It all started in 2020 when Dario Costa, who had already spend more than 5.000 hours in the air in around 20 years of flying experience, had been harboring a dream for years and approached bionic surface technologies with the Words “Can you tell me, using your CFD calculations, how risky a high-speed flight through a long tunnel with a flying machine is?”. It was an undertaking that no one before him had ever dared to undertake. As experts in aerodynamics and long-time companions of many Red Bull Air Race pilots – including Dario himself and his mentor Peter Besenyei – we did not want to miss this opportunity. For a year, we searched and simulated at BST and also locally the perfect conditions that would make a stunt of such magnitude possible. For this purpose, the mammoth project was divided into five different episodes.

To get a feel for the situation, three calculations were initially done with different speeds and wind conditions. These assumed an unmoved, i.e. practically ‘stationary’ in the air, aircraft one in free flight, on the viaduct and once inside tunnel T2. The latter two calculations had a flight altitude of 1m due to the tunnel dimensions. The special feature of these simulations was that a scanned geometry of the racing plane was used for them. Due to the fact that there were no examples for such a flight, the difficulty was that the angle of attack and pitch of the aircraft were unknown – these were necessary for the further calculations. Both could be estimated and finally calculated. Finally, the output was how the attitude of the airplane varied inn the three different scenarios when the three different acting forces were changed.

After the aircraft was simulated stationary during the first calculations on the one hand, a piece of tunnel and open track, i.e. the entrance from the viaduct into the second tunnel, was selected for the moved analysis – for the best possible results – on the other hand. This made it possible to better visualize the changes in forces and moment on the aircraft and to finally clarify whether such a risky stunt could even be within the realm of possibility. Also crucial was the definition of a reaction time. This was the time the pilot had to react to external conditions, such as crosswinds. This was 0.3 seconds and was important for the mental coaching of Dario Costa in the further course. A short sensitivity analysis, i.e. how sensitive the aircraft responds to certain parameters, was also done. This 2D analysis, which again looked more closely at altitude, speed and temperature, revealed three so-called “Bumps” in the air pressure. These were passed on to the pilot so that he could prepare himself optimally.

In April 2021, the important technical location check too place to get an idea of the actual situation. On site, the track and the tunnel were examined again in detail, and wind conditions, solar radiation, and outside and inside temperatures were measured at several times of the day. Because the temperature inside and outside the tunnel was more constant, the wind calmer and Dario had the sun at his back while flying – thus less potential to be blinded when exiting the tunnel – it could be determined with high probability at this point, that the flight would take place in the early morning. In addition, the route was completed several times with a test vehicle at the same speed as Dario would then fly. During these runs, a camera was mounted at the eye level of the flight pilot to serve him in the form of film sequences during later training.

At the same time, further simulations were run to make Dario Costa’s project as safe as possible. For this purpose, further modifications were made to his flying machine, a Zivko Edge 540 V2, such as rudder tips. As a further highlight, the Riblet surfaces developed by BST were also used on the aircraft. For the first time in aviation history, Riblets were used to absorb and compensate ground-level turbulences in the air – also a world premiere. The special structure of the Riblets and the specially designed rudder tips, which had a larger contact surface, increased the stability of the entire aircraft. This was confirmed by Dario Costa, as well as the fact of having more control at low speeds.

The final phase included comprehensive weather monitoring for which BST was also responsible in addition to the modifications and simulations. For this, five weather stations were installed in and around the tunnels, measuring the speed and direction of wind, temperature, humidity and pressure every minute between 4 and 8 a.m. Based on this plan and BST’s CFD calculations, engineers created a plan of action for which weather conditions would be safe to launch. For example, there were no-go criteria in which a flight could not take place under any circumstances: when the temperature would have changed by more than 10°C or an absolute wind speed of more than 7 m/s.