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

 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

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