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Helicopter Axes of Flight – Control Around the Longitudinal and Lateral Axes

Filed Under: Physics

Movement around the longitudinal and lateral axes is handled by the helicopter’s main rotor. In the cockpit, there are two levers that control the main rotor, known as the collective and cyclic pitch controls. The collective pitch lever is on the side of the pilot’s seat, and the cyclic pitch lever is at the front of the seat in the middle. [Figure 3-91]

Figure 3-91. Helicopter cockpit controls.
Figure 3-91. Helicopter cockpit controls.

When the collective pitch control lever is raised, the blade angle of all the rotor blades increases uniformly and they create the lift that allows the helicopter to take off vertically. The grip on the end of the collective pitch control is the throttle for the engine, which is rotated to increase engine power as the lever is raised. On many helicopters, the throttle automatically rotates and increases engine power as the collective lever is raised. The collective pitch lever may have adjustable friction built into it, so the pilot does not have to hold upward pressure on it during flight.

Figure 3-82. Fully articulated main rotor head.
Figure 3-82. Fully articulated main rotor head.

The cyclic pitch control lever, like the yoke of an airplane, can be pulled back or pushed forward, and can be moved left and right. When the cyclic pitch lever is pushed forward, the rotor blades create more lift as they pass through the back half of their rotation and less lift as they pass through the front half. The difference in lift is caused by changing the blade angle (pitch) of the rotor blades. The pitch change rods that were seen earlier, in Figures 3-82 and 3-83, are controlled by the cyclic pitch lever and they are what change the pitch of the rotor blades. The increased lift in the back either causes the main rotor to tilt forward, the nose of the helicopter to tilt downward, or both. The end result is the helicopter moves in the forward direction. If the cyclic pitch lever is pulled back, the rotor blade lift will be greater in the front and the helicopter will back up.

Figure 3-83. Eurocopter 725 main rotor head.
Figure 3-83. Eurocopter 725 main rotor head.

If the cyclic pitch lever is moved to the left or the right, the helicopter will bank left or bank right. For the helicopter to bank to the right, the main rotor blades must create more lift as they pass by the left side of the helicopter. Just the opposite is true if the helicopter is banking to the left. By creating more lift in the back than in the front, and more lift on the left than on the right, the helicopter can be in forward flight and banking to the right. In Figure 3-92, an Agusta A-109 can be seen in forward flight and banking to the right. The rotor blade in the rear and the one on the left are both in an upward raised position, meaning they have both experienced the condition called flap.

Figure 3-92. Agusta A-109 banking to the right.
Figure 3-92. Agusta A-109 banking to the right.

Some helicopters use a horizontal stabilizer, similar to what is seen on an airplane, to help provide additional stability around the lateral axis. A horizontal stabilizer can be seen on the Agusta A-109 in Figure 3-92.

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