One of the challenges to the safe and reliable operation of aircraft is the hazard of in-flight icing. Ice accretions on the aircraft happen when supercooled droplets – droplets that are liquid although their temperature is below freezing – hit the surface. Depending on the position where the ice grows, different problems can arise. Ice accretions on wings reduce the lift and increase the drag of the aircraft. Ice on propellers can reduce the thrust and increase the required power significantly within one or two minutes. When sensors or antennas ice up, their output might be faulty or stop completely. Overall, icing on aircraft has multiple negative effects that can cause a crash in the worst case.
Hence, icing research has been an important field within the aircraft community for more than seven decades. Two different main areas of research can be distinguished. First, understanding the physics behind icing and how ice accretions affect different parts of the aircraft. Second, developing and testing solutions to protect aircraft that fly in icing conditions. Different methods are used to conduct the research. The foundation has been laid by performing theoretical research about the path of droplets in the air, where they impinge, and what factors influence the rate of droplet freezing. This research has been supported by performing experiments. A second, more recent method is the use of numerical simulations.
While numerical simulations allow investigating a wide range of different conditions, they cannot serve as a standalone tool. This is because the equations that are used to do the numerical calculations are often not exact replications of the true processes but require modeling of some parameters. The model tuning is typically done by comparing numerical simulations to experimental results. Hence, experimental tests play a very significant role in the field of icing research. Experiments can have different levels of complexity and similarity to the conditions found in real flights. Especially some of the foundational work has been done in conditions that are different from real applications, for example by using flat plates as the geometry. These experiments still allow gaining insight into the theory behind icing. However, when we aim to understand in more detail how different parts of aircraft are affected or can be protected, the experimental conditions must be closer to the conditions in real flights (real geometries, sizes, ambient conditions, etc.).
The highest similarity to real flights can be achieved by performing flight tests. The goal is to operate the aircraft in real icing conditions and investigate the effect of icing or the performance of the protection solutions. Two big challenges related to flight tests in icing conditions exist. First, it is extremely difficult to find icing conditions and it can take a very long time to conduct enough flight tests to cover the wide range of potential cloud conditions. Second, measuring all important variables, related to both the ambient conditions and the influence of ice accretions, during the flight is challenging or even impossible. Thus, experiments in icing wind tunnels are a popular alternative since they allow testing at different conditions in a controlled manner.
Like conventional wind tunnels, icing wind tunnels have fans to create an airflow through the tunnel and towards a test object, for example, a wing or a propeller. The special feature of icing wind tunnels is that they can recreate conditions as aircraft could find them in icing clouds during a flight. To do so, icing wind tunnels are equipped with a spray system to spray droplets into the airflow. Additionally, the air in the tunnels can be cooled down below freezing to supercool the droplets before they hit the test object. This can be achieved by having a heat exchanger that only cools down the air that flows through the closed return tunnel, or by having an open wind tunnel and cooling down the whole room, see figures above. Hence, testing in icing wind tunnels allows performing experiments that are close to real icing conditions in a controlled environment. This is an important tool since it allows testing at specified conditions without having to wait or search for them for a long time. Additionally, the physics does not have to be modeled, as is the case for numerical simulations.
But unfortunately, there are also shortcomings of experiments in icing wind tunnels. First, the number of facilities in the world is small. This also means that time in icing wind tunnels is limited and expensive. Second, all icing wind tunnels come with limitations in the range of conditions they can test. Except for the largest facilities in the world, the size of the test section is typically only in the range of one to three meters. Hence, testing full aircraft or even full wings for manned aircraft is typically not possible. This is especially challenging because scaling is a very difficult aspect of icing. To meet all similarity requirements of icing conditions, many parameters must be matched, often resulting in contrasting constraints. Hence, scaling of results is not commonly done. Additionally, also the range of ambient parameters that can be tested in icing wind tunnels depends on the facilities. Most facilities can only spray limited amounts of water in the air, only have limited speed variation, or are limited regarding the coldest temperatures they can keep during experiments. Last, but not least, the conditions in icing wind tunnels are no exact replicates of icing conditions in real flights. For example, the experiments in icing wind tunnels are typically performed at constant conditions during the whole run. However, in real clouds, the conditions can change significantly within a few hundred meters.
Summarized, icing wind tunnels are neither a universal tool for icing research nor a niche tool. Yes, icing wind tunnels have some shortcomings that reduce their capabilities. However, they are still a very important tool for performing icing research because they allow testing without the limitations of numerical models and without the very expensive and time-consuming task of finding icing conditions for real flights. Also, the other methods have their strengths and weaknesses. Hence, it is probably the combination of the different methods that leads to the best results. Using numerical simulations to cover wide ranges of icing conditions, verifying the results using icing wind tunnels, and finally confirming these findings in real flights should be the best use of all the different techniques and maximize the learnings.
Text: Joachim Wallisch