Aug. 18, 2025
By Lauren Nagel
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The drone engineering process often operates as a ‘design loop’, which refers to the circular nature of the design process. Building the first version of the drone relies on certain assumptions, many of which will change as components are selected and the design comes together.
In this article we will cover:
We will be using the Series thrust stand to gather data for this article.
The design loop begins when the designer looks at how the first version of the design differs from the assumptions, then goes back to the beginning with the new information (figure 1).
Figure 1: The drone design loop illustrated
In our previous article, How to Increase a Drone's Flight Time and Lift Capacity, we covered the first stage of the design process and reached a first version of our design. In this article we will start where we left off, looking at how our assumptions held up.
We started our design process with the assumption that our drone would weigh 777 g and would be able to fly on its own. Following these assumptions, we predicted we would need 1.9 N of thrust per propeller for hover flight, so we looked for the motor-propeller combination that would be most efficient at 1.9 N. Once we found the most efficient combination, we had the tools needed to estimate our flight time, which is where we will start off today.
For this article we will be more precise with the mass of our components. We will assume the following mass breakdown of our 777 g drone:
Our goal is to maximize our drone’s flight time so that it can hover as long as possible. In our previous article, we modelled the flight time of our drone with varying battery capacity (figure 2).
Figure 2: Flight time vs. battery capacity for the original drone design
We presumed our design would include a Turnigy nano-tech mAh 4S battery and included its mass in our overall calculations. The battery’s capacity is just over 19.2 Wh (14.8 V * 1.3 Ah = 19.2 Wh), which occurs within the growth phase of the graph and gives us only about 4.5 minutes of flight time.
If we increased the battery capacity, we could also increase our flight time, but the trade off would be increased weight. This is where the design loop begins, as we swap components to try and build the drone that best meets our needs.
Up to the 0.2 hour mark there is an increase in flight time with increased battery capacity, but after about 100 - 125 Wh the marginal gains become less significant. For this reason, we will start by swapping our old battery with a new battery that has around 100 - 125 Wh of capacity in order to increase our flight time. The Turnigy mAh 6S LiPo pack nicely fits our criteria with 111 Wh of capacity (figure 3).
Figure 3: Turnigy mAh/ 111 Wh LiPo battery (Photo: HobbyKing)
This new battery weighs a whopping 655 g compared to our old battery that weighed just 155 g. Assuming all of our other components stay the same at 622 g, the total mass of our drone is now 1,277 g.
We will therefore need to produce at least 12.5 N of thrust for the drone to hover (1.277 kg * 9.81), just over 3.1 N per propeller. We would also like to achieve at least double that thrust to have a good control authority, so we will be looking for the propeller that is most efficient at 3.1 N, but can also achieve up to 6.2 N of thrust.
To review, we have three propellers in our list of candidates:
We will work with the assumption that our drone frame is set and we cannot exceed 6” in diameter for our propellers. We can learn about our three propeller candidates by looking through the RCbenchmark database of electric motors, propellers and ESCs. Test data such as thrust, torque, RPM, power, efficiency and more is collected using one of our propulsion test stands, and for this drone the RCbenchmark Series would likely be the best fit.
For our candidates, data from the database tells us that all three propellers reach our hover thrust of 3.1 N, but only the R King Kong nears the maximum thrust of 6.2 N (0.63 kgf) (figure 4).
Figure 4: Thrust performance of the propeller candidates
These results suggest that either our battery is too heavy or our motor/ propeller combination was not producing sufficient thrust. We are aiming to have the longest flight time possible, so rather than looking for a smaller battery right away, let’s explore some more propellers that fit within our frame limits, but produce more thrust.
Further reading: How Imbalance Causes Propeller Vibration
Our frame limits us to propellers that are 6” or less in diameter, but we can still experiment with our pitch, material, and brand. We will use the drone component database to filter for propellers that are 6” in diameter and produce at least 6.2 N (0.63 kgf) of force. This search provided several good options, but for simplicity we will narrow it down to three candidates that produce the most thrust:
Figure 5: Thrust vs. RPM for the new propeller candidates
As you can see in figure 5, all of our propeller candidates can produce 10 N (1 kgf) of thrust or more. For this reason, we can aim for a hover thrust of 5 N and a max thrust of 10 N, which will allow us to lift a larger battery with the same propulsion unit.
As shown in figure 6, at our original hover thrust of 3.1 N (0.32 kgf) and at our new hover thrust of 5.0 N (0.51 kgf) the efficiency of propeller 1 and propeller 2 is very similar, separated by only about 0.1 gf/W. Propeller 2 is slightly more efficient, but it is also heavier. This increased weight could lead to a shorter flight time and leaves less mass available for the battery. In a quadcopter, the total difference would be 3.76 g ((4.32 g - 3.38 g)*4).
