Cyclic and Forward Flight

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Tip Path Plane:

The tip path plane, or TPP, is the plane connecting the rotor blade tips as they rotate. While hovering, the thrust vector of a helicopter is oriented upwards, perpendicular to the tip path plane. In order for the helicopter to travel forward, this thrust vector needs to be rotated slightly in the forward direction. To rotate the thrust vector, it is in turn necessary to rotate the TPP by the same amount, as illustrated below. The hovering TPP is drawn in purple, while the forward TPP is in orange.

Tip path planes and thrust vectors for hovering and forward flight
Tip path planes and thrust vectors for hovering and forward flight [from Gessow and Myers, 1952]

Cyclic Control:

Since tilting the rotor hub or rotor shaft is impractical, an alternative means of rotating the TPP is needed. Most modern helicopters use a system of swashplates. Seen in the following diagram, the swashplate system is composed of upper and lower swashplates.

Cyclic control and swashplates
Cyclic control and swashplates [from Gunston and Spick, 1986]

The red portion of the diagram, including the lower swashplate, remains stationary relative to the helicopter. The upper swashplate (in blue) rotates with the rotor, while remaining parallel to the lower swashplate. By utilizing what is called cyclic control, the swashplates can be angled so as to vary the pitch of the blades depending on their azimuth angle. As the swashplates are tilted in the proper direction, there is an increased lift on the aft portion of the rotor, causing the blades to flap up, which in turn causes the TPP to rotate forwards. As the TPP rotates forwards, the thrust vector does as well, imparting a forward acceleration to the helicopter.





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