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Laminar flow technology

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The European laminar flow investigation (ELFIN) project focuses on laminar flow research for aircraft wing and nacelle applications. ELFIN research focuses on basic flow investigation. Large scale wind tunnel tests with hybrid laminar flow (HLF) wings and nacelles and flight tests are performed to generate experimental data for the validation of numerical stability codes. The wind tunnel tests have given a first insight into hybrid laminar flow stability. They have shown the potential of this technology. Most important for the design of a laminar wing is an accurate transition prediction of the flow. N-factor envelopes calculated from wind tunnel test data were compared with flight test N-factors. Only natural laminar flow (NLF) data are considered. This comparison shows that there is no generally valid calibration procedure for transition prediction based on the N-factor method. A flight test was performed to test the limits of NLF. After a first set of data was accumulated it was recommended that the surface quality of the test glove should be improved. The glove surface was refinished and flights continued. In total 15 flight test hours were logged producing 2 complete sets of data marking the limits of NLF. Numerical work is completely related to laminar flow with heavy emphasis on stability analysis for transition prediction. Boundary layer data generated from surface pressure distributions supply the input for stability calculations. Existing boundary layer codes were modified and curvature terms were included in the stability codes. For the considered laminar profile the N-factor calculated is reduced by approximately 30%. This result makes the comparison of results more reliable. To ease design engineers work a 2-dimensional database method was established. This database is not just a set of experimental data but is a set of precalculated stability data.
A large scale model with boundary layer suction for hybrid laminar flow (HLF) control was designed with tested in the Office National d'Etudes et de Recherches Aerospatiales (ONERA) SIMA wind tunnel. A suction panel on the glove (main suction system (MSS) has been designed and the variable sweep angle range has been increased up to 28 degrees where high cross flow instabilities (CFI) were expected. Futhermore, a retracted Krueger flap resulting in 2 lower side chordwise gaps has been simulated by using suction in the second (Krueger gap suction (KGS)) and in a panel behind the second gap (Krueger panel suction (KPS)). Finally, suction on the wing's root has been applied in order to avoid attachment line transition (ALT) (ALT avoidance suction system (ASS)). Measurement techniques involved in the HLF test were: pressure measurements; infrared imaging (IR); a wake rake mounted at the middle flap; a boundary layer rake and CPM probes; standard hotfilms behind as well as miniature hotfilms on the suction plate; a special designed wind tunnel environment sensing probe (WESP) containing unsteady pressure transducers, microphones and a hot wire for correlating wind tunnel unsteady data with boundary layer data on the model. At 28 degrees sweep and under realistic transonic conditions laminarity was gained up to the end of the HLF glove. Furthermore, at lower sweep angles the disturbance to the laminar boundary layer caused by 2 gaps of a retraced flap has been surpressed by suction, too. Advanced measurement techniques have been applied and tested, so that for future tests sensors can be chosen for any special purpose.
This project specified and designed a wind tunnel tool to investigate the aerodynamic problems induced by the interaction of engine slipstream and airframe at high subsonic to transonic speeds. This tool was a large modular generic wind tunnel model, powered with air turbines for engine simulation. First of all, the aircraft to be simulated with the GEMINI model were identified and fully defined (specification, shape and general arrangement). A number of alternative parts were included for later use. The wind tunnel operators were also involved in designing all the missing or new interfaces with their facilities (balances, stings, electronics). Then the model design started from both structural and instrumentation sides. Its heart was the skeleton, a primary structure on which the complete set of components (wings, fuselages, tails) was to be fitted in the ad hoc configuration. Its skin was also fully defined on manufacturing drawings. Another important part of the project was the power plant simulator itself. This consisted of an air driven turbine, the propellers and all related instrumentation, such as rotating balances designed to read thrust and torque during the tests. These pieces of hardware were specified and designed by specialists. All the technical objectives of the project have been met. Even more, a consistent programme for wind tunnel testing has been initiated, including all the procedures necessary for its successful completion. More than 1000 documents (drawings and memorandums) have been produced.

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