Rotary-wing aircraft, or rotorcraft, is heavier-than-air aircraft with rotary wings or rotor blades that produce lift by rotating around a vertical mast. The most well-known example of such vessels are helicopters, which have become a vital part of naval aviation. The helicopter has many military applications, such as anti-submarine warfare (ASW) and search & rescue functions, as well as minesweeping and amphibious warfare functions.
Helicopters offer a myriad of advantages over conventional aircraft, including the fact that lift and control are relatively independent of forward speed. They have the ability to fly forward, backward, sideways, or hover above the ground. Furthermore, helicopters do not necessitate runways for takeoffs or landings, and they can easily take flight from the deck of a small ship or open fields.
Rotary-wing aircraft adhere to the same aerodynamic principles as fixed-wing aircraft. However, the main difference between the two aircraft types is the way lift is generated. While fixed-wing aircraft acquire lift from a fixed airfoil face, rotary-wing aircraft get lift from rotating airfoils called rotor blades. Furthermore, a helicopter utilizes one or more engine-driven rotors, where lift and propulsion are obtained.
How Flight Is Achieved
A helicopter’s main rotor consists of two or more rotor blades, and the airfoils are perfectly symmetrical, meaning that the upper and lower surfaces are alike. This symmetry is one of the major differences between a fixed-wing aircraft’s airfoil and a helicopter’s airfoil. Airfoils on fixed-wing aircraft have greater camber on the upper surface than on the lower surface, while airfoils on helicopters have the same amount of camber on both surfaces.
Helicopters have symmetrical airfoils because the center of pressure should not move. Meanwhile, on fixed-wing airfoils, the center of pressure moves forward and backward along the chord line. It is important to note that the center of pressure changes with shifts in the angle of attack. By contrast, if this type of airfoil was on a rotary-wing aircraft, it would force the rotor blades to jerk uncontrollably. As such, symmetrical airfoils do not produce this undesirable effect. When rotated, the airfoil travels smoothly through the air.
Theories of Flight
Rotor lift can be outlined by one of two theories. The first theory utilizes Newton’s law of momentum. Generally, lift comes from accelerating a mass of air downward, which is similar to jet thrust that develops by accelerating a mass of air out of the exhaust.
The second theory is the blade element theory. This theory explains that the airflow over the airfoil section, or blade element, of the rotor blade acts the same as it does on fixed-wing aircraft. Keep in mind that the simple momentum theory determines only the lift characteristics, and the blade element theory provides both lift and drag characteristics.
How Lift Is Generated
Normally, lift changes by increasing the angle of attack or pitch of the rotor blades. This action generates ample lift to raise the helicopter off the ground and keep it hovering. When the rotor is turning on a helicopter, the blades are at a zero angle of attack, meaning that no lift is developed. This allows pilots to have complete control of the lift generated by the rotor blades. One major assumption regarding lift is that it depends on the entire area of the rotor disc. The rotor disc area is equal to the length of the rotor blades.
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