Lift jet propulsion allows aircraft to lift vertically without the need for aerodynamic lift. This technology is used in both lighter-than-air vehicles and heavier-than-air vehicles, such as the Harrier jump jet. While lift jet vehicles consume more fuel, they do not require runways and can maneuver like fixed-wing aircraft once airborne. The Harrier is the most durable lift jet vehicle built as of 2011, but is being phased out and replaced by the F-35B or Joint Strike Fighter. Lift jet engines can use various propulsion systems, including turboprop, turbofan, or turbojet engines.
A lift jet is a form of propulsion system for aircraft that does not require the traditional principle of aerodynamic lift to increase its lift, which traditionally occurs through air pressure changes as a vehicle moves horizontally through the atmosphere. The unique propulsion of jet lift vehicles allows them to be aerostatic, meaning they can lift vertically from the ground without having to generate lift by moving horizontally at any significant speed. Aircraft based on aerostatic designs fall into two camps: lighter-than-air vehicles like balloons and blimps that rely on the buoyancy principle for lift, and heavier-than-air vehicles like the Harrier jump jet or vertical takeoff. and landing aircraft (VTOL) that use traditional jet engines or turboprop engines to rise vertically from a stationary starting position.
Most traditional commercial and military aircraft use aerodynamics to create lift as they move horizontally towards the ground. This is done by shaping the body of the aircraft so that air takes longer to pass through the upper than the lower surfaces in the body of the craft, creating a pressure difference around the vehicle that provides atmospheric lift. While this is a reliable method of generating lift, it requires stable atmospheric currents for safe travel and relatively long takeoff and landing runways. The aircraft needs to achieve a minimum speed necessary for the lift and thrust of its horizontally placed engines to counteract the effects of gravity. Planes using a lift jet system consume more fuel to counteract gravity, but do not require runways and can take off like helicopters, but maneuver like fixed-wing aircraft once airborne.
The use of the lift jet principle in heavier-than-air vehicles gives them a hybrid nature and a level of performance between traditional aircraft, helicopters and lighter-than-air vehicles. The main feature that most VTOL aircraft have in common is that their engines rotate once in the air to provide horizontal thrust. Some designs have airframes with additional horizontally built fixed propulsion systems to drive the vehicles forward once airborne.
Various types of jet lift vehicles have been built since the 1950s, but most were considered impractical. The Soviet Yakovlev-38, MIG-23, and Sukhoi-24 were successful designs, but they had high fuel consumption and limited payload and range capabilities, leading to their limited use. The UK-based Harrier or Jump Jet has proven to be the most durable heavier-than-air lift jet vehicle built as of 2011.
The Harrier consists of four models that were deployed as of 2010 by the North Atlantic Treaty Organization (NATO) allied nations of the United Kingdom, the United States, Spain, and Italy. Aircraft lift jet engines are turbofan engines that generate thrust by pushing external air through a fan assembly, heating it with burning fuel, and exhausting it vertically downward through a jet exhaust nozzle during takeoff to generate thrust. As of 2011, the Harriers were being phased out and replaced by a more modern version of the US-designed VTOL aircraft, known as the F-35B or Joint Strike Fighter, with the UK expected to adopt an updated version of F-35C of the aircraft for 2020.
Technically, a lift jet engine can use one of several propulsion systems, including turboprop, turbofan, or turbojet engines. A turboprop engine has an external propeller, and aircraft such as the US V-22 Osprey is a VTOL aircraft using such engines, although jet propulsion only plays a minor role in its lift capability. The turbojets can also be used as a lift jet, and have all the components of a turbofan engine like that in the Harrier, with the added ability that they actively compress incoming air to increase thrust, and are more efficient above speeds twice the speed of sound, or Mach 2.
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