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Helicopters often used to feature 2 main rotor blades, but over time, that number has grown significantly, with some helicopters sporting up to 8 blades. Typically, as a helicopter increases in size, it weighs more, meaning it requires more lift to remain aloft. Using additional blades allows designers to increase the entire rotor system surface area while keeping the size of each rotor blade as small as possible. Read on to learn why some helicopters only have two main rotor blades, while others have many more.
In retrospect, the main rotor blades of a helicopter are designed to lift the vehicle. Constructing a helicopter involves the consideration of a number of factors, such as maintaining low costs, using as few parts as possible, and keeping the weight of the rotor system low. Moreover, it is important to minimize noise, vibration, and drag created by the blades to ensure a smooth ride. As rotor blades grow in size, they produce additional lift, but more material also leads to additional weight, costs, drag, and power requirements. As rotor blades become longer, they droop more when not in rotation, and this centrifugal force will cause their tips to travel more quickly.
Rotor blades have aerodynamic limitations to their design, including blade tip vortices, wake turbulence, static-blade droop, centrifugal loading, and vibration. To start, rotors must spin at an optimal RPM to create the most lift with the least amount of power. The blade tips travel faster as the blade lengths elongate, causing the helicopter itself to increase in speed. This is because, as air meets the rotor blades on the advancing side of the vehicle, it creates its own forward momentum, in addition to the rotational speed of the rotor blade. As the Advancing Rotor Blade (ARB) approaches the speed of sound, it begins to break the sound barrier, creating shockwaves and a large amount of drag. As a result, the lifting power of the helicopter reduces, increasing sound and vibration. To overcome this issue, engineers must design a rotor blade tip that mitigates shockwaves; however, one can also simply reduce blade length or speed.
Another aerodynamic limitation helicopter blades face is wake turbulence. In order for a rotor blade to best function, undisturbed air must be passing over it, meaning that any irregular airflow will reduce the lift it is able to produce. As each rotor blade moves through the air, it creates turbulence that may not be able to be replaced with smooth, undisturbed air before the next blade approaches it. To overcome this on large, engineers of multi-bladed helicopters make changes to the plane in which half the blades rotate in. This results in one blade rotating on a lower plane, while the next blade is on a higher plane, and this pattern will repeat in order to provide more undisturbed air for each blade to work on.
Static blade droop occurs with longer blades that are at rest. While in flight, centrifugal force keeps the rotor blade flat, but it can be difficult to get the blades traveling up to that speed. When the blades are rotating slowly, there is a strong possibility that they will come into contact with the tail boom, causing serious structural damage. For these reasons, many helicopter manufacturers will issue a maximum wind speed for stopping and starting particular rotors.
An additional aerodynamic limitation is centrifugal loading. This means that, as the rotors grow in size, they also grow in mass. As this mass begins to rotate, centrifugal force will grow, inflicting constant stress on the rotor blades and head. In order to prevent these forces from removing the blades, the blade root, grip, and rotor head must be made of strong, durable materials. Finally, rotor blades cause a great deal of vibration, and the larger the blade, the lower the vibration frequency will be, creating a bouncing sensation that can be felt on board the aircraft. To avoid a situation where destructive ground resonance occurs, designers use weights throughout the rotor head and airframe to counteract those vibrations.
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