Have you ever asked yourself what exactly is inside those "big barrels" hanging from your airplane's wings? Or how they work, and how is it possible that they "push" such a "monster" at such a high speed?
Jet engines are based on the physics principle of "action and reaction": they push back a mass of air in order to obtain a forward movement. Just like when you inflate a balloon and then you let it go.
Let's see how they work:
in the forward section of the engine, external air is routed through a series of spinning blades, called the compressor, which compresses such air and increases its pressure.
From here, air goes through one (or multiple) container/containers called the combustion chamber, where it gets mixed with sprayed fuel and ignited. This process transfers high amount of energy to the mix, in the form of pressure and heat. The only way this energy can deplete is through the rear part of the engine, from where the mass of hot air can escape at high speed.
During its transit towards the so called exhaust cone, the air mass bleeds off part of its energy by making another series of blades spin, more or less like a windmill. This is the turbine, which is, in turn, connected to the compressor, making it spin to compress new fresh air.
The air mass flowing out of the engine after the turbine would be sufficient to give forward motion to the aircraft, and this is actually what happens in small jet planes or some military fighter jets.
The problem is that such a small jet stream, in order to have enough power to push a big airplane, should be accelerated to extremely high speeds, and this is not practical, and would require the use of a big amount of energy, which translates in a big quantity of fuel. Imagine this: if we wanted to double the speed of the air mass, we would need about four times the fuel!
For this reason a trick is used: instead of increasing the air mass speed, we accelerate a bigger quantity of it; in this case, the increase in fuel consumption is proportional to the increase in air mass, thus making this method more efficient.
In order to do this, the engine is designed in such a way that almost all the energy of the ignited fuel-air mixture gets transferred to the turbine, which in this case will also be used to spin a much bigger set of blades in the forward part of the engine, called the fan. The fan is what you see looking at the front of the engine, and just like a fan in your house, it blows back a much bigger mass of air, even though at slower speed.
You might then think that the bigger the fan, the more efficient the engine. Well, actually, an engine with a huge diameter would pose an excessive resistance to the airplane's forward motion when flying at high speeds.
Engineers just find the best trade off, based on the type of aircraft and its characteristics.
Engineers just find the best trade off, based on the type of aircraft and its characteristics.
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| A "fan" seen from behind |
The turbine engine is very common in aviation.
Even modern propeller aircrafts use turbojets (see The propeller airplane); the difference is that the energy transferred to the turbine, instead of being used for rotating a fan, is transmitted to a propeller. The advantage of such a setup is to have more efficiency at slow speeds, but the maximum achievable speed is rather limited.
Even the Auxiliary Power Unit, or APU, which gives electrical power and usually provides air conditioning when the engines are off, is a small turbine jet engine (see this link).
Now that you know something more about engines used in aviation, it will be easier for you to understand the reason for the typical jet engine noise (see this link).
How reliable is a jet engine?
Well... A lot! Airliners' jet engines undergo a series of very demanding tests at extreme operating conditions during the design phase, while they are operated at a much lower rate during their normal usage.
Moreover, during their operational life, thorough checks are periodically performed.
In any case, should an engine fail in flight , commercial liners always have at least another one... and aircrafts are designed to perfectly fly with one failed engine!
Don't Worry Fly Happy!
