Final Report Summary - NLF-WINGHIPER (NLF Wing High Speed Performance Test)
Executive Summary:
The EU project NLF-WingHiPer had shown the capabilities of the European Transonic Windtunnel for the high speed testing of natural laminar flow wing configurations. The achieved high data quality was combined with an acceptable low number of turbulent wedges up to a Re number of about 18 mio.
A special aspect of the test campaign was to study the influence of given discontinuities (specified waviness and particles size of a transition fixing) and also any undesirable perturbation on the model surface (gap at the leading edge, escaping sealing material and contamination by inherent particles). The results are showing a high sensitivity on those influences and a need for extremely careful preparation of the model. In contrast to previous laminar test at ETW studies, the fidelity of the test object became dominant compared to the particle issue of the tunnel.
Higher Re numbers and thus a decreasing boundary layer thickness reveal the problem of adhering particles at the leading edge of the wing. Also the unavoidable contamination of the leading edge after a testing time in the order of hours is still a challenge. The measures taken until now to reduce the particle concentration within the tunnel were successful. The particle monitoring equipment is a good indicator for further improvements.
In order to correct the influences of turbulent wedges on the total drag of the model further parameter studies and correction algorithms are needed. The investigation with fixed transition at different height had given valuable instruction in this regard.
The project gave good pieces of advice for understanding of how to design natural laminar flow wings. In this context the testing of NLF configurations at flight Re numbers at ETW plays an essential role. Further research work is needed for a better understanding of transition phenomena at high Reynolds numbers.
Project Context and Objectives:
The objective of the test campaign was the further improvement of the test capabilities of ETW with respect to testing natural laminar flow wings at flight conditions. Several measures should be taken into account reducing the adverse effect of disturbances by particles in the flow resulting in premature transition and the generation of turbulent wedges.
Another aim of the tests is to provide data for the design and construction of models with respect to acceptable manufacturing tolerances for surface steps and surface waviness for maintaining laminar flow at high Re conditions.
Especially the feasibility study regarding the use of large half models in ETW for performance predictions was a core point of interest. For this purpose beside a clean wing the transition of the upper surface flow was forced by transition strips at different positions. This was provided to simulate the effect of imperfections or disturbances at real flight conditions to get information about the sensitivity of the boundary layer at different Re-numbers and sizes of obstacles.
Furthermore the results should be used for validation of CFD predictions on natural laminar flow wing designs.
Beside the technical aspects the dissemination and exploitation of the research work at ETW was part of the project NLF-WingHiPer.
Project Results:
The aim of this test campaign was using the existing model to try and derive a process for predicting the performance levels of a natural laminar flow wing design. As the model was only designed to study imperfections no direct conclusions for high speed NLF geometries can be drawn. Nonetheless, this campaign provided some very useful hints, although some difficulties occurred (see above) and the evaluation of the gathered data is not yet completed.
Reliable and accurate performance predictions are needed to convince the clients of the capabilities of new or modified aircrafts. It is important not to be too pessimistic as this may lose sales to the competitors. On the other hand a too optimistic prediction will soon lose sales once the market discovers that the aircraft does not perform as predicted and, for those sales already made, penalty charges may apply for not meeting performance guaranteed.
A reliable, dedicated CFD derived performance prediction for natural laminar flow aircrafts is presently not available. The back to back process using an existing aircraft as a reference is likely to remain.
Halfmodel testing is the presently preferred approach for specific transition investigations, e. g. steps and waviness effects. For performance (drag) measurements using a full model is recommended. The aerodynamic effect of appearing turbulent wedges is to be corrected numerically.
The objectives of NLF-WingHiPer represent a logical next step with respect to the development of Natural Laminar Wing aircraft configurations. Recent entries with the existing half-model had been devoted to work out the acceptable manufacturing tolerances for relevant wings while this campaign focussed on gaining performance data although it has to be kept in mind that the model used is not a real performance model.
On the way to acquire high quality performance data it had to be proven that those data can be gathered with a big half-model being tested at cryogenic conditions in ETW. Based on recently gained experience upgraded model handling and cleaning procedures had been performed in combination with tuned coating processes for the temperature sensitive paint. The achieved surface finish represents the highest presently achievable quality providing the prerequisite for transition investigations at high Reynolds number flow conditions.
