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 with accuracy, since props with the same diameter and pitch often have different blade shapes and areas and also may be more or less flexible depending on the brand and on the type. So, 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 of the prop's blade. 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 onerevolution 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, so it is typically specified at 75% from the center of the blade. During constant level flight speed, the thrust force is equal and opposite to drag. In order to get reasonable climb and acceleration capabilities, the Static Thrust should be at least about 1/3 of the airplanes' weight. A static thrust to weight ratio greater than 0.5:1 is needed so that the airplane would be able to take-off the ground. However, thrust alone is not enough to guarantee the plane to fly, since other factors such as the prop pitch speed must also to be taken into account. The Static pitch speed is equal to the RPM times the prop's pitch. The Static RPM is less than the RPM during level flight (not hovering). Unless it's a glider, the adequate static pitch speed should be greater than 2.5 times the plane's stallspeed. 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 of a car) Smaller diameter & more pitch = less thrust, more top speed. (like the high gear of 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 85 W to produce 27 oz of thrust at 6000 RPM. To produce the same thrust, a 6x5 prop needs about 195 W at 18000 RPM. You may estimate the power needed if you know the static pitch speed and the thrust you need. | ||||
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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 and 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. The prop with P/D ratio of 1/2 yields higher thrust at low airspeed, but gives lower top airspeed. Whereas the prop with P/D ratio of 1/1 yields lower thrust at low airspeed, but gives higher top speed. |
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