Within the Clean Sky project BinCola effective and efficient cryogenic pressurised NLF testing at flight-relevant conditions have been established, comprising of:
• Temperature Sensitive Paint (TSP),
• Precision TSP coating by milling and polishing,
• Infrared (IR) laser radiation and Carbon Nanotube (CNT) based heating to locate laminar / turbulent boundary layer transition.
New capabilities demonstrated and database collated for an innovative jet design featuring laminar nacelles and a laminar horizontal tail plane (HTP):
• Benefits from use of extended laminarity shown,
• Data vital to advance and validate numerical tools,
• CNT based TSP heating challenging but valuable for specific cases,
• IR based TSP heating suited for high-productivity testing, ca. 5 times faster and less costly compared with common temperature-step method.
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). The data obtained can be used to determine where the 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.
In order to effectively and efficiently localize the transition zone on an aircraft part (such as a nacelle) in the wind tunnel, the first task was to evaluate and determine which of the two types of 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 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.
The BinCola project 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. Besides the main goal of the BinCola project, 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. 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.
Regarding exploitation and dissemination, a press release was published on the ETW website www.etw.de. The performed Clean Sky project is going to be incorporated in the company profile listing ongoing and completed European research projects. As a major dissemination item, a video clip featuring the “Innovative Application of Temperature Sensitive Paint (TSP)” has been produced and published on the ETW website (
https://www.etw.de/publications/learn-more-movie/bincola-measurements(öffnet in neuem Fenster)).
A paper about the CNT methodology of the “Application of Temperature sensitive paint to investigate Laminar-to-Turbulent Transition on Nacelles”, developed within BinCola was presented on the AIAA SciTech Forum in Orlando, Florida by DLR. The “New Efficient Method for Transition Detection by TSP on a Full Span Model by use of Infra-Red Laser” was orally presented by ETW on the same conference. Furthermore oral presentations on the CEAS conference in Bordeaux in February 2020 have been given by representatives of Dassault and ETW.
A further relevant activity is a more comprehensive article published in the periodically released “ETW-News” available for download on the ETW-website.
