Written by Daniel Sanfelice

With over 20 years of combined experience as sUAS operators, the flight testers at ModalAI sure know their way around a drone. In this fourth installment, learn about motors and propellers, one of the five essential components of high flying drones you should be familiar with in order to have a successful flight every time.

1. GPS 
2. Radios and Datalinks
3. Batteries
4. Motors and Propellers
5. IMU and Magnetometer

What are Motors and Propellers?

The motors and propellers (commonly referred to as props) are some of the most essential parts of your aircraft and they are what generate lift so the aircraft can fly. They also control the attitude of the aircraft and allow you to climb/descend, pitch, roll and yaw. They are an essential part of your aircraft’s propulsion system (PS).

While the motors and props are not the only items in the propulsion system, the selection of your motors and props will have a large impact on how your aircraft will perform. You will have to choose the correct combination for endurance, speed, responsiveness and payload capacity for your application. There are a wide variety of motors and props, and an almost infinite combination of the two that you can choose for your aircraft. It is extremely important that you design your propulsion system to the requirements for your aircraft. 

We want to stress that the most important aspect of your propulsion system is that it is properly sized for the requirements of your airframe.

Motors:

The motors are the backbone of your propulsion system. They take energy from your battery to turn your props which generate the lift that controls your aircraft. The most important thing to consider when choosing a motor is to determine what you intend to do with your aircraft. Do you need to fly fast? Do you need to hover in place for a long time? How responsive does it need to be? How much payload do you need to carry? All of these questions will help you determine the proper motor for your aircraft.

• Brushed vs Brushless: There are two main types of Direct Current (DC) electric motors in the RC and drone world, brushed and brushless. We recommend only using brushless motors because brushed motors are typically only suitable for small toy drones and rc cars. Brushless motors have all the advantages over brushed motors in efficiency, power and reliability and there is a large marketplace of motors and ESCs to choose from. 
• Physical Attributes: You want to make sure you get the correct size motor for the requirements of your airframe. Bigger motors will typically create more power but they will also draw more current and weigh more. People tend to think “bigger is better.” However, a DC brushless motor is more efficient the faster it turns, so sometimes it might make sense to use a smaller motor that would be more efficient during your flight. For example, a big motor creating 5lbs of thrust while spinning at 25 percent RPM is less efficient than a smaller motor generating 5lbs of thrust while spinning at 50 percent throttle.

DJI Phantom 4 Enclosed Brushless Motor	EMAX Outer Rotor Brushless Motor

You can find DC brushless motors in all kinds of shapes and sizes to fit whatever requirements you have (Again, this is one of the reasons why we like brushless over brushed) so keep that in mind as you design your airframe and propulsion system.

• Kv Rating: The Kv rating of a motor determines how fast a motor will spin unloaded when 1 voltage is applied. In simple terms, the Kv rating is a correlation to what your highest RPM can be based on what voltage battery you use. To find your top RPM, just take the Kv value and multiply it by the voltage of the onboard battery pack. For example, we use a 4s LiPo battery on the ModalAI VOXL m500 with 880Kv motors. To find our top RPM speed capable with this combination, we need take a look at three numbers
• Fully charged voltage per cell of a lithium polymer battery = 4.2 volts per cell
• The number of cells in the battery = 4 cells
• The Kv rating = 880

The formula for this is the total pack voltage (individual cell voltage multiplied by the number of cells) times the Kv rating, or 16.8v x 880. Which would give the VOXL m500 a maximum RPM of 14,784 RPM.

Remember that the faster a motor spins, the more efficient it is. So size the motors and propulsion system based on your aircraft requirements.

• Amp Draw and Torque: Torque is usually associated with the diameter and length of a motor, which means that a larger motor will usually create more torque. More torque means more responsiveness from your aircraft. However, higher torque will also draw more power. The amount of torque and the amperage being drawn is very closely correlated. Higher amp draw means you need a beefier battery and ESC, and you will also get less flight time (Have we mentioned that you should choose the motor based on the exact requirements of your aircraft?).

• Efficiency: The faster a motor spins, the more efficient it is. The hotter a motor gets the less efficient it will be and DC motors create less heat the faster they spin. 

