Gas Turbine Engine Systems and Propulsion Study Notes

Foundations of Jet Propulsion and the Messerschmitt Me262

  • Historical Milestones:

    • Messerschmitt Me262 Fighter Aircraft: The maiden flight dates varied by engine type:
      • April 18, 1941: Initial flight performed with piston engines.
      • July 18, 1942: First flight performed using jet engines.
    • Power Jet W1: This engine was named after Sir Frank J. Whittle.

  • Engine Applications and Instrumentation:

    • Pratt & Whitney 4000 (PW4000): This engine family is found on the Boeing 747−400747-400.
    • N2 Gauge: This gauge is primarily utilized during engine start or initial starter engagement. It is the first gauge to start turning; fuel addition and ignition follow once a specific N2 is reached. It references the turbine component.
    • Engine Sections: In a modern jet like the PW4000, the combustion section is identified as the 3rd stage of the engine.
    • Micro Jet Engines: These can pack at least 1010 times more energy per volume of fuel than conventional batteries and occupy less space than fuel cells.

  • Physics of Propulsion:

    • Newton's Laws of Motion:
      • Newton's Third Law: "For every action, there is a reaction equal in force and opposite in direction." This law explains the generation of lift by a wing and the production of thrust by a jet engine.
      • Newton's Second Law: Defines force as the change in momentum of an object over time: f=d(mv)dtf = \frac{d(mv)}{dt}.
    • Momentum and Thrust: The thrust of a gas turbine engine is determined by the momentum, which is the change in the mass of air and the speed of that air passing through the engine. Thrust depends on two primary variables: velocity and mass.
    • The Thermodynamic Cycle (Heat Engines):
      • Compression: Characterized by a rise in pressure and temperature and a decrease in volume.
      • Combustion: Occurs at almost constant pressure, increasing both temperature and volume.
      • Turbine: Extracted energy results in increasing volume and decreasing temperature and pressure.
      • Exhaust: Characterized by increasing speed and decreasing temperature.

  • Engine Efficiency and Gas Laws:

    • Thermal Efficiency: The ratio of net work produced by the engine to the amount of energy provided by the burned fuel.
    • Propulsive Efficiency: The percentage of power produced by the engine that is actually utilized to move the aircraft.
    • Boyle's Law: When the temperature of a gas is constant, its volume varies inversely with its pressure.
    • Charles' Law: When the pressure of a gas is constant, its volume changes in direct relationship to its absolute temperature.

  • Aerodynamics and Speed:

    • Thrust: A mechanical force generated by engines to move an aircraft through air.
    • Forces of Flight: Lift, weight, thrust, and drag.
    • Sonic Boom: Sound associated with shockwaves created by an object traveling faster than the speed of sound.
    • Mach Number: The ratio of the speed of an object to the local speed of sound.
    • Hypersonic Speed: Defined as speeds greater than Mach 55.
    • Shaft Horsepower (Brake Horsepower): A measure of the actual mechanical energy per unit of time delivered to a turning shaft.

Inlets, Cowlings, and Ice Protection

  • Inlet and Duct Functions:

    • The inlet duct serves one aircraft function and two engine functions.
    • Diffuser Duct: Usually contains a diffusion section ahead of the compressor to change ram air velocity. The average speed for ram pressure vortex recovery is between Mach 0.10.1 and Mach 0.20.2.
    • Pitot-Type Circular Intake: This shape makes the fullest use of the ram effect due to forward speed and experiences minimum losses of ram pressure despite changes in aircraft attitude.
    • Supersonic Inlets: A disadvantage is that when the aircraft yaws, a loss of ram pressure occurs on one side, causing uneven airflow distribution into the compressor, which may result in a stall.
    • Ram Recovery Point: The airspeed at which the ram-pressure rise equals the friction pressure losses, or where compressor inlet total pressure equals outside ambient air pressure.

  • Engine Cowling and Nacelles:

    • Purpose: Provides an aerodynamically smooth surface over the engine and accessories and controls airflow.
    • Identification: Engines on the Boeing 737737, A320A320, and A340A340 (CF6 or CFM56) use specific numeric/alphabetical identifiers. A14 identifies the nacelle intake anti-ice pipe.
    • Bolted Components: The inlet cowl and exhaust plug are bolted directly to the engine case.
    • Inlet Cowl Chine: Causes air separation at a fixed point to create more stable airflow around the nacelle.
    • Spacing: In low bypass ratio engines, a gap of approximately 1/21/2 of the engine diameter was maintained between the nacelle and the wing.

  • Ice Protection Systems:

    • Turbojet Engines: Typically use hot air supplied from Stage 14.
    • Turboprop Engines: Use electrical power or a combination of electrical power and hot air.
    • Inlet Anti-Ice: Prevents ice formation on components ahead of the compressor.

