What actually is a turboprop?
In the simplest terms – a turboprop is a jet engine with a propeller attached. Rather than converting its energy into thrust coming from a jet pipe, the turboprop engine turns this into power that drives a propeller to create thrust.
Passenger-carrying turboprops themselves tend to be far smaller than their jet counterparts, and typically have between 1-2 engines mounted in the nose or on the wings. There are numerous military turboprop aircraft in service both large and small, but we will focus on the passenger aircraft for the sake of this comparison. The two most common passenger-carrying turboprops in commercial service today are the ATR 42, ATR 72, and Bombardier Dash 8 Q400. They are also prevalent in private aviation, with aircraft such as the Beech King Air and Pilatus PC-12 proving their worth as safe and efficient utility aircraft.
Turboprop aircraft are generally favored on shorter range and regional routes. They fly at slower speeds and lower altitude than jets, and are often more efficient on such routes. The passenger experience is also quite different. Before we take a deeper dive into why, let’s look at the origins of the humble turboprop.
The Development of the turboprop
The history and development of turboprop aircraft trace back to the mid-20th century, marking a significant advancement in aviation technology. The concept of the turboprop engine emerged in the 1930s, aiming to combine the efficiency of propeller-driven aircraft with the increased speed and power brought about by newly developed jet engines.
One of the earliest turboprop engines to be developed was the Rolls-Royce RB.50 Trent. Remarkably, the first turboprop engine was actually tested on Britain’s first jet fighter, the Gloster Meteor. The ‘Trent Meteor’ was the only example of its type and marked the initial step towards integrating turbine engines with propellers.
The Vickers Viscount, which first flew in 1948, became the first turboprop aircraft to achieve substantial commercial success. The Viscount, equipped with the Rolls-Royce Dart engine, demonstrated the advantages of turboprops in terms of fuel efficiency, range, and reduced noise compared to piston-engine aircraft. Its commercial operations began in 1953 with British European Airways, quickly gaining popularity for its smooth ride and lower operational costs. The Viscount was one of the few British-built aircraft to achieve widespread commercial success in the United States.
The success of the Vickers Viscount paved the way for other notable turboprops, such as the Fokker F27 Friendship and the Hawker Siddeley HS 748, establishing turboprops as reliable and efficient aircraft for short to medium-haul flights. Turboprops by nature have a distinctive sound, and the Viscount was no exception. The Viscount is even something of an unsung pop culture icon. The whir of the four Dart engines can be clearly heard in the opening moments of the Beatles’ 1969 record ‘Back in the USSR’. This remains one of the few recordings of this distinctive sound.
Turboprops vs. Jets – what are the differences?
Down to business. Let’s look at the physical and operational differences between jets and turboprops that affect the airlines and, more importantly, you as a passenger.
The physical differences
There are some clear physical differences between turboprops and jets, the most notable being size. Turboprops are mostly used on shorter distance, lower capacity routes, and as such are typically smaller than their regional jet and narrowbody counterparts. Both the ATR and Bombardier turboprops are high winged (have the wing mounted on top of the fuselage) compared to most jets which are low winged, though one notable example of a high winged regional jet is the Dornier 328 Jet.
Turboprops often have a lower profile than jets (they sit lower to the ground) and typically feature a T-tail, whereby the horizontal stabilizer is fitted to the top of the vertical tail fin. The main advantage of this is that the elevator surface is clear of the effect of downwash from the propeller. That said, T-tails are more prone to deep stalling, a stall condition from which it can be incredibly difficult to recover.
The aircraft are also systemically quite different. Whilst jets often use bleed air to remove ice from the aircraft’s surfaces, turboprops employ a system of inflatable ‘boots’ to remove ice from the leading edges of the wing. This is particularly important as turboprops cruise lower than jets, and subsequently spend more time in icing conditions. Mishandling of flaps in icing conditions was cited as a key factor in the crash of American Eagle Flight 4184 in 1994.
In the cockpit, many turboprop aircraft do not feature an autothrottle, leaving manual thrust management to the pilots even when the autopilot is engaged.
The differences for airlines
Turboprops present an array of advantages for airlines.
Turboprop aircraft generally burn less fuel than jets, making them far more cost efficient on shorter routes. Jets have to gain as much altitude as possible to get the most efficiency from their engines. Turboprops do not have this problem. Propellers can maintain high efficiency in the denser air found at lower altitudes, providing good fuel economy without the need for high-altitude operation.
They also have excellent performance in hot and high conditions – i.e. airports at high altitudes or subject to high temperatures. Jet engines have a degraded performance in such conditions for several reasons.
- Air is less dense in hot and high conditions. Jet engines rely on compressing air to achieve efficient combustion. Lower air density means there are fewer air molecules entering the engine, reducing the mass flow rate and, consequently, the thrust produced by the engine.
- High ambient temperatures reduce the temperature difference between the incoming air and the exhaust gasses. This smaller temperature gradient decreases the efficiency of the engine’s thermodynamic cycle, leading to reduced power output and increased fuel consumption.
- At high altitudes, the thinner air provides less lift, requiring jet engines to work harder to achieve the same performance. Combined with reduced thrust, this can significantly impact takeoff and climb performance.
Turboprops, on the other hand, suffer less from these conditions due to their different operating principles.
