Experimental and numerical investigations on the implications of aerodynamic ground effect in manoeuvring flight

• Applicants: Professor Dr.-Ing. Ewald Krämer; Professor Dr.-Ing. Markus Raffel
• Subject Area: Fluid Mechanics
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 282718145

The flow topology around helicopters in ground effect is substantially different from the one out of ground effect. Rotor loads, vibration levels of the airframe as well as the helicopter performance deviate significantly. In addition, when flying over loose ground sand or snow may rise, obstructing the pilot's view eventually. Those effects constitute a serious security problem for ground personnel as well as the helicopter's crew.The proposed project will closely investigate the ground effects on the helicopter numerically as well as experimentally. At IAG, the existing model helicopter will receive new measurement equipement for blade deformations and subsequently flight experiments in and out of ground effect will be undertaken. On the numerical side, the strong fluid-structure coupling will be enhanced to enable the simulation of manoeuvring flight close to ground, as happens for example during start or landing. In order to represent the correct flight path from experiments, a piloting model has to reproduce appropriate control inputs. This new code functionality will be tested and validated using the flight data gained previously.As last and greatest experiment within the project, at DLR Göttingen free-flight data will be gathered using a real helicopter available there. The focus of the investigations lies on the wake behaviour, measured using the BOS method developed at DLR. In addition, the research helicopter is equpped with further sensors, for example for control inputs, rotational speed and mast moments. Those data will be used for validation of the toolchain developed and established at IAG during the project.All experimental studies up to now show that tip vortices are no more detectable after some time. It is supposed that vortex pairing or other dissipative phenomena reduce the vortex strength significantly. However, the exact reasons are yet unknown and shall be explored in the project by said experiments and accompanying numerical simulations.After validation, additional numerical parameter studies are planned, concentrating on various influence factors on the flow topology in ground effect. In simulations the flight path or geometrical properties of the helicopter can be changed easily without new flights necessary. Furthermore, loads and moments on different aircraft parts are to be predicted without needing a full-scale research helicopter for flight data generation.

• DFG Programme: Research Grants