DNW innovations in wind tunnel testing: new moving belt system for Large Low speed Facility
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The German-Dutch Wind Tunnels DNW is one of Europe’s most advanced and specialized organizations for wind tunnel testing. DNW’s 11 wind tunnels include subsonic, transonic and supersonic facilities, and provide experimental aerodynamic simulation capabilities to a global user community at large. DNW provides techniques for aerodynamic, aeroacoustic or aeroelastic simulations and tests of scaled models in a controlled environment. Its experimental simulation techniques capture the essence of the issues to be investigated. The Large Low speed Facility (LLF) in Marknesse (the Netherlands) is an industrial wind tunnel for the low-speed domain. It is a closed circuit, atmospheric, continuous low-speed wind tunnel with one closed wall and one configurable (slotted) wall test section and an open jet. Low speed means testing of aircraft in take-off and landing flight configurations and therefore DNW focusses its investments for the LLF on safety (ground proximity, powered and unpowered) and environmental issues (acoustics) related testing capabilities. Recent DNW-LLF upgrade programs focussed on ground proximity simulation (procurement of a new moving belt system) and reducing of wind tunnel circuit background noise level to improve its capabilities and market attractiveness. The main drive for the latter initiatives is a clear trend in aircraft characteristics, i.e. continuous reduction of aircraft noise levels. Funding support was provided by the Ministry of Economic Affairs (the Netherlands), the German Aerospace Center DLR and the European Commission through EU 7th Framework European Strategic Wind tunnels Improved Research Potential ESWIRP. The paper will further detail the various development steps taken for the new moving belt system and elaborate on the calibration activities conducted.
KeywordsWind tunnel Testing Aeronautics Ground effect Aircraft
1.1 Ground effect
The DNW-LLF experimental capabilities capture the essence of the issues to be investigated in the low-speed regime, i.e. aircraft take-off and landing flight regime. Customers amongst others use this facility to investigate the effect of ground proximity on aircraft performance. Aerodynamic data are required to optimise high lift device (flaps and slats) settings and calculate take-off and landing performance to feed the flight control computer and ensure that the aircraft under development meets its runway length targets.
In the case where either aircraft rotation for take-off or an attempt to conduct a go-around after touchdown is initiated at too low a speed for the aircraft configuration or weight, ground effect may lead to a state that cannot be sustained, as the distance from runway surface increases and the lift from ground effect reduces. Several accident and incident investigations in the past decade reported ground effect to be a critical factor .1
Predicting the aircraft behaviour in landing or take-off configuration under sidewind close to the ground with sufficient accuracy is still challenging.
In-ground proximity, wing lift and drag at a given angle of attack have shown to be dependent on the history of reaching this angle of attack (hysteresis),
and because the effects are rather unique to any specific airplane (wing) configuration.
1.2 Moving belt solution
Buckling of the belt system under aerodynamic loading either due to the vicinity of a wing in high lift configuration or simulated engine exhaust flow and
its limited maximum speed, less than half of the maximum speed capability of the wind tunnel. Consequently tests at wind speeds higher than 40 m/s are performed with a speed difference between with the wind tunnel and the belt.
The high work load of operating (tracking) the moving belt under asymmetrical aerodynamic loading.
Its limited life time due to belt wear.
1.3 Funding support
A grant from the Ministry of Economic Affairs (the Netherlands, balanced by an equal amount from the German Aerospace Center DLR for the system procurement).
Sponsoring from the European Commission through EU 7th Framework European Strategic Wind tunnels Improved Research Potential ESWIRP, covering cost for feasibility studies and interface adaptations to the existing DNW-LLF wind tunnel infrastructure.
The EU-ESWIRP project (http://www.eswirp.eu/) has been funded by the European Framework Programme 7 to support the integration of and access to research infrastructure of pan-European interest. It has significantly enhanced the interoperability of three key world-class European aeronautical wind tunnels, and harmonized, improved and optimized the scientific access conditions thereto: DNW-LLF (Marknesse, NL), European Transonic Windtunnel ETW (Cologne, GE) and ONERA S1MA (Modane, FR).
Central elements of the project were infrastructure improvements, networking and joint research activities and transnational access (TNA) to the facilities to four research consortia with a total of more than 100 scientists from 17 different nations.
2 System development
2.1 System requirements
Belt size 7.92 m × 6 m.
Maximum belt speed corresponding to a wind tunnel speed of Mach 0.25 (80 m/s) keeping the wind tunnel and belt speed equal up to typical aircraft start and landing speeds.
