FLIRET - Flight Reynolds Number Testing
Overview
Background & policy context:
Despite considerable progress in computational aerodynamics, wind tunnels are still the prime tool to measure and to predict aircraft performance for take-off and cruise, design and off-design conditions However, conventional wind tunnels face physical limits in matching Reynolds and Mach number ranges required to realistically simulate cruise conditions. A means to overcome this limit are cryogenic wind tunnels, for instance, the European Transonic Wind tunnel (ETW).
Objectives:
The 'Flight Reynolds number testing' (FLIRET) project's objective was to improve the accuracy of performance measurements at flight Reynolds number in cryogenic wind tunnels where the highest measuring accuracy is needed to predict the flight behaviour and performance of new aircraft. But there is also a considerable improvement for the handling quality and loads testing. The performance guarantees given to airlines trust to a large extent the accuracy of wind tunnel test data and their extrapolation to flight conditions. Similarly, a high fidelity simulation of the aircraft in the wind tunnel is the best way to avoid aerodynamic difficulties during flight testing and hence reduces the time to market and cost.
Teh FLIRET project focussed on aircraft model mounting techniques in cryogenic wind tunnels since they have a significant influence on high Reynolds number performance measurements. FLIRET investigated several model-mounting alternatives and compared the devices with existing state of the art stings. This approach appeared reasonable since most of the stings used to date had been designed more than ten years ago. With support of state of the art CFD tools it was hoped to achieve a reasonable progress in measurement accuracy.
Another objective of FLIRET was to better integrated CFD (Computational Fluid Dynamics) simulation capabilities and wind tunnel testing. It was intended to clarify the advantages and the disadvantages of numerical and experimental work to take maximum benefit of synergy effects. The FLIRET approach offered a lot of opportunities for identifying weaknesses of each method and to combine their strengths.
Methodology:
The following work was done:
- Designing and manufacturing of several model mounting devices (stings);
- Applying and harmonising CFD and prediction tools including the numerical meshes;
- Analysing the test results of each FLIRET work package;
- Analysing the applied model quality, manufacturing and handling strategies;
- Deriving recommendations for industrial testing in cryogenic tunnels.
Based on numerical simulation, new and improved designs were used of straight stings, fin stings and twin stings. Ten test campaigns were performed in the ETW, the ETW pilot tunnel and the Aircraft research association (ARA) tunnel with four new sting configurations and two existing ones. One test with a new 2D-model and three half model tests were performed. One of them required three different peniches, which ended up in nearly three small, but separate measuring campaigns.
For most of the tested configurations improvements were found. It is estimated that FLIRET managed to raise testing accuracy in cryogenic tunnels, but in particular in the ETW by about 10% with reference to the state of the art at the start of the project. This was demonstrated by utilising FLIRET's new sting configurations. Unsurprisingly, the new stings have each to be used under specific model and wind tunnel conditions. A universal sting which allows excellent measurements under any condition is not feasible. For example, the minimum size straight sting provides reference data in a limited loads window and the blade sting guaranties very stable model behaviour in the wind tunnel.
A full design process including a detailed analysis of model/support interferences was performed with two sting designs being selected for detailed analysis of sting interferences. CFD results showed encouraging results showing our ability to reduce the interference between stings and models. This has shown the large benefit from designing a support to the model itself, instead of trying to adapt an existing support. The analysis has also shown the difficulty to define wind tunnel corrections since the efficiency of a support is strongly dependant on the way the correction is defined.
The aero-lines of the Straight and Fin Stings have been worked out leading to the minimum size straight sting and the optimised fin sting designs. Their performance has been assessed with Navier-Stokes codes and compared to the reference stings. This comparison is showing a clear reduction in the level o
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