What is a free-turbine, reverse-flow turboprop engine ?

October 19, 2025  , , , ,  

 Turboprop Engine — Basic Idea 

Free-turbine, Reverse-flow 

A turboprop engine is a jet engine that drives a propeller. It uses gas turbine power to turn a propeller through a gearbox, providing efficient thrust at lower speeds (usually below Mach 0.6).
⚙️ 2. Free-Turbine Design
This describes how the turbine sections are connected inside the engine.

🔹 In a free-turbine engine:

There are two turbine sections:
  • Gas generator turbine (N₁): drives the compressor.
  • Power turbine (N₂): drives the propeller through a reduction gearbox.
These two turbines are not mechanically connected — only linked by airflow (the hot gases leaving N₁ drive N₂).

⚙️ Benefits:

  • Easier starting (since propeller doesn’t turn until engine is stabilized)
  • Smooth operation and better control
  • Quieter and more efficient at low speeds

Examples:

  • Pratt & Whitney PT6A
  • Allison 250-B17

🔄 3. Reverse-Flow Design

This describes the direction of airflow inside the engine.

🔹 In a reverse-flow turboprop:

Air enters from the rear of the engine.It flows forward through the compressor, combustion chamber, and turbine — but in a U-shaped path, the exhaust gases exit backward again.

⚙️ Why “reverse-flow”?

Because the air changes direction inside the engine instead of flowing straight through front-to-back.

Advantages of reverse-flow:

  • Compact size (engine is shorter)
  • Easier installation (especially for small aircraft)
  • Better protection from debris or foreign objects (FOD)
A free-turbine, reverse-flow turboprop engine therefore means:
🔸 The power turbine (N₂) is not mechanically connected to the compressor turbine (N₁).
🔸 The airflow inside the engine reverses direction, making the engine compact and efficient.
Example: Pratt & Whitney Canada PT6A
Classic free-turbine, reverse-flow turboprop Air enters from the rear, reverses direction inside
Drives propeller via reduction gearbox Used in aircraft like the King Air, PC-12, Caravan

Summary Table

Feature                          Description                                        Advantage


Free-Turbine                 Power turbine separate from gas generator         Easier starting,                                                                                                                                 smoother running

Reverse-Flow          Airflow direction turns inside engine               Compact, efficient,                                                                                                                            easy installation