What is a propeller?

A propeller is a device with a hub to which angled blades are attached and set to a pitch to form a helical spiral that, when rotated, ‘screws’ its way through air.

Big words, I know. All we really need to know is that a propeller is the spiny thing at the front of an airplane and it’s what propels the airplane forward through the air.

It’s often compared to driving a wood screw into a piece of wood, which is a good comparison for the sake of understanding, but there’s a lot more that goes into airscrews than the simplicity of a wood screw.

What does a propeller do?

In it’s simplest explanation, a propeller converts the rotational power from the engine into linear thrust.

Since propellers have an airfoil shape just like the wing on an airplane, it ‘lifts’ an airplane forward, as if it is a spinning wing.

Are Propellers Dangerous?

Propellers are arguably the most dangerous part on a Radio Controlled Airplane. When you’re working on glow or gasoline airplanes, tuning the engines require tools to be put really close to the propeller while it’s spinning in order to adjust the needles. It’s important to not wear any kind of loose clothing, and be mindful of what you’re doing and where you are when you’re around a spinning propeller.

You should never use a propeller when it has been damaged, regardless of what the propeller is made up. Any gouges or nicks in the propeller blades will reduce the strength of the blade, and they may not hold up during use, meaning they could come apart and injure someone. Propellers are cheap enough. If you’re not sure about the condition of a propeller, it’s better to put a new one on.

Never stand in either the rotational plane, or in front of a spinning propeller. If something happens and it suffers a catastrophic failure, the parts will get flung forward and outward from the hub.

It’s also important to be sure to hold nitro and gasoline airplanes down while starting them and running them up before flight. After all, propellers are what make the airplane move, so you don’t want it moving when you’re around it!

Last but not least, never stand in front of an airplane when it’s running, or when an electric powered airplane is armed and ready to fly.

How are Propellers Classified and Labeled?

Propellers will be labeled with two numbers, with an “x” in between them. The first number is the diameter, and the second number is the pitch. Propellers can be measured and labeled in either standard or metric.

The diameter of a propeller is the size of the circle it makes when it’s spinning.

Pitch is used to describe how far the propeller would theoretically move forward with one complete rotation. More on pitch later.

A 10×6 (pronounced ten by six) propeller is a 10” diameter propeller that would theoretically move forward 6 inches in one rotation.

We say theoretically because there are some factors that make it that way. One is slip. Since air is not a solid and is compressible the propeller slips in the air and doesn’t get a firm grasp on it like it would if it was going through a solid, like a wood screw. Another factor is flex. Since prop blades have some give in them, they flex causing inefficiencies in the airfoil and therefore lift.

There are different propeller designations that tell you what type of propeller it is. You may see some letters listed on the propellers. If you do, it is telling you what kind of special characteristics have been designed into the prop. Some of the more common designations are:

E – Thin Electric or Electric (electric only) 

F – Folding Blade (electric only) MR Multi-Rotor (electric only) 

SF – Slow Flyer (electric only) 

W – Wide (chord) 

N -Narrow (chord)

P – Pusher (or reverse rotation for electrics) 

RPM Limitations

Propellers have a limit to how fast they can spin. Usually this isn’t much of a concern because they are usually designed to roughly match the engine that is spinning it. Not all manufacturers list the rpm limitations of their propellers. According to APC’s website, their propellers are rated for the following RPM.

Glow Engines are rated for 190,000 RPM/prop diameter (inches). For example, a 10×6 glow engine prop should be limited to 19,000 RPM.

Thin Electric (E) Propellers are rated for 50,000 RPM/prop diameter (inches). A 10×6 thin electric prop would be rated for 5000 RPM. Knowing how much of a difference there is between the thin electric prop and the normal propellers, if your electric motor will spin more than the rated RPM, it’s okay to use a regular propeller that’s rated for the higher RPM.

Slow Flyer Propellers (SF) are rated for 65,000 RPM/prop diameter (inches).

What is Pitch?

Unique to airplanes, pitch changes along the blade to account for the different angle of attack as the speed to which the blade is presented to airflow changes as diameter changes. At the tip, the propeller is much flatter since ‘airspeed’ is much higher and doesn’t need the angle to match the ‘lift’ produced at the root.

A properly designed propeller will have a balanced amount of lift from root to tip. This is called called pitch distribution and shows itself in the form of blade twist. Blade twist keeps the angle of attach uniform throughout the blade. 

For low speed flying, a flat pitch is best – like a low gear in a car.

For high speed flying, a steep pitch is needed – like a high gear in a car.

Constant Speed Propellers

Constant speed propellers have variable pitches. They are designed to be ran at a constant rpm. The amount of thrust generated is changed by varying the pitch of the propeller blades. This isn’t common on RC airplanes yet, but may become more common in the next few years.

Fixed Pitch Propellers

Fixed pitched propellers are what nearly all of us will use and be familiar with. They are the simplest propeller design since there’s no way to change the pitch of the prop. Thrust is varied by changing the speed of the motor driving it. 

Balancing Propellers

What props can I balance?  ALL of them!

What props SHOULD I balance?  ALL of them!

If you get in the habit of balancing a prop as soon as you add it to your ‘stash’, it becomes a standard practice so any prop you select from your inventory will be ready for service when bolted to your power system.

Balancing a propellor isn’t a difficult task.  However, there are several reasons why it needs to be done. An imbalanced propeller:

• causes vibration, which is bad for electronics.  
• will not develop the maximum available thrust because it will not produce max RPM.
• can cause negative flight characteristics.
• can cause component fatigue on linkages which can result in control surface failure.
• can prematurely wear plastic components like hinges, and control horns due to the vibrations and harmonics associated with it.
• causes ugly wear on cowl mounting holes, canopies, etc.
• can cause failures of glue joints, especially around the firewall area.
• can cause excess fuel consumption.
• Can cause propeller failure due to stress cracking within the blades.

There are several tools available to balance your propellers- from a simple spindle with pointed ends that you hold between your fingers, to elaborate devices that sit on your bench- they all provide the same end result: a balanced prop.

The two most common types:

• Roller supported balancer:  A spindle that clamps the prop at its hub, then supported by rollers at either end.
• Magnetic supported balancer: Same type of spindle as above, but supported at either end by magnetism.

There are pros/cons to each type:

• The roller supported balancer is not as precise as the magnetic balancer due to possible drag from the rollers. The larger and heavier the prop, the better it works.
• The magnetic balancers are limited to props that weigh under what the magnetic field can support.  Great for lightweight, small props that would not be able to be balanced by the roller-type.

How to balance a propeller

Choose the right tool.

• Use the magnetic type for small, lightweight props.  Good for electric-specific props up to about 11” diameter.  Also good for boat props!  Anything larger may be too heavy to use the magnetic balancer.
• Use the roller type for everything else! 

Develop your technique.

• Develop a technique that is repeatable for consistent results.  It’s ok if your technique differs from others.  What matters is the end result: A property balanced prop.
• Practice makes perfect.  Balance a prop.  Balance it again.  Do this until it becomes ‘muscle memory’.
• Patience young grasshopper.  It takes time to get it right.  Again, it’s not difficult, but it takes time and patience to get it dialed in.  Some props are ornery.  Stick with it and it WILL balance.
• Be mindful of air currents in your balancing dojo.  Air currents can make balancing impossible.  Your area should be completely still to get proper results.
• Work from a level surface.  Technically speaking, it shouldn’t matter whether your surface is completely level, however, a level surface ensures that your results and technique are repeatable and consistent.

Watch to see how RC Plane Lab balances their propellers.