Definition
A short, shallow propeller spinner that covers only the central hub area of the propeller, leaving the inner portion of each propeller blade exposed at its root. It is a partial fairing rather than a full streamlined cone.
Plain English
A small, dome-shaped cover fitted over the middle of the propeller. It hides the hub but does not extend out far enough to cover where the blades meet the hub.
Context Anchor
Seen during preflight inspection and in descriptions of propeller installations on light aircraft.
Derivation
Named after a skull-cap, the small close-fitting cap that covers only the top of the head. The spinner is called this because it sits over the hub like a cap, without extending down the sides.
Why Pilots Care
Reduces drag at the hub and provides some protection to the propeller mounting hardware.
Intuition Check
Do not read “skull-cap” as protective headgear. Here it means a small, close-fitting spinner on the front of the propeller hub.
Example Sentence 1
The trainer was fitted with a skull-cap spinner, so the mechanic could easily inspect the propeller hub without removing any cowling.
Example Sentence 2
Mechanics installed a skull-cap spinner on the trainer to improve the propeller's appearance and reduce minor drag.