Motor and Propeller

The fundamentals behind the electric motor and propeller configuration are deceptively complicated. Many hobbyists out there need to rely on calculators to perform the mathematical calculations necessary to determine the optimal and efficient setup for their airplane. As with the other sections, the goal of this tutorial isn't the make you an expert on the topic, but rather to simply give you the information you need to buy and replace these items if you choose to. There are a vast number of other tutorials out there that delve into the mathematics and theory behind this topic. If you are really interested in this topic a quick google search will produce results that will be far more in depth than what you are about to read.

Propellers


Your propeller and motor combination will be determined by the type of flying you intend on doing. Jets that fly fast will often have smaller propellers that spin at faster speeds, while 3D/aerobatic planes will have larger propellers that spin at slower speeds. The EDF jets will break speed records, while the 3D/aerobatic planes will be able to turn on a dime. Choosing a propeller that is correct for your application is just as crucial as picking out the right electric motor.

There are two factors that determine and rate propeller performance. They are the diameter and the pitch. The diameter is merely how long the propeller is. Pitch determines how much air is moved with each turn of the propeller. You will usually see propeller sizes expressed something like: 10x7 or 11x6. The first number is the diameter, the second is the pitch.

So, how do these characteristics effect flight? Well, the answer is actually very complicated. The variables of motor speed, motor torque, pitch and diameter are all interconnected. The dumbed down explanation is that if you want speed, you will go with a lower diameter and higher pitched propeller. If you want acceleration, where your aircraft can change direction on a moments notice, you go with a high diameter and lower pitched propeller.

It is important that the propeller you choose is supported by the electric motor that you use. The higher the diameter and pitch, the more stress there is being put on the motor. Motors mounted with an incorrect propeller size can draw massive amounts of power. This extra power draw can damage your battery, fry your ESC and melt your electric motor. It is recommended you view the manufacturer's specifications on your motor to determine what propellers they recommend and support.


Electric Motors


There are several different types of motors used in electric flight. There are two sets of classifications that are the most important when it comes to electric motors. All motors fall into one group from each of the following categories: Brushless OR Brushed, and Inrunner vs Outrunner. The distinction between brushless vs. brushed motors is very easy to summarize. Brushed motors are less efficient, noisier and will eventually wear down to a point of being unusable. Brushless motors are very efficient, quiet and will generally have longer lifetimes. Simply put, brushless motors are superior and the most common type sold today.

     

The distinction between inrunner and outrunner motors are a little harder to explain. An inrunner motor (the above left picture) spins very fast, but lacks rotational power (torque). Outrunner motors (above right picture) are larger in size, spin slower, and have higher torque. If you are using anything other than a small prop, you won't be able to use an inrunner motor without a gearbox. A gearbox will allow your high RPM inrunner motor to work with a propeller that will spin very slowly. A gearbox is essentially a transmission for all you car lovers out there. There is usually maintenance involved, and the gearbox adds extra weight. Due to the extra maintenance, complication and weight, most people fly with outrunner motors. The only major exception is when you use very small, but fast spinning props, like in high performance EDF jets. You will see some of the faster jets have inrunner motors inside of them.

The speed at which the motor turns is decided by its Kv rating. Without getting into the complicated math involved, lets simplify things by saying that the higher the Kv rating, the faster it will turn with every volt of electricity you give it. With all other things being equal, a higher Kv electric motor will be able to spin your propeller faster.

Now, before you rush out and buy the highest Kv motor, you need to understand that there is a lot more to consider. As I said, a high Kv motor spins faster with each volt you give it, but each model motor will require specific voltages. Some motors will work at high voltages (like you'd get with a 6S battery), while others will only work at low voltages (like you'd get with a 2S battery). A 1500 Kv motor that only operates with a 2S (7.4v) battery will rotate nowhere near as fast as a 1000 Kv motor that uses a 5S (18.5V) battery.

