To me, the most beautiful sound in the world is the spool-up of a turbofan engine. It gives me chills every time. Years ago at my home base airport, our flight academy building was located across from our mother FBO. We shared ramp space with transient parking, and if I happened to be walking across the ramp when a jet began its start sequence, I'd always stop and listen. There's just no other sound like it. I'd wait for the "click, click, click..." and then hear the whine start to sing as the turbine began spinning faster and faster. Major cool. I'd always grin, and I still do.
A jet takeoff would bring business to a standstill for a few seconds inside the flight academy building as everyone inside peered out the window and listened to the engines roar. Nothing inspires me more than watching and listening as a jet rockets down the runway and leaps into the sky.
For the turbine-challenged (or unexposed), which many piston pilots are, here are the basics of turbofan operation. Surprisingly, jet engine operation is much more simple than piston engine operation. A turbofan engine consists of three major parts: the compressor section, the ignition/combustion section, and the turbine section. Air is sucked into the engine by a series of compressor fans at the front of the engine. The speed of the first and largest compressor fan is expressed as a value of N1 in the cockpit. After intake air is compressed it enters the ignition/combustion section. Here the compressed air is mixed with fuel and ignited. The expanding gas then accelerates over the turbine causing it to spin very quickly (turbine speed is expressed as a value of N2 in the cockpit). The burned fuel/air exhaust gas then travels out the back of the engine through the exhaust nozzle. The spinning turbine, in turn, spins the compressor fans and the process becomes self-sustained. Bleed air from an auxiliary power unit is often used to set the engine into this self-perpetuating motion on startup.
The hot exhaust gas stream out the back of the engine results in high pressure behind the engine. Because the pressure in front of the engine is much lower than behind it, the engine (and airplane it is attached to) are sucked forward. This works similarly to how an airfoil produces lift. Airplanes fly by altering pressure around them, and they are quite literally being sucked upward and onward. Many older jets express engine power in terms of engine pressure ratio (EPR). EPR (pronounced "e-per," if you want jet pilots to think you're cool) compares the pressure at the back of the engine to the pressure at the front of the engine. The larger the difference, the greater the power.
Modern turbofan engines are high bypass engines, meaning most of the intake air bypasses the engine core and is simply accelerated and directed out the back of the engine. High bypass engines are quieter and more efficient. If you've ever seen an old military fighter takeoff and noticed black exhaust streaks behind it, it was probably a low bypass turbojet engine. Those black streaks are basically unburned fuel.
A byproduct of jet engines is bleed air. Bleed air is compressed air that is "bled" from the engine during some stage of the compression process. It is directed away from the engine and is used for ice control and environmental control purposes. Because the bleed air is hot, it can be circulated through engine nacelles and leading edges of the wings to heat them up for anti-ice functionality. The remaining compressed air is usually directed into the cabin for pressurization and heating or cooling. In some jets, engine bleeds must be left off during takeoff so all available intake air can be used for the production of thrust. Flight crews refer to this as a "bleeds off takeoff," and it's normally performed on shorter runways and/or at high density altitudes and high weights. Just like a good hunter doesn't waste any animal, a jet engine doesn't waste any of its capability to produce power and useful bleed air.
Another cool thing about turbofan engines is that their thrust production is non-linear. On takeoff, for example, as the airplane accelerates down the runway, more and more air is being rammed into the engine because of the increase in speed. This is called "ram recovery." Because the engine is moving through the air faster, more air is being gulped in for compression and the engine produces more thrust. This means the rate at which the airplane accelerates down the runway increases during the takeoff run. It is, indeed, a spirited affair.
Jet engines, they're a thing of beauty. Extremely reliable, efficient, and incredibly sexy. And they sing so beautifully, like a serenade to my ears.
(Citation CJ1 engine start) http://www.youtube.com/watch?v=L8TboIEJyg8
