Chapter 9: Types of Gas Turbine Engines

A set of practice flashcards covering the key concepts from Chapter 9: Types of Gas Turbine Engines, including turbofan characteristics, bypass ratio, spools, turboprop configurations (direct drive, compound, free turbine, reverse flow), turboshaft purpose, and the Auxiliary Power Unit (APU).

What are the four main types of gas turbine engines discussed in the notes?

Turbofan, turboprop, turboshaft, and auxiliary power unit (APU).

What are the two key differences between turbofan engines and turbojets?

A large ducted fan in the air intake and only part of the air (primary) goes through the core while the rest bypasses the core as secondary air.

What is bypass ratio in turbofan engines and how has it changed from early to modern designs?

The ratio of bypass air to core air; early turbofans were about 1:1, modern turbofans are at least 3:1.

Name two main functions of bypass air in a turbofan engine.

Cooling the engine core and reducing noise (by mixing with exhaust); it also contributes significantly to thrust (up to about 80%).

How can a turbofan’s fan be driven in relation to the first compressor stage?

The fan can be driven by the same low‑pressure turbine (forming the low‑pressure spool) or by a dedicated turbine in a three‑spool configuration.

What is a spool in gas turbine engines?

A set of rotating components (fan, compressor, turbine) connected by a shaft that spins together; multi‑spool engines have separate spools for different sections.

What is a direct drive turboprop configuration?

Propeller, compressor, and turbine are on the same shaft, so the propeller is driven directly by the turbine.

What distinguishes a compound turboprop from a direct drive turboprop?

A two‑spool arrangement where the propeller is on the low‑pressure spool and there is a separate high‑pressure spool.

What is a free turbine turboprop?

The propeller is driven by a separate power turbine on its own shaft, independent of the compressor/turbine spool.

What is a reverse flow turboprop and why is it used?

Air enters and travels to the back of the engine, makes a 180-degree turn to the front, allowing a more compact layout; the propeller is still driven by a separate power turbine.

What is a turboshaft engine typically used for?

Driving a shaft to power helicopter blades or other rotor-driven applications.

What is an Auxiliary Power Unit (APU) and where is it commonly located?

A small gas turbine used to generate electricity on the ground and provide air for systems; commonly located at the tail of the aircraft.

What is a key advantage of turbofan engines over turbojets?

Higher efficiency and better fuel consumption (and generally quieter operation).

Why is the APU capable of delivering more air than needed to drive turbines?

APUs are designed to provide excess airflow to support aircraft systems and starting main engines.

What role does bypass air play in reducing noise in turbofan engines?

Bypass air is mixed with exhaust, which helps lower noise levels.