Super Easy RC Prop Size Calculator
This handy guide presents you with an easy-to-read prop size calculator. Not all propellers are equal, though. It pays to have a little knowledge of flight fundamentals. That’s what this short, no-nonsense introduction covers. Keep reading if you want to learn what to look for—specifically. Or, jump straight to the propeller size charts below if you only want to check prop sizes for gas aircraft.
It’s vital to understand the differences between testing props with gas and electric powered aircraft. Knowing how a radio-controlled model takes to the air is as exciting as flying it. RC aviators who understand something of the basic mechanics tend to make better pilots too.
Making Sense of RC Propeller Sizes
This short guide gives a simplified explanation of what’s an otherwise complex topic. Novice pilots are keen to start, but this is one area where slow is the fastest approach. Allow yourself a few minutes to take in the information on this page to avoid newbie blunders. It’s the only way to ensure you get optimal performance from the model’s motor or engine, and plane.
Point to note! Wrong propellers have the potential to cause major damage to other components.
Heed Manufacturer’s Advice
Perhaps you need to replace a damaged propeller? Maybe you’re building a plane from a kit that doesn’t include props, or you want to upgrade the stock options? There should be prop size recommendations from the manufacturer in each case. Heeding such advice is the quickest and most stress-free way. Makers typically put the prop sizes in the tech spec sections of instruction manuals.
How RC Props Work
Everyone knows that propellers on RC planes, drones, and helis are fast-spinning blades. However, not everyone knows how they work. It’s not rocket science, at least not at a basic level. Think of the propellers on your RC aircraft as carefully engineered rotating wings. The aircraft’s motor or engine produces power and the propellers transmit that energy.
Props work by converting the power from the engine or motor into thrust. It’s this “thrust” that pushes and pulls the craft through the air. RC props generate thrust in the same way that wings create lift. That’s why propellers have curved surfaces or airfoil sections like fixed wings.
Understanding Prop Sizes & Labelling
There are two measurements to consider when it comes to propeller sizes, e.g., 8×4”:
- First number: diameter (length) of the imaginary disc (‘arc’)
- Second number: pitch, the length of the angle of the twist on an RC prop
The first number is easy and merely donates the length of the propeller. A bigger diameter results in more thrust and noise. The second number, pitch, is the length of the prop’s twist section. That’s 4 inches in our example. The higher the pitch number, the more bite the prop takes out of the air. Thus, a higher pitch value typically donates a faster aircraft.
There are other factors at play, but this simple account explains the basics of prop numbers.
IC Engine Propeller Size Chart
It’s straightforward to match the right sized prop to an internal combustion engine (ICE). Either follow the maker’s recommendations or use the tables below for guidance. There are two charts here, one for 2-stroke engines and the other for 4-stroke. Each table has three columns that show the engine size, starting props, and alternate props. So, what are ‘starting’ and ‘alternative props?’
Starting propellers are those with a mid-range pitch (see above). They’re ideal for use with any craft made for that engine size. Starting props should always be a newbie pilot’s first choice.
Alternate props are suggestions for skilled pilots who want to fine-turn their model’s performance. There are no guarantees with alternate prop sizes, so one must prepare for trial and error.
PROPELLER CALCULATOR RESULTS FOR 2-STROKE ENGINES
|Engine Size||Starting Prop||Alternate Propellers|
|.049||6-3||5.25-4, 5.5-4, 6-3, 6-4, 7-3|
|.09||7-4||7-3, 7-4, 7-5|
|.15||8-4||8-5, 8-6, 9-4|
|.19 – .25||9-4||8-5, 8-6, 9-5|
|.29 – .30||9-6||9-7, 9.5-6, 10-5|
|.35 – .36||10-6||9-7, 10-5, 11-4|
|.45||10-7||10-6, 11-5, 11-6, 12-4|
|.50||11-6||10-8, 11-7, 12-4, 12-5|
|.60 – .61||11-7||11-7.5, 11-7.75, 11-8, 12-6|
|.70||12-6||11-8, 12-8, 13-6, 14-4|
|.78 – .80||13-6||12-8, 14-4, 14-5|
|.90 – .91||14-6||13-8, 15-6, 16-5|
|1.2||16-8||16-10, 18-5, 18-6|
|1.8||18-8||18-10, 20-6, 20-8, 22-6|
|2.0||20-8||18-10, 20-6, 20-10, 22-6|
The following table uses the same format to show prop sizes for 4-stroke IC engines.
