Helicopters and Active Rotors

From a system dynamic point of view, helicopters and their rotors are very complex and therefore very interesting from a scientific point of view. We work in two directions: (a) Active Rotor Control and the characterization and optimization of flight comfort; and (b) The special dynamics of ultra-light coaxial helicopters and their stabilization with purely electric actuators.

Since the end of 2016, Germany has officially had the Ultralight (UL) helicopter class.
This allows UL helicopters to be approved as aerial sports equipment.
The acquisition of a pilot's license for this is associated with a lower expenditure of time and money than with a normal private pilot's license.
Due to the lower entry hurdle and the cost-effective operation of these helicopters, it can be assumed that the number of UL helicopter pilots will increase in the coming years.
Since a helicopter has a large coupling of its axes and normally shows an unstable flight behavior, mastering the helicopter is a challenging task for a pilot.
At the Institute of Flight Mechanics and Flight Control, a stabilization system was developed that supports pilots and can relieve them on tiring missions.
The aim was and is to make the system as simple as possible without sacrificing function.
Since UL helicopters generally do not use hydraulic amplification of pilot forces in the control train, electromechanical actuators were installed directly in the control linkage.
These actuators can lengthen the control linkage again in addition to the pilot.
The main objective of the research project is to find out whether the pilot is more disturbed than relieved by the force feedback of the actuators to himself when flying.

The stabilization system was installed in a UL helicopter of the type "CoAX 2D" from edm aerotec GmbH in Geisleden in Thuringia.
First flight tests show a clear stabilization of the helicopter in hovering flight, whereby the pilot classified the disturbance as low.

To improve the passenger comfort in helicopters, it is necessary to quantify the comfort by quality measures. Atmospheric turbulence / gusts can occur during a flight and affect the comfort of helicopter occupants. To quantify the discomfort contribution of turbulence, first the turbulence has to be determined. Within a project cooperation with Airbus Helicopters in Donauwörth, various approaches for the reconstruction of turbulence induced accelerations are investigated and the influence on the passenger comfort is evaluated. In order to determine the overall flight quality, accelerations are measured at different locations of the helicopter and the comfort of the passengers is predicated with existing standards. The project aims to investigate and, if necessary, optimize the predictive capacity of existing standards. For this purpose, several experiments were carried out at the Max Planck Institute in Tübingen.

During the nominal rotation of a rotor, the rotor blades move out of the rotor plane (flapping) and in the rotor plane (lead-lag-motion). The lead-lag motion suffers from low damping which can result in resonance phenomena such as ground- and Air-resonance. In order to prevent such resonances, a control concept was developed that makes use of TEFs such that the damping of the lead-lag-motion is increased. This control concept was implemented on a full-scale EC145 and its functionality was demonstrated for the first time (worldwide) through flight tests.

A part of helicopter vibration is originated by blade dissimilarities. These dissimilarities cause the rotor blades to travel on different paths leading to static and dynamic imbalances, which in term lead to vibratory loads in the airframe. To minimize these vibrations a control concept was developed, which a continuous adaptation of the rotor tracks over the whole flight regime through control. It was demonstrated the first time (worldwide) with flight tests on a full-scale EC145.

Individual blade control has shown its potential for various control tasks like helicopter vibration reduction and lead-lag damping enhancement, on an EC145 (four rotor blades, 3,5t), the rotor of which was equipped with active TEFs. Ongoing work focuses on the scaling of this technology to other helicopter configurations with respect to the IBC actuation system, blade number or the size of the rotorcraft.

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