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Turbofan powered model in LWTE

In the Green Regional Aircraft platform of the European aeronautical research program Clean Sky, technologies are investigated to reduce the ecological footprint of future aircraft....

Topics include weight reduction, noise, all electric systems, mission and trajectory management, and new configurations.

Two specific aircraft configurations are studied in more detail – one powered by modern turbofan engines, the other by more conventional turbo-prop engines. A consortium led by IBK was awarded the task to build and test corresponding wind tunnel models in the RUAG LWTE low speed wind tunnel: the ESICAPIA project dealt with the turbofan powered aircraft model and LOSITA with the turboprop. RUAG, as a member of the consortium, was responsible to provide the power simulation system for both models and to perform the wind tunnel tests. While the simulation of propellers has been standard in our wind tunnel for many years, a turbofan presented some new challenges.

The development of the RUAG TFS was started by building a first proof of concept unit, based on the hardware available from the counter-rotating open fan tests performed in 2010 (see Newsletter 19). As a consequence, this first demonstrator featured a counter-rotating ducted fan. The geometry of the nacelle and the core were not representative of a real engine, nor was the demonstrator suitable for use in a wind tunnel application. The primary goal was to validate the design process for the fan and to confirm performance prediction.

In the meantime, specifications for the ESICAPIA model were finalized and the develop- ment of a TFS unit for wind tunnel testing applications could be initiated. It quickly became clear that the mass flow requirements could be achieved using a single stage fan, which was a tremendous benefit in terms of complexity. In order to reduce the risks, it was decided to power the TFS with one of the two motors used for the counter rotating fan. The resulting reduction in length allowed a design of the TFS which would fit into a nacelle with no geometrical deviation from the original.

In propeller powered aircraft, the thrust-drag bookkeeping methodology is rather straight forward. The direct thrust is measured by a rotary shaft balance in the hub of the propeller. For a TFS unit, an a priori calibration must be used to relate the thrust to internally measured parameters such as mass flow, temperatures, fan pressure ratio. An “isolated setup” was designed to obtain the corresponding data. The TFS unit is attached to a balance and installed in the wind tunnel. A non-metric fairing shields the balance, the strut and the TFS. Therefore, only the thrust directly generated by the TFS is measured in this setup.

The first TFS unit was tested in the wind tunnel during the Summer of 2015. Unfortunately, initial performance did not match the expectations. The problem was traced to an error in the design of the stator. After a redesign, the expected mass flow of 4 kg/s, enough to cover the requirements of ESICAPIA, could be achieved. The remaining two TFS units were also calibrated and put into storage, awaiting their use on the model.

Wind tunnel test of the ESICAPIA model were performed in the last quarter of 2016. The TFS unit were integrated into the scale nacelles and installed on the model. In an extensive test campaign, an aerodynamic database of the airplane, including power effects was generated. The TFS units performed flawlessly for the full duration of the test.

The TFS developed for the ESICAPIA model, with their diameter of 250mm are too large for most of the models which are normally tested in RUAG’s Large Wind Tunnel Emmen (LWTE). Therefore, the next logical step was to reduce the size. Again, the enabler for the “mini-TFS” was a new generation of hydraulic motors of yet smaller dimension and higher power. The new concept unit with a fan diameter of 170 mm and a massflow of 3 kg/sec is a completely new design. Its performance has just been proven by extensive tests.