Propellers

Part of the RC Airplane Electric Power System Center

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Quick Propeller Selection Checklist

Use the motor or engine manufacturer’s approved propeller range as the starting point.
Match both diameter and pitch; changing either one changes the load.
Confirm the propeller is intended for electric, glow, gas, or turbine use as applicable.
Check rotation direction, balance, damage, hub fit, spinner clearance, and the manufacturer’s RPM limit.
On an electric airplane, verify the finished combination with a wattmeter before flying.

If you are choosing an entire electric system rather than only a propeller, use the motor, ESC, battery, and propeller matching guide.

RC Propellers Explained Without Making Them Complicated

We devoted Episode 38 to propellers, and my highly technical description on the show was basically that a prop is the spinny thing on the front of the airplane that makes it go forward. Sometimes it is on the back. The joke works because the part looks simple. What it does to the airplane is not.

Diameter, pitch, blade count, RPM, battery voltage, motor Kv, engine characteristics, and the airplane itself all interact. The goal here is not to turn propeller selection into an engineering degree. It is to understand what changes when you swap props and keep from asking the motor, engine, or propeller to do something it was never designed to do.

What is a propeller?

A propeller looks simple, but it is doing real aerodynamic work. Each blade is an airfoil, and as the propeller turns it accelerates air backward to produce the thrust that pulls or pushes the airplane forward.

People often compare a propeller to a screw moving through wood. That picture helps explain pitch, but air is not solid and a propeller always slips. Diameter, pitch, blade shape, RPM, and the airplane’s airspeed all affect the result.

What does a propeller do?

In the simplest terms, the propeller turns the motor or engine’s rotational power into thrust. I think of each blade as a rotating wing. The blade develops an aerodynamic force, and the useful part of that force points forward.

Are Propellers Dangerous?

The propeller is one of the most dangerous parts of an RC airplane, and I treat it that way every time the airplane is armed or running. With glow and gasoline engines, tuning can put a hand and a tool uncomfortably close to the spinning disc. Secure the airplane, remove loose clothing and lanyards, keep spectators behind it, and decide where your hands will go before starting the engine.

I retire any propeller with a nick, gouge, crack, damaged hub, or questionable history. A damaged blade can fail under load, and a replacement propeller costs far less than an injury or a destroyed airplane.

Keep yourself and everyone else out of the propeller’s plane of rotation and away from the area in front of the airplane. Restrain glow and gasoline airplanes during starting and run-up. On electric models, treat the propeller as live from the moment the battery is connected, and remove the propeller while programming the ESC or working on the radio setup.

RC Airplane Propeller Sizes and Types

Propellers are normally labeled with two numbers separated by an “x.” The first number is the diameter and the second is the pitch. A 10×6—pronounced “ten by six”—has a 10-inch diameter and a theoretical pitch of 6 inches.

Diameter is the size of the circle the propeller makes as it turns. More diameter generally moves more air and adds load to the motor or engine.
Pitch is the theoretical distance the propeller would move forward in one revolution. More pitch generally favors speed and also adds load.

The pitch distance is theoretical because a propeller slips through air rather than threading through a solid material. Blade shape, flex, RPM, airspeed, and the airplane itself all affect what the propeller really does.

Common RC airplane propeller types

The letters molded or printed on a prop can tell you something about what it was designed for, but they are not a universal language. An electric prop, slow-fly prop, glow/gas prop, folding prop, and pusher prop may all have the same basic diameter-and-pitch format while being built for very different loads and RPM ranges.

Marking or type	What it means	Typical use
E	Thin electric	Most conventional electric RC airplanes
SF	Slow flyer	Lower-RPM, lightly loaded electric airplanes
F	Folding	Electric sailplanes and powered gliders
MR	Multi-rotor	Multirotors; available in standard and reverse rotation
P	Pusher or reverse rotation, depending on the manufacturer	Pusher installations or counter-rotating setups
Sport/glow/gas	Heavier blade and hub designed for an internal-combustion application	Glow and gasoline engines; some are also approved for electric use

Those letters are not universal across every brand, so check the manufacturer’s description before assuming two similarly marked propellers are interchangeable.