On the facility side enhanced manual cleaning and air filtering as well as specific coating of insulation material generating “Clean Room” conditions in critical areas led to a new higher level of cleanness. Accordingly, tunnel cleaning runs and adapted operating sequences regarding flow acceleration and deceleration led to a major reduction of particles circulating in the main flow as assessed by the reading of a second installed particle counter.
As the absence of any turbulent wedge can’t be guaranteed over longer test periods reference measurements with fixed transition were introduced in the test programme. Gained data allowed the validation of the developed wedge correction procedures with respect to drag measurements. By testing with two transitions bands attached at the same location but featuring different band heights essential knowledge could be gathered about the contribution of band roughness to overall drag.
The performed campaign has remarkably contributed to improve the understanding on how to design laminar flow aircrafts but certainly further steps are required to achieve this challenging goal.
Potential Impact:
With respect to the ACARE 2020 goals the development of aircrafts with a substantial extent of laminar flow on their wings exhibits a promising approach for reducing drag and, hence, being a “green” concept. Starting with the EC project TELFONA ETW has continuously improved its test capabilities with respect to laminar wing investigations up to flight Reynolds numbers. NLF-Wing HiPer has impressively demonstrated the presently existing status allowing high quality performance measurements on large laminar wing half-models including a drag correction procedure. The availability of this worldwide unique capability has opened the door to engagements in the design of future laminar wing aircrafts by providing a qualified test facility.
Regarding exploitation and dissemination a press release will be 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 has been produced providing an overview on the performed project activities at ETW. Here, special emphasis is given to the nowadays mandatory assessment of model deformation illustrating the applied technique in more detail. In the modern design process of an aircraft CFD is playing a major role complementary to wind tunnel investigations. Useful comparisons can only be performed based on a known and identical model shape.
A further relevant activity is a more comprehensive article published in the periodically released “ETW-News” available for download on the ETW-website.
Paper or oral presentations at major international aerospace conferences are under discussion. Due to the confidentiality of data and results the scope of publishable content has to be clarified with the responsible industrial Clean Sky partner and requires approval. This time consuming process did not allow meeting the deadline for a submission of a proposal in 2015. Therefore, a presentation is aimed for the next SkyTech conference in January 2017.
List of Websites:
WWW.ETW.DE
https://www.etw.de/cms/services-research/research
https://www.etw.de/cms/publications/learn-more-movie/deformation-measurements
The EU project NLF-WingHiPer had shown the capabilities of the European Transonic Windtunnel for the high speed testing of natural laminar flow wing configurations. The achieved high data quality was combined with an acceptable low number of turbulent wedges up to a Re number of about 18 mio.
A special aspect of the test campaign was to study the influence of given discontinuities (specified waviness and particles size of a transition fixing) and also any undesirable perturbation on the model surface (gap at the leading edge, escaping sealing material and contamination by inherent particles). The results are showing a high sensitivity on those influences and a need for extremely careful preparation of the model. In contrast to previous laminar test at ETW studies, the fidelity of the test object became dominant compared to the particle issue of the tunnel.
Higher Re numbers and thus a decreasing boundary layer thickness reveal the problem of adhering particles at the leading edge of the wing. Also the unavoidable contamination of the leading edge after a testing time in the order of hours is still a challenge. The measures taken until now to reduce the particle concentration within the tunnel were successful. The particle monitoring equipment is a good indicator for further improvements.
In order to correct the influences of turbulent wedges on the total drag of the model further parameter studies and correction algorithms are needed. The investigation with fixed transition at different height had given valuable instruction in this regard.
The project gave good pieces of advice for understanding of how to design natural laminar flow wings. In this context the testing of NLF configurations at flight Re numbers at ETW plays an essential role. Further research work is needed for a better understanding of transition phenomena at high Reynolds numbers.
Project Context and Objectives:
The objective of the test campaign was the further improvement of the test capabilities of ETW with respect to testing natural laminar flow wings at flight conditions. Several measures should be taken into account reducing the adverse effect of disturbances by particles in the flow resulting in premature transition and the generation of turbulent wedges.
Another aim of the tests is to provide data for the design and construction of models with respect to acceptable manufacturing tolerances for surface steps and surface waviness for maintaining laminar flow at high Re conditions.
Especially the feasibility study regarding the use of large half models in ETW for performance predictions was a core point of interest. For this purpose beside a clean wing the transition of the upper surface flow was forced by transition strips at different positions. This was provided to simulate the effect of imperfections or disturbances at real flight conditions to get information about the sensitivity of the boundary layer at different Re-numbers and sizes of obstacles.
