5 Things to Know Before Buying drone torque measuring

By Lauren Nagel

If you are looking for more details, kindly visit Wing Flying.

A question we get fairly often is: “Why do you need to measure torque when doing motor or propeller testing?” This is often followed by: “How can I measure torque?”

These are both important questions for drone designers who want to get the most out of their designs. It ultimately comes down to measuring your motor’s efficiency by comparing the input to the motor with its output.

Table of Contents:

1. Torque and RPM
2. Motor Efficiency Equation
3. How to Measure Torque
4. How to Calculate Brushless Motor Mechanical Power

Note: Our small thrust stands and larger Flight Stands are both capable of measuring torque.

Torque and RPM

There are two key variables when it comes to the propeller: the first is the rotation speed and the second is torque. When you multiply rotation speed and torque together, you obtain mechanical power.

If we look at how a propeller and motor are connected, we see that the only connection or “information” sent from the motor to the propeller is RPM and torque.

Figure 1: Drone motor connected to three blade propeller

Motor Efficiency Equation

At the other end of the motor, electricity enters from the battery or power source. We can therefore consider the motor a machine that transforms electricity into RPM and torque or electrical power into mechanical power.

This brings us to our key efficiency formula. When we measure torque, we’re able to obtain mechanical power, which we can divide by the electrical power to obtain efficiency:

There are design trade-offs that come with increasing torque and RPM, and testing multiple propellers is the best way to find the most efficient motor-propeller combination for the type of flight you want to do.

Further reading: Brushless Motor Power and Efficiency Analysis

If we can keep the same propeller efficiency, increasing the ratio of mechanical power to electrical power means that air vehicles will be able to fly longer and carry more payload.

How to Measure Motor Torque

There are a few ways to measure torque, and in our test stands we use a steady-state solid system, which means that there are no moving parts. This is key because it minimizes hysteresis and vibration.

In the Flight Stand 15 thrust stand, the motor is mounted on the stand using a motor mounting plate. Behind the motor is the force measurement unit (FMU), which contains a monoblock load cell that measures both thrust and torque. The load cell is calibrated for thrust, torque, and crosstalk to ensure highly accurate measurements. The image below shows a coaxial version of the system, where motors are separated by a minimum safe distance. Up to 8 Flight Stands can be connected at once, allowing for distributed propulsion testing.

Further Reading: How to Calculate Motor Torque Using Formulas

Figure 2: The Flight Stand 15 motor thrust stand

How to Calculate Brushless Motor Mechanical Power

Now that we have measured torque, we also need to measure the motor’s RPM / rotation speed to fill in our equation for mechanical power. 

This is achieved by using a small infrared RPM sensor that can sense when a piece of reflective tape passes in front of the sensor. The accompanying electronics use a counter to determine how many times the reflective tape passed the sensor, which allows it to calculate the rotation speed. The rotation speed can also be measured electrically, using the ESC signal. This method is simpler mechanically, but it is more sensitive to the motor load and size.

Multiply this figure by rotation speed and divide the product by electric power to get mechanical power.

Further reading: Drone Design Calculations and Assumptions

Conclusion

Measuring the system’s torque is essential when designing a propulsion system, as it allows you to measure the motor efficiency separately from the propeller efficiency. Here we covered how you can measure torque and how to use this information to build a more efficient drone. 

If you are interested in testing motor torque yourself, check out our range of test equipment: 

Considerations for Drone Motor Sizes When Choosing a Drone Motor

Choosing the right drone motor sizes is essential for optimizing your FPV drone’s performance, whether you’re aiming for speed, agility, or stability. Drone motor sizes directly impact the drone’s power, efficiency, and overall flight characteristics. Understanding the differences between various motor sizes will help you make an informed decision that aligns with your specific flying needs, from racing to freestyle or cinematic flying.

In this guide, we will be concentrating on the drone motor sizes, which is one of the most important factors you should consider when you are choosing the right motor for your FPV drone. If you want to learn more about how to choose a drone motor, read this guide on How to choose fpv drone motor a detailed guide

What Drone Motor Sizes We Need?

