If you’re just starting to consider the possibility of owning your own aircraft, one of the first things you may want to understand is the difference between piston engine powered aircraft and turboprop aircraft. Understanding the differences - and selecting the right aircraft for your needs - will ensure years of happy missions!
First A little history…
Piston engines, also known as reciprocating engines, are descendents of steam engines which first appeared in the early 17th century. In simple terms, steam engines use heat to produce steam pressure, which in turn is used to generate rotating motion.
Steam engines were eventually used to drive everything from mechanical equipment to the first railway trains. In today’s aircraft piston engines, gas propulsion replaces steam in creating pressure within the engine.
So how do piston engines work?
Piston engines are characterized by one or more cylinders, each containing a snugly fitting piston (a solid cylinder) which is able to move freely back and forth within the cylinder. Each piston is connected to a crankshaft, via a connecting rod (or 'piston rod').
When fuel is introduced and ignited within a cylinder, the hot gases produced expand with incredible force, driving the corresponding piston forward within the cylinder, which in turn moves the connecting rod forward and causes the crankshaft to turn. The full rotation of the crankshaft pushes the piston back into the cylinder and the cycle begins again. It is the crankshaft that converts the reciprocating, linear motion of the piston into the rotating motion that drives the propeller.
In a typical 4- or 6-cylinder aircraft piston engine, pistons rods are connected to the crankshaft at offset intervals to even out the firing of the pistons.
Often a flywheel is used to further reduce vibration and encourage smooth propeller operation. Generally, more cylinders translate to smoother operation.
Piston engines were used to power all aircraft until jet engines were first conceived in the early 20th century, inspired by the quest for aircraft that could fly at greater speeds and higher altitudes. Today, piston engine aircraft remain popular for both personal and business use for their low cost of entry and solid performance.
On to Turboprops…
Turboprop aircraft, which are powered by a close relative of the turbine engine, are another animal entirely…
A turbine – which is simply another word for ‘fan’ – is exactly that: a rotary driven fan that derives energy from fluid flow or airflow. Early ancestors of today’s turbine engines are mill wheels and windmills, which each convert water flow and wind flow into rotational energy, respectively.
So how does a turbine engine work?
A simple turbine design includes a rotor (rotating wheel) to which angled blades are attached. As fluid or air moves through the blades, the fluid or air pressure against the blades will cause the rotor to spin.
In an aircraft turbine engine, high-pressure gases, resulting from the combustion of jet fuel, take the place of water or wind in driving the motion of the rotor. For this reason, aircraft turbine engines are categorized as gas turbine engines. Note that they are also referred to, interchangeably, as jet engines, due to the jet of exhaust they produce.
In simple terms, aircraft turbine engines are characterized by a front-end air inlet, a compressor, a combustion chamber (combustor), the turbine itself, and often, a propelling nozzle. The compressor, comprised of a series of rotating fans, draws air into the engine via the inlet, first compressing it and then forcing it into the combustion chamber, under extremely high pressure.
Fuel is introduced and ignited within the combustion chamber, driving an explosive force of hot air through the turbine and causing it to spin. The rotating turbine, which is connected to the compressor via shaft, sustains continuous engine operation by driving rotation of the compressor fans. Simultaneously, hot air exits the engine at incredible speed, providing a powerful jet stream out the back of the engine. It is this jet stream thrust that propels the aircraft forward. Often a back-end propelling nozzle is used to narrow the jet stream and further intensify speed.
Aircraft turbine engines generally fall into four common categories: turbojets, turbofans, turboprops and turboshafts. We’ll focus on Turboprops, but it is helpful to understand the differences between these engine types:
Turbojets are jet engines that depend exclusively on the thrust of jet exhaust expelled by the engine for propulsion. Turbojet engines are extraordinarily powerful and only efficient at extremely high speeds. As such, they are more likely to be found in a missile, although the now defunct Concorde jet is an example of a turbojet powered commercial aircraft.
Turbofan engines, which power the majority of commercial aircraft, are turbine engines that have been fitted with a powerful front-end fan. The fan sends air into the combustor, similar to a turbojet engine. However, the fan also sends a second stream of air through a larger cylinder entirely outside (and around) the engine core. This second stream of air provides additional thrust, cools the engine, and also serves to reduce engine noise. Turbofans are interchangeably referred to as bypass engines, in reference to this airflow that bypasses the combustor.
Like turbojet and turbofan engines, aircraft Turboprop engines rely on a gas turbine for power. However, in the case of a turboprop aircraft, the turbine drives a rotating shaft, which in turn drives a reduction gear, which ultimately drives a propeller. The reduction gear is necessary to convert the high-speed shaft rotation into slower, functional propeller speed. Most of the power generated in a turboprop aircraft is used to drive the propeller and any remaining jet exhaust actually plays virtually no role in propulsion of the plane!
The Turboshaft engine is similar to the turboprop in that energy produced within the engine is used to drive a shaft rather than to produce jet stream thrust. However, in the case of a turboshaft engine, the shaft is used to drive something other than a propeller - such as a helicopter rotor.
So How Do Piston Aircraft & Turboprops Compare?
From a manufacturing and engineering perspective, the reciprocating engines found in piston aircraft are far less complex than their turboprop counterparts. This is primarily due to the high temperatures and forces unique to turboprop engine operation, which must be accommodated both in materials and engine design – and which come at a price. For this reason, piston aircraft almost always offer a lower cost of entry.
While piston engines are indeed a more simple design, turboprop engines have far fewer moving parts and the smooth, vibration-free operation of a turboprop engine typically offers greater reliability - and longer time between overhaul (TBO) - than a piston aircraft. In other words you can generally log more hours on your turboprop before you need to bring it in for inspection.
The typical TBO for a turboprop engine is 3,000 hours while TBO for a piston aircraft is generally 2,000 hours. So the higher upfront cost of a turboprop aircraft may be somewhat offset by lower maintenance costs over time.
The power of a turbine engine almost always allows turboprop aircraft to travel at higher speeds than piston aircraft. And as turboprop aircraft cabins are typically pressurized these planes also commonly fly at higher altitudes.
Turboprop aircraft are generally most efficient at altitudes of 20,000 to 30,000 feet and at average speeds of 250 to 300 ktas, while piston aircraft, which are typically not pressurized, are usually restricted to altitudes of 12,000 feet or less (unless an oxygen system is used). And as piston engines are less powerful than turboprops engines, piston aircraft speeds are generally limited to about 200 ktas.
Piston aircraft are generally smaller aircraft, seating no more than six passengers, and are well suited for relatively short missions of 300 miles or less, while turboprops tend to be larger aircraft with greater passenger capacity - and more fuel on-board – and are more likely to be found flying distances of up to 1,000 miles.
Both piston engine aircraft and small turboprop aircraft are right at home accessing smaller airports with shorter runways, making them the ideal way to reach many desirable final destinations, where larger aircraft simply cannot land.
Ultimately, there are many factors to consider when investing in an aircraft, but giving careful thought to the types of missions you intend to fly and understanding which aircraft will best support your intended use - while accommodating your budget - will help you arrive at the right choice!
If you are considering purchasing an aircraft and would like to learn how Shoreline Aviation can help, please call contact Keith Douglass or Ann Pollard at 781-834-4928.