A couple of weeks ago I explained reciprocating engines, especially as they relate to aircraft. But, truth be known, I’m all about the turbines. Turbine engines are what got me excited about airplanes to begin with.
Recips are a bit Rube Goldberg-ish, with all the explosions and valve timing, and pistons going up and down to make props go round and round. In comparison, turbines are simple, even elegant.
So, here’s how a turbine works. Newton’s third law of motion states that for every action there is an equal and opposite reaction. This is the basic explanation for all airplane propulsion. With a propeller plane, the action is that of a propeller accelerating a large mass of air through a small change in velocity. With a jet engine, the action is the acceleration and discharge of a mass of air at a very high velocity.
To understand how this happens, it helps to understand Bernoulli’s principle. This is the same principle that keeps wings in the air (mostly). Bernoulli’s principle says that if a column of fluid (air is a fluid) moving at subsonic speed, is forced through a narrowing space, its speed will go up and its pressure will go down. Conversely if that same column of fluid is forced through a diverging space, its speed will go down and its pressure will go up. This seems odd to me, because it seems like air forced through a converging space would slow down and increase in pressure, and vice-versa, but I won’t argue with physics.
So, got that? Converging = speed up, pressure down; diverging = speed down, pressure up. So, with the turbine engine, air enters the front of the engine through inlet ducts (which are stationary blades), that slow it down to ensure that it’s moving at below the speed of sound when it reaches the compressor.
From here it flows into the compressor blades, which are essentially very highly engineered fan blades. The first are convergent, which speed the air up. The next are divergent, which slow it down and increases its pressure. Different engines have different numbers of compressor disks and different configurations of converging and diverging blades, but they all operate on the same principle.
This compressed air then goes into a diffuser duct (essentially a short, wide tube) which increases the pressure even more, then it goes to the “hot section” of the engine – the combustor, where it’s mixed with fuel and set on fire. This flaming air then enters the turbine blades where it spins the turbines. The turbines are, again, similar to fan blades, except instead of having an outside force rotate them, they rotate because of the flaming, high pressure air passing through them.
The turbines are connected through a shaft to the compressor blades and keep them spinning, to keep the whole operation going. After the hot air has done its job in the turbines, it exits through the exhaust nozzle. The force of the exhaust exiting is part of what drives the plane forward.
Depending on the design of the engine, the turbine might also drive a propeller or drive a large fan at the front, which acts like a propeller and contributes to thrust. Most engines on small regional commuter planes like Bombardiers or Embraers are turboprops. So, even though the plane you take from Seattle to Portland looks like a propeller plane, it’s actually a turbine plane – it’s just that the turbine drives the compressor and visible propellers instead of driving the compressor and hidden fan. Some older engines are just jets – compressor and turbine, with all the thrust coming from the force of the exhaust exiting … but these are inefficient so they’re not used much any more.
Most modern large aircraft engines are turbofans. These days the fans are enormous – for example the fans on a 787 Dreamliner engine are about 1.5 times taller than me. And, one blade on a modern turbofan costs around $30,000. Think about that next time you gripe about the price of your airline ticket.