Please find within the following table which describes your problem. Once selected, mark down the troubleshooting points to follow.
Link to Wing Flying
If you are planning to run dual-motor testing, we recommend starting to read the whole document from section 3.1.
In a dual-motor testing environment, you may have two motors and propellers to be mounted on a propulsion system in order to increase the output thrust and neutralize the torque without adding much extra structure on the drone.
In the Series Thrust Stand and Dynamometer, the following versions may allow you to run coaxial tests:
The nature of the dual-motor testing introduce some new parameters:
There are also a few extra parameters when it comes to controlling two motors and ESCs: the rotation speed difference between two propellers and the air speed and pressure generated by the propeller in the front. It is complicated to model all these parameters in a single test and we suggest to first find out the target setup you may need to start a test.
Generally speaking, most of the coaxial propulsion systems on a UAV use the back-to-back design. The advantage of a back-to-back design is mainly that both motors are mounted onto the same drone frame structure and may potentially reduce the material and weight on the drone structure.
If you wish to use the Series Dynamometer to run back-to-back tests, please keep in mind that there is a minimum coaxial distance imposed on the thrust stand. You may not be able to reach the desired distance between your two propellers due to the distance restriction. See section 3.2 for more details about the coaxial distance that you may reach.
When a distance cannot be reached in a back-to-back setup, we recommend using the face-to-face setup as an alternative. Face-to-face setup allows you to reach a minimum distance of 0 in theory. The disadvantage of this setup is that the aerodynamic effect may not be exactly the same as a back-to-back setup.
From what RCbenchmark has tested internally, we found out that the difference between a back-to-back setup and a face-to-face setup is not significant when all propellers are large enough. The smaller the propellers are, the smaller the thrust they may generate and thus make the drag effect more obvious and the test result less accurate. When your propellers are larger than 32” on the 25 kgf and 40 kgf version thrust stand, and larger than 40” on the 75 kgf version thrust stand, you may convert the thrust stand into a face-to-face setup to run the coaxial tests in order to a lower coaxial distance.
As stated in section 3.1, the coaxial distance, defined as Z, is the physical distance between two propellers on the axial direction. This parameter is significant to the dual-motor testing because it determines the airflow inlet from the upstream propeller towards the downstream propeller.
When doing coaxial testing, it is extremely important to have two motors counter-rotating. This means one propeller in clockwise direction (CW) and another in counterclockwise direction (CCW). They must generate either push or pull (thrust) in the same direction, while attempting to neutralize the torque.
Figure copyright: NASA@SC15
When it comes to airflow, the upstream propeller will generate air flowing through both axial and radial directions as you may see in the figure above. This will require the downstream propeller being able to attack the incoming airflow in both directions.
The smaller the Z is, the closer those two propellers become. After the acceleration and twisting, air flows out from the upstream propeller with axial and radial speed and pressure and the closer the downstream propeller is placed, the more turbulence and more severe radial resistance it must suffer.
In coaxial testing, we use the term Z/D factor to represent the relationship between the axial distance Z between two propellers and the propeller's diameter D. If you have two propellers in different sizes, use the smaller one. The Z/D factor is a significant factor to be controlled and observed in coaxial testing and we suggest this factor shall not be smaller than 0.15.
The smaller the Z/D factor is, the more easily the downstream propeller being affected by the turbulence. You may observe negative thrust or torque and other phenomena related to the low Z/D factor.
To start the test, we suggest to first try testing in a face-to-face setup and reach the minimum Z/D factor of 0.15. Gradually increase this factor and make measurements until you can reach the best efficiency, mark down the Z distance and see if you may reach it with the back-to-back coaxial:
It’s hard to predict the overall performance of a coaxial system without testing realistically different Z/D factors. Please also keep in mind that when two propellers have different diameters and pitches, the optimum Z/D value may also differ.
In a common application, some theories indicate that we shall have the downstream propeller larger in diameter and higher in pitch: it can theoretically avoid the downstream propeller under full turbulence and an increased attack angle can also help to tackle the counter-rotating air flow. You may see some of our tests on the RCbenchmark database:
https://database.tytorobotics.com/tests
Radial offset distance, shown as e in the figure located in section 3.1, is the offset of two central axes in any radial direction. In most of the dual-motor or coaxial propulsion systems used on drones, designers tend to keep both motors on the same axis thus a zero radial offset.
Generally speaking, we use the term “overlapping percentage” to characterize potential aerodynamic effects of the airflow impact on the downstream propeller. When two propellers are at the same size and without any radial offset, we can say the overlapping percentage to be 100%. The smaller the overlapping percentage becomes, the smaller turbulence the downstream propeller may suffer and can potentially increase the thrust generated.
