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Clean Sky's POLITE model gets its second wind

Testing in wind tunnels before new aircraft start their flight test regimes typically entails examining the performance of various types of models in different types of wind-tunnels to replicate the varied types of air pressure, altitude, speed and other aerodynamic parameters. Clean Sky's POLITE (powered, modular wind-tunnel model for low and high Reynolds tests) project, completed in 2019, is a gamechanger in wind-tunnel testing, bringing time savings and cost efficiencies to European aeronautics R&D development, in line with ACARE objectives and the EU’s Green Deal ambitions.

POLITE model in the LWTE wind tunnel in high-lift configuration
POLITE model in the LWTE wind tunnel in high-lift configuration

‘The most innovative aspect of this project is having one propeller-powered wind tunnel model able to perform in two types of wind tunnel, a non-pressurised one and a pressurised one, reproducing maximum engine power conditions,’ says Clean Sky project officer Costin-Ciprian Miglan. ‘Normally, wind tunnel models are particular for each wind tunnel. In this case, although there are some customisations, the parts can be easily interchangeable, and then you can have the wind tunnel model in several configurations.’

The project set out to use one model to undergo low-Reynolds number tests in the non-pressurised Large Wind Tunnel at RUAG in Emmen, Switzerland (which provides ambient temperature and pressure conditions). For the high-Reynolds tests the model will be installed at the F1 Low Speed Pressurised Wind Tunnel at ONERA's Le Fauga-Mauzac Center in the framework of another Clean Sky 2 project, PERTURB (Reynolds numbers are parameters used to measure the significance of inertial effects over viscous effects). 

Beyond the technical challenges, the underlying objectives of the project were to support Clean Sky's Flight Testbed 2. Topic manager Luis Pablo Ruiz-Calavera, Technical Expertise Leader for Aerodynamics, Flight Physics CoC (IG), Airbus Operations, says the plan was to confirm, or otherwise, the ‘aerodynamic benefits of some of the devices that we are going to flight test on the testbed so that we could have an early warning of whether we needed to introduce any modification in the design and to make sure that they work properly.’

A second main objective, explains Ruiz-Calavera, was the generation of an aerodynamic database of the aircraft ‘that can be used to justify to the airworthiness authorities that the aircraft is safe for flying so that they provide a “Permit To Fly” for the flight test campaign.’

Overcoming technical challenges

The ambitions of the project meant addressing a complex series of technicalities:

‘One of the problems with designing such a model was the fact that, due to the different infrastructures inside the two wind tunnels, we couldn't install the same kind of motors inside the model. So, the model was equipped with hydraulic motors where you have oil running at 400 bars for the RUAG wind tunnel, and with pneumatic turbines operating with air running at 100 bars for the ONERA F1 tests,’ explains Nicola Paletta, R&D Manager at IBK Innovation and coordinator of the Clean Sky POLITE project, which is funded in part by the European Commission’s Horizon 2020 programme. 

‘This posed a lot of challenges and a lot of design difficulties – the whole instrumentation chain and testing architecture changes, because if you install pneumatic turbines or hydraulic engines, that's a completely different story.’

Another challenge was related to the two very different interfaces needed to install the model on the 'balance' in each respective wind tunnel. (The balance is a device used to measure the aerodynamic forces to understand the aerodynamic shape of the model under different conditions). 

In the RUAG facility the model was attached to a balance to measure the lift, drag and side force, as well as the three components of the moments (the turning effect of a force): pitch, roll and yaw. The balance was placed inside the fuselage of the model and the model was actuated – to pitch and roll the model and provide some sideslip angle to the model to ensure that the aerodynamic forces are measured in different conditions.

An innovative balance

However, at the ONERA wind-tunnel this same balance was incompatible with the requirements the POLITE team had for the pneumatic turbines (the air-powered engines on the wings). 

‘It was not possible to have a balance inside the model, so we had to exploit a balance that was outside the model, under the wind-tunnel floor. And since the connection of the balance was a rigid connection without the possibility to place any kind of mechanism inside, we had to insert a mechanism inside the fuselage to move the model,’ Paletta says.

A mechanism like this inside the model, especially for ONERA F1 where the wind-tunnel is pressurised and there are huge loads on the model, required the design of highly compact mechanisms and actuators to provide the required kinematics. And, all of this had to be hooked up to an external console station to monitor the testing process.

‘It was quite a complex task to have this same fuselage able to do these very two different tasks,’ remarks Paletta, but the solution was to design a highly modular model with built-in provisioning so that it could be adapted to the different infrastructural prerequisites of operating in different wind tunnels.

Therefore most of the fuselage was common and parts of the outboard wings, winglets and ailerons were common to both configurations of the model – while motors and parts of the wing were unique for each wind-tunnel. Overall however, the major elements of the model were common in both settings.

This is a significant saving, according to Paletta, who emphasises that ‘in 99% of programmes involving the use of different wind tunnels you cannot save anything from the last model to the new one, because each wind-tunnel has different infrastructures, different interfaces, different conditions.’ 

That observation is reinforced by Clean Sky's Miglan who says ‘it's a worthwhile saving because wind tunnel models cost a lot. So, from this perspective it's better to have a wind tunnel model, which you can use in several wind tunnels.’

POLITE model in the LWTE wind tunnel with running engines
POLITE model in the LWTE wind tunnel with running engines

Collaboration is key

The project was a highly collaborative effort. Coordinator IBK designed and developed the main elements of the model, its remotely controlled control surfaces, and its implementation into the wind-tunnels. POLITE was supported by ARA (Aircraft Research Association) which designed the outboard wing of the model and the design of propeller blades. 

Campania-based R&D and consulting SME Dream-Innovation SRL provided computational fluid dynamics design support, contributing specific safety analysis to understand the strut influence on aerodynamic coefficients for the two different configurations in low- and high-Reynolds tests.

The support of the wind tunnel testing facilities was also invaluable. RUAG contributed with the development of the powertrain (hydraulic motors) to be used in the wind tunnel in Emmen, and with the design, manufacture and calibration of the 6-component rotary shaft balances, enabling accurate and direct measurements of propeller forces and moments during WT operations.

The results were presented at AERODAYS 2019, showing an exploitation pathway for the concept's future use.

Commenting on the outcome, Airbus's Ruiz-Calavera says that the ‘test campaign, performed in a wind tunnel in Switzerland, was successfully completed and we got the data, and we have been able to use it to generate databases to help us obtain the permit to fly, (for Flight Testbed 2) and also to help us confirm that the design that we had done was right.’ 

He notes that ‘there have been important lessons learned, and that has been recognised by everybody, that in a project as challenging as this one you need to have all the main stakeholders as part of the consortium.’ 

Moving forward, he adds that the beauty of the POLITE model is that it would present the possibility of carrying out high thrust and high pressure testing simultaneously. ‘This is the part that is still missing, and it would be very interesting if we could do something of that sort – that would be the basis to start a new project in the future. It would not start from scratch, we already have a significant amount of parts of the model that are available and they could be used for this.’

Miglan adds that ‘the wind tunnel testing will bring important information for the preparation of the flight test, because this was the aim of the project, to build the wind tunnel model and then assess the performance of the movable wing surfaces, to enable modifications [needed] in Flight Testbed 2.’