Definition 1 of 2
Definition
In a constant-speed propeller system, the operating state in which the engine RPM matches the RPM selected by the pilot through the propeller control. The flyweights in the propeller governor are balanced against the speeder spring, so the pilot valve sits in its neutral position and no oil flows to or from the propeller hub. Blade angle is held constant.
Plain English
The propeller is turning at exactly the RPM the pilot asked for, so the governor is sitting still and not changing the blade angle.
Context Anchor
Seen in constant-speed propeller operation, governor adjustment, and propeller system troubleshooting.
Derivation
“On-speed” uses “on” in the sense of “at the target,” and “speed” here means rotational speed, or rpm, not aircraft airspeed. That helps because the term is about the propeller and engine turning at the selected rpm.
Why Pilots Care
Confirms the governor is functioning correctly to maintain selected RPM; deviations signal potential maintenance needs.
Analogy
It is like a car’s cruise control after it has settled at the selected speed. Once the target is reached, the system mainly holds steady instead of making large changes.
Intuition Check
Do not read “on-speed” as meaning the airplane is at the correct airspeed. In this powerplant context, it means the propeller and engine rpm are at the selected setting.
Example Sentence 1
With the propeller control set for 2,400 RPM and the tachometer reading 2,400 RPM in level cruise, the propeller is in an on-speed condition.
Example Sentence 2
The mechanic verified the on-speed condition by confirming stable RPM readings during a ground run.