Definition
A helicopter rotor system in which the blades are attached to the hub below the point where the hub pivots on the rotor mast. This geometry places the blades' center of gravity below the teetering (rocking) axis, which reduces the tendency for the blades to shift fore and aft as they speed up and slow down during each revolution.
Plain English
A rotor design where the blades hang slightly below the point the rotor head tilts on. Hanging the blades a little lower than the pivot keeps them steadier as they spin, so they don't try to lead or lag as much.
Context Anchor
Seen in helicopter rotor-system descriptions, especially for two-blade rotors that rock as one unit.
Derivation
Underslung literally means 'hung from underneath.' The blades are slung (hung) below the pivot point, which is exactly what the term describes.
Why Pilots Care
Affects rotor response, stability margins, and the specific inspection points needed during preflight.
Intuition Check
Underslung does not mean the rotor is underneath the helicopter. It means the rotor blades are mounted below the pivot point they rock around.
Example Sentence 1
The Bell 206 uses an underslung rotor, which is one reason its two-bladed system can get away without lead-lag hinges.
Example Sentence 2
Light helicopters often use an underslung rotor to reduce stress during blade flapping.