Mach number control improvement in ONERA s1ma large sonic wind tunnel

I. Introduction Improvement of wind tunnel productivity and data base quality is a key issue for all the major wind tunnel in the world. As a consequence, accurate Mach number control for large wind tunnels is essential. Indeed, a significant part of the tests performed in these wind tunnels deals with civil aircraft for which test matrix consists in a large number of polar in angle of attack at different Mach numbers. During pitch motion, pressure losses change in the wind tunnel inducing Mach number drifts which are penalizing both for data base quality and test productivity. ONERA, on the framework of the 7th European Programme, launched in 2008 the study of a new S1MA Mach number controller based on hard and software improvements. This work is part of the European project ESWIRP (European Strategic Wind tunnels Improved Research Potential) which deals with the improvement of European strategic wind tunnels (DNW LLF, ETW and ONERA S1MA). For S1MA new Mach controller, the objective is very ambitious since it aims to obtain Mach number stability during a pitch polar within ± 0.001at least up to Mach 0.9. II. S1MA Mach number drift without control and Mach stability in steady conditions During aircraft tests, the model is pitched through a series of angles of attack at a requested Mach number (pitch polar) and drag strongly changed with pitch angle. Drag increase induces a stagnation pressure disturbance then propagates downstream to the fans and sets up a dynamic unbalance on their equilibrium. Finally, without an efficient Mach number controller, a Mach number deviation is observed. To fix some order of magnitude, Mach number drifts, during pitch polar without any Mach control, for a civil aircraft mounted on a Z sting, can reach ±0.030 at Mach number 0.94. A perfect Mach number controller has to cancel these drifts but, of course, residual drifts will still be present. Original objective, which was very ambitious, was to maintain Mach number drifts during pitch polar as small as the Mach stability in steady conditions. III. Initial improvements of S1MA Mach number controller Since 1985, to increase test productivity, ONERA continuously improves its S1MA Mach number controller using a rough control of fan's RPM by increasing or decreasing the water mass flow on the Pelton turbines. With this first Mach number controller, a significant decrease of the Mach number drifts was obtained for Mach numbers ranging from 0.72 to 0.86. However, at high Mach numbers, drifts were still very large. It was obvious, that an additional device, with a transfer function involving short delay and time response between action and Mach response was mandatory. The idea to use an air intake to generate pressure losses, opposite to the ones induced by the model incidence, was selected. IV. S1MA new Mach number controller ONERA launched in 2008, on the framework of the 7th European Programme, the study and the manufacturing of a demonstrator for a new S1MA Mach number controller. The idea was to use fan's RPM control to compensate large pressure losses with some time delay in association with an air intake which can generate limited but fast pressure variations in phase opposition with those of the model during its pitch motion. A parameter identification by applying determined entries and observing associated responses in amplitude, delay, time response was done (Transfer function between the model pitch angle and the Mach response, transfer function between fan RPM and the Mach response, transfer function between air intake opening and Mach response) After these identifications, a long and complex optimization process using numerical simulations was carried out and process ended with an experimental adjustment of the new Mach number controller. After this complex adjustment, Mach number drifts during pitch polar using the final controller were found as small as the natural fluctuations of Mach number in steady conditions, which is a great success. V. Improvements generated on data base quality and test productivity For data base quality, as Mach number drifts can be considered as negligible with the new Mach number controller, model pressure distributions, dynamic measurements and acoustics measurement are now provided at the exact requested Mach number. Furthermore, this improved Mach number control will be of great interest for wake survey tests and laminarity tests for which aerodynamic stability is crucial. Considering wind tunnel productivity, loads interpolations are "easier", leading to the cancel of all the pitch polar which were requested for load interpolations purposes only. Then productivity gains around 25% have been obtained. VI. Conclusion With the new controller, residual peak to peak Mach number drifts, during pitch polar, do not exceed the natural fluctuations of the Mach number in steady conditions which is a very impressive result. This impressive improvement on Mach number stability generates an increase of the tunnel productivity of around 25% which is of great interest for the attractiveness of the S1MA World Class wind tunnel. It is equally important to underline that this Mach stability induces a great improvement of the aerodynamic data base quality for all the data than cannot be interpolated. This new Mach number controller is now used on a regular basis in the ONERA S1MA large sonic wind tunnel, for the financial and technical benefits of all our Clients. Acknowledgments The authors would like to thank the European Commission for the funding of the improvement of the S1MA Mach number controller, in the frame of the ESWIRP project under the 7th European Programme.