Definition
A turboprop engine design in which the propeller, reduction gearbox, and gas-generator turbine are all mounted on a single common shaft, and engine speed (RPM) is held constant during normal operation. Power output is varied by changing fuel flow and propeller blade angle while the shaft continues turning at the same selected RPM.
Plain English
A turboprop layout where the propeller and the engine's turbine are joined on one shaft that always spins at the same speed once the engine is running. To make more or less power, the pilot doesn't change the RPM — fuel and the propeller blade angle change instead.
Context Anchor
Seen when learning turboprop engine types, especially the difference between fixed-shaft and free-turbine turboprop engines.
Derivation
Fixed-shaft' means the propeller and turbine are locked together on one shaft (fixed to each other, not free to spin independently). 'Constant-speed' means the shaft is held at a constant RPM during operation. Together they describe both how the engine is built and how it is run.
Why Pilots Care
Keeps the engine running at its most efficient RPM for the entire flight, reduces vibration, and gives smooth thrust response.
Intuition Check
Do not read “fixed-shaft” as “fixed-pitch.” The shaft connection is fixed; the propeller blade angle can still change. Also, “constant-speed” means nearly constant RPM, not constant aircraft speed.
Example Sentence 1
In a fixed-shaft constant-speed turboprop, the pilot adjusts power by moving the power lever, which changes fuel flow and propeller blade angle while RPM stays the same.
Example Sentence 2
During cruise the fixed-shaft constant-speed system automatically fine-tuned pitch to maintain the selected RPM as fuel burned off.