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Four-engined jet aircraft
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Four-engined jet aircraft
A four-engined jet, sometimes called a quadjet, is a jet aircraft powered by four engines. The presence of four engines offers increased power and redundancy, allowing such aircraft to be used as airliners, freighters, and military aircraft. Many of the first purpose-built jet airliners had four engines, among which stands the de Havilland Comet, the world's first commercial jetliner. In the decades following their introduction, their use has gradually declined due to a variety of factors, including the approval of twin-engine jets to fly farther from diversion airports as reliability increased, and an increased emphasis on fuel efficiency.
The engines of a 4-engined aircraft are most commonly found in pods hanging from pylons underneath the wings. This can be observed in the Airbus A340, Airbus A380, and Boeing 747. Many military airlifters also feature this design, including the Antonov An-124, Boeing C-17 Globemaster, and Lockheed C-5 Galaxy. In this location, the engines can act as a relieving load and reduce the structural weight of the wing by 15%. They are also in a more accessible location for maintenance or replacement. However, disadvantages include a higher risk of the engines ingesting foreign objects as they have a lower ground clearance, and a larger yawing moment during an engine failure. The supersonic airliner Concorde had its engines mounted in rectangular pods conformal to the underside of the wing, without any pylons. The omission of pylons reduces drag and eliminates the risk of them being overstressed.
The four podded engines can also be attached to the rear fuselage, necessitating a T-tail. This reduces cabin noise and frees up more space on the wings for high-lift devices and fuel storage. The airflow over the wings is also undisturbed due to the absence of pylons. However, the rear-mounted engines shift the centre of gravity aft, and are located further from the fuel supply. The Ilyushin Il-62 and Vickers VC10 both have their four engines mounted in this configuration.
Jet aircraft can also be designed with engines buried within the aircraft structure. The de Havilland Comet incorporated four turbojets buried inside its wing roots, the most common location for buried engines. This design reduces both drag and the risk of ingesting foreign objects, but increases the difficulty of maintenance and complicates the wing structure. The Northrop Grumman B-2 Spirit stealth strategic bomber has all four turbofans buried within its wing (as a flying wing, the wing is the main structural component). This reduces the heat signature of the engines by concealing the fans and minimizing the exhaust signature.
The Hawker Siddeley Trident 3B not only has two engines in rear fuselage external nacelles, but also has other two engines mounted vertically in the tail. The aircraft was initially designed as trijet, but the Trident 3B added a fourth engine as additional power was required by the stretched fuselage, increased wing chord and raised gross weight.
A major advantage of having four engines is the redundancy offered, leading to increased safety. A single engine failure is much less significant as the three remaining engines can usually provide sufficient power to comfortably reach a diversion airport or continue the journey, depending on factors such as the severity of the malfunction, altitude, fuel load, and weather conditions. With the increased reliability of jet engines, engine failures rates can be as low as 1 in-flight shutdown per 100,000 engine-hours, reducing the significance of this advantage.
During a single-engine failure, the amount of power lost with four engines is proportionately lower than three or two engines. This is because three of the four engines will still be functioning, constituting a 25% reduction in thrust, compared to 33% for trijets and 50% for twinjets. This can be observed in the following example involving the Boeing 747-400 quadjet, McDonnell Douglas MD-11 trijet, and Boeing 767-300ER twinjet. With all engines operative at maximum takeoff weight, all three aircraft have the power to weight ratios of approximately 1 to 3.4. Following the failure of one engine, the power to weight ratio drops to 1 to 4.7 (747-400), 1 to 5.5 (MD-11), and 1 to 6.6 (767-300ER). The Boeing 747-400 experiences the least degradation in performance, making it safer during an engine failure.
Fitting an aircraft with four engines also increases power, enabling more passengers, heavier payloads, and increased performance. This was especially important for early jet airliners, as the low-bypass turbofans and turbojets of the time were much weaker compared to modern high-bypass turbofans. The Pratt & Whitney JT3D from 1958 had a thrust output of 76 kilonewtons (17,000 lbf), while modern engines like the General Electric GE90 can produce over 440 kilonewtons (100,000 lbf) of thrust, making this advantage less significant nowadays as larger airliners no longer necessarily need four engines.
