In order to improve low-altitude flight security of single-rotor helicopter, an experimental model of a helicopter antitorque device is developed for wind tunnel test. The model is based on the flow control technology of the cross-flow fan (CFF). Wind tunnel tests show that the model can produce side force. It is concluded that the influence of the CFF rotating speed, the rotor collective pitch, and the forward flight speed on the side force of the model is great. At the same time, the numerical simulation calculation method of the model has been established. Good agreement between experimental and numerical side force and power shows that results of numerical solution are reliable. Therefore, the results in actual helicopter obtained from Computational Fluid Dynamics (CFD) solution are acceptable. This proves that this antitorque device can be used for a helicopter.

The basic advantage of the conventional tail rotors is that they require relatively little power, produce good yaw control, and contribute significantly to yaw damping and directional stability in forward flight [

A new antitorque system should have a little impact on helicopter aerodynamic characteristics to avoid complex dynamic phenomena such as the aerodynamic coupling with other components of the helicopter, reduce the aerodynamic noise, and avoid the aerodynamic force which is difficult to control. Finally, the antitorque system should be easy to operate and control. The aim of the present work is to build and test an antitorque device and to use CFF and verify that it has the ability to produce side force. It is also necessary to analyze the relationship with the side force and power by changing the rotating speed, the rotor collective pitch, and the forward flight speed. In order to simulate and analyze the aerodynamic characteristics of the device in a real helicopter, it is necessary to establish a CFD method, the results of which are validated with the experimental data presented in wind tunnel test.

The CFF consists of a drum-like rotor with forward curved blades, encased within housing walls. The inlet and outlet have rectangular cross sections. The advantage of CFF is its ability to extend lengthwise, producing a uniformly distributed inflow and outflow and approximately 90° flow turning from the inlet to the outlet. Therefore, the antitorque device we proposed is shown in Figure

Schematic diagram of CFF antitorque device.

Principle of side force generation

The installation position of the antitorque device

This experiment is based on the low speed open return flow wind tunnel (Figure

Parameters of wind tunnel.

Parameters | Value |
---|---|

Size of test area (m | 3.4 |

Maximum wind speed (m/s) | 40 |

Minimum stable wind speed (m/s) | 5 |

Shrinkage ratio | 4 |

Definitions of geometric parameters.

Definition | Value |
---|---|

Opening angle | 90, 110, 130, 150 |

Outer radius of CFF | 100 |

Fan outlet height | 45 |

Blade mounting angle | 0, 10, 20, 30 |

Blade chord | 25 |

Number of blades (piece) | 10, 12, 14, 16 |

Blade length (mm) | 500 |

Clearance between CFF and shell | 5 |

Parameters of rotor test bench.

Parameters | Value |
---|---|

Number of blades (piece) | 4 |

Blade radius (m) | 1.25 |

Blade chord (m) | 0.072 |

Geometric torsion (°) | 0 |

Rotor solidity | 0.077 |

Blade airfoil | NACA 0012 |

Lift of rotor in wind tunnel test.

Free stream velocity (m/s) | 0 | 5 | 10 | ||||||

Collective pitch (°) | 8 | 10 | 12 | 8 | 10 | 12 | 8 | 10 | 12 |

Rotor lift (kg) | 35 | 45 | 59 | 37 | 49 | 62 | 41 | 53 | 65 |

Experimental model and equipment.

Wind tunnel

Definition of section geometry parameters

Experimental model

The position of the experimental model in wind tunnel

Six-component balance

Principle verification

The experiment includes three states, namely, static state, hover state, and forward flight state. Static state experiments are defined as the antitorque device has no rotor downwash flow and static forward flight inflow. The hover state experiment is defined as the antitorque device is in the rotor downwash flow, without the forward flight inflow. The forward flight state experiment is defined as the antitorque device is in the rotor downwash flow and forward flight inflow. First of all, the principle experiment of the side force generation has been carried out in static state. A smoke generator was placed on the top of the model. CFF rotational speed is set through the controller (Figure

Control block diagram of the rotation speed of CFF.

The experimental uncertainty is mostly expressed as fractional uncertainty of the measured value (Table

Summary of the measurement uncertainties.

Parameters | Uncertainty |
---|---|

Force balance (kg) | ±0.03% |

Data acquisition resolution | ±0.05% |

Free stream velocity (m/s) | ±0.5% |

Free stream deflection angle (°) | ±0.1% |

Model installation angle (°) | ±1% |

Rotor collective pitch angle (°) | ±1% |

Figure

Influence of blade number.