Long life-time (infinite design operating time for a steel belt; so no need for a replacement within next 30 years).
Belt to remain flat under the influence of aerodynamic forces introduced by wind tunnel models during testing (± 1 mm).
Compatibility with the existing boundary layer removal scoop (in front of the belt) and re-injection system (aft of the belt).
2.2 Development approach
The selected system consists of a 1-mm thick metal sheet running on large hardened steel rollers. Based on DNW operational usage and assuming there will be no accidents with respect to the belt, the expected lifetime is endless. A sophisticated suction/blowing system keeps the belt flat under large aerodynamic loading when testing aircraft in ground proximity. A boundary layer removal system and reinjection scoop complement the system. To prevent the belt from accidently being damaged during, e.g. wind tunnel model configuration changes, a protective mat is used to protect the belt against damage from dropped objects (FOD). Only simple periodic inspection of the belt edges and surface is to be conducted.
The MTS metal belt system uses advanced (proprietary) vacuum pre-loaded air bearing technology (with more than 750 NewWay air bearing units) for stable operation of the belt under load. A dedicated air compressor is used to supply pressurized dry air to the air bearings. The selected air compressor system delivers pressurized air up to a mass flow of about 2 kg/s at 10 bar with a dew point of – 30 °C. Two 55 kW vacuum pumps with a capacity of 72 m3/min complement the system.
The new metal belt system is much heavier than the old system with a fabric belt. In parallel to the system development, DNW, therefore, had to modify the modular wind tunnel test section, since the moving belt is interchangeable with the solid floor of the test section. Four additional spindles had to be installed in the solid floor and the floor rails, used to remove or install tha floor section, had to be strengthened. As the dimensions of the metal belt system are also slightly different from the existing system (increased length), a new boundary layer removal system was designed and manufactured.
Furthermore a dedicated test preparation hall was constructed at DNW-LLF for storage, functional testing and test preparation of the new stand-alone system, equipped with all necessary supplies and a fully functional control room.
The Factory Acceptance Testing (FAT) of the new metal belt system took place in the first half of 2012 in a stepwise approach, allowing for minor malfunctions of the new system to be solved. During the first FAT the system was already capable of running at all required operational conditions, except for external (point) loading of the belt surface.
In February 2013, final commissioning at DNW-LLF started with successfully repeating all checks and tests as performed at the isolated system during the FAT at MTS.
Wind-off: belt speed up to 80 m/s.
Wind-on: belt speed and tunnel speed up to 80 m/s.
Wind-on and belt point loading by blown nacelles (symmetrical and asymmetrical) above the belt: belt speed and tunnel speed up to 80 m/s.
Since the new moving belt system performed completely according to specifications, the new moving belt was officially accepted by DNW after a critical inspection of all individual subsystems.
3 DNW-LLF flow quality with moving belt operational
Having completed the acceptance testing, the system was now ready for tuning and optimization of the scoop duct, test section downstream breather and flap settings, due to the moving belt hardware upgrade (especially the lower surface outside of the test section). The purpose of this optimization was to achieve a homogeneous static pressure distribution above the major part of the belt for the whole belt speed range.
DNW’s 6 m wide new moving belt system is the largest of its kind, operating at speeds up to 80 m per second. Even at maximum speed, the system’s moving belt is designed to remain flat to ensure simulation accuracy. The moving belt also exhibits exceptional reliability and durability, especially critical for withstanding the rigors of powered aircraft take-off simulations.
With the new system, DNW is able to offer its aerospace customers the most realistic simulations possible, as shown by the very flat homogenous pressure distribution. DNW is particularly satisfied with the system’s flatness and reliability, which enables its customers to simulate required test conditions with very high levels of precision and efficiency. Wind tunnel model can be positioned very close above the surface of the moving belt.
British Airways Boeing 747-400 departing Johannesburg (http://www.skybrary.aero/index.php/B744,_Johannesburg_South_Africa,_2009_(LOC_AW)), Europe Airpost Boeing 737-300 taking off from Montpelier (http://www.skybrary.aero/index.php/B733,_vicinity_Montpelier,_France_2011_(LOC_AW)), Air Cargo Carriers Shorts SD3-60 attempted to land at Oshawa (SH36 Oshawa ON Canada 2004), Gulfstream G650 undertaking a pre-type certification experimental test flight take off (G650 Roswell NM USA 2011).
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