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Some notes: A tachometer is needed in order to measure the prop's RPM. The propeller converts the engine's torque force into thrust force. It's rather difficult to predict the thrust produced by a propeller, as props with the same diameter and pitch often have different blade shapes and sizes and also may be more or less flexible depending on the brand and type. The results here are therefore only approximate. The thrust produced depends on the density of the air, on the propeller's RPM, on its diameter, on the shape and area of the blades and on its pitch. Geometric Pitch is the distance an element of a prop should advance in one revolution if there was no slip. Mean Geometric Pitch is the mean of the geometric pitches of the several elements. Slip is the difference between the prop's Mean Geometric Pitch and the actual pitch, which is called Effective Pitch. Virtual Pitch is the distance a propeller would have to advance in one revolution in order that might be no thrust. Pitch Speed is the Mean Geometric Pitch times RPM, which means the speed the aircraft would make if there was no slip. The Virtual Pitch Speed is usually 20 to 30% higher than the Pitch Speed. Often the pitch angle is not constant along the prop's blade and is typically specified at 75% from the center of the blade. During constant level flight speed, the thrust force is equal and opposite to the drag. Static thrust should be at least about 1/3 of the planes' weight in order to get reasonable climb and acceleration capabilities. A static thrust to weight ratio greater than 0.4:1 is needed to allow the plane to take-off from the grass. However, thrust alone is not enough to guarantee whether the plane will fly, as other factors such as the prop pitch speed also has to be taken into account. Static pitch speed is equal to the RPM times the prop's pitch. The static RPM is less than the RPM in flight. Unless it's a glider, the adequate static pitch speed should be greater than 2.5 times the plane's stall speed. Prop's Output Power = Thrust x Pitch speed Thus, with a given power, the more thrust you have, the less top speed you get. Assuming the same power: Larger diameter & less pitch = more thrust, less top speed. (like the low gear in a car) Smaller diameter & more pitch = less thrust, more top speed. (like the high gear in a car) A smaller prop requires more power to produce the same thrust as a larger one. For instance, a 12x7 Aeronaut prop takes about 85W to produce 27 oz of thrust at 6000 RPM. To produce the same thrust a 6x5 prop needs about 195W at 18000 RPM. You may estimate the power needed if you know the static pitch speed and the thrust you need. | ||
The recommended prop P/D (Pitch/Diameter) ratio for sport models is 1:2 to 1:1. With a too large pitch, the prop becomes inefficient at low forward speed and high rpm, as when during the take-off and/or climb. Whereas a propeller designed for greatest efficiency at take-off and climb (with fine pitch & large diameter) will accelerate the plane very quickly from standstill but will give less top speed. The graph below shows Thrust & Drag vs Speed for 3 props with different P/D ratios. The plane reaches max level flight speed when the Thrust becomes equal to Drag. 
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