The helicopter as we know it today is a complex aircraft capable of flight maneuvers of hover, vertical, forward, backward, and sideward flight. In spite of the fact that the helicopter is capable of maneuvers that are not possible for fixed wing aircraft, it still operates on of the same basic principles.
The need for maintenance personnel to study these principles may not be apparent at first. However, it is very necessary because a thorough knowledge will be required to understand the maintenance and troubleshooting practices of the various systems.
Like fixed wing aircraft, the helicopter flies because of its airfoils. The airfoils of the fixed wings are primarily their wings. However, the tail surfaces amd sometimes the fuselage, as well as the propeller may also be airfoils. The primary airfoil of the helicopter is the MAIN ROTOR. For this reason, the helicopter is often referred to as a ROTARY WING aircraft. An airfoil, by definition, is any surface which gets a useful dynamic reaction from the air.
For our purposes, this reaction is the lift and thrust which will be necessary for flight and maneuvering.
AERODYNAMIC PRINCIPLES
The blades of the main rotor are the airfoils. With theses airfoils certain nomenclature is used.
The span of the blade is the distance from the root of the blade to the tip of the blade, measured along the center line.
If a cross section of the blade is shown, it may have an imaginary line drawn from the leading edge to the trailing edge. This line is referred to as the chord of the blade.
The shape of the airfoil section may take many different forms. This shape actually affects the flight characteristics of the aircraft. Certain airfoils are noted for high speed, while others are known for low speed, high lift, and supersonic characteristics.
The airfoils which are used for helicopters are usually referred to as symmetrical airfoils, meaning that the airfoil section has the same shape above and below the chord line. This curvature of the airfoil is referred to as the camber. Some successful designs have been built with an unsymmetrical airfoil, meaning that the top and bottom camber are not the same shape.
Some efforts are being made to change the airfoil shape along the span to achieve better flight characteristic in the blade.
1. Relative Wind
As the rotor blade moves, it is subjected to relative wind. The relative wind is the direction of the airflow with respect to the blade. This is always opposite the flight path of the blade. For example, if the blade moves forward horizontally, the relative wind moves backward horizontally. If the blade moves backward horizontally, the relative wind moves forward horizontally. If the blade moves forward and upward, the relative wind moves backward and downward. If the blade moves backward and downward, the relative wind moves forward and upward.
At first, one might wonder how the blade can move backwards. It must be remembered that this is in relation to the nose of the helicopter. For this reason, the forward moving blade is referred to as the ADVANCING BLADE, while the backward blade is called the RETREATING BLADE. The relative wind may be affected by several factors such as movement of the rotor blades, horizontal movement of the helicopter, flapping of the rotor blade, wind speed, and direction. The relative wind of the helicopter is the flow of air with respect to the rotor blade. For example; when the rotor is stopped, the wind blowing over the rotor blades creates a relative wind. When the helicopter is hovering in a no-wind condition, the relative wind is created by the motion of the rotor blades. If the helicopter is hovering in a wind,the relative wind is a combination of the wind and the rotor blade movement. When the helicopter is in forward flight, the relative wind is created by the rotor blades, the movement of the helicopter, and possibly a wind factor.
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