SL Helicopter Flying Handbook/Introduction to the Helicopter
SECTION 1. Introduction to the Helicopter
A helicopter is an aircraft that is lifted and propelled by one or more rotors, each consisting of two or more rotor blades. Helicopters are one of two classes of rotorcraft along with gyrocopters. Unlike fixed-wing aircraft, helicopters are capable of vertical flight giving them great versatility to fly into small confined areas that are not accessible to most other aircraft.
While early prototype helicopters were built early in the 20th century, Igor Sikorsky is credited with producing the first production helicopter, the R-4 in 1942. It was the R-4 that set the standard of using one main rotor and a tail rotor in helicopter design.
The unique capabilities of the helicopter -- specifically its ability to takeoff and land vertically, and to hover for an extended period of time -- enable the helicopter to perform tasks no other aircraft can handle. Helicopters have found use in transportation, construction, firefighting and search and rescue.
2 Rotor System
The rotor system is the rotating part of the helicopter that generates lift. A rotor system may be mounted horizontally, to produce vertical lift as in the main rotor; or vertically, as in a tail rotor, to counter torque from the main rotor or provide yaw control of the aircraft. Helicopters come in a variety of rotor configurations as described in the following sections.
2.1 Single Main Rotor
Most helicopters use a single main rotor with a single tail rotor to counteract thrust. The tail rotor is required because the main rotor produce a torque, or twisting force, as it rotates under power which causes the aircraft to want to turn in the opposite direction of the blades. The tail rotor is usually mounted on a boom of some sort to position it out from under the main rotor. The tail rotor produces sideways thrust that counter-acts the torque from the main rotor.
For most US-made helicopters, the main rotor turns counter-clockwise when viewed from above, but Russian-made helicopters and some European helicopters have clockwise rotating rotor systems. Throughout this manual, we will assume counter-clockwise rotating blades which produce a torque making the helicopter tend to yaw right. For helicopters with clockwise rotating rotor systems, this yaw, and thus the actions needed to counteract it, would be reversed.
2.2 Tandem Rotor
Tandem rotor helicopters have two main counter-rotating main rotor systems, usually mounted one in front of the other. Since the rotors turn in opposite directions, the torque from each rotor cancels each other, thus eliminating the need for a tail rotor. This allows all of the engine power to be used to generate thrust, unlike a standard single-rotor design where some of the power must be diverted to drive the tail rotor. Yaw control in a tandem rotor helicopter is achieved by vectoring the front and back rotors in opposite directions.
2.3 Coaxial Rotor
A coaxial rotor helicopter has counter-rotating blades stacked on top of each other. Like the tandem system, the torque from each rotor cancels the other eliminating the need for a tail rotor. Yaw control in a coaxial helicopter can be achieved by altering the amount of collective pitch in the upper versus the lower rotor system. This causes the torque from the two systems to become unbalanced allowing the helicopter to turn.
2.4 Intermeshing Rotor
In an intermeshing design, the two counter-rotating rotors are mounted in such a way that the rotor blades intermesh with each other, often in a side-by-side configuration. The blades are usually mounted at a slight angle relative to each other, and are synchronized so as to prevent the blades from colliding with each other.
3 Controlling Flight
A helicopter has four primary flight controls:
- Antitorque pedals
The cyclic is a stick that can be moved like a joystick forward and back, and left and right to control the direction of thrust from the main rotor. The cyclic is usually a stick mounted on the floor between the pilot's legs, though some helicopters such as the Robinson models use a T-stick cyclic mounted in the center of the cabin with a T-bar on the top.
The control is called the cyclic because it varies the pitch of the rotor blades as a function of their angle with the rotor mast over the course of a revolution. This results in an unequal lift/thrust from the blade is it rotates causing the rotor disk to tilt in the direction the pilot pushes the cyclic. When the rotor disk tilts, it diverts some of its total thrust to a horizontal component giving the helicopter a thrust in the direction the cyclic was pushed.
The collective is located to the left of the pilot, and is usually a lever that can be raised and lowered. The collective increases the pitch of the blades "collectively" around the entire rotation of the blades thus increasing the total thrust developed by the rotor system. The collective is usually used to control altitude in a helicopter.
3.3 Antitorque Pedals
Antitorque Pedals are located on the floor in the same position rudder pedals would be in a fixed-wing aircraft. They are typically controlled with the feet and move together, pushing one causes the other to move forward and vice-versa. In a standard single main rotor helicopter, the anti-torque pedals control the pitch of the tail rotor, and thus the thrust from the tail rotor. This causes the helicopter to yaw left or right in the direction on which the pedal was pressed.
Helicopters are designed to operate at a specific RPM, or a narrow range of RPMs. As the pilot increases or decreases the collective, the amount of engine power needed to maintain the target RPM will increase or decrease. In single engine helicopters, the throttle is usually a twist grip mounted on the collective. As the pilot increases collective, they simultaneously roll on throttle to increase engine power. When the pilot decreases the collective, they roll off throttle to prevent the rotors from overspeeding.
Many helicopters have systems to assist the pilot in controlling the throttle as described below.
A correlator is a mechanical linkage that automatically adds throttle when the collective is increased, and reduces throttle when the collective is lowered. The adjustments from a correlator are usually approximate and must be fine tuned by the pilot.
A governor an electronic device that measures the rotor RPM, and increases or decreases the throttle through an electric servo to maintain a specific RPM. In helicopters equipped with a governor, the governor can be engaged through a switch, or may come on automatically when the RPM enters a specific range. In all cases, it is possible for the pilot to manually override the governor by manually moving the throttle.
4 Flight Conditions
There are two basic flight conditions for a helicopter: hovering and forward flight. Hovering is the most difficult skill to be mastered when learning to fly a helicopter. This is because the helicopter is inherently unstable, and the pilot must give continuous control inputs in order to maintain control. Furthermore, the beginning pilot has a tendency to overcorrect, which usually results in the aircraft becoming more and more erratic.
When in hovering flight, the pilot uses the cyclic to control horizontal drift, the collective to control altitude, and the pedals to control yaw. When in forward flight, the pilot uses forward and backward cyclic for airspeed, left and right cyclic to turn, the collective for altitude, and the pedals to maintain coordination, similar to how the rudder is used in an airplane.