Brushless Drone Motor Components

There are two major components to a brushless motor:

• Rotor – the rotating component with the magnets arranged in a radial manner
• Stator – the portion with electromagnets that is non-rotating

Brushless Drone Motor Sizes Explained

Just like the meps motor for example:

• length of rotor shaft: 30mm±5
• inner diameter: 16mm±05
• outer diameters: 29.31mm±2

A group of four numerals, such as , , or whatever number it may be, designates a motor. It indicates the rotor’s height and diameter in millimeters (mm). The thrust that a motor produces increases with size.

• Taller stator: greater maximum speed and appalling low speed control
• Wider stator: improved handling at lower speeds and a lower top speed

Brushless Motor: 22 represents the stator width (or stator diameter), 08 represents the stator height, both measured in millimetres.

Brushless Motor: 22 represents the stator width (or stator diameter), 17 represents the stator height, both measured in millimetres.

Comparing Taller and Wider Stators

Because the mass of a wider motor is farther from the rotational axis, it takes more energy to alter the RPM when the motor is spinning, resulting in greater inertia. Therefore, even if they have the same stator volume and torque output, broader and shorter motors are typically less responsive than narrower and taller motors. Smaller magnets on the motor bell of wider and shorter motors can also lower the motor’s output.

However, because they have more surface area on top and bottom, wider motors provide better cooling. Motor performance is highly dependent on temperature. A motor’s capacity to produce magnetic flux diminishes with temperature, which affects torque output and efficiency.

Contact us to discuss your requirements of drone torque measuring. Our experienced sales team can help you identify the options that best suit your needs.

A motor stator’s width and height essentially serve as a compromise between cooling and responsiveness. Depending on how you fly, you should make a choice. For example, bigger stator motors may provide greater cooling for slow cinewhoops hauling a heavier GoPro. Taller stator lengths may be preferable for sports drones or racing drones that react quickly. Larger stators also make it possible for larger bearings, which can enhance performance, lifespan, and smoothness.

Not necessarily are larger stators better. For instance, motors can manage standard 5” propellers. However, utilizing considerably heavier motors with the same KV may not yield any appreciable advantages since they would still generate the same thrust with the same propellers, or the weight may even result in less responsiveness. Higher KV motors are an option if you want to increase performance without gaining weight. However, because of the higher torque requirements, the motor in this case would probably perform better with 6” propellers than the .

Choosing the Right Drone Motor Sizes for Your FPV

To ascertain the optimal drone motor sizes for your FPV drone, proceed as follows:

• First: Size of Frame
• Second: Size of Prop
• Third: Size of Motor

You can determine the suitable motor size by determining the frame size. The prop size is limited by the frame size, and in order to generate thrust efficiently, each prop size requires a varied motor RPM. This is the application of motor KV. Verify the motors’ torque to ensure it can turn the propeller of your choosing. One aspect of this is the stator size. Higher KV and bigger stator diameters frequently result in higher current draw.

Drone Motor Sizes Chart:

Matching Drone Motor Size to Propeller Size

Matching Drone Motor Size to Prop Size for Optimal Performance: The size of your propellers should match the motor size for optimal efficiency and performance. Larger motors generally work better with larger props (e.g., a motor with 5-inch props), providing more thrust and stability. Conversely, smaller motors are suited to smaller props (e.g., a motor with 3-inch props), offering faster response times but less overall thrust. Choosing the right combination ensures your drone performs as expected, without overloading the motors or underutilizing their potential.

Battery Compatibility

Voltage Considerations (3S, 4S, 6S): The voltage of your battery (measured in S, where each cell is 3.7V) significantly impacts motor performance. Higher voltage (e.g., 6S) can produce more power and efficiency, but requires motors with a lower KV rating to maintain balance and avoid overheating. For instance, a 6S setup might pair well with a KV motor, while a 4S setup could use a KV motor for similar performance. Ensuring your drone motor’s KV rating matches your battery voltage will help prevent issues like excessive heat, poor efficiency, or even motor burnout.

Recommendations Drone Motor Sizes for FPV

Drone Motor Size Guide:

Best Motor Sizes for Micros:

TinyWhoops: 

65mm: For a micro drone of this motor sizes, often classified as a tiny whoop, the perfect drone motor size varies on the application of your drone. If weight is your priority, a motor will fit your needs with it also being zippier and coming in a large range of ultrahigh KVs. If torque and power at the cost of efficiency is your priority, for a 65mm, is the best choice, with being for the chunkier builds like a meteor 65 pro. For these drones, the KV options are vast ranging from 19,000 up to 32,000 so for more control choose a lower KV but for raw thrust, high KV is the only way. The MEPS motors are a great choice for this.