How to choose an RC airplane propeller size

Start with the propeller sizes tested or recommended by the motor, engine, or airplane manufacturer. That gives you a safe range instead of making you guess from airplane weight alone.

If you increase diameter, expect the motor or engine load to rise.
If you increase pitch, expect the load and potential pitch speed to rise.
If an electric motor, ESC, battery, or connector is getting too hot, reducing diameter or pitch may reduce current—but diagnose the complete setup rather than treating that as an automatic fix. The electric RC motor and power-system guide explains how those parts affect one another.
Make sure the propeller clears the ground and the airframe throughout normal takeoff and landing attitudes.
Use the correct propeller type and rotation. Installing a propeller backward or using the wrong rotation will produce poor thrust even though it may still move air.

On an electric setup, check every propeller change with a wattmeter. The same motor can draw very different current with what looks like a small change in diameter, pitch, or battery voltage.

Propeller RPM Limits

Every propeller has an RPM limit, and it needs to be treated as a real safety limit. Do not assume that a propeller is safe simply because its diameter looks appropriate for the motor or engine. Use the limit published by the manufacturer for that exact propeller series.

As one example, APC currently lists the following maximum-RPM formulas. Divide the number by the propeller diameter in inches:

APC propeller series	Maximum-RPM formula	10-inch example
Glow, Sport, Pattern, and Speed 400 Electric	190,000 ÷ diameter	19,000 RPM
Thin Electric and Durable FPV	150,000 ÷ diameter	15,000 RPM
Folding Electric	120,000 ÷ diameter	12,000 RPM
Multi-Rotor and Multi-Rotor Folding	105,000 ÷ diameter	10,500 RPM
Slow Flyer	65,000 ÷ diameter	6,500 RPM

These formulas apply to the listed APC series, not automatically to every propeller brand. Recheck the manufacturer’s current limit whenever you change the propeller, and remember that a motor’s KV multiplied by battery voltage is only a no-load estimate—not the actual propeller RPM.

What is Pitch?

Pitch is the theoretical distance a propeller would advance in one revolution if it moved through a solid material. A 6-inch-pitch propeller would theoretically advance 6 inches per turn, although real propellers slip in air.

The blade is twisted because the tip travels much faster than the portion near the hub. That twist helps each section operate at a useful angle of attack. Lower pitch generally favors acceleration and lower-speed thrust, while higher pitch can favor speed, but either choice still has to stay within the motor or engine’s approved load and RPM range. On Episode 38 we compared it to the gears in a car: a flatter-pitch prop is more like a lower gear when you need to get moving, while more pitch is more like a higher gear when you are after speed. That is an analogy, not a prop-selection formula, but it is a useful way to picture what pitch is doing.

Constant Speed Propellers

A constant-speed propeller changes blade pitch to hold a selected RPM as load and airspeed change. These systems are common in full-scale aviation but unusual on everyday RC airplanes because they add weight, cost, and mechanical complexity.

Fixed Pitch Propellers

A fixed-pitch propeller is what nearly all of us use. Its blade angle cannot change in flight, so thrust is controlled mainly by motor or engine speed. Changing the propeller means physically installing a different diameter, pitch, blade count, or design.

Balancing Propellers

I balance every propeller before it goes into service. Doing it when the propeller is new means the next one I pull from the box is ready to install, and it keeps an avoidable source of vibration out of the airplane.

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.

When a Propeller Problem Shows Up in Flight

A propeller that is damaged, unbalanced, installed incorrectly, or poorly matched can appear as vibration, overheating, weak performance, takeoff yaw, or a power loss. Use the symptom guides for motor or ESC overheating, electric motor cutout, takeoff yaw, or glow and gasoline engine problems. Stop after any propeller strike and inspect the entire propulsion installation before running it again.