By Lauren Nagel
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In the world of drones, UAVs and eVTOL aircraft, having an optimized motor-propeller configuration not only allows your aircraft to fly, but to perform optimally.
Manufacturers’ data can give you an idea which motors and propellers will work in your design, but testing is not standardized, so it is impossible to compare parts across manufacturers.
Here are a few ways that a thrust stand can help you improve drone performance:
• Increase your flight time • Acquire diagnostics • Increase your payload • Improve safety • Increase your range • Get certification • Check your noise levels • Improve structure design • Reduce vibration • Quality assurance • Increase reliability • Study throttle response • Prevent in-flight icing • Learn from flight replication • Preventative maintenance • Prevent overheating and failuresA major reason for testing your motors and propellers is to increase your drone’s flight time. Increasing your vehicle’s air time will allow you to shoot longer videos, collect more data, maintain visual contact on a target, and fly farther on a single charge. Simple tests and modifications can add precious minutes to your flight time, giving you a competitive advantage over competitors. A great example comes from two mechanical engineering Masters students at the University of Ottawa who were able to more than double the flight time of their reconnaissance helicopter drone by testing various motor-propeller combinations.
Throughout the testing process, the students were able to determine that lighter electrical components would perform equally well and found a more efficient motor than the one they were previously using. The result was that they increased their helicopter’s flight time from 3 minutes to 7 minutes without compromising on noise or payload (and they came up with an idea for a great aerospace company). This example goes to show that flight time can be improved through basic modifications to your design that centre around increasing efficiency.
Further reading: How to Increase Your Drone's Flight Time
Many up-and-coming drone applications require vehicles to carry all kinds of payloads longer and farther than ever before. Meeting demands for payload capacity often requires testing multiple motors and propellers, but the initial investment will almost always pay off due to the improved operation of your UAV. Maximizing a drone’s payload capacity is important for industries such as eVTOL design, shipping and delivery, videography, cargo carrying, human transport, and more.
Hobby drones frequently have a payload up to 2kg, while drones in the “heavy lift” category may carry hundreds of kilograms of cargo. Whatever your payload requirements, testing multiple motor/ propeller configurations can ensure you’re getting the most bang for your buck. SkyDrive, a Tokyo-based aerospace company, makes use of such testing to optimize the geometry, size, and components of their drones. Most recently, they were able to build a heavy-lift drone capable of flying 15 minutes with a 30kg payload. Their final product, “CargoDrone”, contains 4 coaxial rotors for a total of 8 propellers and motors.
Improved antennas and range extenders have greatly enhanced our ability to fly UAVs into uncharted territory. The limiting factor is no longer how far we can communicate with the drone, but how long the vehicle can stay airborne on a single charge. Testing a drone’s propulsion system can help to extend its range by maximizing powertrain efficiency, contributing to a longer air time. This is especially important when flying missions into inaccessible terrain or over water.
If a drone cannot muster the power for the return trip, it could be lost completely. Ambitious videography projects and reconnaissance flights require a guarantee that the vehicle and footage collected will not be lost. With this guarantee, fantastic footage can be collected with confidence, such as this stunning tour of the Matterhorn in the Swiss Alps. Testing and optimizing your propulsion system can make these flights possible with the added benefit of knowing exactly what to expect from your UAV.
While many eagerly await the future of drones and eVTOL aircraft, one of the biggest societal concerns remains the increased noise levels and their impact on our environment. With the potential for drones to be flying overhead delivering packages, inspecting buildings and taking people to work, this is a fair and valid concern. For many drone applications, the amount of sound produced will be an important factor in deciding whether or not they are put to use. This is true not only for everyday applications, but especially for surveillance and reconnaissance operations that demand silence. Testing your drone’s propulsion system allows you to anticipate the noise levels produced and resolve any issues before it takes its first flight. This insight can ultimately lead to a more effective and competitive UAV solution.
An inspiring example of this technology at work is in wildlife monitoring and conservation efforts. Ocean Alliance’s “SnotBot” program utilizes modified consumer drones to collect organic samples from whales in order to better understand the health of the population. The drones used in these expeditions are exceptionally silent so as to not disturb or frighten the whales, a paramount requirement for the sustainability of their research. Motors and propellers produce the majority of the noise in a UAV, so testing and comparing motors and props is your best bet for conceiving the quietest version of your design.
All powertrains generate some degree of vibration, but excessive reverberation can cause damage to your components and is generally indicative of a lack of efficiency. Running a vibration test for your propulsion system is a great way to balance your propeller, detect inefficiencies and streamline your design. In doing so, you will likely notice that parts last longer and you get more performance out of a single battery charge.