75mm: For a larger micro drone with a more efficient and powerful performance, 75mm with 1s is the best as it’s good for indoors and can fly outdoors with ease while getting more flight time. In a 75mm light-weight build, with a KV ranging from 19,000 to 25,000 is more than enough for high performance and efficiency. If power is your true goal or you are using a 75mm frame with larger ducts such as the meteor 75 pro or the Fractal 75 pro max, a motor with the same KV range will work well at the cost of some weight and efficiency. The MEPS motors are well suited for these builds as well.

85mm: These ducted whoops are truly no longer for indoor and are no outdoor rippers, mainly running 2s batteries with an xt30 connector. There are many different sizes for motor which can be employed but the ideal ones used on most bnf drones are xx03, meaning something like a , or will work fine with these larger whoops. The recommended KV is from 10,000 to 14,000 max for 2s. The MEPS motors are a great fit for this drone class.

Best Motor Sizes for Toothpicks:

2” 1s: A toothpick of this specifications will be using smaller motors for a lighter weight with motor sizes ranging from at minimum with being on the high end. The recommended KV would be from 16,000 to 22,000 at the high end.

2” 2s: These toothpick drones are truly powerful and can harness a larger outdoor area. Motor recommendations range from xx2.5 to xx3 such as .5 and motors. Use the same KV as the 85mm drones which employ the same sized propeller. The MEPS motor is a great option for this size.

2.5”: These drones almost always use 2s and 3s so for a build of this spec, larger motors must be used such as xx03 to xx04 such as a , or and similar sizes. The recommended KV would be anywhere from 10,000 to 14,000 on 2s and 6,000 to 10,000 on 3s. The lower KV MEPS motor is a great option for 3s power.

Best Motor Sizes for Freestyle&Racing Flight:

Small Sized Drones：

3”: This is where drones start to leave the micro class as they grow larger and much more powerful. These drones commonly run on 3s-4s batteries and use larger motors. The recommended drone motor size would be something like an xx04, such as an , or motor. For 3s, the recommended KV would be anything from 4,000 and 7,000 and for 4s anything from 3,000 to 6,000 should provide plenty of power for this class.

3.5”: This class of drones is the limit for a micro and these drones commonly use 4s-6s batteries. For drone motor size, the recommended size for this class of large micros is xx04 motors, such as , and similar sizes. The recommended KV for 4s is 3,500 to 5,000 and for 6s, the recommended KV would be ranging from 2,500 to 4,500. The MEPS motors would be a great choice with a build of this size for drone motor.

Medium Sized Drones:

4”- 4.5”: These drones are almost full-sized freestyle drones. They may be similar size, but the performance is dramatically different, with the 4-4.5” drones being much lighter and nimble. The recommended drone motor sizes for this frame would be xx04.5-xx05, such as a or .5 motor. The recommended KV for 4s is 2,500-3,500 and for 6s, the recommended KV would be 2,200 to 3,300.

Full Sized Drones:

5”-5.5”: What size motor should I use for a 5-inch drone? A 5 inch FPV drone typically uses or motors with a KV rating between KV-KV, depending on battery voltage and flight style. The motor is overall larger and gives more torque and thrust while the motor is generally smoother and more efficient, due to the wider and shallower stator for better cooling. There are many variations that are both smaller and larger than these sizes such as the Racerstar Racing Edition motors made for the 5 inch and the larger SZ motors from MEPS. There are many motor sizes in the middle as well such as the Diatone Mamba Toka .5 motors sitting in the middle of the standard and sizes. The recommended KV for 4s is 2,100-2,750 and the recommended KV for 6s would be 1,700-2,100.

6”: This drone sits in the middle of a 5” freestyle drone and 7” long range drone. A 6” drone is versatile as a cruiser and light freestyle drone, mainly used with 6s power. These drones use slightly larger motors with the recommended size being xx08, such as a motor. The recommended KV for 6s is anywhere from 1,300-1,800. A great motor for this application is the MEPS motor.

Best Size for Large Drone Motors

7 Inch Drone Motor

7 inch drone motors are commonly used for long-range FPV flights or efficient cruising, often equipped with high-capacity batteries to extend flight time. Compared to 5-inch and 6-inch drones, 7-inch builds require larger motors with higher torque to drive the longer propellers.

