Infrastructure development and provision of RTA for innovative uses in the field of drone icing.

All-weather capability of UAVs, especially in the area of icing, respectively its understanding and possible countermeasures are currently the focus of international research and development efforts. The necessity of activities in these areas manifests itself in the fact that currently civil UAVs are not certified for operation below 5°C worldwide. AIRlabs Austria has therefore already taken some innovative approaches in this area.

In November 2020, icing tests were conducted in RTA’s climatic wind tunnel. The AIRlabs tests were planned and carried out in cooperation between FH JOANNEUM, RTA and AIIS (Austrian Institute for Icing Sciences). As part of the icing tests, a multicopter with a maximum take-off mass of 25 kg was operated in defined icing conditions at a constant speed in a cruise flight scenario in order to investigate the effects on the flight characteristics. For this purpose, the “small” climatic wind tunnel (CWT) of the RTA was prepared and made available for the first time for aerodynamic and icing tests with a complete, functional multicopter.

One goal of the test was to optimise the utilisation of the test time: In the course of the preparation, a great deal of attention was paid to optimising the test procedures. In addition to planning activities, this also included the use of efficient methods for documenting the results and for de-icing the multicopter more quickly (reducing downtimes) by means of a newly developed, very finely controllable hot air method, which was used for the first time. The state of the art in other Icing Wind Tunnels (IWT) is to use steam for this, which in turn requires post-drying and would therefore take longer. In order to extract as much added value as possible from the experimental investigations carried out, it was necessary to integrate various measuring equipment into the test setup. This includes high-speed cameras, thermal imaging cameras, various sensors for monitoring the drone and a detailed documentation of the results.

To date, only the large CWT could be converted to an Icing Wind Tunnel (IWT). The main difference for icing tests between the small and large CWT of the RTA is the maximum achievable wind speed, which is adjustable up to 80 m/s in the large CWT and up to 50 m/s in the small CWT. Especially for the UAV market, 50 m/s is sufficient and therefore it was decided to implement this possibility in several steps also in the small CWT. In this way, CWT and IWT costs can be reduced for future users and higher availability can be offered.

In order to be able to work even more cost-efficiently in the area of small test objects (such as UAVs), a “double” occupancy with test objects was also prepared and tried out for the first time. For this purpose, two independent experiments were integrated into the CWT in a staggered manner. The necessary control, data recording etc. had to be carried out twice. The support of the two different users also had to be realised by two independent project management teams.

It became apparent that technical challenges emerged in the area of the degree of blockage, among other things. This ratio is used to ensure that the wind tunnel model does not “block” too much of the wind tunnel cross-section and deflect the airflow too much against the wind tunnel walls or accumulate it altogether. This must be taken into account when staggering the two wind tunnel models. Parallelism of the approach flow: The experiment located downstream in the wind tunnel only delivers qualitatively and especially quantitatively evaluable results if the air-“trail” of the upstream experiment does not disturb the flow too much. In the present case, this is also extremely challenging and innovative in approach because the large multicopter with running rotors and thus also the corresponding downwind fields was integrated in the channel. This influences not only the aerodynamics but also the icing condition (LWC, MVD, etc.).

During the tests carried out, it was possible to realise a test setup in which the behaviour of a multicopter in icing conditions could be observed in a well-founded manner and subsequently assessed. It was observed that icing can lead to critical situations even in very short periods of less than 100 seconds. Scaling to other areas: The innovative methods developed, which are a global novelty, can also be applied to other test scenarios with different test objects. Further steps to be able to utilise the small IWT also in the future for approval processes in the icing but also snow area are in concrete planning.

Behind the main measuring level, where the primary experiment with the multicopter was located, clearly defined climatic inflow conditions are also ideally required at the measuring point of the second IWT. This means that water drops, wind, temperature, angle of attack, etc. should also arrive at the rotor test stand as unchanged as possible. Initial measurement results from the users are promising. However, this innovative approach to infrastructure provision by AIRlabs should and must be further validated and verified through further experiments. The findings were used in two scientific publications at the AIAA Aviation Forum 2021 and were incorporated into the All-Weather Drone research project.

• • Kozomara, D., Neubauer, T., Puffing, R., Bednar, I., and Breitfuss, W., “Experimental Investigation on the Effects of Icing on Multicopter UAS Operation,” AIAA Aviation Forum 2021 – Virtual Event, August 02-06-2021, 2021, DOI: https://doi.org/10.2514/6.2021-2676.

• • Tramposch, A., Thomann, M., and Kozomara, D., “Determination of Droplet Impingement on an Octocopter at different Flight and Icing Conditions with CFD Methods,” at AIAA Aviation Forum 2021 – Virtual Event, August 02-06, 2021, DOI: https://doi.org/10.2514/6.2021-2501.