The introduction of unmanned aerial vehicle (UAV), commonly known as drone has made a big leapt towards a smaller aircraft with greater mobility. The flights of UAVs may operate with various degrees of autonomy: either under remote control by human operator or autonomously by onboard computers. Since UAV is an aircraft that travels with lower speed, hence most of the UAVs are installed with propeller. The propeller of UAVs is also called as spinning wing. As it rotates, it creates a force called lift that allows the UAV to rise into the air as well as giving high manoeuvrability to the aircraft itself. Hence, in this case study, we shall focus on how to simulate the helicopter rotor as well as the flow around the it. The reference for this case study is from Hawk Ridge Systems.
Assume that now you have a RC helicopter model and you would like to know the performance of the helicopter by varying the rotor spinning rate. As mentioned, RC helicopter flies when the lift is sufficiently high to rise the RC helicopter. Lift is defined by the component of force generated by solid body that is perpendicular to the flow direction. To better understand the lift, a pressure difference around the aerofoil is used. As the aerofoil is moving toward the airflow, the air at the upper surface of the wing experience higher velocity and reduced pressure. Whereas, for lower surface of the aerofoil, the air velocity is lower than the upper surface and producing higher pressure. The pressure difference between upper and lower aerofoil surface produces a perpendicular force which is known as lift.
From Hawk Ridge Systems, the RC helicopter is designed to have a mass of 2kg. To allow the helicopter to lift, the spinning rotor needs to generate the lift more than 20N (4.5 Ib) to make the lifting possible. Helicopter blades are designed to operate at a constant RPM. The amount of that the rotor able to produce is dependent to the pitch angle of the rotor. Let the pitch angle range from -10 degrees to +10 degrees where zero being neutral or no lift produced. The diagram below shows the pitch angle at +10 degrees. Different pitch angle will produce different lift due to the aerodynamic lift contribution. Hence, flow simulation is to be done to test the maximum lift the configuration can produce as well as the maximum torque the rotor need to withstand during the spinning.
Here come the suggested set up to perform the helicopter rotor simulation. Before jumping into Flow Simulation tool, we first need to create a couple of parts that represent the rotating region of our study.
Theoretically, the rotor blades should produce flow fields that may not axially symmetric. Therefore, the local rotating region (sliding mesh) technique will be used. The setting of the mesh is as shown below. Since the simulation is an external region of the rotor, the analysis type must be set to “external”. To make the simulation run faster, stuffs that no direct influence to the flow may ignore by exclude cavity and internal space.
Of course, the working fluid is also important. In this case, the fluid is air and all the other conditions are maintained to be standard. The main study for this rotor blade is the driving force that produced by the spinning of the rotor blades. As mentioned, the focus the flow around the rotor blades. Hence, all the components that are irrelevant can be “switched off”. theoretically, to obtain accurate result, canopy and main structural components for completeness must be included. Other details like the inner frame or little bots and gears will not give any accuracy also can be ignored. Otherwise, the mesh around the bolt and gears will refine and massively increase the cell count and solver time.
After identifying the important components in the assembly, the next step will be setting the boundary conditions. The set up for this case is rather simple. Only the two rotating regions need to be define and we are ready for the simulation. Hereby, the author set the rotational speed at 2000 RPM, which is the general speed for most of the RC Helicopter motor. After setting the boundary conditions, here come one of the most important part which is the “meshing” process.
The simplified model is left with the components that will have direct impact to the flow of the helicopter. Since the business and of the case study will be taking place at the rotor, hence the mesh is refined in those area, particularly at the blade tip which contribute the trailing vortices. The bulk of the surrounding volume can be much coarser because it has least impact to the lift produced. This can be done by specifying local refinement regions and overall course setting (author set at 2). By keeping the overall cell count low with enough small volume cells where you need them you can get good results having to let your computer run overnight. In some complex case, more mesh is needed and hence more time is required to solve them.
After the simulation, it’s time to let the simulated results speak the words. When the pitch angle is zero-degree (neutral) position, the flow is not being directly axial through the blades and is this not developing any significant thrust. From the diagram below, the contours at upper and lower regions are almost the same. This means there is no significant resultant force acting downward to provide lift to the helicopter.
There is about 1N of thrust produce, nowhere near the 20N threshold that required to lift the RC helicopter. When the pitch angle has been adjusted to 10 degrees, there is a huge contour difference between upper and lower blades. All the flow is being directed downward. The details are lost in this plot since the direction of the velocity id not being recorded. Therefore, the flow trajectory plot would be needed to further study the flow behaviour.
The flow trajectory plot for 0 degree and 10 degrees as shown below. The compare tool can generate plots from multiple studies, allowing you to quickly view and compare the performance of various design configurations.
0 degree inclination
10 degree inclination