Reducing vibration is especially important in the world of drone videography, where vibration can cause shaky or blurry videos, symptoms of the Jello effect. Stabilizers and post-production editing can improve the quality of your videos, but reducing the amount of vibration you are contending with in the first place can save time and money. Realizing these smoother flights and videos can easily be achieved with a bit of testing.
There is great incentive to increase reliability in the drone industry as the drone failure rate is about two orders of magnitude higher than that observed in commercial aviation. For UAVs to take over functions currently occupied by manned aircraft, their reliability, or mean time between failures (MTBF), must increase. Testing your propulsion system can help to prevent and predict failures as the data can provide insight into the state of your components.
Performing a Reliability, Availability, Maintainability, and Safety (RAMS) assessment, for example, is a great way to prove the reliability of our drone as it is an industry-recognized test that consumers recognize and trust. Once a system has been optimized, data from reliability tests can be a useful resource to reference or even publish as part of a marketing strategy.
Testing your motors and propellers can help understand how environmental factors affect your drone, such as the risk and impact of in-flight icing. In-flight icing or ‘atmospheric icing’ can be a major hindrance to drone operations, as ice build-up changes the aerodynamic properties of the aircraft. Ice accumulation results in increased weight and drag leading to loss of lift, thus inhibiting the flight capabilities of your drone. The NRC’s wind tunnel experiments also demonstrated how temperature and propeller type impact ice accretion, and that torque increases linearly with time exposed to icing conditions. Cold temperature resistance is key for drones operating in cold climates, high altitudes, and in challenging weather conditions.
Knowing how a drone responds to cold temperatures can help to design it accordingly and determine whether its operations require an additional investment in de-icing products. UBIQ Aerospace, a world leader in de-icing technology, studies the patterns and effects of ice accumulation on UAVs by testing the aerodynamics of propellers in wind tunnel experiments. Their tests have allowed them to refine their D•ICE technology, which mitigates ice accumulation for fixed-wing UAVs. For any cold weather drone operations, propeller testing can be an invaluable source of information for anticipating performance.
Further reading: What Conditions Cause Drone Icing
One of the best ways to save time and money on drone maintenance, and any vehicle’s maintenance for that matter, is to repair wear and tear before it becomes a problem. It is infinitely better to invest in replacement parts than to have to replace an entire drone due to a fatal failure.
Testing your motors and propellers is an important part of this protocol as damage is not always evident. Minor erosion from water damage or debris can affect motor efficiency, thus limiting the drone’s performance. Propellers can also become unbalanced over time and take on increased vibration, wearing down the entire propulsion system. Testing your powertrain as part of your preventative maintenance schedule can help detect these inefficiencies, resulting in improved drone performance and great savings in time and money.
In addition to the design phase, motor and propeller data can be useful throughout your vehicle’s lifetime. Recording diagnostics at scheduled intervals, every 50 flight hours, for example, can help to monitor a drone’s performance over time. Such tests are useful for detecting wear and tear as well as lost efficiencies.
Diagnostic motor and propeller testing can also provide insight as to why a UAV hasn’t been performing optimally, or give warning that it may not achieve the performance you are expecting due to unforeseen damage. Drones operating in environments with high humidity, high temperatures, or dust and debris may wear out faster than expected and surprise you with shorter-than-expected flight times or less-than-expected max throttle performance. Diagnostic testing of the propulsion system can detect these weaknesses before a failure occurs, preventing uncomfortable or potentially dangerous situations.
Safety is perhaps the number one concern of investors and regulators in the drone and electric vehicle industry. Without certain guarantees, aerial vehicles will not be permitted to enter the market and serve their purpose. Achieving a safer design can be accomplished relatively painlessly by testing the system’s components, especially the motors and propellers that complete thousands of revolutions per minute.
Better understanding these elements can help prevent overheating, engine failure, power loss and more, thus preventing accidents and injuries. As our skies become increasingly populated by aerial vehicles, citizens must feel confident that they will perform as prescribed, posing no threat to human activities. Safety tests can significantly increase the credibility of individual vehicles and the industry as a whole, guaranteeing performance and promoting peace of mind for investors, government officials and the public alike.
Special permission is required for many common drone operations in the form of a waiver or exemption from a regulative authority. In the USA, for example, a waiver is required for any operation not included in Part 107 of the FAA’s Small UAS Rules. The list of operations requiring a waiver includes flying at night, beyond visual line of sight (BVLOS), over people, more than 400ft above ground level (AGL), or more than 100 miles per hour. This list naturally overlaps with the activities of many hobby and commercial drone operations.