  • Thrust Reversers and Specialized Components:

    • Weight: A total thrust reverser assembly weighs about 1,5001,500 pounds (approx. 750750 pounds per half). They are opened via a hydraulic system.
    • Status Indication: The cascade-type thrust reverser is typically indicated by Green or Yellow on the flight deck when open.
    • Vortex Dissipaters: High-velocity compressor bleed air streams directed into vortices forming in front of pod-mounted engines.
    • Spill Valves (Bleed Doors): Used in moveable wedge systems to prevent engine stall margin and "buzz" during high-speed flight.

Compressors and Combustion Systems

  • Centrifugal Flow Compressors:

    • Secondary Purpose: Supplies engine bleed air for internal cooling, pressurization, air conditioning, and anti-icing.
    • Mechanical Action: The impeller rotates at high speeds, slinging air radially outward into the diffuser, increasing velocity and pressure (Dynamic Compression).
    • Materials: Impeller blades are often made of titanium.
    • Disadvantages: Large frontal area for a given airflow and low mass flow capacity.

  • Axial Flow Compressors:

    • Performance: Modern compressors have ratios over 25:125:1, efficiencies above 90%90\%, and handle approx. 400 lb/s400\,lb/s of air.
    • Components:
      • Rotor: Pushes airflow rearward, increasing pressure and velocity.
      • Stator Vanes: Further increase pressure by decreasing velocity.
    • Drum-Type Rotor: Used in low-speed compressors to keep blade tips below Mach 0.850.85.
    • Dual-Spool: The front compressor is the low-pressure (N1) compressor.

  • Fans and Bypass:

    • The engine fan is the first stage of compression. Mass flow through the fan is typically six times greater than through the core.
    • Fan Blades: Fabricated from titanium skins with a honeycomb core to reduce weight. The Dovetail base is the most common design for front stages.
    • Tip Clearance: The distance separating the fan blade from the inner surface of the inlet cowling.

  • Compressor Stall Management:

    • Indications: RPM fluctuations, EGT (Exhaust Gas Temperature) increase, vibration, fluttering, rumbling, or violent backfires/explosions.
    • Automatic Bleed Valves: Prevent stalls or surges by venting air overboard to prevent "piling up" in high-pressure stages.

  • Combustion Chambers and Fuel Nozzles:

    • OCN (Orbiting Combustor Nozzle): Eliminates the compressor diffuser and turbine stator.
    • Combustor Materials: Internal sections may be made of Ceramic.
    • Fuel Nozzles: Use atomized sprays (cone-shaped) for rapid mixing.
      • Tulip Shape: Formed at intermediate fuel pressures when the film breaks at the edges.
      • Bubble Shape: Formed at low fuel pressures as a continuous film.
    • Pressurizing and Dump Valve:
      1. Prevents flow until sufficient pressure is reached in the fuel control.
      2. Divides flow into primary and secondary.
      3. Drains the manifold at shutdown to prevent post-shutdown fires.
    • Vaporization: Two independent discharge orifices provide better fuel vaporization.

Turbines and Exhaust Systems

  • Turbine Theory and Design:

    • Location: Downstream of the burner to extract energy and turn the compressor.
    • Types: Impulse, Reaction, and Impulse-Reaction.
    • Axial vs. Radial: In axial turbines, the rotor is impacted; in radial turbines, flow is oriented at 90∘90^{\circ} by the compressor.
    • Blade Shrouds: Form a band around the perimeter to reduce blade vibration.
    • Design Examples:
      • PW4000: Straight vanes with turbine case cooling (bleed air).
      • Rolls-Royce Trent 500: Bowed vanes and 77 turbine stages.
    • Free Power Turbine: A turbine that does not drive the compressor but provides shaft power for a propeller or rotor.

  • Maintenance and Material Failure:

    • Stress-Rupture Cracks: Appear as hairline cracks on or across the leading edge of blades at right angles due to excessive temperature.
    • Blade Untwist: Occurs at the trailing edge; causes efficiency loss and may lead to blade tip rubbing on the turbine case.
    • Metal Fatigue: Weakening of material due to repeated cycle loading.
    • Corrosion/Oxidation: Atoms reacting with oxygen at high temperatures.
    • Seals: Carbon seals are used in bearing housings; leaks may necessitate an engine change.

  • Exhaust Systems:

    • Jet Nozzle (Exhaust Nozzle): The opening at the end of the tail pipe.
    • Convergent System: Collects gases and converts them into a solid jet. A cone prevents gases from flowing across the turbine disk.
    • C-D (Convergent-Divergent) Duct: Effectively controls gas expansion and captures energy. Also known as a Laval nozzle (after its Swedish inventor).
    • Iris Nozzle: Uses hydraulically adjusted overlapping pedals to change the angle of thrust.
    • Chevron Exhaust: Triangular extensions that define diverging slots to reduce noise.
    • Noise Reduction: Corrugations or lobes reduce noise but increase diameter, drag, and weight.

