4.4 Turbofan, Turboprop & Auxiliary Power Units (APUs)

4.4 Turbofan, Turboprop & Auxiliary Power Units (APUs)

Gas turbine engines are engineered to deliver propulsion and secondary power in diverse mechanical configurations. While pure turbojets convert internal gas energy almost exclusively into high-velocity exhaust thrust, modern civil and military aviation relies on turbofans, turboprops, and Auxiliary Power Units (APUs). Each system optimizes thermodynamic efficiency for specific flight envelopes by varying how mechanical work is extracted from the expanding gas stream.

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Turbofan Engines: Bypass Ratios & Propulsive Efficiency

A turbofan engine consists of a central core gas generator (compressor, combustor, and high-pressure turbine) that drives an oversized, ducted front bypass fan. The fan discharges air into two separate streams:

1. Core Airflow ($\dot{m}_{core}$): Passes through the core compressor, combustor, turbines, and primary exhaust nozzle.
2. Bypass Airflow ($\dot{m}_{bypass}$): Bypasses the core engine completely, flowing through the outer fan duct to generate propulsive thrust directly.

Bypass Ratio (BPR) Formulations

The Bypass Ratio (BPR) is mathematically defined as the ratio of mass airflow passing through the bypass fan duct to the mass airflow passing through the core engine:

$\text{Bypass Ratio (BPR)} = \frac{\dot{m}_{bypass}}{\dot{m}_{core}}$

Turbofans are categorized into distinct operational classes:

• Low-Bypass Turbofans (BPR < 2:1): Early civil transports (e.g. Pratt & Whitney JT8D with BPR ~1:1 to 1.7:1) and supersonic combat aircraft. Delivers high thrust per unit frontal area and high exhaust velocities, but suffers higher specific fuel consumption (SFC) and severe jet exhaust noise.
• Medium-Bypass Turbofans (BPR 2:1 to 4:1): Transports developed during the early 1970s.
• High-Bypass Turbofans (BPR 5:1 to 8:1): Standard modern civil airliners (CFM International CFM56, General Electric CF6, Pratt & Whitney PW4000). Bypass air produces 75% to 85% of total engine thrust.
• Ultra-High Bypass Turbofans (BPR 9:1 to 15+:1): State-of-the-art engines (CFM LEAP, Pratt & Whitney GTF, Rolls-Royce Trent XWB). Produces massive fuel savings, ultra-low emissions, and quiet community noise profiles.

The Physics of Propulsive Efficiency

Propulsive efficiency ($\eta_p$) represents how effectively kinetic energy imparted to the air is converted into usable forward aircraft thrust:

$\eta_p = \frac{2 V_0}{V_j + V_0}$

Where $V_0$ is aircraft forward flight speed and $V_j$ is jet exhaust discharge velocity. In accordance with Newton's second law ($F = \dot{m}(V_j - V_0)$), an engine can generate a given thrust force by either:

1. Accelerating a small mass of air to an extreme exhaust velocity (pure turbojet), or
2. Accelerating a very large mass of air to a moderate exhaust velocity (high-bypass turbofan).

Because kinetic energy lost to the wake is proportional to the square of exhaust velocity ($\text{KE} = \frac{1}{2} m V_j^2$), accelerating a larger air mass to a lower velocity requires drastically less fuel energy. High-bypass turbofans achieve substantially higher propulsive efficiencies and lower specific fuel consumption at subsonic cruise speeds (Mach 0.75 to 0.85).

Ultra-High Bypass Geared Turbofans (GTF)

In conventional twin-spool turbofans, the low-pressure turbine (LPT) directly drives the front bypass fan via a shared inner concentric shaft. This architecture imposes an aerodynamic compromise:

• The front fan operates most efficiently at low rotational speeds, preventing fan blade tips from exceeding the speed of sound, which induces shockwave boundary layer separation, loss of thrust, and excessive noise.
• Conversely, the low-pressure turbine operates at peak thermodynamic efficiency at very high rotational speeds.

To resolve this conflict, the Geared Turbofan (GTF) (e.g. Pratt & Whitney PW1000G series) introduces a precision planetary reduction gearbox (FDGS - Fan Drive Gear System) between the LPT shaft and the front fan. With a reduction ratio of approximately 3:1, the gearbox decouples rotational speeds. The low-pressure turbine spins at high, peak thermodynamic RPM (requiring fewer turbine stages and saving substantial weight), while the front fan rotates at slow, aerodynamically optimal RPM. This design enables bypass ratios exceeding 12:1 and reduces fuel burn by up to 15% to 20%.

Turboprop Propulsion Systems

A turboprop engine is a gas turbine engine optimized to deliver shaft horsepower (shp) to drive an aircraft propeller. Unlike a turbojet or turbofan where exhaust gases produce significant jet reaction thrust, a turboprop extracts 85% to 90% of the total gas energy across the turbine section. Only 10% to 15% remains as residual jet exhaust thrust.

Fixed-Shaft vs. Free-Turbine Configurations

Turboprops operate under two primary mechanical configurations:

1. Fixed-Shaft Turboprop (e.g. Honeywell TPE331)

• In a fixed-shaft turboprop, the compressor, turbine stages, and propeller reduction gearbox are all mechanically locked together on a single common drive shaft.
• Starting Characteristics: The starter motor must turn the compressor, the turbine, and the entire reduction gearbox and propeller simultaneously during engine start. To reduce starter torque loads, the propeller blades are placed in flat pitch (ground idle start pitch) so they do not load the starter.
• Operational Hazard & NTS: If a fixed-shaft engine suffers a flameout in flight, the propeller will windmill at high speed, back-driving the compressor and creating massive asymmetric aerodynamic drag. This severe drag can cause loss of aircraft directional control. To prevent this, fixed-shaft engines incorporate a Negative Torque Sensing (NTS) system.