Furthermore the results should be used for validation of CFD predictions on natural laminar flow wing designs.
Beside the technical aspects the dissemination and exploitation of the research work at ETW was part of the project NLF-WingHiPer.
Project Results:
The aim of this test campaign was using the existing model to try and derive a process for predicting the performance levels of a natural laminar flow wing design. As the model was only designed to study imperfections no direct conclusions for high speed NLF geometries can be drawn. Nonetheless, this campaign provided some very useful hints, although some difficulties occurred (see above) and the evaluation of the gathered data is not yet completed.
Reliable and accurate performance predictions are needed to convince the clients of the capabilities of new or modified aircrafts. It is important not to be too pessimistic as this may lose sales to the competitors. On the other hand a too optimistic prediction will soon lose sales once the market discovers that the aircraft does not perform as predicted and, for those sales already made, penalty charges may apply for not meeting performance guaranteed.
A reliable, dedicated CFD derived performance prediction for natural laminar flow aircrafts is presently not available. The back to back process using an existing aircraft as a reference is likely to remain.
Halfmodel testing is the presently preferred approach for specific transition investigations, e. g. steps and waviness effects. For performance (drag) measurements using a full model is recommended. The aerodynamic effect of appearing turbulent wedges is to be corrected numerically.
The objectives of NLF-WingHiPer represent a logical next step with respect to the development of Natural Laminar Wing aircraft configurations. Recent entries with the existing half-model had been devoted to work out the acceptable manufacturing tolerances for relevant wings while this campaign focussed on gaining performance data although it has to be kept in mind that the model used is not a real performance model.
On the way to acquire high quality performance data it had to be proven that those data can be gathered with a big half-model being tested at cryogenic conditions in ETW. Based on recently gained experience upgraded model handling and cleaning procedures had been performed in combination with tuned coating processes for the temperature sensitive paint. The achieved surface finish represents the highest presently achievable quality providing the prerequisite for transition investigations at high Reynolds number flow conditions.
On the facility side enhanced manual cleaning and air filtering as well as specific coating of insulation material generating “Clean Room” conditions in critical areas led to a new higher level of cleanness. Accordingly, tunnel cleaning runs and adapted operating sequences regarding flow acceleration and deceleration led to a major reduction of particles circulating in the main flow as assessed by the reading of a second installed particle counter.
As the absence of any turbulent wedge can’t be guaranteed over longer test periods reference measurements with fixed transition were introduced in the test programme. Gained data allowed the validation of the developed wedge correction procedures with respect to drag measurements. By testing with two transitions bands attached at the same location but featuring different band heights essential knowledge could be gathered about the contribution of band roughness to overall drag.
The performed campaign has remarkably contributed to improve the understanding on how to design laminar flow aircrafts but certainly further steps are required to achieve this challenging goal.
Potential Impact:
With respect to the ACARE 2020 goals the development of aircrafts with a substantial extent of laminar flow on their wings exhibits a promising approach for reducing drag and, hence, being a “green” concept. Starting with the EC project TELFONA ETW has continuously improved its test capabilities with respect to laminar wing investigations up to flight Reynolds numbers. NLF-Wing HiPer has impressively demonstrated the presently existing status allowing high quality performance measurements on large laminar wing half-models including a drag correction procedure. The availability of this worldwide unique capability has opened the door to engagements in the design of future laminar wing aircrafts by providing a qualified test facility.
Regarding exploitation and dissemination a press release will be 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 has been produced providing an overview on the performed project activities at ETW. Here, special emphasis is given to the nowadays mandatory assessment of model deformation illustrating the applied technique in more detail. In the modern design process of an aircraft CFD is playing a major role complementary to wind tunnel investigations. Useful comparisons can only be performed based on a known and identical model shape.
A further relevant activity is a more comprehensive article published in the periodically released “ETW-News” available for download on the ETW-website.
Paper or oral presentations at major international aerospace conferences are under discussion. Due to the confidentiality of data and results the scope of publishable content has to be clarified with the responsible industrial Clean Sky partner and requires approval. This time consuming process did not allow meeting the deadline for a submission of a proposal in 2015. Therefore, a presentation is aimed for the next SkyTech conference in January 2017.
List of Websites:
WWW.ETW.DE
https://www.etw.de/cms/services-research/research
https://www.etw.de/cms/publications/learn-more-movie/deformation-measurements