The advantage of radial offset is a decreased overlapping percentage in order to avoid direct turbulence effect towards the downstream propeller. This may potentially increase the thrust the downstream propeller can generate.
There are multiple constraints for a radial offset or a smaller overlapping percentage. One of the major disadvantages of radial offset is mainly about the complexity when designing the flight control system. The extra motor and propeller in a different position may increase significantly the complexity of the flight control design as each motor may be regarded as independent. Drone’s frame and position restrictions can also impose obstacles for a larger radial offset.
If you are looking for more details, kindly visit coaxial thrust stand.
On the thrust stand Series , you may be able to adjust the radial offset with certain setups. Please keep in mind that the diameter difference between two propellers, and the axial offset distance may also affect the optimum thrust. See section 3.4 to learn more about how to start a coaxial design and testing.
As we have introduced different parameters in the previous sections, we will then focus on how to put everything together and to start planning dual-motor propulsion system design.
First, to summarize all the factors that we have mentioned earlier:
To start a design:
Unfortunately, the Series / does not support dual-motor control and testing. If you wish to test dual-motor, we suggest using the Series .
The Series / can only be communicating with ONE computer at a time, the software cannot support multiple opening sessions.
If you must test two motors on the Series /, you may then:
When you have propellers larger than 18”, we suggest using the Series for your coaxial testing.
If you still cannot solve your problem, us at . We support all issues related to the measurement tools itself, and we can provide limited support regarding your test setup and power-train selection. You may go to our online database to obtain more information:
Motors generate a significant amount of electromagnetic interference (EMI), which can introduce noise into measurements and even cause USB dataloggers to crash. Identifying and resolving these issues can be challenging, emphasizing the importance of careful design and simulation of electrical connections from the outset. It's essential to allocate time for diagnosing EMI problems, and consulting with an electronics engineer skilled in power electronics, EMI mitigation, and grounding is highly recommended.
Our research indicates that sourcing a commercial multi-axis load cell capable of simultaneously measuring torque and thrust with the appropriate ratio of thrust to torque is challenging. Typically, one of the two measurements may not utilize the load cell's full scale of measurement, leading to reduced accuracy. Additionally, our load cells are engineered to minimize airflow restrictions while ensuring a minimum thickness, allowing for testing of coaxial systems.
After acquiring suitable load cells, the process continues with designing a calibration method and verifying its accuracy. In powertrain testing, thrust and torque are seldom isolated, highlighting the need to calibrate the load cells under conditions that closely resemble actual operation. This entails calibrating both thrust and torque simultaneously and addressing crosstalk when generating the polynomial. Without a dedicated calibration apparatus, recalibration becomes impractical after prolonged use.
In our endeavor to create the next iteration of test equipment, we found that revamping the software was as time-consuming as upgrading the hardware. The intricacy of the software varies depending on the desired features, but at the very least, it must encompass functionalities for controlling the test stand and performing data logging.
Beyond mere data recording, the processing of data significantly affects your workload. To simplify data handling and analysis, we've incorporated several data processing features into our software:
1. Integration of a low-pass filter option to enhance data smoothness.
2. Data resampling at varying frequencies to cater to diverse analysis needs.
3. Implementation of powertrain mapping functionality.
While programming these features requires time and effort, they ultimately save users time and prove highly practical.
Designing a test stand can present unexpected challenges, such as identifying the stand’s resonance frequencies and mitigating vibration. Without knowledge of these frequencies, there's a higher risk of damaging both the stand and the propulsion system. Balancing stability and measurement accuracy amid vibration poses a challenge, requiring a delicate equilibrium to be struck.
Once you've mastered a single-rotor testing setup, you may wish to test dual motors in a coaxial or offset configuration. While face-to-face propeller configurations are relatively straightforward, back-to-back configurations, common in many quadcopters, pose a greater challenge. Achieving optimal rotor proximity demands meticulous design and planning.
Developing a compact test solution poses a significant challenge. If the test stand interferes too much with the airflow from the propellers, it can affect the accuracy of the readings. Creating a compact design that maintains stability without compromising airflow requires extensive development time.
One of our primary objectives in redesigning our test stands was to ensure a compact design that minimally interfered with airflow, ideally less than or only slightly more than the motor itself.
Experiencing imprecise measurements that result in underperformance is a nightmare scenario for drone builders. Relying on data presumed to be reliable, only to find that your prototype's actual performance falls short of expectations, can be frustrating and challenging to rectify.
While our test stands may not always align perfectly with your project requirements, we trust that this article has equipped you with the knowledge to make an informed decision on whether building your own test stand or purchasing one is the best choice for your project.
If you seek a reliable, professional test stand to ensure results you can trust, we welcome you to explore our range of products further.
Are you interested in learning more about uav motor test? Contact us today to secure an expert consultation!