Thursday, December 13, 2018

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The Front Office | Suck, Bang, Blow

by Chas Hines

The Front Office | October 2018

“The Front Office” is your worldly salvation when it comes to answering questions about jump pilots and piloting. We talk about what exactly pilots do behind the scenes to make your favorite time of week happen. We talk about what they see, what decisions they face and why they might be in a bad mood between loads. We talk about why you are wrong if you haven’t seen “Top Gun.” Mostly, you get a one-of-a-kind inside view from the one seat in the airplane you never get to be in.

 

This month’s subject—engines—is one that jump pilots get a lot of questions about. Whether you are at a big DZ with high-performance multi-engine airplanes burning Jet-A or are lucky enough to be blessed with a 100LL Avgas-burning Cessna 182, there’s one thing that’s certain: Your jumpship needs horsepower to get to altitude.

First, some very basic principles of internal combustion: An engine generates mechanical power (in jump airplanes, this drives propellers) by burning fuel. This burning fuel either pushes a piston or forces a turbine to move the necessary parts. In either case, when an internal combustion engine burns fuel, the process can be summed up by this choice combination of words: suck-bang-blow.

The piston method is most completely illustrated by the four-stroke internal combustion engine, where suck-bang-blow happens step by step, in order.

Suck (Intake): The piston moves down and draws fuel and air into the cylinder.

Bang (Compression and Power): The piston moves back up the cylinder and compresses the fuel and air to a predetermined pressure. At the top of its movement toward the cylinder head, a spark (or diesel ignition) ignites the fuel-air mixture, which drives the piston down (creating our precious mechanical motion).

Blow (Exhaust): The piston then moves back up, pushing the waste gas and air out of the cylinder, and the cycle begins all over again.

When an engine runs under its own power, this process is self-perpetuating until someone kills the engine by removing the fuel, spark or air (the necessary components of a fire).

This exact cycle occurs inside any piston aircraft, whether it’s a Cessna 182 or 206, Robinson Helicopter or DC-3. The Cessna 182 has six pistons and cylinders arranged horizontally (three opposite three), which all push a common crankshaft. Some aircraft, such as the DC-3, have nine cylinders per engine! In either case, the common crankshaft attaches to a propeller, which is what drives the airplane forward.

Suck-bang-blow occurs in big, powerful turbine engines, as well. And the principles of operation are actually considerably simpler than the four-stroke engine.

If we look at the engine of a common turbine jumpship, the SC-7 Skyvan, we can see each of our suck-bang-blow steps all at once by looking at the cross-section of a running engine. Let’s assume the air is entering our diagram from the left:

Suck: Air is pulled straight into the compressor section of the engine, where pressure builds beyond the compressor blades.

Bang: The high-pressure air then mixes with vaporized fuel and ignites.

Blow: The hot, expanding gas pushes its way out of the engine past turbine blades, which connect to the same shaft (at least in this particular engine) that the compressor fan is on. In the case of our illustrated engine, this shaft also connects to a gearbox—and eventually the propeller—in order to (you guessed it) move the airplane forward.

When the engine is running and suck-bang-blow is working its magic, this system is also self-perpetuating. In fact, because there are so few moving parts, seals and metal-on-metal fittings, turbine engines are incredibly powerful and reliable, especially relative to their weight.

There are simpler piston engines (such as two-strokes) and more complicated turbine engines (such as afterburning turbofans) that weren’t covered here. The good news is that the principles are essentially unchanged: Bring fuel and air in, light them on fire and get mechanical motion as a result. The last 100-plus years have given us amazing improvements in both efficiency and performance. I think we will be blessed to see them continue for some time to come.

Chas Hines | C-41147
FAA Certified Flight Instructor and Airline Transport Pilot 

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