Hi, this is Wayne again with a topic “Maker Hangar: Episode 2 – Motors/Propellers”.
Welcome to maker hanger, my name is lucas weekley and today we’re going to be talking about the motors that we use in the rc hobby, which is usually the brushless motor. We’Re also going to be talking about the props that go with these motors and why their sizes matter, but first, let’s learn about motors. There are two kinds of motors used in the rc hobby brushed and brushless motors brushless motors are the most common and used most of the time in rc. Brushed motors are found in electric toothbrushes, power, drills and pretty much anything else. That’S motorized inside your house, they’re.
Pretty much extinct in the rc hobby because they’re very heavy and inefficient brushed motors are powered by dc current, meaning that they have two wires and can be directly connected to a battery to spin the way they work is they have a coil of wire that spins Inside of a can lined with magnets, because the powered part of the motor is spinning, brushes are used to transfer power to the coil. This is where the name comes from. This is very inefficient and wastes. A lot of power.
Also brushed motors generally have more torque and not very much speed. On the other hand, brushless motors are powered by ac curt and have three wires coming off of them. A brushless motor has a stationary coil wire with the magnets spinning around them and because it’s powered by ac current, the coils of wire, can draw the magnets to them in shorter distances, which is more efficient, and it can make the motors a lot faster. Now that you know how these motors work, we need to learn what the values, on the mean there are several numbers to classify a motor by its size and rotational speed. The first set of numbers is two two digit numbers that will be separated by a dash, for example, 28 by 36. These numbers correlate to the dimensions of the motor can, which is the outer metal casing of the motor. The first number is the diameter of the motor, and the second number is the height of the can all these measurements are in millimeters. The second number that you’re going to want to watch out for is kv, which stands for rpms per volt, so say.
If we have an 1800 kv motor we’re running it off of a 12 volt battery, then the motor is spinning at 21, 600 rpms or revolutions per minute. Something else you’ll find when looking for motors is a suggested prop size. This is the propeller that the manufacturer recommends to use with that motor. Now before we go any farther, i want you to understand how important the propeller is to an rc scratch build they’re so important, because they are the starting point to which you choose your electronics.
When you have an idea of how big the plane you want to build, is then you have to pick a proportionate prop to go with that plane? Now, what i mean by proportion is you don’t want to pick a 12 inch prop for a plane with a 20 inch wingspan for obvious reasons, but you equally don’t want to choose a four inch prop for a six foot wingspan plane. Well, once you find a prop which will be easier to understand, once i explain the differences between them in a moment, you can choose the motor to go with it. The propellers are labeled, with a series of numbers to explain their specs. For example, a propeller can be labeled with 8 by 3.8.
The first number in the sequence is going to be a one to two digit whole number, which is the diameter of the prop in inches, and then the second number is also going to be a one to two digit number, but it most likely it’s going to Have a decimal place after it. This is the pitch of the prop. Now the pitch is measured by how many inches the prop will move forward in one revolution under perfect circumstances. So an 8 by 3.8 prop has a diameter of 8 inches and will move 3.8 inches forward every revolution. Another thing you’ll see when you’re searching for propellers is puller and pusher props. Now this simply means counterclockwise or clockwise rotation to produce thrust. It doesn’t matter which one you get, because the motor’s rotation can be changed really easily and we’ll discuss how to do that when we connect all the electronics in a later episode. Okay, so even though there are many different combinations of prop diameters and their pitches, there are only two main types now these are the small diameter high pitch props and the large diameter shallow pitch props, and even though both of these can produce about the same amount Of thrust each one has very different characteristics: we’ll start with large diameter, shallow pitched props because they are bigger, they are harder to spin, and this means that they’re paired with motors with low kvs, these motors have a higher torque and can spin the larger props.
Also because their surface area is much larger, they can produce adequate thrust at low rotational speeds. This also means a large prop can grab the air and pull the plane into the sky. Think of this as the lower gears on a bike. However, the large props, with shallow pitches, do have some limitations they’re pretty much only good for slow flying planes.
This is because, when the plane is moving through the air and the motor’s at full throttle now you have this really big disc, on your plane and it’s creating drag, but even though it’s producing thrust that drag will eventually counter that thrust. This means planes with these props, have limited speed and can only go so fast under their own power. Just like a bike at its first gear. On the other hand, small propellers with high pitches have completely different characteristics because they’re small they can be spun without much power.
So they’re paired with high kb motors small props, are very good with speed. Think of them as the highest gear on your bike. They have little static thrust, meaning if you keep the motor stationary and run it up. This is because they’re small surface area and it can make planes with them a little bit hard to hand launch, but once they’re in the air they compound power by using the fast moving air around them and the only limiting factor on how fast they can go Is how fast they can be spun so for what we’ve been talking about so far with the different props, they can both be powered by motors of the same size.
Well, what happens if you have a propeller with a large diameter and also a steep pitch? Well, then, you have a large motor which can produce much more thrust. And similarly, if you have a small prop with a very shallow pitch, then you have a small motor for a very small plane. So the last thing that we’re going to talk about today is amps, as you probably already know, amperes is the measure of electricity that you put into something like a brushless motor and with differences in kv prop and motor sizes. The amp values can change dramatically.
So, let’s go in order. Remember this is just general statements be sure to check the data sheets for each motor to see the actual amp draws. So first we have the low kv motor with the high diameter prop with a shallow pitch. Now this is going to produce anywhere between 10 to 20 amps, depending on the motor which is very low for a brushless motor.
Then we have high kv motors with small props and high pitches, because these motors spin faster they use more power generally about twice as much as the lower kv motors. This also means faster planes will run for a lot less time on the same batteries. Then you have the large motors with the large props, and this can range anywhere between 30 to a couple hundred amps, depending on how big the motor is and how much power you want, and the little motors can range from anywhere between 1 to 10 amps or More depending on the motor remember, this is just a general statement to give you an idea on amp draws for different motors, most distributors will post up how many amps a motor will draw with a certain prop. This is that suggested prop size we were talking about earlier anyway. The electric component that transfers power to a brushless motor is called an esc or an electric speed control and we’ll talk about those next time. Thanks for watching .