You need to think about what type of plane you will be flying (3D, sport, EDF, glider?). What is the diameter and pitch of the propeller that you will be attaching? Maybe it will be a 3D plane and require a lot of torque with a very large diameter propeller. In this case you might be using a relatively big electric motor to supply the needed rotational power. Larger motors will usually have a lower Kv rating, and accept different voltages, but you might be turning a much larger propeller that would burn out a smaller motor.

In general its best to check with the motor manufacturer's specifications to ensure you are buying the right motor for what you intend to do. The manufacturer will give a list of recommended propeller sizes and accepted voltages. Exceeding these specifications is generally a bad idea, and can result in both damaged equipment and dangerous crashes. Also be mindful of the manufacturer's estimated power draw from the propeller size and voltage you will be operating it with. The specified amperage will be very important when determining the needed specifications for your ESC and battery.


Mounting the Motor and the Propeller

     

You may be wondering how these high powered electric motors and propellers are mounted to the plane. For the motor itself, it it is usually screwed in to a specially made plate or mount. The picture to the upper right shows such a motor mount This mount is typically wedged inside the aircraft so that there is some sort of physical obstruction preventing it from pulling away from the plane. It is usually glued or epoxied as well, but its worth noting that glue is not enough to secure this mount.

Once the motor is mounted, you will then need to load the propeller. In order to do this, you will need an adapter of some kind. The picture to the above left is a collet prop adapter, which is a common adapter used today. Electric motors come with different shaft sizes, so the prop adapter be sized to fit that motor. The base of the adapter must fit snuggly onto the motor shaft, or it will fail to grip onto the motor shaft. Once the base of the prop adapter is fitted onto the motor shaft, there is a second piece that will slide onto the base of the collet adapter. This piece puts pressure on the bottom of the adapter, clamping it onto the motor shaft. The propeller will slide into this piece, and force it in place. Finally, the propeller and the base of the prop adapter are secured into place with some sort of tightening mechanism. Some times it will be just an ugly looking bolt, other times it will be a bullet style spinner like the one shown in the picture. After this is tightened, you are ready to fly.


Propeller Direction and Pusher vs Puller Props


I've actually seen and heard of quite a few planes crashing due to an incorrectly mounted propeller. This notion sounds absurd to a lot of people. If the propeller was facing the wrong way, would it push the plane backwards? Actually, no. The plane will only move backwards if the motor direction is reversed (see the electronics training module for more on this). If the prop is on backwards, it will just lack thrust and fly very poorly. The lack of forward velocity from an incorrectly installed propeller will usually cause the plane to stall very easily. Most people fall into the trap due to the fact that some planes have their propellers mounted in the rear of the plane while others are mounted in the front of the plane. It is very common for people to think that the propeller must face in different directions, depending on whether it is a front mounted prop or a rear mounted prop... but this is completely incorrect. The propeller has a definitive front and a definitive back. The propeller's front must always face forward. The simple rule of thumb to tell which way is the prop should be facing is to look at the writing on the propeller. The writing will always be on the front of the propeller. Some times it will have a logo, like in the above picture, and other times it will just have the prop size in small writing. THE WRITING SHOULD ALWAYS FACE THE DIRECTION OF FLIGHT.


Do not be confused by the fact that there are "pusher" and "puller" props. You can buy either one, and the lettering still needs to face forward. Puller props are the standard propeller, and pusher props are used in specific circumstances. These are not actually designed for pushing or pulling the aircraft. They are actually designed for twin engine planes due to something called torque roll. All planes create a torque roll effect due to the rotation (for a puller prop, it's a counter-clockwise rotation). Torque roll with a puller prop will cause the plane to roll left on its own. This usually only happens at low speeds. The pilot must counter this effect by rolling to the right.

This is usually manageable on single engine planes, but on multi-engine prop planes it becomes a problem. In order to solve this, a puller prop is mounted on one side of the plane, while a pusher prop is mounted on the other side. The motor on the pusher prop is reversed so it spins in the opposite direction (clockwise), while the puller prop rotates normally (counter-clockwise). The torque roll of these motors will now cancel each other out. This is necessary because if both propellers were spinning in the same direction, you'd have a much stronger torque roll effect.



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