Quadcopters and multirotors usually use brushless outrunner motors as their powerplants. When selecting the motor and propeller for your multirotor or quadcopter it is important to know what it is that you are planning to do with it. Are you planning on racing? Casual flying? Do you want to do acrobatics or aerial videos and carry a heavy payload? If you want speed and acrobatics then you need to select a very fast turning motor with small propellers (under eight inches). If you want to carry a payload such as a video camera and gimbal then you need to select a slower turning motor with a large propeller (eight to 12+ inches). Unfortunatley, you just can't have both and you need to have at least some basic knowledge of motors and propellers in order to pick the right motor/propeller combination for your quadcopter or multirotor craft.
The Basics of Propellers and Motors
One of the first things to consider when choosing a motor is the weight that it must be able to comfortably lift. Remember, your multirotor is literally hanging in the air totally dependant that everything works the way that it is supposed to. The basic rule of thumb is that the combined motor/propeller combinations should be able to generate twice the flying weight of the craft in thrust, which will provide more than enough thrust and a nice safety margin. So then the basic equation is this: required thrust per motor equals (total weight x2)/4 motors. If you already know the total weight of the craft you are off to a great start! But if you haven't built the aircraft yet and don't know the weight we can estimate what it is going to weigh. For the average quadcopter, the total weight is usually around 1.3kg (2.86 lbs or 45.85 oz). Using our equation for selecting a quadcopter (four motor) we know that we are looking for a total of 2.6 kg or 650g of thrust per motor/propeller combination. If you are building a multirotor craft that deviates from the average four motor quadcopter, such as a three motor tricopter or six motor hexacopter and need to estimate the weight, a good starting point for your estimates is as follows: the average weight of a motor/propeller combination is about 100 grams each, the average frame weighs around 400-500 grams, a 3s LiPo battery approximately 300 grams, ESCs and wiring about 25 grams each and the combined radio receiver/flight controller around another 50 grams or so. If you are planning on carrying a payload such as video equipment then you need to add in the weight for that as well.
Brushless outrunner motors are rated in kV per RPM (K=RPM, v=per volt) which is usually specified as just kV. For racing and acrobatics (or just plain fun) a good starting point in motor selection is to consider motors that are rated over 1200kV and for heavy payloads motors that are rated under 1200kV (850kV is a nice starting point for heavy payloads). Actually, we can break this down a little more accurately and say for a unit that weighs more than 1kg use a motor in the range of 700-900kV, between 500grams-1kg use motors between 900-1300kV and for craft below 500grams motors around 1300-2200kV. Granted there is often a bit of overlap to this basic rule of thumb, especially when propeller variations are considered. But we all need a basic rule of thumb as a starting point to make life easy!
Motors also have a different numbers of poles. Some motors have only two poles while others can have as many as fourteen poles. Smaller, faster motors for racing and acrobatics have fewer poles than the bigger heavier motors used for carrying video equipment. Another specification to pay attention to when choosing your motor is the working current and maximum current. These are rated in amps and need to be known in order to choose the right electronic speed controller which is also rated in amps. For example, if a motor is rated at a working current of 19 amps and a maximum current of 23 amps then you would want to use ESCs that are rated at a maximum of 25 amps.
Now on to the shaft size, the diameter of the shaft needs to be known so that the propeller fits properly. Fitting a propeller on a shaft that is too small (without inserts/bushings) can be dangerous and can result in unstable flight!
Finally, one last point to consider is the motor mounts and mounting screw pattern. Some motors come with motor mounts others do not. In choosing your motor mounts be sure to consider the mounting hole patterns on both the motor and boom of the frame! With some booms you may be able to bolt the motors directly and eliminate the need for an additional mount between the two.
Propellers come in a variety of diameters and pitches as well as materials such as plastic, reinforced plastic, carbon fiber and wood. For the most part plastic and carbon fiber are the most popular. Smaller propellers under 8 inches are used for racing and acrobatics along with smaller motors rated with a high kV. While larger propellers over 8 inches along with motors with a low kV rating are used for carrying payloads such as video equipment. Whatever material you choose there are two main specifications to consider; diameter and pitch. Let's begin with the pitch. In simple terms, the pitch can be defined as the travelling distance per a single revolution of the propeller. For example, if the pitch is 4.7 on a 9 inch diameter propeller, the propeller is capable of 4.7 inches of travel per revolution. When deciding on the diameter and pitch of a propeller, first consider what you want the craft to do and then find the balance between the diameter and the pitch. Generally speaking, a lower pitch will generate more torque (and less turbulence) for lifting and the motors don't have to work as hard to carry heavier loads. As a result, a motor that doesn't have to work as hard will draw less current from the battery which results in increased flight time. One simple way to increase flight time on a heavier unit is to use a lower pitch propeller on your aircraft! A propeller with a higher pitch can move a greater amount of air but creates more turbulence and less torque. If you notice that your craft wobbles alot when hovering chances are that the pitch of the propeller is too high for that particular unit.
Flight efficiency is closely related to the amount of air contacting the surface of the propeller. A larger diameter is equal to more air contact. A small increase or decrease in the diameter of the propeller can alter how efficiently your quadcopter or multirotor flies. For example, swimming with flippers on your feet is more efficient than barefoot BUT it is also more tiring because it takes more effort to move more water per stroke. Likewise, a smaller prop requires less effort to speed up and slow down but is less efficient than a larger one. Therefore, smaller props will speed up and slow down faster (inertia of movement) making the quadcopter or multirotor more responsive. If operating your aircraft is "hair trigger" or "twitchy" then your propellers are too small for that unit, try moving up a size in diameter! On the same note, larger propellers take more effort to speed up and slow down which makes them less responsive but more stable when hovering. For this reason, larger props with a low pitch are ideal for aerial videography and photography. While smaller props with a high pitch are ideal for fast quick maneuvers.
When deciding on a propeller size the first number is always the diameter and the second number is always the pitch. For example, a prop with the following numbers 7x3.5 is a 7 inch diameter prop with a low pitch of 3.5, while a number like 11x4.7 is an 11 inch diameter prop with a higher pitch of 4.7 inches.
One last point about propellers for quadcopters and multirotors is the direction of rotation. For stable flight you need an equal number of clockwise and counterclockwise propellers (the tricopter is the exception) so when you purchase your props be sure to understand if the pair is a pair of clockwise, counterclockwise or one of each. Which leads us to one final question, "How do you tell the two apart?" The answer is easy, the top rib of the propeller always points to the direction of rotation. The top rib of a clockwise prop points in a clockwise direction and a counterclockwise prop has the top rib pointing in a counterclockwise direction.