SMAUG — what is happening, if an aircraft collides with a drone?

Validated simulation models give a deep insight into possible collisions of airplanes and helicopters with drones.

With the increasing availability of drones to a wider public, the probability of a collision with an airplane or a helicopter increases. A comprehensive understanding of the impact event is therefore an essential requirement to simulate and assess possible collisions.

Currently, there are more than 400,000 drones in Germany, most of them are privately used. In fact, a license is needed to pilot a drone having more than 250 grams, a collision with an aircraft cannot be excluded in any case. Thus, it is of large scientific interest to assess the response of aerostructures during a collision with a drone. Relevant collision scenarios are aligned to typical take-off and landing velocities of airplanes and on cruise speeds of helicopters. The speed range is therefore roughly between 50 and 150 meters per second. In SMAUG — Simulation Methods to Analyze Collisions between Drones and Aircraft — simulation models for respective collision scenarios were developed. The project was financed by the Federal Ministry for Economic Affairs and Climate Action (BMWK) within the aviation research program LuFo VI.
 

Selection of the drone

Experimental and numerical investigations in SMAUG were conducted on the popular quadcopter DJI Mavic 2 Zoom. Besides the wide distribution, its mass of 907 grams (which corresponds to two pounds) was one selection criterion. So, by selecting this drone, an analogy with the certification specifications for small commuter aircraft regarding bird strike could be drawn, since in these requirements a bird mass of two pounds is taken as reference. The selected drone consists of four arms, to where electric motors and rotors are attached, as well as a fuselage that contains a camera and a battery. First experimental investigations on the components motor, camera and battery showed that the motors, but especially the battery are a potential risk. The battery is of special interest due to its compact, however very complex construction, and its comparatively high mass of 292 grams. 

Characterization of the battery of the drone 

For the characterization of the mechanical properties of the battery of the drone, a building block approach was used. Thereby, the understanding of the whole drone is deduced from the behavior of the single components and their materials. The battery of the drone basically consists of a polymer casing, four battery cells as well as a circuit board and cables. To determine the material parameters for the simulation model, specimens were extracted and afterwards tested mechanically. In a next step, the mechanical behavior of the total battery was identified in length and width direction. Based on static compression tests, the stiffness and the failure behavior of the battery could be determined. 

Battery model: development, validation and application

The comprehensive experimental data builds a good basis for the development of a detailed simulation model of the battery with a high degree of complexity. However, with the outlined approach, one essential phenomenon cannot be caught sufficiently up to now: the shattering of the battery of the drone at higher impact velocities. Therefore, using a specifically adapted test stand, impact experiments were conducted in the relevant velocity range. The breaking of the casing and the fragmentation of the battery cells could be documented with a high-speed camera. In the simulation, the fragmentation of the battery cells was modeled by switching the numerical method. In the early stage of the impact, the battery cells, as well as all other components, are modeled with a mesh-based method (FEM, finite-element method). For larger deformations, finite elements are replaced by particles (SPH, smooth particle hydrodynamics). This adaptive change of the numerical method allows to simulate the experimentally observed impact behavior both qualitatively and quantitatively. After the validation of the model against the impact experiments, the applicability of the developed model was tested. For this purpose, the impact of a battery against a windshield of a generic helicopter structure was modeled. Helicopters are of particular relevance due to their comparatively low flight altitude and their operation in urban areas. In the virtually investigated collision scenarios, a strong deformation but no failure of the windshield occurred.

After completion of the SMAUG project, a validated simulation model of the battery of the drone is available. In a next step, it is planned to develop a model of a full drone, so that in the future the response of aerostructures during a collision with a drone can be investigated virtually.