Воздушный отбор в авиационных газотурбинных двигателях: тенденции и преимущества безвоздушных систем.

Aircraft gas turbine function
Bleed air in aerospace engineering is compressed air taken from the compressor stage of a gas turbine, upstream of its fuel burning sections. Automatic air supply and cabin pressure controller (ASCPC) valves bleed air from low or high stage engine compressor sections; low stage air is used during high power setting operation, and high stage air is used during descent and other low power setting operations. Bleed air from that system can be utilized for internal cooling of the engine, cross starting another engine, engine and airframe anti icing, cabin pressurization, pneumatic actuators, air driven motors, pressurizing the hydraulic reservoir, and waste and water storage tanks. Some engine maintenance manuals refer to such systems as "customer bleed air". Bleed air is valuable in an aircraft for two properties: high temperature and high pressure (typical values are and , for regulated bleed air exiting the engine pylon for use throughout the aircraft).

Bleed air systems have been in use for several decades in passenger jets. Recent improvements in solid state electronics have enabled pneumatic power systems to be replaced by electric power systems. In a bleedless aircraft such as the Boeing 787, each engine has two variable frequency electrical generators to compensate for not providing compressed air to external systems. Eliminating bleed air and replacing it with extra electric generation is believed to provide a net improvement in engine efficiency, lower weight, and ease of maintenance. According to Boeing internal documents, eliminating the use of bleed air as a source of cabin air also translates into the “elimination of engine contaminants potentially entering cabin air supply.”

A bleedless aircraft achieves fuel efficiency by eliminating the process of compressing and decompressing air, and by reducing the aircraft's mass due to the removal of ducts, valves, heat exchangers, and other heavy equipment. The APU (auxiliary power unit) does not need to supply bleed air when the main engines are not operating. Aerodynamics are improved due to the lack of bleed air vent holes on the wings. By driving cabin air supply compressors at the minimum required speed, no energy wasting modulating valves are required. High temperature, high pressure air cycle machine (ACM) packs can be replaced with low temperature, low pressure packs to increase efficiency. At cruise altitude, where most aircraft spend the majority of their time and burn the majority of their fuel, the ACM packs can be bypassed entirely, saving even more energy. Since no bleed air is taken from the engines for the cabin, the potential of engine oil contamination of the cabin air supply is eliminated. Lastly, advocates of the design say it improves safety as heated air is confined to the engine pod, as opposed to being pumped through pipes and heat exchangers in the wing and near the cabin, where a leak could damage surrounding systems.