Propellers

The propellers (props) of your aircraft take the rotational energy of the spinning motor and turn that into lift. The props are usually the easiest item to change when it comes to fine tuning the performance of your propulsion system. We will discuss some of the important attributes to consider when choosing a prop.

• Pitch: the pitch of your prop is the biggest factor in how much lift you can generate. The pitch is the angle of the blade on the prop. The higher the pitch of the prop, the more lift you can generate at a given RPM, but you also will be less efficient. If your aircraft needs to fly really fast, you will need a higher pitch prop, and if you need to hover in place for a long time, then a lower pitched prop is better suited for that. A high torque motor combined with a high pitch prop will make your aircraft faster and more responsive in flight. The opposite will make your aircraft docile and easier to fly.

• Length: The diameter is the number one factor for prop efficiency. The longer the prop, the more efficient it will be. You should try to fit the longest prop that your airframe will accommodate.

• Number of props: This is a very complicated subject and there are no hard and fast rules for this, so we recommend just sticking with the standard “dual” prop 

  2 blade propeller	3 blade propeller	4 blade propeller

• Material: The material that a prop is made from can have a big impact on how it performs. Flimsy, plastic props are cheap and easy to find, but they can be less efficient and create excess vibration in the airframe that can induce errors in other sensors. Carbon fiber props are more stiff but will cost much more. 

• Balance: Having properly balanced props will reduce vibration but will usually increase the price. Unbalanced props will decrease efficiency of your propulsion system.

How to choose:

The best way to nail down exactly what combination of motors and props to use is to identify exactly what your aircraft will be required to do. This decision is always a question of power vs efficiency. Make sure you are asking questions like

• How much payload capacity will you need? Size the motors so you have enough lift to carry your payload for the desired flight time with your aircraft flying in the desired mode.
• How much endurance will you need? This will drive what RPM and prop pitch you need. Less pitch and higher RPM at hover will typically be most efficient.
• What is the required speed at cruise? How about top/sprint speed? Higher cruise speeds will require higher pitch props, which decreases efficiency
• How will density altitude affect your propulsion system? You will need more lift to fly at higher DA than at sea level.

Tips and Best Practices 

• Make sure the motors are spinning the right way: If you find your motors are spinning the wrong way, you can easily switch the motor direction on brushless motors by switching any two of the three motor leads that connect the ESC to the motor

• Keep the motors free and clear of debris: Dirt and debris are the arch enemies of electric motors. Keep them free from dust and dirt to prolong their service life and prevent motor failures that can lead to crashes. You can do a quick check for debris in the motor by spinning the motor while listening for any grinding and making sure it spins smoothly. When doing this check, spin the motor while pushing down, pulling up and pressing side to side on the prop shaft. 

• Ensure everything in your propulsion system is secure: The components in your propulsion system spin at high speeds and create a lot of vibration, which can lead to screws and bolts coming loose. Give each motor a periodic wiggle to make sure the motor mounts haven’t come loose.

• Inspect props before each flight for cracks or chips: Run your fingers down the leading and trailing edge of each prop before every flight to inspect for cracks or chips. Any compromise to the structural integrity of the prop can cause adverse flight performance or failure of the prop. If your props sound louder than normal, you may have damage to one of your props.
• Make sure the props are on the right way: All multi-rotor aircraft have motors that will spin clockwise and counterclockwise. Some props are only meant to spin one way, so make sure you have the correct prop for the corresponding motor direction.

• Density altitude reduces lift: Remember to consider density altitude when choosing your propulsion system components. Higher elevation, temperatures and humidity will increase density altitude and reduce the amount of lift your props create. Even if you can still lift the payload, your aircraft performance might suffer greatly. (More info here). 
• Use dyno testing to help determine what works best for you: The easiest way to test different off the shelf motor and prop combinations is to use an online calculator or bench mounted dyno. Here at ModalAI, we use dyno testing to log the efficiency and power of different motors and props for custom aircraft builds. We save this data and can go back and use the database as we design future platforms.

As drone operators, it’s important to understand the essential components of whatever autonomous vehicles we fly. This foundational knowledge of drone motors and propellers will help reduce troubleshooting time and ensure you have a safe and successful flight every time. Stay tuned for the next blog in this series to learn about IMUs and magnetometers.

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