Obtaining a waiver is largely dependent upon the applicant’s ability to convince the FAA that their operation is safe, and extensive testing of a UAV’s propulsion system is an important part of the evaluation of a safe vehicle. Performing and demonstrating the replicability of propulsion tests can greatly support a waiver application while providing the designer with valuable information. While regulations vary worldwide, many designers will find propulsion data useful or mandatory for drone certifications all over the world.
Improving a drone’s efficiency is a circular process that begins with certain assumptions. These assumptions can include the total weight of the drone and the weight of individual components, as well as its intended use. Once these initial assumptions are made, propulsion testing can help determine whether your design will meet the requirements for its proposed purpose. For racing drones, you can determine if your design will meet speed requirements or for a delivery drone, whether it will achieve flight time minima.
The best part is that if your initial design doesn’t meet your needs, you can swap in new motors and propellers to find an ideal configuration. Once you have found your ideal powertrain set-up, you can also go on to try new batteries or modify your frame. The initial assumptions guide the optimization process, but once you have completed one round of review, you can make informed decisions about other modifications to your design.
Further reading: Drone Design Calculations and Assumptions
Propulsion testing can strengthen and legitimize the quality assurance offered to potential investors and clients. Backing up a marketing pitch with rigorous performance data is an excellent way to foster confidence in your product and give your sales team a competitive edge. In addition to sales, testing your motors and propellers can provide valuable information for data sheets and allow designers to breathe easy knowing the vehicles will perform as advertised.
Propulsion testing will also allow you to further standardize products, ensuring each unit performs equally. This translates to less time spent on customer support requests as the additional level of security will ensure a lower number of defective products leaving the facility. Whether your operation is large or small, testing motors and propellers is a great way to legitimize your quality guarantee and ensure consistency between products.
It is often necessary to learn how quickly a propulsion system can react to a change in control input. Data from such tests provides insight into how quickly the UAV can react to disturbances such as wind gusts. A typical way to physically test the reactivity of a propulsion system is to subject it to a frequency sweep control signal. A frequency sweep is a sinusoidal signal whose frequency is constantly varied to cover the whole spectrum of frequencies to be tested. With the data collected, the UAV designer can determine how fast the propulsion system can react to sudden changes in throttle.
Another method to learn about the reaction time of a powertrain is to subject it to a step input. While data is recorded at high speed, a sudden change in throttle is applied. After some time, the propeller stabilizes to a new rotation speed. The time it takes to stabilize the speed is known as the settling time. Finally, a proportional integral derivative (PID) test can measure your propulsion system’s consistency over an extended period of time by commanding a consistent, targeted thrust. These tests can be performed using a dynamometer with sufficient scripting capabilities, allowing for a complete understanding of a UAV’s throttle response.
Propulsion testing can contribute to simpler, more efficient flight replication for applications such as agriculture, surveying, research and more. The ability to record flights using propulsion testing equipment allows designers to plan and save a route automatically, then reproduce the throttle data later on. Without even leaving the lab, designers can optimize their flight plan based on data collected on motor and propeller performance. Agricultural applications of autonomous UAV technology are often based on the principle of efficiency, minimizing labour and input costs with the added benefit of protecting human health.
Airboard Agro’s agriculture drone is a great example, as it sprays crops on pre-programmed routes in challenging terrain, conducting the laborious and repetitive work previously performed by humans. To truly reap the benefits of this technology, drones should be optimized for the specific task they will carry out, taking into consideration the speed, thrust and stability required. Propulsion testing is a great resource for improving these flight replications and ensuring UAVs will attain their maximum efficiency for autonomous missions.
One of the most common causes of drone failure is engine overheating leading to engine failure. Maximum temperature and voltage ratings are often provided with electric motors, but it can be unclear when your motor is approaching these limits. Additionally, despite the fact that engine cooling depends on current, current ratings are not standard in the industry. One way to test a motor’s limits is to measure its temperature at various speed intervals using thermal probes, a useful strategy for circumventing failures.
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At Kent State University in Ohio, Dr. Blake Stringer’s lab is performing such thermal tests with electric motors to investigate their thermal properties and to look at management of sUAS eVTOL motors under high-power conditions. This video produced in their lab shows the unfortunate consequences of thermal runaway, resulting in the overheating and destruction of the motor. These studies are increasingly important as drone operations become longer and move into harsher, hotter climates. Testing motors early in the design process can prevent overheating and ultimately save paying the cost to replace them.
Further reading: How to Automate Propulsion Tests for Your Drone
We hope that this article has demonstrated the multitude of benefits that come with testing your drone's propulsion system. We offer several tools that can perform these testing functions and help you take your design to the next level:
Please find within the following table which describes your problem. Once selected, mark down the troubleshooting points to follow.
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.
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:
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