![]() ![]() ![]() The leading edge is the first part of the airfoil to meet the oncoming air. This is necessary because rotational velocity increases toward the blade tip. Twisting a rotor blade causes it to produce a more even amount of lift along its span. Blade twist refers to a changing chord line from the blade root to the tip. The blade span is the distance from the rotor hub to the blade tip. Since the angle of attack of the rotor blades is constantly changing during each cycle of rotation, the blades tend to flap, feather, lead, and lag to a greater degree. If it moves ahead of the pivot point, the pitch of the rotor disc decreases. As the angle of attack increases, the center of pressure moves forward. When the center of pressure lifting force is behind the pivot point on a rotor blade, it tends to cause the rotor disc to pitch up. One of the reasons an asymmetrical rotor blade is not as stable is that the center of pressure changes with changes in angle of attack. Using this type of rotor blade allows the rotor system to operate at higher forward speeds. Normally these airfoils would not be as stable, but this can be corrected through reflexing bending the trailing edge to produce the same characteristics as symmetrical airfoils. In addition, airfoils are asymmetrical in design, meaning the upper and lower surface do not have the same camber. Modern design use thinner airfoils and obtain the required rigidity by using composite materials. Center of pressure is the imaginary point on the chord line where the resultant of all aerodynamic forces is considered to be concentrated. This stability is achieved by keeping the center of pressure virtually unchanged as the angle of attack changes. Symmetrical blades are very stable, which helps keep blade twisting and flight control loads to a minimum. The airfoils were also designed to be symmetrical, which means they had the same camber (curvature) on both the upper and lower surfaces. This prevented excessive blade droop when the rotor system was idle, and minimized blade twisting while in flight. Because the rotor blades were very long and slender, it was necessary to incorporate more structural rigidity into them. Although there are many different rotor blade airfoil designs, in most helicopter flight conditions, all airfoils perform in the same manner.Įarly helicopter designs used relatively thick airfoils for their structural characteristics. An airfoil is any surface, such as an airplane wing or a helicopter rotor blade, which generates aerodynamic force when it interacts with a moving stream of air. ![]()
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