Propeller performance is affected by several factors, among them are diameter relative to RPM, and blade area relative to power absorption and pitch.

Diameter is the measurement (usually in inches) of the prop from tip to tip.

Pitch is defined as the theoretical advancement of a propeller in one revolution (usually measured in inches) and defines the speed and maneuverability characteristics of flying. For example, a 10x6 describes a diameter of 10" and a pitch of 6", or forward movement of 6 inches per revolution. In metric measurement this would be 30x15. Sometimes a particular series letter is used, such as 1260S (12" diameter, 6" pitch, Scimitar Series).

Now, more about pitch, which is the hardest part to visualize. Imagine you turn a 6" pitch prop in a tub of something like butter. It should advance 6" for each turn along its axis. Naturally, the faster you turn the prop, the more rapidly you will advance.

Model propellers have a practical limit on how fast they can turn (RPM) based on the power curve of the engine and the diameter of the prop. Another practical limit is due to noise considerations. Prop tip speed limits should be at 600 to 650 feet per second. Tip speed is explained below.

Slow speeds, aerobatics, great take-off's and landings can all be accomplished with low pitch propellers. Higher pitches lead to less maneuverable but faster flying. Because modelers do not have the luxury of variable pitch, most select a pitch based on how they like to fly or compromise speed and maneuverability with a pitch somewhere in the middle - around 65 -70% of the prop diameter. A limiting factor which will decrease propeller efficiency is engine horsepower and aircraft drag, i.e. a high pitch prop can't make an airplane any faster than it's capable of being and too low of pitch can result in lower power/thrust. It should be noted that industry standards are that pitch is measured at 75% of radius.

Thrust refers to the force exerted by the rotating propeller in the direction of travel of the airplane. This is the whole purpose of the propeller - to convert the power of the engine which appears as a rotating force, or torque, into a linear force to propel the airplane. Thrust is usually measured in pounds is a function of air density, rpm, diameter, advance ratio and Reynolds number. It is a long, complicated process to get this number, but what is important to remember is that thrust is different for every shape of propeller and changes with flying conditions.

Power Absorption refers to the power output curve of the engine. Power is the product of torque times rpm. As rpm increases, an engine produces less force (or torque) because the air/fuel mixture is not as efficient at higher rpm's. This is why a power curve becomes flat or decreases at higher rpm's, and means that the most efficient propeller is the one that allows the engine to operate at its optimum power band.

An interesting point in understanding power absorption is that propeller power varies as the cube of the rpm. Consequently, twice the rpm requires 8 times the power.

Tip Speed is measured in feet per second and a formula is provided below to find this measurement.

For model airplane purposes, the best tip speed for efficiency and noise requirements is 600 feet per second. This is due to compressibility losses and the fact that subsonic airfoils do not work well in transonic/sonic speeds with required sound levels.

Feet Per Second (ft/s) = RPM x diameter in inches x .00436

For example, to find the tip speed of a 10x6 on a .40 size engine running at 13,500 RPM, the equation would be 13,500 x 10 x .00436 = 588.6 ft/s.

To find the correct diameter at 600 ft/s, use this formula:

Diameter in inches = 138,000 / RPM

Using a .40 engine running at 13,500 RPM, the equation would read as follows:

138,000/13,500 = 10.22

Rounding down, the correct diameter is 10"

For both of the above formulas, use RPM for the optimum power band of your engine. Consult your owner's manual if you do not know this number.

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