Figure 6: Propeller efficiency vs. thrust for the new propeller candidates
After a quick look at the online marketplaces, it is evident that 4g makes no difference in terms of capacity for batteries of this size. For this reason, and the negligible effect of 4 g of mass for our drone, it makes sense to use propeller 2 due to its higher efficiency.
Our next step will be to find the brushless motor that is most efficient with this propeller at our new hover thrust of 5 N. In general, we are looking for a motor that can exceed our max thrust of 10 N, but not by too large a margin. We don’t want to operate the motor at its maximum speed for too long, but we also don’t want to haul a motor that produces more thrust and torque than we need.
Of the two motors we tested previously, MultiStar Elite Kv and EMAX RSII Kv, only the Kv motor meets our max thrust requirement (figure 7). We will therefore have to use the motor database to find a new candidate.
Figure 7: Motor efficiency vs. thrust for Kv and Kv motor candidates
From the database we find the Hypetrain Blaster Kv, which meets our criteria. We ran a test with each of the two motors paired with propeller 2, and the results are shown in figure 8. Motor 2, EMAX RSII Kv, is the most efficient with propeller 2 at our hover operating point of 5 N (0.51 kgf) and it also happens to be more efficient at our max thrust of 10 N (1.02 kgf). The efficiency difference at hover thrust is about 2.2% (55.6% vs. 53.4%), but the Kv motor is also lighter (32.37 g vs. 36.96 g), so it makes our decision easy.
Figure 8: Motor efficiency vs. thrust for Kv and Kv motor candidates
Now is a good time to summarize the mass of our components since the mass of our propellers and motors has changed as well as our hover thrust. Here is the new breakdown:
Based on these new values, we have .7 g of mass available for our battery.
Since we also have our motor and propeller picked out, we can also determine our discharge (C rating) needs, which will also be a consideration for picking out the battery. We want to be sure that our motor will not draw more current than our battery can provide, or else the battery could rapidly degrade or overheat. The formula for determining current draw for a battery is: Current (A) = C rating * Capacity (Ah).
Further reading: Brushless Motor Power and Efficiency Analysis
There is no information on continuous or burst current for the EMAX RSII Kv online, but we can look at data in the RCbenchmark database and compare all tests done with this motor. As we can see in figure 9, the max current reached during various tests was about 42 A.
Figure 9: Current vs. Rotation speed for EMAX RSII Kv motor
The Turnigy High Capacity mAh 4S 12C Lipo Pack has the highest capacity in Wh of all the batteries in our weight range, giving us 4 * 3.7 * 16 = 236.8 Wh. It weighs 1,366 g, has a 12 C discharge rating and 16 Ah of capacity, so it can handle a current draw of 192 A, which is more than we need.
The main consideration for choosing an ESC is that it can deliver the motor’s peak current. In our case we do not expect our motor to exceed 42 A, so an ESC like the HobbyKing 60A ESC 4A SBEC will work great. It can deliver a constant current up to 60 A and a burst current up to 80 A, while providing 4 A to the BEC. This gives us a bit of a safety margin, so this ESC will be a good choice for our drone.
Figure 10: HobbyKing 60A ESC 4A SBEC (Photo: HobbyKing)
As we learned in our previous article, flight time is dependent on the capacity of the battery and the power drawn by the propulsion system. Many factors thus come in to play, summarized in the formula below (see previous article on increasing flight time for more details):
Where
E = capacity
σ = energy density
M = mass in grams (g)
We can copy+paste our propulsion test data into this handy flight time calculator, plug in our weight and battery capacity, and it will give us the best estimate of our flight time based on our data. Our estimated flight time is 15.2 minutes (figure 11), which is a significant improvement compared to our original design, which had only about 4.5 minutes of flight time.
If you are looking for more details, kindly visit drone torque measuring.
Figure 11: Using the flight time calculator to estimate our drone’s flight time
As we have seen, the drone design process is cyclical and there’s almost always room to improve a design.
If you want to take your design a step further, consider adding custom sensors to your test setup.
Collecting propulsion data is one of the best ways to determine where there is room for improvement in your drone, and we offer many test stands and tools to help you do so:
Drone motors are the heart of any drone's propulsion system, directly influencing the performance, efficiency, and reliability of the aircraft. The type, design, and specifications of a drone motor play essential roles in defining the drone's capabilities, from endurance and speed to payload capacity and agility. Understanding the intricacies of these motors is crucial for enthusiasts and professionals alike. This article explores the main types of drone motors, their operation, the key factors to consider in selection, and recent advancements in motor technology.
