Selecting propellers


Written by Lucas Weakley
New Technology
Column
As seen in the Spring 2018 issue of
Park Pilot.


>> When I first began designing my own RC airplanes, determining the size of the propeller was probably the most confusing thing that I had to learn. Over time, I began to intuit which propeller to use. I’ll share how I learned this and show my process for selecting a propeller for an airplane design.

A good place to start is propeller theory. Wait! Don’t leave yet! I promise I won’t get too technical; I just think it’s important to know why propellers are classified the way that they are.

Propellers are classified by a set of two numbers. For example: 6 x 4, 8 x 3.8, or 10 x 5. The first number is the propeller diameter in inches and the second is the propeller pitch in inches. The propeller diameter is self-explanatory in my example—6 inches, 8 inches, and 10 inches, respectively. The pitch is where things become confusing. Pitch is usually an angle measurement, but now it’s a distance. Here’s why.

A propeller is a spinning, twisted airfoil. This twist in the airfoil allows for even force distribution across the entire propeller blade. The pitch angle at the blade’s tip is shallower than the steep pitch that is closer to the hub because the tip is spinning much faster than the hub, and therefore doesn’t need to “bite” the air as much to create the same thrust. The twist also means that the pitch angle is not constant over the span of the propeller blade. Therefore, we can’t rely on pitch angle for propeller classification.

So now what do we do? Well, we look at pitch distance. Imagine a screw being driven into a piece of wood. The screw teeth have an angle, and when we turn the screw 360°, it goes into the wood a certain distance, which is constant. A propeller does the same thing.




The Maker Trainer 2 has a 30-inch wingspan and an 8-inch propeller. This makes the propeller slightly more than 25% of the overall wingspan, so the airplane rolls from the torque on takeoff. Photo by Kent Weakley.


Imagine spinning a propeller in a thick fluid. If it never slipped, it would follow the path carved out by the angle of its blades. In this theoretical case, the distance a propeller would move forward in one revolution is the pitch distance. So, for my example, it would be 4 inches, 3.8 inches, and 5 inches, respectively. That’s it! The percentage of difference between this theoretical pitch distance and the actual pitch distance of the propeller in flight is known as propeller efficiency, but now I’m getting too technical.

In my aerospace engineering classes, when we design a full-scale airplane, we determine which propeller we’re going to use through a number of fancy calculations. Luckily, we don’t have to go this crazy in-depth for an RC airplane, although it is fun to know that all of the same equations apply to either case.

There are several ways that you could go about selecting a propeller for your RC airplane. The first, and most obvious, is by looking in the product description of the motor you purchased. There will usually be at least a manufacturer’s suggested propeller diameter and pitch. Sometimes you’ll get lucky and the shop owner has done testing and can give you a list of propellers and the static thrust that they create.




Looking down the end of a propeller, the twist that Lucas Weakley discusses can be seen.


This is how I do 90% of my propeller selecting, although you probably knew all of this already (if not, I just blew your mind!). What do you do when you have an airplane without a motor yet or one that you can’t identify? Now that’s more interesting!

There are a few leading questions that you can ask about your airplane to derive which propeller size you should use.

First, how does the airplane fly? Propellers are like the gears on a bike or a car. Large-diameter propellers have their greatest thrust at close to zero airspeed, and eventually lose the ability to create more thrust when the airspeed of the airplane exceeds the pitch distance rate of the propeller.

Small-diameter propellers have low static thrust and produce the most thrust when they are operating at higher airspeeds. Because smaller propellers usually have much higher proportional pitches, they are also able to continue accelerating the airplane into much higher airspeeds. So, if the aircraft is meant to fly slowly, it should have a larger-diameter propeller and vice versa.

To finalize the propeller diameter, look at the airplane’s wingspan. If the propeller is too large, the torque and gyroscopic effects coming from it will cause adverse flight characteristics that we don’t want.

It’s generally a good idea to keep the propeller at approximately 25% of the overall wingspan. This can be difficult to follow, for example, when you have a 6 x 4 propeller for your 18-inch wingspan park flyer jet.

These are just rules of thumb. On a slow-flying airplane, the wings won’t have enough air going over them to counter the torque, but the park flyer jet in this example will be going fast enough that the torque won’t really have an effect at cruising speed.

Taking off with that park flyer jet will be sketchy because you will probably need nearly full throttle to get enough thrust to take off, and there won’t yet be air over the wings to counter the torque. You might want to have a friend help you launch.




Here, the spirals follow the motion of the propeller through one revolution and the cylinder shows the pitch distance traveled by the propeller.


Pitch can be determined by looking at the current draw and Kv of your motor. The more amps that your motor and ESC can handle, the steeper the propeller pitch you can use. If your motor is rated at only 12 amps and has a low Kv (below 1,500), it should have a large diameter and shallow pitch propeller such as a 10 x 4. If your motor is rated at 40 amps and it has a high Kv (more than 1,500), the motor can likely handle a small, steep-pitched propeller such as a 6 x 5 or even a 7 x 4.

Knowing when to use a propeller with more than two blades is just as simple. A three- or four-blade propeller will produce more thrust (while also being less efficient) than a two-blade propeller of the same diameter. They are used when more thrust is needed, but the diameter can’t be increased. This is why they are popular on racing quadcopters.

Speaking of selecting propellers for multirotors, it is probably best to review propeller test data or do your own testing. Multirotors operate differently from airplanes because they mostly use static thrust to stay aloft. This means the intuiting way of determining your propeller that I explained doesn’t work for quadcopters.

A big reason why manufacturers often include propeller test data with their motors is to cater to this market of VTOL (vertical takeoff and landing) aircraft.

