Definition
The length of an imaginary line drawn from the tip of one propeller blade, through the center of the hub, to the tip of the opposite blade. It is the diameter of the circle the blade tips trace as the propeller rotates.
Plain English
How wide the propeller is from tip to tip, measured straight across through the middle.
Context Anchor
Seen in aircraft and propeller specifications, engine performance discussions, and maintenance records for approved propeller installations.
Derivation
Diameter comes from the Greek diametros, meaning 'measuring across.' It is the straight-line distance across a circle through its center — in this case, the circle the blade tips sweep through the air.
Why Pilots Care
Larger diameter moves more air and produces more thrust but increases torque, requires more engine power, and can reduce ground clearance on takeoff and landing.
Analogy
Think of the spinning propeller as drawing an invisible circle in the air. The propeller diameter is the width of that circle.
Intuition Check
Do not read propeller diameter as the length of one blade. It is the full distance across the spinning propeller’s circle, through the center.
Example Sentence 1
The propeller diameter on the trainer was 76 inches, giving the blade tips plenty of clearance above the ramp.
Example Sentence 2
Before approving the new propeller, the mechanic verified that the larger diameter still provided safe ground clearance with the tail down.