Definition
A helicopter rotor system, typically with two blades, in which the blades are rigidly attached to a hub that is free to tilt (teeter) on a single trunnion bearing relative to the rotor mast. The blades cannot flap independently of one another, but the hub-and-blade assembly as a whole rocks like a seesaw, allowing one blade to rise as the opposite blade descends.
Plain English
A two-blade helicopter rotor where the blades are fixed to a hub that tilts as one piece on the mast, like a seesaw. The blades don't move up and down on their own — they only move together, with one going up as the other goes down.
Context Anchor
Seen in helicopter systems, rotorcraft aerodynamics, preflight discussions, and training for two-blade helicopter designs.
Derivation
Semi- (Latin, 'half') plus rigid. The rotor isn't fully rigid (the hub can tilt) and isn't fully articulated (the blades can't flap or hunt independently) — it sits halfway between the two designs.
Why Pilots Care
Determines how the helicopter handles control inputs and maintains rotor stability without complex individual blade hinges.
Analogy
Think of a seesaw bolted to the top of a pole. The seesaw can rock side to side as one piece, but the two ends can't move independently of each other.
Intuition Check
Semirigid does not mean the rotor is loose or flimsy. It means the blades are held firmly together, while the rotor hub is allowed to rock as one unit.
Example Sentence 1
The Robinson R22 uses a semirigid rotor, so the pilot must avoid low-G maneuvers that could lead to mast bumping.
Example Sentence 2
Many light training helicopters use a semirigid rotor because it provides predictable handling with fewer moving parts.