Validation of CFD Codes for the Helicopter Wake in Ground Effect

When a helicopter takes off, lands, or makes hovering or taxiing flights in ground effect, its downwash interferes with the ground. Encounters with such highly turbulent helicopter wakes have been blamed for two fixed-wing aircraft crashes in the United Kingdom. Additional incidents including tents blown away are reported in Japan. Due to these accidents, the Japan Aerospace Exploration Agency (JAXA) and the University of Glasgow (UoG) are investigating the helicopter wake structure in ground effect, especially during taxiing, by means of computational fluid dynamics (CFD). In this study, CFD codes of UoG and JAXA are validated through comparing numerical results of each party and flight experiment data. As a result, it is found that the CFD codes show qualitatively the same results each other and they are also close to the experiment

JAXA-ONERA-DLR cooperation: results from rotor optimization in hover

A cooperation between JAXA, ONERA and DLR puts the focus on the aerodynamic optimization of helicopter rotors. This paper represents the conclusions from the first phase: optimization of a hovering rotor. The HART-II blade is first investigated with low-fidelity tools and compared against state-of-the art CFD simulations. Afterwards, the chord distribution and twist of the HART-II blade are optimized using the low-fidelity tools as well as CFD. Since the partners observed differences in the outcome of the CFD simulations for the low-fidelity optimized blades, a deeper investigation of the effects of the turbulence modelling approach, elasticity and grid topology is conducted. The findings show that the chosen flight condition is close to the thrust of the maximum Figure of Merit and that the vortex-triggered separation on the outboard sections of the blade has to be modelled correctly. In this study, the blade grids had the most noticeable effect on the results, followed by the turbulence model and elasticity. With respect to the optimization, low-fidelity methods require special care, whereas CFD optimized blades were found to lead to more robust designs, even though they have only been optimized for a single point. This is explained by the more accurate modelling of the stall phenomenon with respect to geometrical changes

JAXA-ONERA-DLR COOPERATION: Results from Rotor Optimization in Forward Flight

This paper presents the results of a cooperative study by JAXA, DLR, and ONERA on the optimal design of helicopter blades for high-speed forward flight. Optimizations and simulations are carried out by each agency with their own analysis codes using both blade element theory-based methods and computational fluid dynamics. These results are cross-compared and show common trends identified for optimum rotor blades obtained by each agency and the mechanism for improving forward flight performance are discussed. From the effective drag distributions, it is confirmed that, in order to improve forward flight performance, it is first important to reduce drag on the advancing side, and that a blade with a relatively small twist angle and planforms with a smaller chord length at the root and tip compared to the mid-span section is generally a suitable blade

Overview of the Novel Intelligent JAXA Active Rotor Program

The Novel Intelligent JAXA Active Rotor (NINJA Rotor) program is a cooperative effort between JAXA and NASA, involving a test of a JAXA pressure-instrumented, active-flap rotor in the 40- by 80-Foot Wind Tunnel at Ames Research Center. The objectives of the program are to obtain an experimental database of a rotor with active flaps and blade pressure instrumentation, and to use that data to develop analyses to predict the aerodynamic and aeroacoustic performance of rotors with active flaps. An overview of the program is presented, including a description of the rotor and preliminary pretest calculations

Investigations of a boxed rotor: The STAR II rotor in DLR's test hall

The second Smart Twisting Active Rotor (STAR II) project aims at investigating active twist on a conventional blade design in the DNW-LLF wind tunnel. Prior to the wind tunnel test, the rotor is first tested for correct operation in the rotor test hall at the German Aerospace Center (DLR) in Braunschweig. This test shall ensure that all the instrumentation, but also the active twist, works correctly along with setting up the tracking of the blades before shipping the whole test setup to the DNW-LLF wind tunnel. From previous rotor tests in this hall, it was observed that vibration levels increased drastically above certain thrusts. Since there were no PIV tests carried out within this test chamber, it was decided to carry out CFD simulations to obtain visualizations of the flow and to find the origins of these vibrations. The rotor test hall was approximated by a simple box. With the help of CFD simulations carried out by DLR and JAXA, the initial guess of potential recirculation could be confirmed. The recirculation effect and vibrations become stronger with increasing thrust and therefore limits the maximum thrust in the test hall. This paper details the results of the CFD simulation and presents them along with initial experimental data from the rotor test hall

Vortex-Induced Stall on an Actively Twisted Highly Loaded Model Rotor Blade

As preparation for a future wind tunnel test of a rotor with twist actuation blades (STAR II ), a prediction effort by the contributing partners has been executed. In this paper, the results of the planned vortex-induced stall operating condition are presented. The current conclusion is that a noticeable spread of the results is given when searching the maximum attainable thrust, depending on the onset of stall once perceived by the individual simulation methodology. Some general trends for the best suited actuation settings reduce vibrations and required power could still be identified. Generally, most CFD-based results allowed to capture this physical phenomenon, but carefully tuned low-fidelity aerodynamics also managed to capture the same trends at a fraction of the computational cost

Vortex-Induced Stall on an Actively Twisted Highly Loaded Model Rotor Blade

In preparation for a future wind tunnel test of a rotor with twist-actuated blades (STAR II), numerical predictions of this test have been carried out by the contributing partners. In this paper, the simulated results of the vortex-induced stall operating condition, synonymous with a highly loaded flight condition, are presented. The current conclusion is that a noticeable spread of the results is given when searching for the maximum attainable thrust, depending on the onset of stall once perceived by the individual simulation methodology. Some general trends among the simulations could still be identified with respect to the actuation settings that reduce vibrations and the required power. Generally, most CFD-based results allowed to capture this physical phenomenon, but carefully tuned low-fidelity aerodynamic tools also managed to capture the same trends at a fraction of the computational cost

Smart Twisting Active Rotor (STAR) - Pre-Test Predictions

A Mach-scaled model rotor with active twist capability is in preparation for a wind tunnel test in the large low-speed facility (LLF) of the German-Dutch wind tunnel (DNW) with international participation by DLR, US Ar-my, NASA, ONERA, KARI, Konkuk University, JAXA, Glasgow University and DNW. To get the maximum benefit from the test and the most valuable data within the available test time, the tentative test matrix was covered by predictions of the partners, active twist benefits were evaluated, and support was provided to the test team to focus on the key operational conditions