The primary types of drone motors are brushed and brushless motors, each with unique structures, performance characteristics, and applications. Here's a breakdown of each:
●Structure: Structure: Brushed DC motors consist of a simple design with a rotor that has windings and a commutator, along with brushes that maintain contact with the rotor to supply current.
●How they work: Brushed motors use carbon brushes to conduct electricity to the rotating armature. The brushes wear down over time, reducing the motor's lifespan.
●Performance: Brushed motors are less efficient and powerful than brushless motors, and they generate more heat. They are typically used in low-cost or toy drones where high performance is not critical.
●Structure: Brushless motors have a stator (stationary part) with copper windings and a rotor with permanent magnets. Instead of brushes, they use an electronic speed controller (ESC) to control the switching of current in the windings.
●How they work: Brushless motors use electronic speed controllers (ESCs) to switch the current to the stator windings, creating a rotating magnetic field that spins the rotor.
●Performance: Higher efficiency, durability, less heat generation, more power, and greater torque for weight. They are also quieter and support high-speed operation. They are used in most modern drones, from small quadcopters to large delivery drones.
When selecting a motor for a drone, understanding the specifications and factors that influence thrust and efficiency is crucial. Here’s an outline of the main factors to consider:
●Motor Size: Drone motors are often specified by a number format like , where the first two digits refer to stator diameter (in mm) and the last two refer to stator height. Larger stators generally produce more torque, which supports larger propellers. Smaller motors are lighter and more responsive but have lower thrust and power output.
●KV Rating: Measured in RPM per volt (RPM/V), this indicates the motor speed at a given voltage. Lower KV motors provide more torque, suitable for larger props and heavier drones; higher KV motors suit smaller, faster drones with smaller props.
KV Rating and RPM
The KV rating (RPM per volt) indicates how many revolutions per minute (RPM) a motor will turn for each volt applied. For example, a motor with a KV rating of will spin at RPM when supplied with 1 volt.
KV Rating and Torque
A higher KV motor produces less torque for the same current because the windings are typically optimized for high speed rather than high torque. Conversely, a lower KV motor has more windings, which increases resistance but allows for greater torque at lower speeds.
Higher torque enables faster changes in propeller speed, allowing the drone to respond more quickly to control inputs, which is essential for agile maneuvers and maintaining stability in windy conditions. However, excessive torque can lead to jerky movements and potential instability, especially in delicate maneuvers.Thus, balancing torque with efficiency is key for optimal drone performance, especially in designs focused on extended flight time.
●Maximum Continuous Current and Power Rating: This indicates the maximum amount of current the motor can handle continuously without overheating. A higher current rating allows for more powerful motors and longer flight times. Ensure that the motor’s current draw and power output match the battery and ESC (electronic speed controller) specifications. Exceeding these limits can overheat and damage the motor or ESC.
●Voltage Compatibility: Choose a motor that supports the battery voltage of the drone (e.g., 3S, 4S, 6S batteries, where ‘S’ indicates the number of cells). Higher voltages generally allow higher power outputs but must be compatible with the motor's design.
●Internal Resistance: Lower internal resistance leads to higher efficiency and better power output.
●Propeller Compatibility: The choice of propeller size and pitch must match the motor specifications to optimize thrust and efficiency. Larger propellers typically generate more thrust but require more power from the motor.
●Weight: The motor weight impacts the overall weight of the drone, which in turn affects flight time and maneuverability. Choose a motor that balances power with an acceptable weight for the drone’s purpose.
We usually see terms like 12N14P in the motor parameters of drones. What does it mean? In fact, 12 represents the number of electromagnetic poles in the motor's stator. 14 indicates the number of permanent magnets installed on the rotor. In the context of drone motors, the terms "poles" and "magnets" are often used interchangeably, but they represent distinct components within the motor's construction.
●Poles: These are the electromagnetic coils in the stator (the stationary part of the motor) that generate a magnetic field when energized.Commonly, drone motors have between 4 and 24 poles, depending on their application. A higher pole count generally means smoother, more precise motor control, which is essential for applications like drone stabilization.
●Magnets: These are permanent magnets embedded in the rotor. The number of magnets is often close to the number of poles, though not necessarily identical, as it depends on the motor's design to create synchronous or asynchronous rotation patterns.
The configuration of poles and magnets impacts motor efficiency and performance in several ways:
●Smoothness and Control: More poles and magnets can lead to smoother operation and better torque characteristics. This is because they allow for more frequent magnetic interactions, which can reduce cogging torque (the resistance to movement when the rotor is stationary) and enhance responsiveness during flight.
