Bypass Ratio – the ratio of incoming air that bypasses the core to the amount of air that passes through the engine core.
TURBOFAN ENGINES Fan Pressure Ratio – the ratio of air pressure leaving the fan to the air pressure entering the fan.
Turbofan – Bypass Ratio • The ratio of the incoming air that bypasses the core to the amount of air that passes through the engine core. • Generally divided into classifications based on bypass ratio: • Low bypass (1:1) • Medium bypass (2:1 or 3:1) • High bypass (4:1 or greater)
Fig. 3-11. (A) Bypass air is ejected directly overboard in forward-fan engines with a short fan duct. (B) With a ducted fan, bypass air is ducted along the engine’s entire length. Short Duct Fan
Fig. 3-12. High-bypass turbofan engines use large diameter fans with a bypass ratio of 4:1 or greater and use the largest diameter fan of any of the bypass engines. High bypass turbines offer higher propulsive efficiencies and better fuel economy than low or medium bypass turbines. With most modern day high bypass engines – most of the engines thrust is produced by the fan rather than from the jet exhaust thrust.
FAN PRESSURE RATIO • Is the ratio of air pressure leaving the fan to the air pressure entering the fan. The fan pressure ratio on a typical low bypass fan is approximately 1.5:1, whereas the fan pressure ratio for some high bypass fans can be as high as 7:1. To obtain high fan pressure ratios, most high bypass engines are designed with high aspect ratio blades. (Thin blades)
ASPECT RATIO Is the ratio of a blade’s length to its width or chord. High Low
UNDUCTED FAN CLICK HERE Bypass ratios of 30:1 are obtainable
ENGINE COMPONENTS • Air inlet duct • Compressor section • Combustion section • Turbine section • Exhaust section • Accessory section Systems necessary for starting, lubrication, fuel supply, and auxiliary purposes such as anti-icing, cooling and pressurization. Page 3-8
TWO MAIN SECTIONS OF A TURBINE ENGINE: HOT & COLD SECTION Air Inlet & Compressor make up the Cold Section Combustors, Turbines & Exhaust make up the Hot Section
AIR INLET DUCT Considered part of the airframe, not the power plant. The air inlet to a turbine engine is designed to recover as much of the total pressure of the free airstream as possible and deliver it to the compressor. This is known as ram recovery or pressure recovery . In addition to recovering and maintaining the pressure of the free airstream, inlets can be shaped to increase air pressure above atmospheric pressure. This ram effect causes air to “pile up” in piles up and the higher the inlet air pressure increases above ambient.
The air inlet duct also provides a uniform supply of air to the compressor for efficient operation. The inlet duct is designed to cause as little drag as possible. It takes only a small obstruction to the airflow inside a duct to cause a severe loss of efficiency.
SUBSONIC INLETS Subsonic inlet ducts are usually of a fixed geometry and diverging in shape.
SUPERSONIC INLETS Supersonic aircraft typically utilize a converging – diverging inlet duct.
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- Spring '14
- Turbofan, Gas turbine, Gas compressor, Axial flow compressor