PROPELLER CALCULATOR RESULTS FOR 4-STROKE ENGINES
|Engine Size||Starting Prop||Alternate Propellers|
|.20 – .21||9-6||9-5, 10-5|
|.40||11-6||10-6, 10-7, 11-4, 11-5, 11-7, 11-7.5, 12-4, 12-5|
|.45 – .48||11-6||10-6, 10-7, 10-8, 11-7, 11-7.5, 12-4, 12-5, 12-6|
|.60 – .65||12-6||11-7.5, 11-7.75, 11-8, 12-8, 13-5, 13-6, 14-5, 14-6|
|.80||13-6||12-8, 13-8, 14-4, 14-6|
|.90||14-6||13-6, 14-8, 15-6, 16-6|
|1.20||16-6||14-8, 15-6, 15-8, 16-8|
|1.60||18-6||15-6, 15-8, 16-8, 18-6|
|2.40||18-10||18-12, 20-8, 20-10|
|2.70||20-8||18-10, 18-12, 20-10|
EP Propeller Sizes
The prop selection for electric powered (EP) RC is not as straightforward as it is with IC engines. That’s because the operating speeds and loads can vary significantly between models. Batteries, ESCs, and motor combinations are often unique to each craft. Therefore, there’s little to no room for flexibility or trial and error. Following the manufacturer’s specific size recommendations is advisable in these cases.
Beware of Over-Propping an EP Aircraft
OVER-propping on an IC plane is not a serious concern and does one of two things:
- Fly but underperform
- Stall before takeoff
OVER-propping an EP aircraft is a different matter. The problem here is that the motor is forced to work harder than the makers designed it to. The EP motor won’t stall like the IC engine either. It just keeps trying until it eventually overheats or catches fire. That’s the problem. UNDER-propping EP motors is less of an issue. All that happens in this case is that the craft underperforms.
The Simple Watt Meter to the Rescue
There are two means to ensure the correct RC prop size for an EP motor. You have an easy way and a complex approach. The simple method—if you have a pre-built model—is to check with the maker. But what if you want to experiment with different sized props than the stock suggestions? The makers are unlikely to give you alternates to their recommended blades.
Different props can still enhance the performance of a regular RC model. There may not be official recommendations, but that doesn’t deter RC enthusiasts from testing. The approach is more complicated but perfectly doable using a simple watt meter. The Tenergy High Precision RC Watt Meter & Power Analyzer is an excellent example.
You may need to purchase appropriate plugs for the meter if you don’t already have them. The watt meter tells you if the EP prop is a good match for your bird. It does this by reading the propeller’s current-draw through the model’s ESC and its motor. It’s totally safe and 100% accurate.
How Many Propeller Blades?
Most radio-controlled craft at the hobby level use two blades, but they’re not the only options. The 3 and 4 blade propellers also have their place, but is more better? No, is the short answer. Two-bladed props are cheap and efficient and therefore most in demand. Extra blades reduce efficiency as there’s more turbulent air to cut through from the preceding prop.
Some hobbyists still prefer 3 and 4 bladed props, usually for style reasons. There’s a general rule for choosing 3 or 4 bladed propellers that looks like this:
- Reduce the prop’s diameter by an inch
- Increase the prop’s pitch by an inch
It may not be that simple, though. There’s both ground and fuselage clearance to consider, and that’s something pilots need to work out depending on the model. It’s all about trial and error.
OK, that about concludes this RC prop size calculation guide. I’ll just end with a short section on safe practices for attaching and testing new propellers to an RC model aircraft.
Propeller Installation & Safety Measures
It’s easy to overlook safety concerns when we’re eager to install new props and take to the air. Most of these are good common sense routines, but there’s no harm in repeating them.
- ALWAYS install the propeller with the curved side facing forward
- ALWAYS use the correct sized wrench to tighten the bolts or nuts
- ALWAYS recheck the bolt or nut’s tightness, especially with wooden props
- ALWAYS test the new props AT LEAST 20ft. away from onlookers
- ALWAYS keep hands well clear of spinning props
- ALWAYS keep faced & body clear of the prop’s arc after starting the engine/motor
- ALWAYS make fine adjustments from behind the prop (except pusher props)
- ALWAYS keep loose clothing well clear from spinning propellers
- NEVER throw something into the path of spinning props to kill the engine
- ALWAYS discard damaged props. NEVER attempt to repair
- ALWAYS test new props well away from loose gravel and sand
- ALWAYS keep the glow driver wire away from the prop’s path
Accidents happen all the time, yet RC mishaps are avoidable with a little foresight. Try not to let excitement and enthusiasm make you impatient or rush you along.