Definition
The aerodynamic resistance produced by a propeller when it is being driven by the airflow rather than driving the aircraft forward. This occurs when engine power is reduced or lost and the propeller blades, still set at a low (fine) pitch, act like flat discs being pushed through the air. The result is a large braking force that significantly increases total aircraft drag.
Plain English
When an engine quits or is throttled back, the wind keeps spinning the propeller. Instead of pulling the airplane forward, the propeller now acts like a brake, slowing the airplane down and making it harder to glide.
Context Anchor
You will see this term in multiengine and constant-speed propeller discussions, especially when learning why a failed engine's propeller may need to be feathered.
Derivation
Propeller comes from words meaning “to drive or push forward.” Drag comes from older English words meaning “to pull.” Together, the term points to the problem: the part normally meant to pull the airplane forward can also pull against it when it is not set correctly.
Why Pilots Care
Unreduced propeller drag after engine failure can cause rapid loss of airspeed and make continued flight impossible on the remaining engine.
Analogy
Imagine sticking your open hand out the window of a moving car versus turning it sideways. Open hand catches the wind and pushes your arm back hard — that's an unfeathered propeller. Edge-on hand slices through the air with little resistance — that's a feathered propeller.
Intuition Check
Propeller drag does not mean the propeller is scraping or physically dragging on something. It means the propeller is creating air resistance that slows the airplane.
Example Sentence 1
After the right engine failed, the pilot feathered the propeller immediately to eliminate propeller drag and maintain climb performance on the remaining engine.
Example Sentence 2
High propeller drag on the failed engine made it hard to hold altitude until the blades were aligned.