• Recommended Motor Size: xx08-xx09, such as , , or .5
• Recommended KV Ratings:
  • 4S Battery: 1,600-1,900KV
  • 6S Battery: 1,000-1,400KV
• Key Features:
  • Low KV motors with larger propellers improve efficiency and reduce current draw
  • Higher torque enhances load capacity and stable cruising
  • Ideal for long-range FPV or extended flight applications

A great motor choice for 7-inch drones is the MEPS SZ.5, which offers a perfect balance of power and efficiency. Another great option for this is the GEPRC SPEEDX2 motors as they are great value and provide great thrust to weight ratios.

8 Inch Drone Motor

8-inch drone motors are designed for even larger drones, primarily used for professional aerial photography, mapping, and industrial applications. These drones typically run on higher-voltage batteries (such as 6S or above) for improved efficiency and use high-thrust motors to support larger propellers.

• Recommended Motor Size: xx09-xx10, such as , , or
• Recommended KV Ratings:
  • 6S Battery: 900-1,200KV
  • 8S-10S Battery: 600-900KV
• Key Features:
  • Higher torque for heavy-lift applications such as professional aerial cinematography
  • Low KV motors maximize efficiency, reduce power consumption, and extend flight duration
  • Ideal for endurance flights, mapping, patrolling, and search-and-rescue operations

For 8 inch drone motors, a solid choice are the MEPS NEON and NEON , which delivers reliable thrust and optimized flight efficiency for long-duration operations.

Best Motor Sizes for Cinewhoops:

2”: This small indoor Cinewhoop uses the same sized motors as the other 2” prop spinning drones, with the ideal motor size being xx03 such as and . And the ideal KV being 10,000 to 14,000. A great choice is the MEPS motor.

2.5”: This is the step up from the 2.5” Cinewhoop using a 4s battery, with it being one of the most versatile drones you can get. They do well both indoors and outdoors with the recommended drone motor size being xx04, such as and the recommended KV being 4,000-6,000. The MEPS motor is a great fit for this.

3”: This is a Cinewhoop to be mainly used outdoors in some larger indoor applications. The recommended drone motor size would again be xx04, such as a motor. The Recommended KV would be from 3,500-4,300 for 4s and 3,000-3,800 for 6s. The MEPS motors would work well on this build as well.

3.5”: This is a Cinewhoop for outdoor use with much better performance and efficiency than the others. The recommended motor size would be xx04-xx05.5 such as a or motor. The recommended KV for 4s would be 2,300-2,800 and 1,700-2,200 for 6s. The MEPS motors are a great option for a build like this.

Drone Motors Matching from MEPS

For detail informtion, you can check the article: MEPS Motors Size Choosing Guide

FAQ-What’s a Brushless Motor

What’s a Brushless Motor? Simply put that an FPV drone motor that is brushless has no brushes, as the name suggests. The rotor and the stator are the two distinct parts that make up a brushless motor. The core component that the rotor is attached into is called the stator. The stator is composed of a network of radial electromagnets that, when a current flows through the windings, alternately turn on and off to create a momentary magnetic field. A set of permanent magnets housed in the rotor are positioned near the electromagnets of the semi-permanent stator. Rotation is produced by the stator and rotor magnets’ attractive and repulsive interactions. After the rotor is constructed, its shaft is placed into two ball bearings in the stator to keep the rotor rotating smoothly and linearly.

The brushless motor cannot be driven directly, even though it is powered by DC current. Rather than requiring brushes or a commutator, the brushless motor is directly connected to the control electronics. Because the rotor and stator do not come into direct touch, brushless motor longevity is exceptional. In terms of efficiency, the brushless motor outperforms the brushed motor as well. In applications requiring high power outputs and efficiency—such as tiny and micro multicopter applications—brushless motors are widely used. If you want to learn more about the differences between brushed and brushless motors, check out the guide

Brushed vs Brushless Motor: What’s the Difference?

Typically, mini drones and lightweight configurations use brushed motors. Because of the brushes on its lower half, it may be powered directly by direct current DC; nonetheless, it is not very effective in scenarios when maximum power is required. Brushless motors, on the other hand, might be a better choice if you’re designing a racing drone, which will place a high demand on the motor because of its weight and capacity.

The company is the world’s best drone test equipment supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.