The choice of propeller in aeromodelling aircraft is actually a flexible thing and usually requires trial and error and experience in the field. In this article the general concept will be explained in determining the right propeller for aeromodelling aircraft. It can be said, the selection of propellers is very important in determining the flight performance of the aircraft, even the worst possibility that might occur is damage to aircraft components, this can especially occur in electric aircraft. The most appropriate propeller selection is following the recommendations of the motor / engine provided by maker of these components, but an understanding of the propeller concept is important for aeromodellers.
The working principle of the propeller is actually identical to the wing, that is by utilizing an airfoil that moves in a rotating manner so as to produce an aerodynamic force (similar to an elevator on a wing) which is called thrust or thrust. The rotating motion of the propeller results in airfoil motion speeds at the tip and base of the propeller being different, therefore the angle of attack (AOA) of the propeller blade from base to end is made smaller, so the force generated is the same (the higher the airfoil speed, the greater the force produced )
Then, two important things that must be known in choosing a propeller are diameter and pitch. Diameter, as the name suggests is the diameter of the propeller when rotating, or it can be said to be the length of the propeller from end to end. Then, pitch is a measure of how much distance the propeller / plane travels will move in the air in one round of propeller in inches. The value of this pitch is only a theoretical number, because in real conditions, there are a lot of factors that affect the distance traveled by each propeller round, such as propeller material, efficiency, air condition and others. The greater the pitch value, the faster the plane will move.
Then the diameter of the propeller generally affects the thrust produced and affects the RPM of the engine, besides that the propeller diameter also greatly affects the noise that is driven by the propeller, sometimes the propeller is more noisy than the engine. The large diameter of the propeller reduces the rotational speed (RPM) of the engine because it requires large power, because of the decrease in RPM, the large diameter propeller is less noisy than a small propeller.
Meanwhile, the pitch and diameter of the propeller are usually written on the propeller in the form of an x pitch diameter. Suppose the propeller with the writing 10 × 8 indicates that the propeller has a 10 inch diamater and an 8 inch pitch.
In aircraft with engine / combustion engines, the following are examples of propeller selection based on charts taken from top flight. In the following example, the A.90 engine is selected so based on the graph, propellers are selected with specifications between 13 × 6 to 15 × 8.
As for electric aircraft, propeller selection is very important. On engine aircraft, the wrong propeller selection only causes the engine to stall and reduce performance, whereas in electric aircraft, the wrong propeller will not make a stall machine, which forces the motor to continue working until perhaps ESC can overheat. Smaller propellers will be safer for electronic components but of course the performance will be less. The best thing to do is to follow the recommendations of the motorbike maker. Theoretically, the selection of propellers can be done using the following equations and graphs:
The issue that is often considered in choosing a propeller is the number of blades. In general, there are two aeromodelling aircraft propeller blades. However, a propeller can be found with three to four blades. In general, the more bars, the propeller will be more inefficient because the turbulent produced is greater. In theory, the most efficient propeller is a propeller with one blade, but the propeller cannot be stable when spinning, unless given a ballast. The propeller with more than two blades is used for scale aircraft so that it gets the perfect scale form, and with more and more blades in general can be chosen smaller diameter and pitch. However, the issue that is most often considered is the ground effect and the height of the propeller to the land which limits the size of the propeller.