Definition
On a typical single-engine airplane with a propeller turning clockwise as viewed from the cockpit, the descending propeller blade is the blade on the right side of the propeller arc that is moving downward as it rotates. When the airplane is in a nose-high, high-power attitude, this descending blade meets the relative wind at a higher angle of attack than the ascending blade on the left side, producing more thrust on the right side of the propeller disc. This asymmetric thrust is the cause of P-factor, which yaws the airplane to the left.
Plain English
It's the propeller blade that is on its way down as the propeller spins. On most American single-engine airplanes, that's the blade passing the right side. When the nose is high and the engine is working hard, this downward-moving blade bites into the air harder than the upward-moving blade on the other side, pulling the nose toward the left.
Context Anchor
Seen when learning how to establish a climb and why the airplane may need right rudder as power and nose-up attitude increase.
Derivation
“Descending” comes from words meaning “to climb down.” “Propeller” comes from a word meaning “to drive forward.” Together, the phrase points to the blade that is moving downward while the propeller drives the airplane forward.
Why Pilots Care
It produces more thrust than the opposite blade in high-angle-of-attack flight, creating a yaw force that must be countered with rudder to maintain coordinated flight.
Intuition Check
Do not read “descending” as meaning the airplane is descending. Here it means one propeller blade is moving downward as the propeller rotates.
Example Sentence 1
During the climb, the descending propeller blade on the right produces more thrust than the ascending blade on the left, so the pilot adds right rudder to hold heading.
Example Sentence 2
The pilot adds right rudder to offset the yaw caused by the descending propeller blade at high angle of attack.