Background

One of the most important components in an aircraft capable of sustaining flight without the use of thermals is of course the engine. One of the fundamental forces acting on an aircraft is drag. Drag is countered by thrust and when these two forces are the same, the aircraft enters an “equilibrium”. If in this state the thrust force increase, the aircraft accelerates until the amount of drag again equals the force of the thrust. Similarly, if the thrust is reduced, drag decelerates the aircraft until the thrust equals drag again.

The 4 Fundamental Forces in Powered Flight

How an Engine Works

Now given the stark contrast between the different types of engine, it might come as a surprise to know that they all operate the same on a fundamental level. In order to generate power an engine needs to perform the following tasks:

• Suck or draw in air from its environment.
• Compress the air it’s sucked in.
• Inject fuel into the compressed air and ignite the mixture.
• Expel the exhaust from the ignited mixture.

This process can be simplified in the commonly used expression of “suck, squeeze, bang, blow”.

An Illustration of “Suck, Squeeze, Bang, Blow” in a Piston Engine

Types of Engine

There are two main types of engine in the aerospace industry, Gas Turbine Engines and Piston Engines (also known as reciprocating engines). The main difference between these to engine types is how they generate their thrust.

Piston Engine Operation (typically connected to a propeller for thrust):

Gas Turbine Engine Operation:

Gas Turbine vs Piston

Gast turbine engines are by far the most favoured in the commercial aviation industry over piston engines because they’re:

• More efficient at high power operation
• Produces a lot of thrust
• Have a wide variety of variations for different operating conditions (hint at a future post?)
• Produce less vibration and mechanically more efficient (everything moves around a single point)
• Better operational characteristics at high altitudes
• Lightweight

Piston engines still have some advantages over Gas Turbine though:

• More efficient at low power operation
• Significantly cheaper
• Use less fuel idling
• Easier to overhaul
• Probably less catastrophic if something goes wrong (i.e. components aren’t going to spin off and embed themselves into the fuselage, thus potentially severing other components.)

These differences in effectivity somewhat isolate the two types of engines to different markets. Airliners and military aircraft typically experience harsh operating conditions (flying at high speeds and altitudes). Weight saving and efficiency is often the key factor on whether an aircraft is suitable for a particular role. Comfort (a lack of vibration) is also a particularly important factor for the commercial airline market. Price is also less of a concern when compared to the private consumer markets. Thus, Gas turbine engines are the natural choice for these roles.

By contrast, aircraft owned privately (by one or two people, or a small company like a flight school) often don’t need or benefit from high altitude performance, especially for short flights such as typical training flights or recreational flights. In these instances, flying below the clouds for nicer views, etc is more beneficial. Owners of private aircraft are generally also a lot more restricted in terms of how much they can afford to spend on an aircraft as they don’t have the backing of a large company with millions to throw around. These owners often don’t have speed as a priority either. Thus, the piston engine is a natural choice for private owners.

Conclusion

While there are exceptions and partial crossovers in delivery of thrust (e.g. Turboprop engines. A gas turbine that drives a propeller, instead of a piston engine driving a propeller), both main types of engine clearly have their place in the aviation industry with each sporting advantages and disadvantages, making them more effective in their own environments and so attractive to different markets.

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