R/C Propeller Size Basics
To an R/C novice trying to make sense of the hobby, a list of 10x5, 9x6 and 8x4 propellers looks a bit like, well, a multiplication table pop quiz. This is unfortunate because the two numbers tell you most of what you need to know about a propeller.
The first number is the diameter of the circle a propeller makes as it rotates, usually in inches. Each propeller blade is, first and foremost, just a rotating wing that generates lift (Some call a propeller an airscrew). But where the wings on the side of the plane lift vertically, a propeller lifts horizontally, converting an engine/motor’s torque into the thrust needed for forward motion.
The second number describes the propeller’s pitch, or the distance it theoretically moves through the air in one rotation. Thus, 8x4 is an 8-inch diameter propeller that moves 4 inches through the air in one revolution. What determines pitch? If you look at a propeller, you can see that it is not simply flat and angled, but twisted along its length. This shaping has a good reason: Because the propeller tip moves considerably faster than closer-in parts of the blade, this twist allows a propeller to maintain a correct angle of attack along its length and “bite” into the air efficiently. A relatively flat-looking propeller has a low pitch. Pretty simple, right?
To an R/C engine, the matter is simpler still: A propeller is simply a load. The higher the load, the more work it has to do. The load presented by a propeller can be calculated by the square of the diameter multiplied by the pitch. For example, an 8x4 propeller presents a load of 256 (8x8x4=256). Increasing diameter a single inch – moving to a 9x4 propeller increases load to 324 (9x9x4=324), a not-so-trivial increase. Increasing load causes an engine to work harder, which drops engine revolutions per minute (RPM). Decreasing load raises engine RPM.
When it comes to propellers, engines know what they like. That’s because engines operate most efficiently and produce the most horsepower at a particular operating speed. The manufacturer will recommend propellers that make use of the engine’s power band. If the engine is asked to swing too large a prop, revs won’t get high enough -- if the plane can even get in the air, performance will suffer. By the same token, too little prop does an engine no favors. The under-loaded engine will over-rev, causing overheating and possibly engine damage.
So, there you have it: Propellers are not complicated or arcane. Most of what you need to know is right up-front. Happy flying.