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Turbofan engines have two exhaust components: high pressure/temp exhaust coming from the turbine, and unburnt low pressure air coming from the bypass, which is

Turbofan engines have two exhaust components: high pressure/temp exhaust coming from the turbine, and unburnt low pressure air coming from the bypass, which is forced through by the first turbine but not actually burned. Military craft have low bypass engines, while large civilian craft have high bypass. These different designs trade off engine size and efficiency; large jets are willing to have large engines if they’re more fuel efficient, while military craft have high needs and very little room to spare. My suspicion is that these techniques will need significant tweaks to work on the very different large engines. Already we’re seeing external nacelle changes to wide body aircraft for noise reasons, rather than the internal design shown here.

graycat 5y ago
Here's some of the story on bypass for jet engines: Yes, high bypass: With a jet engine, what you pay is kinetic energy, and what you get and want is momentum. For mass m and velocity v, the kinetic energy, that costs you (1/2)mv^2, and for the momentum you get mv. So, you want to maximize the resulting momentum mv for the paid kinetic energy (1/2)mv^2. So, you want to select m and v to maximize momentum for a given kinetic energy. Since for a given velocity v, in kinetic energy are paying for v^2 but in momentem getting only v, clearly want to make mass m large and velocity v small. So use the kinetic energy of the pressure of the burning fuel to turn a big turbine that moves a large mass m by a small velocity v. That works so well that GE took one of their early jet engines and put on an aft-fan, that is, put a fan, a turbine, free wheeling, not driven by any shafts, on the back of the engine. So the blades of the fan played two roles: (1) Close to the axis of rotation, the blades got hit with the fast, hot gas from the engine, and (2) the rest of the blades, that extended past the engine, pushed on cold air. The Dassault FanJet 20 used these GE fanjet engines, and those were the planes, modified for carrying cargo, used at the start of FedEx. BUT! My understanding is that this whole story only works for subsonic flight! So, jet engines developed for military fighter jet planes and supersonic flight didn't have fans. But when the US military wanted a really big cargo airplane, the C5A, they paid for the development of high bypass turbofan engines and used them. With the engines developed (took big bucks), commercial aviation took advantage.

graycat 5y ago

Here's some of the story on bypass for jet engines: Yes, high bypass: With a jet engine, what you pay is kinetic energy, and what you get and want is momentum. For mass m and velocity v, the kinetic energy, that costs you (1/2)mv^2, and for the momentum you get mv. So, you want to maximize the resulting momentum mv for the paid kinetic energy (1/2)mv^2. So, you want to select m and v to maximize momentum for a given kinetic energy. Since for a given velocity v, in kinetic energy are paying for v^2 but in momentem getting only v, clearly want to make mass m large and velocity v small. So use the kinetic energy of the pressure of the burning fuel to turn a big turbine that moves a large mass m by a small velocity v. That works so well that GE took one of their early jet engines and put on an aft-fan, that is, put a fan, a turbine, free wheeling, not driven by any shafts, on the back of the engine. So the blades of the fan played two roles: (1) Close to the axis of rotation, the blades got hit with the fast, hot gas from the engine, and (2) the rest of the blades, that extended past the engine, pushed on cold air. The Dassault FanJet 20 used these GE fanjet engines, and those were the planes, modified for carrying cargo, used at the start of FedEx. BUT! My understanding is that this whole story only works for subsonic flight! So, jet engines developed for military fighter jet planes and supersonic flight didn't have fans. But when the US military wanted a really big cargo airplane, the C5A, they paid for the development of high bypass turbofan engines and used them. With the engines developed (took big bucks), commercial aviation took advantage.