The 2020 ACARE targets present a challenge to aircraft manufacturers to reduce CO2 emissions through engine efficiency and aircraft design improvements. A 'pro-green' aircraft configuration has been proposed that has a significantly higher aspect ratio wing and lower wing sweep than today's standard designs. This reduction in sweep opens the possibility to design a wing for natural laminar flow (NLF). Such a wing could enable a 20% wing drag reduction in comparison to today's designs.
The major objective of TELFONA was the development of the capability to predict the in-flight performance of an NLF aircraft using wind tunnel tests and CFD calculations.
A number of supporting objectives have been defined:
- transition prediction tool calibration for NLF aircraft testing in ETW using a specially designed wind tunnel model;
- improvement of transition receptivity models using wind tunnel test data from ETW and small-scale facilities to understand better how surface quality and atmospheric conditions influence transition mechanisms;
- development of methods for predicting the in-flight performance of an NLF aircraft, including understanding whether conventional scaling approaches using low Reynolds number wind tunnels can be used;
- validation of the developed methods through the design, manufacture and test of an NLF wing designed for high performance;
- development of technology for wind tunnel testing of hybrid laminar flow control wings.
A pathfinder wing was designed to determine the N-factor levels within ETW with the emphasis on cross-flow and TS instabilities. The design was then made into a wind tunnel model with pressure tappings and sensors for transition detection. The ETW test included measurement of boundary layer data and oncoming flow quality.
The PETW pilot facility was used to prove the proposed measuring techniques before the wind tunnel model was made. The pathfinder model was tested in a wide range of conditions to build a large database of results and these test results were used to determine the N-factor characteristics of ETW. The transition calculations using test data were compared to results from the design phase.
Test data was also used to develop the means of linking the flow characteristics in the wind tunnel with the measured transition behaviour. This activity was supported by university wind tunnel tests. Pathfinder test data was used to determine performance-scaling methods for NLF aircraft and the design of the performance wing used the new calibration data. The performance wing was used to demonstrate NLF drag reductions and was also tested in ETW. The range of test conditions of this test were reduced compared to the first test and were representative of flight conditions.
The results from the performance test were analysed to validate the N-factor calibration method and the performance scaling methods, and hence provided a validated in-flight performance prediction for a large NLF aircraft. In addition, a small activity was undertaken to identify a means of testing a future HLFC suction system.
The successful completion of the project was expected to result in two wind tunnel model tests and improved transition prediction methods. The first 'pathfinder' model was tested in ETW with the results being used to calibrate transition prediction methods, and to provide insight into the receptivity problem. The second 'performance' model was designed using the calibrated methods and will be used to demonstrate the potential of a large NLF aircraft.
Other expected project results:
- Experience in the laminar wing design process;
- Validation of CFD methods for laminar flow technology;
- Validation of wind tunnel testing (ETW) of laminar flow wing (NLF);
- Reliable scaling method(s) for wind tunnel to flight extrapolation;
- Knowledge of receptivity of B/L for noise and turbulence;
- Knowledge of performance of NLF HARLS wing;
- TELFONA results are also applicable on laminar nacelle.