FAA Powerplant Written Practice Test

What does "free turbine" mean in the context of turboprop and turbo shaft engines?

1. The turbine is fully enclosed to maximize airflow

2. The turbine wheel is not physically connected to the turbine wheel

3. The turbine does not require external power for operation

4. The turbine operates independently from the engine controls

The correct answer is: The turbine wheel is not physically connected to the turbine wheel

In the context of turboprop and turbo shaft engines, "free turbine" refers to a configuration where the turbine wheel is not physically connected to the compressor wheel. This means that the turbine can rotate independently of certain other components in the engine, which allows for more flexibility in the engine's operation and design. In free turbine designs, the turbine takes energy from the exhaust gases to help drive the propeller or shaft without being mechanically linked to the high-pressure compressor that compresses the incoming air. This allows the turbine to operate at a different speed than the compressor, which optimizes performance for various flight conditions or operational requirements. By decoupling the turbine from the compressor, the engine can achieve greater efficiency and responsiveness, particularly in applications requiring variable speed operations, such as in helicopters or certain aircraft. The other choices do not accurately describe the concept of a free turbine. Enclosures may be present for aerodynamic purposes, but it does not define a free turbine. While external power for operating an engine is a relevant concept, it doesn't pertain to the definition of a free turbine. Independence from engine controls may also be a characteristic of certain systems, but it is not the defining trait of a free turbine setup.