Jet Propulsion Air Bleed Systems: A Comprehensive Technical Dive

Jet propulsion air bleed systems are critical components in gas turbine engines, responsible for regulating the pressure and temperature of the airflow within the engine. These systems extract a portion of the high-pressure air from the compressor stage and direct it to various locations, such as the fuel system, lubrication system, and combustion chamber, ensuring optimal engine performance.

Měření tlaků v cestě plynu

The measurement of gas path pressures is a crucial aspect in evaluating the performance of the engine. In practice, a free air jet is used to calibrate recovery ratios and temperature differentials. However, for non-adiabatic expansions, a properly designed free air jet is necessary. The recovery ratio, which represents the ratio of the total pressure at the measurement location to the total pressure in the free stream, is typically around 0.98 for well-designed probes in high-speed flows. For low-speed flows, the recovery ratio can be as low as 0.90, requiring careful calibration.

Core Performance Evaluation

jet propulsion air bleed systems

The core performance of the engine should be presented separately, and errors in pressure ratios and efficiencies must be accounted for during data reduction. An overall characteristic curve, based on total mass flow, is essential for rig testing, particularly for low-pressure ratio fans. In such cases, a torque meter could be considered as an alternative to the direct measurement of mass flow, as it can provide a more accurate representation of the engine’s power output.

Combustion Chamber Instrumentation

For combustion chambers, instrumentation requirements are particularly challenging due to large gradients of temperature at the combustor discharge. The combustor pressure drop, which is the difference between the inlet and outlet pressures, is a critical parameter that must be accurately measured. The combustor pattern factor, which represents the non-uniformity of the temperature distribution at the combustor exit, is another important characteristic that requires precise measurement. Additionally, the radial profile shape, which describes the variation of temperature and pressure in the radial direction, is a crucial parameter for evaluating the combustion process.

F-22 Life Support System

In the case of the F-22 Life Support System, engine bleed air is conditioned to the right pressure, temperature, and humidity before being supplied to the pilot’s mask. The OBOGS (On-Board Oxygen Generation System) concentrates the breathing air to the necessary PPO2 (Partial Pressure of Oxygen) by controlling the cycling of the OBOGS, unlike other aircraft oxygen generation systems that dilute cockpit air with breathing air. This ensures that the pilot receives the appropriate oxygen concentration, enhancing safety and performance during flight operations.

Technical Specifications and Instrumentation

The technical specifications of jet propulsion air bleed systems are complex and multifaceted, requiring precise measurement and control of various parameters. These include:

  1. Gas Path Pressures: Measurement of total and static pressures at various locations within the engine, with recovery ratios typically around 0.98 for high-speed flows and 0.90 for low-speed flows.
  2. Core Performance: Accurate measurement of pressure ratios and efficiencies, with an overall characteristic curve based on total mass flow for rig testing, particularly for low-pressure ratio fans.
  3. Spalovací komora: Measurement of combustor pressure drop, combustor pattern factor, and radial profile shape, with large gradients of temperature at the combustor discharge.
  4. Engine Bleed Air Conditioning: Precise control of pressure, temperature, and humidity for the F-22 Life Support System, with the OBOGS concentrating the breathing air to the necessary PPO2.

These technical specifications require advanced instrumentation and data analysis techniques to ensure the proper functioning and optimization of jet propulsion air bleed systems.


  1. Instrumentation for Turbine Engine Component Testing
  2. AFSAB Report on Oxygen Systems
  3. S.1939 – F-22 Raptor Restoration Act