Definition
Two propellers mounted on the same axis (one behind the other) that rotate in opposite directions, driven through concentric shafts from a single engine or gearbox. The counter-rotation cancels the torque and asymmetric thrust effects that a single large propeller would produce.
Plain English
Two propellers stacked on the same shaft line, spinning in opposite directions. They share one centerline but turn against each other so the twisting forces cancel out.
Context Anchor
Seen in aircraft design, engine, and propeller discussions, especially where two propellers are installed at the same nose or tail location.
Derivation
Coaxial comes from Latin co- (together) and axis (axle or pivot). Literally 'sharing the same axle' — which is exactly what these two propellers do.
Why Pilots Care
The opposing rotation removes most torque effect on the airframe and can improve climb and cruise efficiency.
Analogy
Think of two fans placed one behind the other, both centered on the same straight line. They share the same centerline, even though each fan has its own blades.
Intuition Check
Coaxial does not automatically mean the propellers turn opposite directions. It means they share the same axis; opposite rotation is a separate design choice.
Example Sentence 1
The contra-rotating coaxial propellers on the Tu-95 allow the aircraft to absorb enormous engine power without the handling problems a single propeller of that size would cause.
Example Sentence 2
During preflight the mechanic verified that both forward and aft coaxial propellers turned freely in opposite directions.