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In this blog I will be utilizing the amazing SOLIDWORKS Flow simulation CFD (computational fluid dynamics) tool to find out what motor specs I would need to get my RC Helicopter airborne.
The Flow Simulation tool is built into the software interface directly, so there is no time wasted exporting or importing your model. This also means that design changes can be easily and efficiently implemented and tested. As we will see, the set up for this type of problem is a breeze when using this tool, as is finding the data we need.
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BACKGROUND
In order to get my RC Helicopter to fly the rotor blades must produce sufficient Lift. To understand lift, consider a solid body immersed in a non-stagnant fluid. Lift is defined by the component of force generated by the solid body that is perpendicular to the flow direction. In our case it is the force that is normal to the ground generated by the rotor blades.
The rotor blades generate lift by creating a pressure differential which in turn generates the upward force needed to get airborne [see Appendix]. Considering that the weight of the RC Helicopter is 2 kg, I will have to generate more than 20N (4.5 lbf) of thrust to get airborne.
Helicopter blades are designed to operate at a constant RPM. The amount of thrust produced at any given time is controlled by the pitch angle of the rotor. My RC Helicopter has a pitch angle range from -10 degrees to +10 degrees (zero being neutral, or zero lift).
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The image above illustrates the rotor blades at 0 degree pitch. The following image shows the rotor blades at a 10 degree pitch.
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The different configuration of pitch angles allows the pilot to control the amount of thrust at any given time.
An important fact to consider is that the highest demand on the motor will be when you are generating lift. By testing the configuration that is producing maximal lift, the torque on the blades will represent the maximal torque requirement for my motor.
SET UP
Before jumping into the Flow Simulation tool, we first need to create a couple of parts that represent the rotating region of our study.
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Theoretically, the rotor blades should produce flow fields that may not be axially symmetric. Therefore, a local rotating region (sliding mesh) technique will be used.
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The working fluid will be air, unsurprisingly, and all other conditions will be standard. The main driving force of the flow will be dictated by the spinning of the rotor blades.
The next step is to ‘switch off’ any components that are irrelevant to our study. Theoretically, I could just study the rotor blades, but will include the canopy and main structural components for completeness. Other details like the inner frame or little bolts and gears will not give you any more accuracy. In fact, your mesh will have to be refined around those areas which would massively increase your cell count and solver time.
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BOUNDARY CONDITIONS
The set up for this particular problem is rather simple. We only have the two rotating regions to define, and we’re done.
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I set the rotational speed at 2000 RPM, which seemed to be the general speed for most RC Helicopter motors.
GOALS
Since we are interested in solving for the motor specs and determining the thrust characteristics, I will define some Engineering Goals to enable easy calculation of desired values.
- A Surface goal on the faces of the rotors to calculate total Torque.
- A global goal of the Y (vertical) component of force (thrust).
That should encompass all the data we need.
MESH
Once the model is simplified, it’s time to focus on our mesh. Since the business end of our study will be taking place at the rotors, I will need to make sure that the mesh is refined in those areas, particularly at the blade tip. The bulk of the surrounding volume can be much coarser.
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This can be done by specifying local refinement regions and an overall course setting (mine was at 2). As a rule of thumb, it is a good idea to have 2-3 elements between your model and the rotating region boundary.
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By keeping the overall cell count low with enough small volume cells where you need them you can get good results without having to let your computer run overnight (although this is unavoidable in some complex cases).
RESULTS
- Zero degree pitch (neutral) position
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As can be seen from the Velocity Cut Plot, the flow is not being directed axial through the blades and is thus not developing any significant thrust.
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By reviewing the Engineering Goals set earlier, it is shown that there is only about 1 N of thrust, nowhere near the 20N threshold we need to overcome (which is good news for a neutral position).
- 10 Degree Pitch
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As can be seen from the Velocity Cut Plot, the flow is being directed downward. The details are lost in this plot since the direction of the velocity id not being recorded. Therefore, a Flow Trajectory Plot would be needed to further study the flow behavior.
The Compare tool would allow me to quickly create the two plots seen above.
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The compare tool can generate plots from multiple studies, allowing you to quickly view and compare the performance of various design configurations, as I did with the 10 degree and zero degree rotor pitch configurations.
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The Flow Trajectory Plot clearly illustrates the direction and sense of the flow. It is also interesting to view the blade tip vortices around the blades.
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The results of the study show that the blades are producing 95.6N (21.5 lbf) of thrust. This is more than enough to get airborne.
The torque on the blades is evaluated to be 15.86 Nm (11.7 ft-lb), which translates to a power requirement of 3321.7 Watts (power = Torque x Rotational Velocity).
Looking at available RC Helicopter Motors, one viable solution is to use the Turnigy HeliDrive SK3 Competition Series motor which is rated to 3770 Watts and only weighs about 400 grams.
Recall that we worked with a motor speed of 2000 RPM. For a more complete analysis, we could utilize a Parametric Study. This would allow us to run the same study with a range of RPMs to ensure the motor is the right choice across the entire power band.
Thank you for reading this blog!
APPENDIX
For those of you who are interested in how the rotor blades generate lift consider the following:
0 Degree Rotor Pitch
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As can be seen in the images above, the pressure distribution across the blade is symmetrical about its chord. This trend mimics the velocity profile of the streamlines. The increased velocity creates areas of lower pressure. However, due to the symmetric nature of the profile the net lift is zero.
10 Degree Rotor Pitch
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As can be seen in the images above, the pressure distribution is no longer symmetrical about the chord. As before, the pressure distribution closely mimics that of the velocity profiles of the streamlines. In this case, the pressure differential creates a resultant force oriented perpendicular to the flow direction (lift).
30 Degree Rotor Pitch
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Eventually, as the angle of the rotor pitch continues to increase, the flow can no longer keep attached to the blade. The detached flow increases the drag force (the force component parallel to the flow) and reduces lift.
This phenomena is known as Stall, and should be considered during your design process. This can easily be detected by utilizing the Streamlines option when creating a Velocity Cut Plot.
