What does a free-turbine engine mean?

A free-turbine engine is characterized by having its turbine section not mechanically connected to the gas generator, which is the part of the engine where the incoming air is compressed and mixed with fuel before being ignited. This distinction allows the turbine section to operate independently, which is particularly advantageous in applications such as turboprop engines.

By not having a direct mechanical connection, the turbine can spin at a different speed than the gas generator, providing flexibility in performance and efficiency. This configuration allows for better optimization of propeller speed and engine operation under various flight conditions.

The other options do not accurately define a free-turbine engine. For instance, a direct connection to the propeller or operation solely on fuel pressure does not encapsulate the essence of what makes the free-turbine design unique. Additionally, a mechanical starter is not a defining feature of free-turbine engines, as many engines can utilize various forms of starting mechanisms, whether they are free-turbine or not.