Why you should test your drone's motors and propellers
By Lauren Nagel, last updated: 30-10-2020
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. In this young and competitive industry, achieving maximum performance is essential in order to have a competitive edge.
Testing your motors and propellers throughout the design process can save valuable time and resources in the long run. 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 just a few of the reasons why you should test your motors and propellers (expanded below):
|• Increase your flight time||• Diagnostics|
|• Increase your payload||• Safety|
|• Increased your range||• Certification|
|• Noise level testing||• Structure design|
|• Vibration testing||• Quality assurance|
|• Reliability||• Throttle response|
|• In-flight icing||• Flight replication|
|• Preventative maintenance||• Motor thermal testing|
Increase your flight time
A 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 company). This example goes to show that flight time can be improved through basic modifications to your design that center around increasing efficiency. Continue reading here for an in-depth look at how you can increase your drone’s flight time by testing your motors and propellers.
Increase your payload
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.
Increase your range
Improved antennas and range extenders have greatly enhanced our ability to fly UAVs into unchartered 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.
Noise level testing
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. A Reliability, Availability, Maintainability, and Safety (RAMS) assessment, for example, 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. NASA’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.
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.
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.
Motor thermal testing
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. 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.