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Flight Reynolds Number Testing


FLIRET is closely related to aerodynamics and the wind tunnel testing of aircraft models, which has to guarantee a maximum of similarity to the flying aircraft. Only pressurised cryogenic wind tunnels can perform tests at Reynolds numbers, needed for a medium or large-sized aircraft. In contrast to conventional wind tunnels, they provide complete dynamic similarity to the flying aircraft. This type of testing still needs to be improved to get the full benefit of the new technology.

Project objectives

FLIRET’s objective is to improve the accuracy of performance measurements at flight Reynolds numbers in cryogenic wind tunnels. The project focuses intentionally on model mounting techniques under cryogenic conditions. Model mounting devices have a significant influence on high Reynolds number performance measurements, which are currently compensated by empirical correction methods.

As far as the aircraft wing is concerned, testing at the flight Reynolds number provides the opportunity to reduce the weight of the wing as the profile thickness can be increased at the rear spar position, due to the thinner boundary layer and its high potential to act against adverse pressure gradients. The profiles can be optimised and tested in the Re-range occurring for the aircraft. There is no need to design wings for lower Reynolds numbers because of the constraints of conventional wind tunnels.

FLIRET will provide the missing links for industrial use of cryogenic testing. This includes contributions to special problems for complete and half-model testing, and the interactive use of advanced CFD tools for cryogenic testing.

Description of the work

Specialists from 16 partners in seven European countries are participating in FLIRET. The spectrum ranges from model design and manufacture to wind tunnel testing. The application of fluid dynamic design tools for optimisation purposes of supports and of advanced CFD is included in the work share to get the full benefit of all relevant disciplines.

The work is done in four Work Packages. Work Package 1 is the most important one. It is based on a support development strategy, which will provide improved supports for different test cases and model types.

Work Package 2 is devoted to high-speed buffet onset and model vibrations for complete models. Again, CFD specialists are strongly involved, to contribute to the testing process and to validate their tools for this class of test problems.

Special low-speed problems (related to take-off and landing of aircraft) are solved in Work Package 3 for half models. Clear recommendations concerning model roughness are expected in task 3.1 reducing costs and avoiding erroneous effects on flow similarity and accuracy of wind tunnel data. Task 3.2 will clarify special wall effects in cryogenic wind tunnels to ensure the consistency of half and complete model results under these special conditions.

Work Package 4 provides the final analysis and integration of all results.

Expected results

FLIRET will provide improved tools to increase the accuracy of cryogenic testing and to reduce test times. Both are necessary for an industrial use of cryogenic testing in the European aircraft industry.

Based on the expected results, a new wind tunnel strategy is under development allowing a high Reynolds number design of new aircraft. This strategy can make full use of the dynamically similar simulations of aircraft offered by cryogenic testing, leading to reduced wing weights and fuel consumption.