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Four-engined jet aircraft
A four-engined jet, sometimes called a quadjet, is a jet aircraft powered by four engines. The presence of four engines offers increased power and redundancy, allowing such aircraft to be used as airliners, freighters, and military aircraft. Many of the first purpose-built jet airliners had four engines, among which stands the de Havilland Comet, the world's first commercial jetliner. In the decades following their introduction, their use has gradually declined due to a variety of factors, including the approval of twin-engine jets to fly farther from diversion airports as reliability increased, and an increased emphasis on fuel efficiency.
The engines of a 4-engined aircraft are most commonly found in pods hanging from pylons underneath the wings. This can be observed in the Airbus A340, Airbus A380, and Boeing 747. Many military airlifters also feature this design, including the Antonov An-124, Boeing C-17 Globemaster, and Lockheed C-5 Galaxy. In this location, the engines can act as a relieving load and reduce the structural weight of the wing by 15%. They are also in a more accessible location for maintenance or replacement. However, disadvantages include a higher risk of the engines ingesting foreign objects as they have a lower ground clearance, and a larger yawing moment during an engine failure. The supersonic airliner Concorde had its engines mounted in rectangular pods conformal to the underside of the wing, without any pylons. The omission of pylons reduces drag and eliminates the risk of them being overstressed.
The four podded engines can also be attached to the rear fuselage, necessitating a T-tail. This reduces cabin noise and frees up more space on the wings for high-lift devices and fuel storage. The airflow over the wings is also undisturbed due to the absence of pylons. However, the rear-mounted engines shift the centre of gravity aft, and are located further from the fuel supply. The Ilyushin Il-62 and Vickers VC10 both have their four engines mounted in this configuration.
Jet aircraft can also be designed with engines buried within the aircraft structure. The de Havilland Comet incorporated four turbojets buried inside its wing roots, the most common location for buried engines. This design reduces both drag and the risk of ingesting foreign objects, but increases the difficulty of maintenance and complicates the wing structure. The Northrop Grumman B-2 Spirit stealth strategic bomber has all four turbofans buried within its wing (as a flying wing, the wing is the main structural component). This reduces the heat signature of the engines by concealing the fans and minimizing the exhaust signature.
The Hawker Siddeley Trident 3B not only has two engines in rear fuselage external nacelles, but also has other two engines mounted vertically in the tail. The aircraft was initially designed as trijet, but the Trident 3B added a fourth engine as additional power was required by the stretched fuselage, increased wing chord and raised gross weight.
A major advantage of having four engines is the redundancy offered, leading to increased safety. A single engine failure is much less significant as the three remaining engines can usually provide sufficient power to comfortably reach a diversion airport or continue the journey, depending on factors such as the severity of the malfunction, altitude, fuel load, and weather conditions. With the increased reliability of jet engines, engine failures rates can be as low as 1 in-flight shutdown per 100,000 engine-hours, reducing the significance of this advantage.
During a single-engine failure, the amount of power lost with four engines is proportionately lower than three or two engines. This is because three of the four engines will still be functioning, constituting a 25% reduction in thrust, compared to 33% for trijets and 50% for twinjets. This can be observed in the following example involving the Boeing 747-400 quadjet, McDonnell Douglas MD-11 trijet, and Boeing 767-300ER twinjet. With all engines operative at maximum takeoff weight, all three aircraft have the power to weight ratios of approximately 1 to 3.4. Following the failure of one engine, the power to weight ratio drops to 1 to 4.7 (747-400), 1 to 5.5 (MD-11), and 1 to 6.6 (767-300ER). The Boeing 747-400 experiences the least degradation in performance, making it safer during an engine failure.
Fitting an aircraft with four engines also increases power, enabling more passengers, heavier payloads, and increased performance. This was especially important for early jet airliners, as the low-bypass turbofans and turbojets of the time were much weaker compared to modern high-bypass turbofans. The Pratt & Whitney JT3D from 1958 had a thrust output of 76 kilonewtons (17,000 lbf), while modern engines like the General Electric GE90 can produce over 440 kilonewtons (100,000 lbf) of thrust, making this advantage less significant nowadays as larger airliners no longer necessarily need four engines.
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