Propellers

 Propellers

These things are evil and will hurt you if you don't take care. So make sure to never go near a rotating propeller with any body part you want to keep.

Specifications

A propeller has two important numbers that define the basic characteristics - the diameter and the pitch.

Most propellers are marked with these two numbers in a format like 8x4.5". This means the propeller has an 8" diameter and a 4.5" pitch.

The diameter quite obviously defines the length of the blades and the disc area in which air is moved. This determines the power of the pull and the torque that is generated, but not the speed.

This is what the pitch stands for. It is defined as how far the propeller would travel in one full rotation while being in a perfect fluid. You can visualize this by thinking of a screw. A higher pitch screw will go deeper into the wood than one with a lower pitch if you turn both exactly one revolution. Since air is not a perfect fluid, this number does not translate into the real world. But it still helps in determining the speed of the air transported by the propeller.

A high pitch propeller is used for speed, but takes longer to accelerate and has poor climbing abilities. Compared to a car it's like starting to drive in 5th gear. You will get fast, but it takes a while and you will probably not get up a hill. These are not good for a multirotor.

A low pitch propeller on the other hand is slower, but accelerates faster, has a better climbing rate and better speed control. As four our car comparison - it's like driving in a low gear: You will be slower, but can accelerate quicker and climb hills.

To make things more difficult there are variable pitch propellers. Usually those are reserved for helicopters, but there are multirotor models out there that use these - and can do crazy maneuvers like flying upside down...

Also important is the type of the propeller. You can have clockwise (CW / pusher) or counter clockwise (CCW / tractor) props. For a multirotor you usually have to use both kinds because the motors rotate in different directions to minimize torque that wants to turn the copter around. Always make sure you use the right type of propeller on each motor and have the numbers that are written on the blades point in the direction the motor is supposed to travel (that is "up" for multirotors).

There are discussions on the number of blades though. Two blades are more efficient than three, because more blades might disturb the air for the next blade. But if you want to build a smaller model and have to reduce the disc area, you can go for a three or four blade prop.

It's important that these numbers match the motors you use. Quite often the manufacturer of a motor mentions an "engine comfort zone" and suggest what propellers you should use. For low kv motors that usually means a bigger diameter, but flat pitch. High kv motors are used with smaller diameter props with a bigger pitch which results in higher speeds.

Materials

As the frame itself, propellers come in a big variety of materials.

• Wood
  These can be lighter than other materials if they get bigger and can tolerate higher RPMs, but they break easily. They are not common on multirotors.


• Fiberglass


• Carbon Fiber
  These are the most rigid ones. They give you the best performance, but are really expensive and don’t handle crashes well.


• Nylon
  These are the cheapest and come with quite a list of negative properties. They are flexible and might deform at higher speeds which will change the pitch and make them loose power. They are good for beginners though, because they will not break that easily.


• Fiberglass-reinforced Nylon
  These are the heavier ones, but also the most durable. They are less efficient than fiberglass or carbon fiber props, but still pretty good for a beginner.

No matter what material your props are made of - if they show any sign of damage or cracks, throw them away and get new ones. Because they will mutate into high speed projectiles if they break at high speeds.

Balancing Props

When you buy new props the first thing you should do is balance them. Because of the way they are manufactured they are more or less out of balance. If you don't fix this you will have vibrations as a result. These not only rob power from the engines, but might even destroy your frame or the electronics as well as reeking havoc on your flight controllers gyros.

Repeat after me: Vibrations are bad!

Balance your props and motors. Well balanced propellers will give you a better flying experience and are more efficient.

Finally, check out the Flite Test Beginner Series episode on power systems which includes information on props and motors.