the temperature, ‘calming down’ the gas particles to reduce the flow viscosity, in sum recreating realistic flying conditions.
In order to effectively and efficiently locate the transition zone on an aircraft part (such as a nacelle) in the wind tunnel, the BinCola team's first task was to evaluate and determine which of the two types of temperature sensitive paint (TSP) technologies, developed and applied by the German Aerospace Center DLR for cryogenic conditions (down to -163 °C), would be most appropriate for the testing process.
One option was to use a fixed infrared laser beam distributed via a lens to heat up the (Temperature Sensitive Paint) TSP-coated model parts. The TSP then shows where the surface heat flux is higher or lower, so that it is possible to distinguish where the flow stays laminar or becomes turbulent, revealing the transition zone.
The other option is to embed a layer of carbon nanotubes below the same TSP. Then, by attaching two wires and feeding electricity through it, it heats up, creating a temperature difference between the surface of the model and the air flow around it. From that it is also possible to observe where the laminar and turbulent boundary layer occur.
‘What was important to understand was which option would be easier to collect data from, which one is more controllable, how reliable one method was over the other, and how usable it would be, because embedding a layer of carbon nanotubes is not easy. Also, with temperature sensitive paint, if the application is too thick, it can delaminate and detach from the surface. If it's too thin then it just cracks on the surface. So, those are the kinds of problems that we faced,’ says Dr. Shroff.
Dr. Dietz commented on the investigation into the two potential methods, saying that ‘although in some cases, nanotubes might be the superior technology, after investigating both techniques in our facilities, we now think that the infrared radiation is more attractive because you just install the TSP-coated model in the wind tunnel and radiate on its surface. An additional layer of carbon nanotubes increases the risk that the paint will crack and peel off due to the temperature differences that we have in our wind tunnel. Thus, in the main ETW test we just applied the infrared laser heating because it simply works, and it's likely to be better suited for future industrial testing than the nanotube technique.’
‘BinCola successfully verified and quantified the benefits that an innovative aircraft design may gather from making use of natural laminar flow (NLF) on engine nacelles and horizontal tail planes,’ says Dr. Dietz. ‘Besides BinCola project’s main goal, ETW enhanced the productivity of its NLF test methods by establishing infrared laser heated TSP. Compared with the classical method, which provokes a heat flux by step-changing of the flow temperature, the infrared laser method is about 5 times quicker and less costly.’
Another relevant factor is the use of a business jet model in the project, says Dr. Dietz. ’On a business jet, laminar flow is easier to be exploited than on large commercial jets. Due to their smaller size and the associated lower flight Reynolds numbers, it is easier to maintain laminar flow over large portions of the aircraft surface in a ’natural’ manner, in other words by proper shaping of the aircraft surface. More laminarity means lighter engines, less energy consumption, and cleaner aircraft.’
Beyond the specific mission of the BinCola project, Dr. Dietz accentuates the broader advantages of collaborations with the Clean Sky JU: ’The Clean Sky ecosystem enables ETW to contribute its unique features to advance aeronautics research into product innovation, to advance its own test methods and tools, and to establish these in the aircraft design processes to the benefit of ETW’s industrial clients. It also enables ETW to team up with academia, other research establishments and SMEs to boost European aviation R&D.’
Finally, from the Clean Sky perspective, Dr. Sonell Shroff says that ’the results are extremely useful. When they did the final tests it was with a complete model of a scaled aircraft with a nacelle on it, and we collated a unique database to advance the exploitation of laminar flow control towards clean aviation.’