How a Turbofan Moves Air

Stand behind an airliner engine at the gate and you can see straight through it around the middle: a ring of daylight between the fan and the core. That ring is where most of the thrust is made. The jet engine in the center, with its burning fuel and spinning turbines, exists mostly to turn the fan. Here is one on a test stand. Push the throttle and watch the two streams of air:

Two streams leave the engine. The wide, cool one from the fan duct carries most of the air and most of the thrust. The narrow, hot one from the core is the jet engine's own exhaust, and in an engine like this one the fan duct passes about seven and a half times as much air as the core does. That number, the bypass ratio, is what the shape of the engine is about.

A wing that goes around#

A fan blade makes thrust the way a wing makes lift. Here is the air flowing past a wing section, drawn as ribbons that each follow their own streamline. Air meeting the curved upper surface speeds up, which lowers its pressure; the pressure underneath stays higher, and the difference pushes the wing up and the air down. Tilt the wing and watch the ribbons over the top turn pale:

Now bend that wing into a ring of blades and spin it. Each blade meets air coming from two directions at once: straight in through the inlet, and sideways, because the blade itself is moving. Near the hub the blade moves slowly, so the air arrives nearly head-on; at the tip it moves at about 450 m/s, faster than sound, and the air arrives almost sideways [1]. A blade that met both correctly has to be twisted. Spin it up and watch the arrows:

What turns the fan#

The fan is turned by the core, which is a jet engine of the kind that powered the first jets: a compressor squeezes air to thirty or forty times atmospheric pressure, fuel burns in it, and the hot gas blows through turbines on its way out. Cut the engine open and point at the parts:

The gas leaving the combustor is at about 1,700 K, hotter than the melting point of the turbine blades, which survive because cool air is bled from the compressor and blown through passages inside them [2]. The first turbine, right behind the combustor, drives the compressor. The turbine stages behind it drive the fan.

Two shafts, one inside the other#

The fan wants to turn slowly, because its tips are already near the speed of sound; the core compressor wants to turn fast, because its blades are small. So the engine has two shafts, one inside the other. The outer, high-pressure shaft joins the compressor to its turbine and turns at about 10,000 rpm; the inner, low-pressure shaft runs the whole length of the engine, from the fan to the last turbine stages, at about 3,000 rpm. Set the fan speed and watch the two shafts:

Why the fans got so big#

A jet pushes air backward, and the plane moves forward because of it. The thrust is the mass of air per second times how much its speed was raised. The energy spent is proportional to the square of that speed rise. So for the same thrust, a big fan that raises a lot of air by a little spends far less energy than a small jet that raises a little air by a lot. The propulsive efficiency says how much of the jet's energy ends up pushing the plane. Drag the flight speed and see where each kind of engine sits:

A jet that leaves no faster than the plane is flying would make no thrust at all, so the curve ends at 100% with nothing to show for it; real engines sit on its shoulder. Airliner fans have gone from a bypass ratio of 1 in the 1960s to 10 and more today, and most of the fuel saved along the way is this curve [1]. The limit is the size of the fan: a wider fan means a heavier engine, more drag, and longer landing gear to keep it off the runway.

All the parts#

Take the engine apart and the layout is plain: fan and its case at the front, the core as a stack of rings in the middle, the long low-pressure shaft threading through all of it, and the nacelle around the whole thing:

Nothing in it is there for its own sake. The core exists to turn the fan, the fan exists to move a great deal of air a little faster, and the whole point of the design is to leave as little energy as possible behind in the wake.

References#

1. N. Cumpsty and A. Heyes, Jet Propulsion, 3rd ed. (Cambridge University Press, 2015).
2. Rolls-Royce, The Jet Engine, 5th ed. (Wiley, 2015).