●Torque Production: A higher pole and magnet count generally increases the torque output and makes the motor more suitable for applications requiring greater thrust, like lifting heavier payloads or steady hovering in stable flight , as it allows the drone to respond quickly to control inputs without significant lag.
Efficiency and Speed: Motors with fewer poles and magnets typically spin at higher speeds with less torque. For high-speed drones, a low pole/magnet count is often chosen to achieve faster RPMs (revolutions per minute).
Larger or higher-pitched propellers generate more lift but require more torque, meaning they pair best with low-KV, high-torque motors to avoid overload and maintain efficiency. Conversely, smaller or lower-pitched propellers work well with high-KV motors, favoring speed over lift. Larger propellers generally produce more thrust but may reduce speed and efficiency, while smaller propellers offer higher speeds but lower thrust. Propeller type—such as material (carbon fiber for rigidity, plastic for flexibility) and blade count—also impacts stability, thrust, and efficiency.
Voltage and current requirements are fundamental in motor selection and battery pairing, as they determine the motor’s power output and efficiency. Motors rated for higher voltage can achieve higher RPMs, delivering more power, but they also demand a compatible battery with sufficient voltage output, such as a higher cell-count LiPo battery (e.g., 6S, 12S). Current, on the other hand, affects the motor's torque and responsiveness; higher current draw increases torque but also generates more heat, requiring both efficient cooling and a battery that can supply the necessary current without quickly depleting. Mismatching voltage or current capabilities can lead to reduced performance, motor damage, or even battery failure.
Electronic speed controllers (ESCs) are essential in drone motor systems, serving as the link between the flight controller, battery, and motors. ESCs regulate the power supplied to each motor by adjusting the voltage and current, which directly controls motor speed and ensures precise synchronization. They interpret signals from the flight controller to modulate motor RPMs, enabling smooth acceleration, braking, and directional adjustments, which are vital for stability and maneuverability. In brushless motors, ESCs convert direct current (DC) from the battery into three-phase alternating current (AC), which is necessary for motor operation. Each motor typically requires its own ESC, enabling independent speed adjustments that contribute to stable flight and maneuverability. Additionally, ESCs can incorporate features like active braking and battery management, enhancing overall performance by ensuring efficient power use and preventing battery damage.
Recent advancements in drone motor technology focus on enhancing efficiency, power density, and control precision to improve flight performance. Innovations include the development of brushless motors with higher magnetic strength, which increase torque without significantly raising size or weight, resulting in more compact and powerful designs. Improved cooling mechanisms, such as integrated airflow systems and heat-resistant materials, allow motors to operate at higher current levels without overheating, crucial for sustained high-performance flights. Additionally, sensor-based technologies, like field-oriented control (FOC) and motor position sensors, provide smoother and more precise motor control, enhancing stability and responsiveness. Besides, improvements in electronic speed controllers (ESCs) have enabled more precise control and responsiveness, facilitating better flight performance. These improvements allow drones to achieve longer flight times, better stability in challenging conditions, and optimized performance in specialized applications like heavy lifting, racing, and aerial photography.
Drones are now widely used in various applications such as racing, aerial photography, industry, and logistics. How do you choose the right motor for different drone applications?
●Racing Drones: High-KV, high-RPM motors for agility and speed.
●Aerial Photography: Low-KV, high-torque motors for smooth, stable flight with large props.
●Industrial Drones: Low-KV motors with high torque for lifting payloads and maintaining stability.
●Delivery Drones: Efficient, high-thrust motors for carrying payloads over long distances.
In each case, proper motor selection ensures that the drone meets the demands of its intended use, balancing speed, efficiency, and thrust capacity.
Drone motors come in a variety of types and configurations, each with unique advantages. Understanding how each motor type functions, along with key specifications like KV rating, thrust, and ESC compatibility, is essential in selecting the right motor. As drone technology continues to evolve, motor advancements will likely play a pivotal role in achieving higher efficiency, greater power, and more specialized functionality across a wide range of drone applications. Grepow offers UAV batteries and semi-solid state batteries ranging from 4S (14.8V) to 18S (68.4V) with capacities up to 84Ah, designed to support a wide variety of applications and compatible with drones equipped with diverse motor setups. If you have any questions or needs, please feel free to contact us at .
Related Articles:
Understanding Drone Payload: A Comprehensive Guide
What is a Drone ESC and Is it Important?
How to Choose the Right Drone Propeller?
What Is A Drone Flight Controller?
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