Installing Propellers on Your Quadcopter

Propeller Rotation Direction

Props are designed to produce maximum thrust when turning in only a single direction. Quadcopters are equipped with two sets of two different props: one set of “clockwise” (CW) props which rotate like a clock, and another set of “counter-clockwise” (CCW) props. Normally when you buy props for quadcopters, you receive a full set of 2 CW and 2 CCW props.

You can determine the direction of a prop in one of several ways. Most props have labels on them that look something like “5045” and “5045R”. When labeled like so, the “R” stands for CCW while the lack of an R means the prop is a CW prop.

The better way to determine the intended direction of a prop is to find its leading edge. The leading edge of a propeller blade is the side of the blade that is raised above the other side with a slight concave curve underneath it. All props have this, whether or not they are labeled, so learning to recognize it pays off.

In the above image, the white dots mark the leading edge of the six-bladed prop. It is obvious when the prop is mounted on the quadcopter, but a bit more difficult when it is not. The reason is that the prop, when flipped upside down, looks very similar. The difference is the shape of the curve from the leading edge to the trailing edge of the prop.

Installing Your Propellers

Once you can determine the intended direction of rotation of your props, you can install them on your quadcopter. Pretty much every quadcopter these days uses the Multiwii prop configuration. This looks like this:

In the diagram, the red arrow points to the front of the quadcopter. The blue circular arrows represent the rotation direction of the props. You just need to install your props so they are oriented to rotate in this diagrammed direction.

Rather than having to look this diagram up every time you install your props, I’ll offer you my favorite way of remembering this orientation: All front props on a Betaflight/Cleanflight/Multiwii quadcopter spin into the FPV camera. The rear props, likewise, spin into the rear of the quadcopter.

Propeller Safety

I’m sure most folks who have gone through the trouble to do some research into this hobby already know this, but miniquad propellers can seriously hurt you. While they are only plastic, the centrifugal force of the prop tips at 27,000 RPM causes the plastic to take on near-metal hardness and turns even a dull leading edge into a potential knife. Think about what the much lower RPM of the string trimmers used by gardeners can due to weeds and multiply both the RPM and plastic hardness by several factors.

While you are working through your honeymoon phase with your new ARF quadcopter, I strongly advise you to treat it like a loaded gun whenever it is plugged in to a battery. Point the quadcopter away from your face at all times and try to keep body parts and clothing out of the prop arc. Remember that if it does arm, it will very likely spin about in a rapid, uncontrolled fashion — be prepared to drop it and run! Quadcopters rarely (if ever?) arm for no reason whatsoever, but there quite a few human errors that can cause them to go off unexpectedly. Be prepared.

While we won’t link anything here, you might consider googling “quadcopter injury”. The images can get gory but they are sobering and can help motivate you to take a few extra safety measures.

Buying Replacements: Quadcopter Propeller Performance

If you are new to flying quadcopters, you’re going to need replacement props in short order. You could just buy direct replacements for the props sent by your ARF quadcopter’s manufacturer but many times these are hard to come by and can take a long time to get shipped to you. A better (and often cheaper) option is to buy after-market props. To do this properly, though, you’ll need to understand a few things about propeller performance.

I don’t want to dig too deep into performance in this article because we already have a fantastic article available on this topic: our Miniquad Propellor Buyer’s Guide. Please follow that link to learn everything you will need to know about what the different “numbers” that go alongside props means.

Puffed lipo

A “puffed” LiPo battery is the primary side-effect of a too-aggressive prop choice. The battery on the right is permanently damaged. Image courtesy of LearningRC.

What I would like to talk about is how to determine what kind of props you can safely use on an ARF quadcopter. “Safely” doesn’t refer to your personal safety, but to the longetivity of your quadcopter and spare parts. The limiting factors here are:

  1. How much power your batteries can deliver.
  2. How much power each motor can continuously take.

Fitting larger, higher-pitch and higher-blade-count props will all cause your quadcopter to use more power to fly. It is similar to fitting larger, more sticky tires onto a car — only it has a much more significant impact. For example, you can pretty easily find two 5″ props that have a power gap of greater than 300% (e.g. one prop will cause your quadcopter to consume more than 3x the power).

Batteries are the greatest limiting factor for miniquads. We fly with small, lightweight batteries and ask them to deliver more power than your average microwave or vacuum cleaner runs on. LiPo batteries that are stressed tend to get very warm and expand like a sponge, commonly called “puffing”. When this happens, the batteries are damaged and will soon need replacement. One of two things can happen: either they will rapidly lose their capacity, or they can puff too far and rupture the cells, causing a small fire in flight. This is not something you want to happen to you.

Motors are a bit more durable. They are generally overbuilt and there are four of them taking the electrical load. However, a quadcopter which has vibration problems can cause the motors to be overworked by the flight stabilization software. In this case, the motors will get very hot after a flight. If they get too hot, the epoxy insulation on the motor windings will melt and the motor will emit the “magic smoke” and no longer work. The smell of a burned motor is un-mistakable: it smells like $20-$30.

Our recommendations for picking after-market propellers

How do you avoid these problems when choosing a prop for your quadcopter? It depends on your flying style. If you fly conservatively with mostly controlled, slow-speed flight, you can likely fit just about any propeller on your quadcopter without worry. All motors and batteries can survive short bursts of high-powered flight, it’s when you do this over and over again that you start to get into trouble. If your flying style involves coming out of every turn at full power, you should consider limiting your prop pitch to save your batteries. How much you should limit it depends on the motor kV on your quadcopter.

To summarize:
Conservative flying – Use any prop. We recommend tri-blades with quad-specialized airfoils, like the RaceKraft 5051s.

Aggressive flying, 2300kV or less motors – We recommend 5040 tri-blades. You can get away with less-aggressive 5045 props like the DAL TJ5045s.

Aggressive flying, >2300kV motors – Stick with two-bladed props, a cheap option is the RayCorp 5045. You can get away with 5040 tri-blades too but you should try to reign in your flying style if so.

 

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