The way that we name and categorize our propellers is hopelessly broken. Here is a simple fix.
Propellers on both full-size and model airplanes have always used the same marking system, as far as I know. It must have originated in the early days of aviation. Everybody knows that you should not mess with a good thing. A system that has been in place for so many years must be a good one, right?
Actually, we are surrounded by inefficient systems that are in desperate need of replacement. The layout of our keyboards, called QWERTY, was originally designed to slow down typists. Talk about a system begging to be replaced! Our U.S. Customary unit system is confusing to use and causes international trade to be slower and more expensive.
Problem is, both of these systems have been in place for so long, the cost of replacing them is huge. There is just an enormous amount of inertia in place. For example, once you have learned to touch type on a keyboard, you do not want to have to learn again with an entirely different key layout.
Fortunately, I do not believe that fixing the naming standard for propellers needs to be a painful process. As you will see in a moment, my proposed transitional labeling would still be very easy to understand by those unfamiliar with the new system.
The first number printed on a propeller is always the diameter. It is anyone’s guess whether the number is in inches or centimeters. An inch is roughly two and a half centimeters, so if the number looks unusually large, then it must be in centimeters. If you live in a country that uses a sane units of measurement system, then if the number looks unusually small it must be in inches. A lot of model airplane propellers now come with both inches and centimeters printed on them, so in practice I have not found this to be a problem.
The second number printed on propellers is the pitch. The diameter and the pitch can be separated by either a “x” or “-” symbol. This is the problematic number.
The pitch is the number of inches (or centimeters) that the propeller would move forward if it made one complete revolution in a solid material like soft butter. Explained like this it does not sound so bad, does it?
The problem is that, measured this way, the pitch value makes it hard for the end user to reason about how the propeller is going to behave. It also makes it very hard to compare the pitch values across propellers of different diameters.
Blade Angle Proposal
A far more intuitive way to label the pitch of a propeller blade is by using the angle of attack of the blade measured in degrees. Even though the SI unit of angle measurement is the radian, the degree is an acceptable SI unit. The blade angle should be measured from the zero lift angle and not from the geometric zero angle line.
Since propellers are twisted, measure the angle at the 75% radius point. This point is a great facsimile for how the propeller as a whole behaves.
To avoid confusion, use a different symbol to label the angle. For the sake of argument, let’s use the “@” symbol, since it is very different from what is being used today.
To help with the transition, print both the pitch and the blade angle on the propellers. So a 12×4 propeller would become a 12×4@8. Eventually this could be simplified to just 12@8.
Let’s try a simple quiz to prove my point. Are the following statements true or false?
- A 12×8, 8×5, and a 16×10 propeller all have the same pitch angle.
- A 12×6 propeller has a 12 degree blade pitch angle.
- A 17×7 propeller has a 10 degree blade pitch angle.
- An 11×8 prop will probably be stalled if the airspeed is zero.
- Are a 4×2.5 and a 4.5×3 propeller roughly equivalent in pitch?
Not so easy, you say? Here is another version of the same quiz with the blade angles added:
- A 12×8@16, 8×5@16, and a 16×10@16 propeller all have the same pitch angle.
- A 12×6@12 propeller has a 12 degree blade pitch angle.
- A 17×7@10 propeller has a 10 degree blade pitch angle.
- An 11×8@17 prop will probably be stalled if the airspeed is zero.
- Are a 4×2.5@15 and a 4.5×3@16 propeller roughly equivalent in pitch?
No problem telling that all the statements are true in this second version? Exactly.
Computing the blade angle from the pitch value is easy, but you do need a calculator with trigonometric functions. Here is the formula: arc tan(pitch/(diameter*Pi*0.75)). The arc tan is the inverse tangent function, sometimes labeled tan-1.
I do not expect the propeller manufacturers to start labeling their propellers this way overnight. After all, it takes almost no work on their part to do so. It would also only benefit their customers. Better let a competitor start doing it first and then play catch-up in a hurry after you start losing customers. Until then, just ignore the complaints from your customers.