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Helicopter Aerodynamics The helicopter, as we know it today, falls under the classification known as rotorcraft. Rotorcraft are also known as rotary wing aircraft, because instead of their wing being fixed like it is on an airplane, the wing rotates. The rotating wing of a rotorcraft can be thought of as a lift producing device, like the wing of an airplane, or as a thrust producing device, like the propeller on a piston engine. Helicopter Structures and Airfoils The main parts that make up a helicopter are the cabin, landing gear, tail boom, powerplant, transmission, main rotor, and tail rotor. [Figure 3-81]
Main Rotor Systems The classification of main rotor systems is based on how the blades move relative to the main rotor hub. The principal classifications are known as fully articulated, semi-rigid, and rigid. In the fully articulated rotor system, the blades are attached to the hub with multiple hinges. The blades are hinged in a way that allows them to move up and down and fore and aft, and bearings provide for motion around the pitch change axis. Rotor systems using this type of arrangement typically have three or more blades. The hinge that allows the blades to move up and down is called the flap hinge, and movement around this hinge is called flap. The hinge that allows the blades to move fore and aft is called a drag or lag hinge. Movement around this hinge is called dragging, lead/lag, or hunting. These hinges and their associated movement are shown in Figure 3-82.
The main rotor head of a Eurocopter model 725 is shown in Figure 3-83, with the drag hinge and pitch change rods visible.
The semi-rigid rotor system is used with a two blade main rotor. The blades are rigidly attached to the hub, with the hub and blades able to teeter like a seesaw. The teetering action allows the blades to flap, with one blade dropping down while the other blade rises. The blades are able to change pitch independently of each other. Figure 3-84 shows a Bell Jet Ranger helicopter in flight. This helicopter uses a semi-rigid rotor system, which is evident because of the way the rotor is tilted forward when the helicopter is in forward flight.
With a rigid rotor system, the blades are not hinged for movement up and down (flapping) or for movement fore and aft (drag). The blades are able to move around the pitch change axis, with each blade being able to independently change its blade angle. The rigid rotor system uses blades that are very strong and yet flexible. They are flexible enough to bend when they need to, without the use of hinges or a teetering rotor, to compensate for the uneven lift that occurs in forward flight. The Eurocopter model 135 uses a rigid rotor system. [Figure 3-85]
Anti-Torque Systems Any time a force is applied to make an object rotate, there will be an equal force acting in the opposite direction. If the helicopter’s main rotor system rotates clockwise when viewed from the top, the helicopter will try to rotate counterclockwise. Earlier in this chapter, it was discovered that torque is what tries to make something rotate. For this reason, a helicopter uses what is called an anti-torque system to counteract the force trying to make it rotate. One method that is used on a helicopter to counteract torque is to place a spinning set of blades at the end of the tail boom. These blades are called a tail rotor or anti-torque rotor, and their purpose is to create a force (thrust) that acts in the opposite direction of the way the helicopter is trying to rotate. The tail rotor force, in pounds, multiplied by the distance from the tail rotor to the main rotor, in feet, creates a torque in pound-feet that counteracts the main rotor torque. Figure 3-86 shows a three bladed tail rotor on an Aerospatiale AS-315B helicopter. This tail rotor has open tipped blades that are variable pitch, and the helicopter’s anti-torque pedals (positioned like rudder pedals on an airplane) control the amount of thrust they create. Some potential problems with this tail rotor system are as follows:
An alternative to the tail rotor seen in Figure 3-86 is a type of anti-torque rotor known as a fenestron, or “fan-in-tail" design as seen in Figure 3-87. Because the rotating blades in this design are enclosed in a shroud, they present less of a hazard to personnel on the ground and they create less drag in flight.
A third method of counteracting the torque of the helicopter’s main rotor is a technique called the “no tail rotor" system, or NOTAR. This system uses a high volume of air at low pressure, which comes from a fan driven by the helicopter’s engine. The fan forces air into the tail boom, where a portion of it exits out of slots on the right side of the boom and, in conjunction with the main rotor downwash, creates a phenomenon called the “Coanda effect." The air coming out of the slots on the right side of the boom causes a higher velocity, and therefore lower pressure, on that side of the boom. The higher pressure on the left side of the boom creates the primary force that counteracts the torque of the main rotor. The remainder of the air travels back to a controllable rotating nozzle in the helicopter’s tail. The air exits the nozzle at a high velocity, and creates an additional force (thrust) that helps counteracts the torque of the main rotor. A NOTAR system is shown in Figures 3-88 and 3-89.
For helicopters with two main rotors, such as the Chinook that has a main rotor at each end, no anti-torque rotor is needed. For this type of helicopter, the two main rotors turn in opposite directions, and each one cancels out the torque of the other. |
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