  • Thrust Management and Reversal:

    • Danger Zone: Behind a running engine at takeoff power, this zone can exceed 1,000 feet1,000\,feet.
    • Thrust Reverser Types:
      • Cascade: Aerodynamic blockage type.
      • Clamshell/Bucket: Mechanical blockage type (Bucket doors are often hydraulically actuated).
    • MEL (Minimum Equipment List): Usually allows flight with one thrust reverser deactivated.
    • Water Injection: Increases fuel flow by reducing turbine temperature; disadvantages include thermal shock and heavy system weight.

Fuel and Lubrication Systems

  • Fuel Types and Characteristics:

    • Kerosene: Preferred for higher energy per gallon, better lubrication for fuel system components, and lower volatility compared to gasoline.
    • APU Fuel Supply: Typically supplied from the left wing tank.
    • Fuel Filters: The Micron filter provides the greatest filtering action among common turbine filters.

  • Fuel Control Systems:

    • Hydro-mechanical Fuel Control: Uses mechanical cams, levers, and fluid pressure; requires trimming after engine change, fuel control change, or throttle adjustment.
    • FADEC (Full Authority Digital Engine Control): Digitally calculates and controls the precise fuel flow rate for specific thrust.
    • Lean Die-out: Caused by closing the fuel valve too quickly when reducing power.
    • Trimming Restrictions: Do not trim the engine if wind speed is above 25 mph25\,mph.

  • Lubrication Systems:

    • Synthetic Oil: The only oil used for gas turbine engines (e.g., Turbo Oil 2380, 2197). Exxon 2389, used on APUs, is often in a Yellow can.
    • Servicing Window: Accomplished within 5−10 minutes5-10\,minutes after shutdown and up to 2 hours2\,hours later.
    • System Types:
      • Wet Sump: Used on early turbines, small engines, or APUs. Often no pressure relief valve (variable pressure).
      • Dry Sump: Oil supply is carried in an external tank.
      • Hot-tank vs. Cold-tank: In a cold-tank system, the oil cooler is located on the scavenge side.
    • MCD: Master Chip Detector.
    • Properties: Oil must have a High flash point to withstand high temperatures.

Accessory Drives, Ignition, and Starting

  • Accessory Gearboxes:

    • External Gearbox: Generally located on the underside of the engine for ground access. Takes between 400400 and 500500 horsepower from the engine.
    • Intermediate Gearbox: Uses bevel gears to redirect drive when radial shafts cannot align with the external gearbox.
    • Crank Pad: Located at the front view of the engine.

  • Ignition Systems:

    • Usage: Primarily used during start and then turned off. Switched to CONT (Continuous) during takeoff, landing, icing, or flame-out threats.
    • Components:
      • Low Tension: Uses 28 V DC28\,V\,DC.
      • High Tension: Uses 115 V AC115\,V\,AC and provides up to 12 Joules12\,Joules for cold weather or high altitude.
      • Glow Plug: Contains a resistance coil.
      • Igniter Plug: Constricted air gap type is most common. Outer coil grounded to the body; replaced due to progressive erosion of electrodes.
    • Maintenance: Output voltage is lethal; parts must be grounded. Threads are covered with anti-seize compound grease.
    • Spark Rate: Between 6060 and 100100 sparks per minute.

  • Starting Procedures:

    • Standard Operation: Switch is placed in GND/GRD. At 55%55\% N2, the switch is moved to OFF to disengage the starter.
    • Monitoring: During start, monitor N2, EGT, Oil Pressure, N1, and Fuel Flow.
    • Hung Start: A situation where the engine accelerates to an intermediate RPM below idle and remains there.
    • Starter-Generator: Combines two functions to save weight; permanently engaged via a drive spline, common in small jets.
    • Manual Override: Pneumatic start valves can be driven manually by a 3/8 inch3/8\,inch drive.

Questions & Discussion

  • Q: What is the primary cause of arcing in ignition connectors?
    • A: Insufficient force pushing contact buttons together. Contributing factors include lack of proper torque on the connector nut, wear on parts, and lack of high-tension lead spring tension.
  • Q: What should you do if the #1 ignition system fails an audible test?
    • A: Disconnect high tension cable, clean cable ends, replace grommet, clean igniter plug well, clean exciter well, and lubricate sealing surface of the grommet.
  • Q: What happens when the fuel control switch is moved to cutoff?
    • A: The firewall fuel cutoff valve is closed and electrical power is blocked.
  • Q: What engine system is referenced by Scooby Doo?
    • A: (No specific technical detail provided in transcript beyond the mention of Test 2).