Bereits 2022 führten wir den dynamischen Crashversuch für die Insassensicherheit unseres Mü-32-Rumpfes erfolgreich durch. Im letzten Jahr konnten die dabei gewonnen Daten nun für die Zulassung des Rumpfes verwendet werden.
The project consisted of three parts:
- Development of efficient and reliable approval procedures to increase crash safety
- Creation and validation of a material model for dynamic material behavior
- Expansion of the competencies of the Akaflieg for the development of aircraft structures and the performance of structural tests
Work completed
A crash structure was developed and manufactured for the fuselage of the Mü 32. This consists of two lateral reinforcements on each side and a layered structure in the nose, which has been optimized for a frontal impact. The reinforcements, known as crash beams, stiffen the cockpit and are designed to ensure that a survival space is maintained. Furthermore, the fuselage is designed to be deflected and converted into forward motion. The layers in the nose area are graded, allowing for gradual failure and energy dissipation. This acts as a crumple zone and reduces the accelerations that occur. As a further measure, the frames have been optimized for a crash scenario.
The structure was developed in close collaboration with the Chair of Carbon Composites at TUM. A detailed FE model was used to validate the structure and identify potential weak points. For example, high stress was detected on an adhesive seam in the upper nose section, which is why additional reinforcement was added here.
The full-scale crash test was carried out together with our project partners from Hanover in the summer of 2022 at the Oberschleißheim Airfield, which was ideal for our project due to its proximity and local conditions.
A steel structure was designed to achieve the desired impact parameters of a 45° angle of impact and a 5° slip angle. This also replaces the wing and tail masses without causing any disruptive aerodynamic effects. The test vehicle was held in place by a crane via a tensioned crossbeam and deflected by a second crane. The impact energy can be determined by selecting the appropriate height of the suspensions or the length of the cables. Overall, the test setup resembles a pendulum hanging too low. The loads to be achieved are based on the applicable certification regulations and are intended to represent a typical crash scenario. The load should be as high as possible without reaching a level at which survival is impossible under any circumstances.
When it broke free, the fuselage swung downwards and hit the ground nose first. The nose was visibly dented. It then briefly became completely airborne again, only to hit the ground again with its underside along its entire length. This resulted in the highest loads on the pilot and severe damage, especially to the non-critical rear fuselage section. The fuselage then moved forward and slid across the asphalt. Although asphalt is unlikely to be the surface in a crash, it was a compromise between feasibility and comparability with the simulations.
The Astir from Akaflieg Hannover, with its retrofittable safety cockpit, was successfully tested together with our test vehicle. For further information on the NaSiCo project run by Akaflieg and the University of Hannover, please refer to the publications by the project partners.
The evaluation
The test fuselage was extensively instrumented in order to obtain as comprehensive a picture of the kinematics as possible. This included rotation rate and acceleration sensors in the fuselage and an "H3 50th percentile male" dummy as the pilot. Together with seat belt force sensors, this allowed conclusions to be drawn about the expected injuries. The test was monitored from the outside by several high-speed cameras and a DIC system. In the end, the test was carried out without any major problems. We would like to thank the many supporters for their contributions to this project.
We were then able to review the research objectives based on the data obtained. The expected failure behavior in the nose area was confirmed. This can be clearly seen in the test fuselage, which is currently being used as an exhibit at our trade fair stands.
Various injury criteria can be calculated from the measurement data provided by the dummy sensors. Most of the usual values are not critical in our scenario. Only the lumbar load criterion for the lumbar spine is critical, which is consistent with the injuries observed in similar crashes. This value could be improved with a cushion, as used by many pilots. The ankles are also exposed to high loads, which was also expected. The simulations also correspond well with the experimental data. This means that the 2024 project was completed with complete success.
(source: Akaflieg Hannover)
Next steps
The data from the crash test was also successfully used in the certification process for the fuselage of the Mü 32. This made an important contribution to development under conditions close to real-life. Since then, Akaflieg Munich has focused on certifying the wing, incorporating the experience gained from the crash test into the load test.
Author: Clemens „Lötlurch“ Lippmann