2. Free-Turbine Turboprop (e.g. Pratt & Whitney Canada PT6A)

• In a free-turbine engine, the engine consists of two mechanically independent sections:
  1. Gas Generator ($N_1$ or $N_g$): Consists of the compressor and the compressor turbine on a shared shaft.
  2. Power Section ($N_2$ or $N_p$): Consists of one or more power turbine stages mounted on an independent coaxial shaft that drives the propeller reduction gearbox.
• There is no mechanical connection between the gas generator shaft and the power turbine shaft; energy is transmitted purely through aerodynamic gas coupling.
• Starting Characteristics: Starting torque is extremely low because the starter motor only turns the gas generator spool ($N_1$). The propeller remains completely stationary during initial light-off, or can even be held stationary with a propeller rotor brake while the gas generator runs at idle.

Turboprop Reduction Gearboxes (RGB)

Aircraft gas turbine spools operate at speeds between 20,000 and 45,000 RPM. Aircraft propellers cannot operate at such high speeds; if propeller blade tips exceed the speed of sound, efficiency collapses and extreme noise and vibration ensue. Propeller tip speeds must remain subsonic, operating between 1,200 and 2,200 RPM.

• Gear Reduction: The Reduction Gearbox (RGB) utilizes compound planetary gear systems and helical reduction sets with gear ratios ranging from 12:1 to over 25:1 to step down high turbine RPM into high-torque propeller shaft rotation.
• Torquemeters: The gearbox houses a hydraulic or electronic torquemeter that measures torsional deflection or oil pressure reaction against helical ring gears, providing cockpit instruments with an accurate measurement of engine shaft horsepower.

Critical Turboprop Safety Systems: NTS and TSS

• Negative Torque Sensing (NTS): When an engine flames out or loses power in flight, the propeller begins to windmill, driving the reduction gearbox and engine rather than being driven by them. The reversal of gear tooth thrust forces the gearbox ring gear to shift axially against heavy helical springs. This mechanical displacement actuates an NTS valve that commands the propeller governor to drive the propeller blades toward coarse pitch (feather), eliminating windmilling drag.
• Thrust Sensitive Signal (TSS): An automated safety system that monitors positive engine thrust during takeoff. If an engine suffers power loss during high-power operations, TSS automatically initiates full propeller feathering without pilot intervention, preventing control loss.

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Auxiliary Power Units (APUs)

An Auxiliary Power Unit (APU) is a self-contained, automated gas turbine engine installed in an unpressurized compartment (typically the aircraft tailcone or wheel-well bay). Its purpose is to provide electrical power and pneumatic bleed air to aircraft systems independently of the main propulsion engines.

Primary Operational Roles

1. Pneumatic Bleed Air: Delivers high-volume, low-pressure compressed air to aircraft pneumatic manifolds for environmental control systems (cockpit/cabin heating and air conditioning) and pneumatic starting of the main propulsion engines.
2. Electrical Power: Drives an engine-type AC generator (typically 115V AC, 400 Hz, 3-phase) to power flight instruments, avionics, fuel boost pumps, and passenger services on the ground without running propulsion engines or connecting ground support equipment (GPU).
3. In-Flight Emergency Backup: Certified APUs can be started in flight to provide emergency backup electrical power and bleed air following main engine failure or electrical generator tripping.

Mechanical Architecture

APUs typically utilize a single-spool architecture incorporating a centrifugal compressor, a reverse-flow annular combustor, and a radial or axial turbine. The engine operates at a constant governed rotational speed—100% design RPM—under the management of a dedicated Full Authority Digital Electronic Controller (FADEC / APU ECU). As electrical or pneumatic loads fluctuate (e.g. air conditioning packs cycling on), the ECU modulates the fuel metering valve and compressor guide vanes to maintain precisely 100% RPM.

APU Automatic Safety Shutdown Protection

Because APUs routinely run unattended on the airport ramp during passenger boarding and cargo loading, FAA airworthiness standards (14 CFR Part 33 and Part 25) mandate comprehensive automatic emergency shutdown protections.

Unattended Ground APU Automatic Shutdown Parameters:
1. Overspeed: Critical primary protection; commands instant mechanical/electronic fuel shutoff.
2. High Exhaust Gas Temperature (EGT) / Over-temperature.
3. Low Lubricating Oil Pressure.
4. High Oil Temperature.
5. APU Fire Detection: Triggers automatic shutdown, closes bleed valve, and discharges fire extinguisher bottle.

• In-Flight Logic Override: In many transport-category aircraft, the APU ECU switches to an in-flight protection mode when airborne (sensed by the landing gear squat switch). To guarantee emergency electrical power to fly-by-wire flight control computers and critical instruments, non-critical automatic shutdowns (such as low oil pressure or high EGT) are inhibited in flight. The APU will continue running to exhaustion unless an actual overspeed or fire condition occurs.

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Propulsion System Comparison Matrix

Independent FAA AMT Powerplant prep by OpenExamPrep. In fixed-shaft turboprops, negative torque sensing (NTS) checks are mandatory ground maintenance procedures. An inoperative NTS system renders the aircraft unairworthy due to the catastrophic risk of uncontrollable asymmetric drag in the event of an in-flight engine flameout.