Flight Reynolds Number Testing


FLIRET is a Specific Targeted Research Project (STREP) funded under the sixth framework programme of the European Commission. An international consortium of 16 partners from industry and research has decided to take the effort to improve the accuracy of performance measurements at flight Reynolds number in cryogenic wind tunnels. 

Wind tunnel test are indispensable for predicting the aerodynamic performance of new aircraft designs. In order to produce realistic results the test has to guarantee an optimal similarity to the real flying aircraft. Pressurised cryogenic wind tunnels can perform such tests creating the Reynolds numbers needed for a realistic simulation of medium for large sized aircraft. In contrast to conventional wind tunnels, cryogenic tunnels operate at very low air temperatures allowing fluid dynamics conditions similar to the flying aircraft at cruising altitudes to be created. However, the highly accurate flow conditions require adequately accurate testing conditions of the model and the model support. Otherwise the full benefits of the costly cryogenic testing technology are not achieved.

Testing aerodynamic performance in wind tunnels is a decisive element in aircraft design and marketing despite the advanced possibilities of computational aerodynamic analysis. Cryogenic tunnels in which air temperature can be brought as low as -163°C offer the possibility of realisitc simulation of Reynolds and, at the same time, Mach numbers.

One of the cruical points of measurements in wind tunnels still is the attachment of the aircraft model within the tunnel. This is why FLIRET has set its focus on the problem of support corrections and prediction and on the testing of various model mounting devices.


The major objectives of FLIRET are to

  • advance capabilities of cryogenic testing and measurement technology,
  • reduce the cost of wind tunnel tests,
  • improve competitiveness of large European testing facilities.

FLIRET’s objective is to improve the accuracy of performance measurements at flight Reynolds number 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, the flight Reynolds number testing 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 still missing links for the 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.

Expected indirect benefits from the project might be a possible reduction of wing weight by 10% and a consequent fuel reduction of up to 3% per passenger for large aircrafts.  Another aspect of the project is the intense combination of advanced CFD tools and wind tunnel testing.

Project Approach

Within the project the most promising model mounting alternatives will be investigated and compared to existing state-of -the-art stings. Technologies to be investigated are:

  • designing and manufacturing of several model mounting devices (stings),
  • applying and harmonising CFD and prediction tools including the necessary meshes,
  • analysing the test results of each FLIRET work package,
  • analysing the applied model quality, nanufacturing and handling strategies,
  • deriving recommendations for the industrial testing in cryogenic tunnels.

The work spectrum ranges from model design and manufacture to wind tunnel testing and data analysis. Aerodynamic design tools are used to optimise a set of model supports. In conjunction with practical testing the use of the CFD tools ensures that optimum geometries in a fast and efficient approach can be defined.

The work is split into 4 workpackages. WP 4 provides the final analysis and integration of all results. WP 1 is the most significant one. It is based on a support development strategy, which will provide improved supports for different test cases and model types.

WP 2 is devoted to the high-speed buffet onset and model vibrations for complete models. Again, CFD specialists are highly involved in supporting the testing process and in validating the tools for this class of test problems.

Special low speed problems (related to landing and take-off of aircrafts) are solved in WP 3 for half models. Clear recommendations for the model roughness quality are expected in task 3.1, which will help 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 conditions.

Dr.rer.nat. Christian Klein
Deutsches Zentrum für Luft- und Raumfahrt (DLR)

Institut für Aerodynamik und Strömungstechnik
, Experimentelle Verfahren
Tel: +49 551 709-2440

Fax: +49 551 709-2830

E-Mail: Christian.Klein@dlr.de
URL dieses Artikels