One last thing. There is a handy calculator for propeller sizes that you can use called WebOcalc (archive.is/IDDdd). It will give you a list of suggested propellers based on several aircraft parameters. This version does, however, require the use of metric units.

Anyway, that’s how I’ve been figuring out my motor/propeller combinations for several years. I hope you’ve found some of the information here useful for your future projects! Although you could get much more technical with analyzing the efficiencies between using a 10 x 4 or a 10 x 3.8 propeller, take it from this aerospace engineer in training—for park pilots, that isn’t really necessary.

-Lucas Weakley
lucas.weakley@gmail.com






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10 comments

Why back 'when'.....the 50s......airplanes and propellers were taught differently at Embry Riddle Aeronautical U. Years of model flying verified new designs until the 80s when computers did the thinking. This caused a radical change in prop design mainly because of the better plastic and composite materials. What never will change is the affects of the atmospheric conditions of air on wings and props. When props are static tested, the result of thrust produced at different RPMs has a problem.......the air in front of the prop is stagnant and has to be pulled to the prop....a model flying is moving into the air and can create additional thrust to increase real speed 10% greater then the RPM / pitch theory. Imagine a boat starting and the cavitation bubbles which are the same as the stall air flow on a static test.
Another phenomenon occurs when the prop tip is moving faster then 200 MPH....... air compressibility begins. If the prop had power to get it 700+MPH, then the phenomenon is the familiar MACH one conditions. For 10" ~ 14" props, the 200 MPH threshold begins at roughly 12,000 RPM. Smaller diameter props can go much higher RPM to be at 200 MPH.
Model airplanes are like playing golf for the hole in one goal......without all the walking. God, I still love this hobby.

Choosing the wrong pitch and/or diameter will lead to disappointment.

Slow, draggy airplanes need large diameter and low pitch.
Fast airplanes need shorter diameter and higher pitch.

Estimate (or measure) the normal flying speed of the model.
Compare to a basic "pitch speed" calculation: Pitch (inches) X RPM/ 1000 = MPH (its off a little, but close)

If pitch speed is 150% of airspeed in level flight, you are trying to beat the air into submission, and it does not work. Go lower pitch and probably larger diameter.

Target pitch speed for most sport aerobatic models is appx 115% of level flight airspeed.
Slow draggy models, target is appx 125%.
Pylon racers with low drag can approach maintaining level flight with airspeed = pitch speed.

EDF modes can have sustained level flight airspeed exceed calculated pitch speed of the fan due to the tapering of the "eflux" ducting.

Here is another link for Web O Calc. You can choose metric OR imperial.

https://web.archive.org/web/20161012020813/http://flbeagle.rchomepage.co...

Also remember to check your motor...Too much prop too much heat.

Several years ago, Bob Boucher of Astro-Flight fame, put out a booklet on fundamentals of Electric Flight. One section was devoted to propellor characteristics and selection. The charts included a broad selection of power, in watts, absorbed by various propellors. He also gave the basic equation from which they were established. The parameters are prop diameter and pitch, selected against rpm. This 'look-up' table allows one to determine how much power a prop uses in selected conditions.

Based on 80% efficient motors (most brushless motors), the power input is about 125% of the values noted, e.g., the power required by the battery and system to drive the motor/prop combination.

The general parameters in Mr. Weakley's article are spot-on, relative to flight characteristics and prop sizing. Mr. Boucher's charts then give you the power requirements for such a selection.

These calcs are NOT high level aerodynamic data; they just give you the general (but accurate) requirements for selecting a prop to drag your bird around the air. You still have to get a general assesment of power required, from general 'watts per pound' or 'watts per ounce' of flying airframe weight. There are a few more steps to optimized selection than space allows here, but 8-10 watts per ounce of flying weight will move you smartly!

There are two kinds of pitch, each may be measured in two different ways. Geometric pitch is the distance a propeller would screw itself through a solid medium in one revolution, like a screw going through wood. This is the pitch that is labeled on the prop. This is related to the angle the blade chords make to the plane of rotation. Aerodynamic pitch is the actual distance the propeller moves through the air in one revolution. Aerodynamic pitch is less than geometric pitch. The two are different because the blade chords must move along the helical path through the air at an attack angle to produce lift. The geometric pitch angle is greater than the aerodynamic pitch angle by the attack angle.

The ratio of aerodynamic pitch to geometric pitch is not the efficiency of the prop. The efficiency of a prop is the ratio of power out to power in. Power out is equal to thrust times velocity. Power in is torque times revolution rate. These are not the same.

A larger diameter propeller is more efficient than a smaller diameter propeller generating the same thrust because the larger diameter propeller is accelerating the air less over a larger disk area. Too large a diameter propeller will break on landing. A good rule of thumb is to make the propeller diameter equal to the square root of wing area. The wing area produces the lift that supports the airplane weight. The swept disk area generates the thrust that balances the drag of the airplane. The relationship is between the area of the wing and the area of the propeller swept disk.

Quad copters use rotors that are not propellers. They have a uniform blade angle along the radius. A propeller has a diminishing blade angle going from hub to tip. Helicopter rotor aerodynamics is very different from propeller aerodynamics.

A good introduction to propeller engineering is Chapter XI The Propeller in Richard von Mises Theory of Flight.

http://store.doverpublications.com/0486605418.html

Does the 25% of wingspan rule apply to bi-planes or is there a different percentage?

I learned some things I didn't know so certainly worth the time to read the article. Thank you!

Good article. will vbe saving it it for further reference.

Okay I know I need two props for a P38 and I know the diameter is limited by the space available between the engines and the nose. I'm making it even more fun by reducing the size of the plane to 66% of the original Ziroli plans. I do know the proposed wing length and area. I have no clue as to expected speed except that it's probably going to be fast. The engines I have on hand are OS 1.20 4-strokes. How do I calculate props? Rex

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