Curves of side force with rotation speed

Curves of power with rotation speed

Figure

Influence of blade mounting angle.

Curves of lateral force with rotation speed

Curves of power with rotation speed

Figure

Hover state.

Curves of side force with rotation speed

Curves of power with rotation speed

Figure

Forward flight speed is 5 m/s.

Curves of side force with rotation speed

Curves of power with rotation speed

Figure

Forward flight speed is 10 m/s.

Curves of side force with rotation speed

Curves of power with rotation speed

The numerical simulations are performed using the commercial general-purpose CFD code FLUENT 14.5 by Fluent Inc. The ANSYS ICEM software was used for grid division. Numerical calculation results will be compared with the experimental data. A two-domain model for the numerical calculation is shown in Figure

Computational grid generation.

Calculation domain definition

Interface definition

Mesh generation complete

There is measurement uncertainty in the experiments and numerical convergence in simulation: purely two-dimensional simulation in CFD and three-dimensionality of flow near the wing ends in the experiment, with inevitable mechanical losses in experiment but not in CFD. Domain size influence has been checked by extending the domain in each direction by an additional 50 per cent in length, respectively. Results (in terms of the side force) agree well with those from the baseline domain, confirming that the size of the current domain is large enough for an accurate prediction.

Grid independency studies were also conducted with three successive grids: coarse, medium, and fine meshes. The coarse mesh has 368,632 elements in total without any inflation layer near the walls. The medium mesh has 599,146 elements with a small element cluster around the blades and the central area. The inflation layer of 1 mm from the first element to the wall is added to both housing and shaft walls. The fine mesh has 1,163,316 elements with more strict mesh controls at the same locations as those used in the medium mesh. In particular, the inflation height is decreased considerably from 1 mm down to 0.1 mm. The study has shown that differences in results from the coarse and medium meshes are significant, while differences in results between the medium and fine meshes are almost negligible. Hence, it is decided to use the medium mesh as the baseline for further simulation.

Figure

Blade mounting angle is 10 degrees.

Curves of side force with rotation speed

Curves of power with rotation speed

Figure

Curves of power with rotation speed.

Curves of side force with rotation speed

Curves of power with rotation speed

Figure

Opening angle is 110 degrees.

Curves of side force with rotation speed

Curves of power with rotation speed

Test conditions are as follows: the CFF rotating speed is 1400 r/min, the blade number is 12, the blade installation angle is 20 degrees, and the rotor collective pitch is 10 degrees. Opening angle is 90 degrees, 110 degrees, 130 degrees, and 150 degrees. Figure

Opening angle changes.

Curves of side force with the opening angle

Curves of power with the opening angle

The experimental conditions are as follows: rotation speed of CFF is 1400 r/min, the blade number is 12, the blade installation angle is 20 degrees, the rotor collective pitch is 10 degrees, the flow speed is 0 m/s, and the opening angle is 110 degrees. It is shown in Figure

Left and right interface changes.

The interface effect on side force curve

Streamlines of both interfaces

Streamlines of right interface

Streamlines of no interface

Forward flight state comparison.

Curves of side force with rotation speed (5 m/s)

Curves of power with rotation speed (5 m/s)

Curves of side force with rotation speed (10 m/s)

Curves of power with rotation speed (10 m/s)

Figures

Figures

The paper drew the following conclusions based on the wind tunnel test and numerical calculation method:

Wind tunnel tests proved that the antitorque device can produce side force. CFF rotation speed, rotor downwash flow, and the forward flight flow have a great impact on side force. In this paper, the optimal geometric parameters for reactive torque device are as follows: the blade number is 10, with half interface, and the opening angle is 110 degrees. The test calculation power load of reactive torque device has a little difference with common helicopter and it can take place of a single-rotor helicopter tail rotor mechanism.

We carried out numerical calculations at different blade installation angles, different blade numbers, different opening angles, with and without interface, different CFF rotation speeds, different rotor downwash flow speeds, different forward speeds, and compared them with the wind tunnel experiment result. Finally, this proves that the calculation result is credible.

Numerical simulation flow pattern revealed initially the principle of producing side force. One part is that CFF accelerated the airflow, the other part is that the airflow bypassed the external surface of reactive torque device, formed velocity circulation, and generated the side force.

According to the shape characteristic of the reactive torque device, the system can be arranged on the tail beam of the single-rotor helicopter to balance reactive torque and control course by controlling the rotation speed of the CFF.

The authors declare that they have no competing interests.

The authors would like to acknowledge the financial support of the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Nanjing University of Aeronautics and Astronautics Innovation Foundation (Grant no. 201501049).