Balancing Props

We are now forced to draw another comparison between tyres and propellers. Your props need to be balanced in order to spin smoothly at high speeds, just like your tyres do. Although it is a delicate process, everybody who enjoys this hobby must learn it. By balancing your props, you ensure that each blade is the same weight and that the vibration that results from the prop spinning at a high RPM is minimised (RPM).

In theory, the procedure is pretty straightforward: install the balanced prop you require on a free-spinning axle. There are numerous techniques to achieve this free-spinning axle. You may create a nearly frictionless fulcrum point by suspending a metal rod between two magnets through the centre of your prop mounting hole. Every heavy point on the propeller is able to rotate gently in the direction of the ground due to the lack of friction.

Does the propeller want to "lean" in one direction or the other, or is it totally still? One can add weight to the light side of the prop or take weight away from the heavy side by using this technique to locate the heavy locations in the prop. Do not forget that they are usually fairly minor alterations. While adding weight can be accomplished with a few tiny clips of Scotch tape or label stickers, taking weight away typically requires some patience and sandpaper. The propeller should be able to be changed to any position once it is perfectly balanced without one blade or the other sagging towards the ground.

Modern propellers are nearly perfectly balanced when they leave the factory, but it's a good idea to check them again yourself. The likelihood that a prop will be balanced out of the factory increases with the quality of the prop. Although you might save a little money by using less expensive props, you will have to take the time to balance them yourself.

Figure 3-3. When perfectly balanced, the prop should be able to sit in the balancer in any position without falling in one direction or the other.

Motors

Our quadcopter's power train differs significantly from your car's in that it utilises a direct-drive technology as opposed to a transmission. The engine in your car serves as the sole source of power, and the transmission distributes that power to the wheels that require it. Each wheel, or prop in this case, is directly connected to its own power source when using direct drive. A brushless motor serves as the source of power for our quads.

Sizes

Most contemporary brushless motors used in the hobby sector can be identified by the width and height of the motor housing (see Figure 3-4). A 2216 motor has dimensions of 22 mm in width and 16 mm in height. It's always a good idea to consult a spec sheet whenever feasible because, of course, not every organisation follows the exact same naming practises.

Figure 3-4. These 2204 motors are rated at 2300 kV and work with 5- or 6-inch props.

kV Rating

The kV rating is a different specification that you have undoubtedly seen stamped on the side of brushless motors. This information reveals the motor's RPM for each volt of power applied. As the motor is not under any load when this test is made, friction and load will have an effect on RPM. Now that you have this information, you can see how the RPM rating is a sliding scale based on battery voltage. The motor will spin more quickly with a higher battery voltage, but the kV rating won't change.

​ Calculating RPM

A 900 kV motor with a battery putting out 12 volts will spin at 10,800 RPM (12 x 900) under no load. The same motor using a larger battery (16.8 volts) would spin at 15,120 RPM.

Pairing with the Right Props

The performance of each motor will vary depending on the propeller it is connected with. Nowadays, there is typically little room for speculation because the majority of motor manufacturers publish the suggested prop specifications for their motors online. You should have an idea of the final weight of your drone in order to choose the proper prop for your design. According to the manufacturer's recommendations, the larger prop you should utilise the heavier you intend your design to be.

Total Lift

The amount of upward thrust that a motor/prop combo produces in the real world is referred to as a motor/prop combo's total lift, which is a term you will frequently hear in relation to them. A weight value that represents the lift produced at a 100% throttle command is typically used to document this. (The majority of manufacturers offer this details.) With a 6 3 prop, the 2204 motors we used to build the Little Dipper can produce a total lift of 539 grammes. The lift number would change if we switched out the props. Of course, each motor and propeller are measured individually. By dividing your results by the number of arms on your copter, you may determine the lift of the whole aircraft (four for our quad). It is (4 x 539 kilos) 2156 grammes of lift for the Little Dipper!

Calculate Your Payload Capacity

One spec that every UAV designer has to define is the payload capacity. This is the difference between the aircraft’s total lift and its all up weight (AUW). The AUW is exactly what it sounds like: the weight of the aircraft as it is outfitted for flight, with motors, battery pack, computer, and so on. Now that we understand how to calculate our total lift, we can weigh all of our components to find our AUW and determine our aircraft’s payload capacity.

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