- Turboprops generate thrust primarily through their propellers, which are more efficient at converting engine power into thrust at lower speeds. In less dense air, propellers can maintain their efficiency better than jet engines, which rely on high-speed exhaust gasses.
- Turboprop engines are generally designed to operate efficiently across a broader range of temperatures and altitudes. They can maintain better performance in hot and high conditions because the power produced by the turbine drives the propeller directly, rather than relying solely on jet exhaust for thrust.
- Turboprops typically operate at lower speeds, where the effects of reduced air density are less pronounced. This allows them to maintain performance and efficiency even when conditions are less than ideal.
The same advantages mean turboprops can take off and land in shorter distances than their jet counterparts, making them ideal for operations out of smaller, regional airports with shorter runways.
The lower cruising levels of turboprop aircraft also reduces the stress on the airframe caused by the pressure when pressurizing the cabin, increasing the service life of the aircraft.
What differences will you notice as a passenger?
The passenger experience on board a turboprop aircraft is strongly influenced by its physical and flight characteristics.
Being smaller than jets, turboprop cabins are also, in general, smaller. This has led to turboprops being anecdotally less popular with passengers than regional jets, particularly in the North American market. Turboprop manufacturers have made strides to develop more spacious cabins with more overhead bin space in recent years, with the latest developments to the ATR72-600 focussing on a renewed cabin design.
Are turboprops quieter than jets?
In short – yes. Externally, turboprops tend to be quieter than jet-powered aircraft. The A320-200 typically generates noise levels up to 120 dB near the runway, while the A320neo, with its quieter, more efficient engines, reduces this to around 110 dB. In contrast, turboprops like the ATR 72-600 and Bombardier Dash 8 Q400 are generally quieter, producing noise levels up to 100 dB and 105 dB respectively. Those lower noise levels mean turboprops are less likely to fall foul of increasingly strict noise regulations at some airports.
Let’s take a look at why turboprops are so much quieter.
There are several factors that make turboprops quieter than jets.
A propeller has a larger surface area that rotates at a lower speed than the fan blades in a turbojet or turbofan engine. Lower rotational speeds result in less noise. Modern turboprops are designed with advanced propeller blade shapes and materials that reduce noise. The blade designs help minimize aerodynamic noise produced during operation.
Another factor is bypass ratio. An engine’s bypass ratio is the ratio of air bypassing the engine core to the air passing through it. This has a huge amount of influence on the performance of an engine, and new generation engines such as the CFM LEAP and Pratt & Whitney PW1000 have bypass ratios of around 9:1 to 11:1. Older commercial aircraft engines had lower ratios, which is why they appear thinner than modern engines. This high bypass ratio contributes to improved fuel efficiency and reduced noise levels compared to previous generations of engines. When it comes to turboprops, more air is passed through the propeller than goes through the combustion process, making the engine quieter overall.
What about noise on the inside?
Noise levels inside the Airbus A320-200 typically range from 85 to 95 dB at takeoff thrust, whilst the ATR 72-600 turboprop generally maintains cabin noise levels between 75 and 85 dB, considerably lower. That said, noise levels inside the A320neo also average around 75 to 85 dB, showing the significant strides that the newer generation of narrowbodies have made to reduce noise.
That said, turboprop passengers do encounter more cabin vibrations than jet passengers. This is a notable problem on the Dash 8 Q400, which has an innovative Automatic Noise and Vibration Suppression system (ANVS) installed to counter its naturally higher cabin noise and vibration.
The ANVS system employs active noise control techniques, using microphones placed throughout the cabin to detect noise levels. These microphones send signals to an onboard computer that analyzes the noise patterns. The system then generates anti-noise signals through speakers embedded in the cabin, creating sound waves that are 180 degrees out of phase with the detected noise. This effectively cancels out the unwanted noise through destructive interference. The Q400 is also equipped with dynamic vibration absorbers and tuned vibration dampers placed strategically on the airframe and engine mounts. These components help to absorb and dissipate vibrations generated by the engines and propellers, preventing them from being transmitted into the cabin.
Where are turboprops operating?
According to data from IBA Insight, there are a total of 1,631 active DHC8 and ATR family turboprops globally as of July 2024 (excluding military types). The rugged versatility of the passenger turboprop means its global footprint is vast and varied. Turboprops connect isolated communities to major cities, and make some more unique locations accessible by air.
Here’s a great piece of research from The Air Current that looked at the global footprint of ATR operations in late 2023. As you can see, the diversity of operations is vast and varied.
Turboprops also come in handy for Arctic and Antarctic operations, with the British Antarctic Survey utilizing the rarely seen De Havilland Canada Dash-7 (DHC-7) in their Arctic operations.
The future of turboprops
Turboprops are increasingly seen as an avenue for decarbonizing aviation, with their lower overall emissions and increased efficiency on some routes proving a driver for further development. Most existing concepts and schemes involve supplementing jet fuel with electric and hybrid electric technologies where possible, reducing overall emissions at the tailpipe.
ATR’S ‘Evo’ program aims to “incorporate innovative technologies to enable significant improvements in operating costs”, using the existing ATR72-600 as a platform. Meanwhile Embraer’s future turboprop concept uses their successful E-jet range as a basis, aiming to bring a more efficient aircraft into the regional market and take advantage of the larger, more comfortable cabins in regional jets (though this project is on hold at the time of writing).
