The acquisition of an experimental database, which is transferable from the model investigated under wind tunnel conditions to the full-scale helicopter, needs a complex test technique. The development and improvement of these techniques are the main objectives of the activities on the helicopter simulation in the wind tunnel. Therefore, the Institute operates two facilities for wind tunnel testing on model helicopters, the rotor test stand ROTEST and the rotor on sting ROTOS. They are the indispensable prerequisites for meeting the increasing requirements of clients and users. The extensive use of both test facilities has demonstrated their excellent suitability for:
For an optimal benefit, the test facilities have to cater to basic research programs as well as to industrial projects.
The rotor test stand ROTEST is used mainly for basic research on Mach scaled helicopter rotors. It is designed for model rotors with a maximum diameter of 4 m to provide good correlation with the corresponding full-scale rotor. The test facility, shown in Fig. 1, consists of the drive system, the rotor balance, the control system, and the measuring system. The rotor is powered by a nine-piston axial hydraulic motor connected by hydraulic lines to a remotely located electric drive pump. The hydraulic motor drives the rotor shaft by a tooth belt and has a performance of 130 kW at a maximum rotational speed of 1050 rpm. ROTEST is able to support different hubs for hingeless rotors like the Bo 105 as well as for articulated rotors. The rotor hub is mounted on a five-component rotor balance, that measures the pitching and rolling moments of the rotor and the horizontal, lateral, and vertical forces. For a separate measurement of static and dynamic load components, the balance contains load cells and piezoresistive transducers connected to the transmission rods in a serial arrangement. The rotor power is measured by a torque tube mounted between the upper and lower part of the rotor shaft. The rotor control is realized by a swash-plate system consisting of three electrodynamic actuators attached to the fixed system, the swash-plate, and the rotating blade pitch rods. The computer controlled actuators provide collective and cyclic blade pitch control by adjusting the non-rotating part of the swash-plate. Potentiometers at the blade roots measure the actual incidence angles of all rotor blades during rotation. Since 1990, the actuators for the swash-plate control can be exchanged for electrohydraulic HHC (Higher Harmonic Control) actuators to give the corresponding HHC inputs to the rotor blades. The measuring system provides data from both the body fixed sensors and the instrumentation in the rotating system of the rotor. It is a PCM system that can handle 64 channels of rotor data with signal frequencies of up to 250 Hz. Since 1992, it can be replaced by a new unit with miniaturized amplifiers in the rotating frame and a 256-channel slip ring system with a maximum frequency of 10 kHz. Since then, it is possible to pre-amplify a large number of low-level sensor signals such as bending, torsion and in particular pressure distributions at the rotor blade with high resolution, before transmitting them as analogue signals to the fixed system. Here, the data can be sampled simultaneously at a rate of 2048/rev using a powerful data acquisition system (TEDAS I). Complementary to ROTEST, the ROTOS is used for investigations on helicopter configurations consisting of the main rotor, the fuselage, and a tail rotor. Based on the experiences made with ROTEST, this test facility was designed by the Institute in cooperation with ECD for experiments in the DNW. Therefore, the performance of the subsystems concerning the balance, the control, and the measuring system for the main rotor are similar to ROTEST. These elements are located in the central module, which was built in a compact manner, so that it fits to the scaled fuselage of any considered helicopter including the narrow contours of attack helicopters. As a consequence of this space saving design, the already mentioned HHC actuators cannot be used for ROTOS. Furthermore, the rotor drive system of ROTOS drives the rotor shaft by a bevelgear. The corresponding hydraulic motor has the same performance as the one used for ROTEST. A second six-component balance can be adapted to the central module in order to measure the aerodynamic forces and moments originating from the fuselage and the empennage of the helicopter model. In combination with a tail rotor, this arrangement makes it possible to trim the helicopter model to free flight conditions, providing information about the Flight Research of new configurations from wind tunnel tests during the design period.
Application of the test facilities
The rotor test stand ROTEST was used in December 1998 within the aeroacoustic rotor optimization program ERATO, see Fig. 2. These tests were performed in cooperation with ONERA. It focused on the investigation of a completely new rotor design with respect to the power consumption and the BVI noise characteristics in descent flight conditions. The next application of ROTEST is planned for the HART II wind tunnel tests by end of 1999.
The ROTOS was used for the NH90-II tests in December 1996 to investigate the infrared signature of the full-scale helicopter under different operational conditions, see Dynamic Wind Tunnel Research and Simulation. Fig. 3 shows the corresponding helicopter model mounted in the DNW. For 1999, ROTOS is planned to be used within the wind tunnel tests of the HELIFLOW project. These tests concentrate on the investigation of flow phenomena originating from the interactions between the helicopter rotor, its fuselage, and the tail rotor, see Dynamic Wind Tunnel Research and Simulation.