Lift jet
Encyclopedia
A lift jet is a jet engine
angled to provide an aircraft
with aerostatic (i.e. not requiring the movement of air over an airfoil
) lift
instead of (or in addition to) thrust. On a fixed-wing aircraft, lift jets may be installed as auxiliary engines, with a separate engine to provide forward thrust, or, as in the Harrier jump jet
, may be vectored
in flight to provide both.
Lift jets were first designed by German
engineers during World War II
, but none saw operational service. Many nations had experimental programs involving such engines by the early 1950s: one typical example was Rolls-Royce
's Thrust Measuring Rig
(TMR), nicknamed the "Flying Bedstead
," which took its lift solely from engine thrust. An early dedicated lift jet was the Rolls-Royce RB108
, first run in 1955.
In the early 1960s both the USSR and western nations considered the possibility of lift engines as a way of providing STOL
or even VTOL
capability to combat aircraft. The Soviets did side-by-side testing of versions of combat aircraft using variable geometry wings
and lift jets (the results became the Mikoyan MiG-23 and Sukhoi Su-24
). The problems with the auxiliary lift engine are high fuel consumption, extra weight (which is simply dead weight when the engines are not needed for lift), and taking up fuselage volume that could be used for fuel or other systems. The Soviets decided that variable-geometry wings provided comparable advantages in take-off performance without as many penalties.
One of the few operational military aircraft with auxiliary lift engines was the Soviet Yakovlev Yak-38
, a VTOL fighter used by the AVMF's small aircraft carrier
s, which were not large enough to support conventional fixed-wing aircraft. The Yak-38 proved disappointing in service for most of the above reasons, having limited payload and range as well as dismal handling.
A novel alternative to the lift jet for auxiliary vertical thrust is to be employed by the VTOL Lockheed F-35B
version of the U.S. Joint Strike Fighter program. This does not have a jet lift engine, but a central lift fan that is powered by the main engine via a driveshaft
. When not needed, it is declutched from the engine so that it does not consume power (analogous to the transfer case
of a four-wheel drive
vehicle). The operational viability of this system remains to be seen. It has, however, in pre-operational testing proven the ability to take off vertically and go supersonic in level flight in the same mission, a first for a combat aircraft.
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
angled to provide an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
with aerostatic (i.e. not requiring the movement of air over an airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
) lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
instead of (or in addition to) thrust. On a fixed-wing aircraft, lift jets may be installed as auxiliary engines, with a separate engine to provide forward thrust, or, as in the Harrier jump jet
Harrier Jump Jet
The Harrier, informally referred to as the Jump Jet, is a family of British-designed military jet aircraft capable of vertical/short takeoff and landing operations...
, may be vectored
Thrust vectoring
Thrust vectoring, also thrust vector control or TVC, is the ability of an aircraft, rocket or other vehicle to manipulate the direction of the thrust from its engine or motor in order to control the attitude or angular velocity of the vehicle....
in flight to provide both.
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
engineers during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, but none saw operational service. Many nations had experimental programs involving such engines by the early 1950s: one typical example was Rolls-Royce
Rolls-Royce plc
Rolls-Royce Group plc is a global power systems company headquartered in the City of Westminster, London, United Kingdom. It is the world’s second-largest maker of aircraft engines , and also has major businesses in the marine propulsion and energy sectors. Through its defence-related activities...
's Thrust Measuring Rig
Rolls-Royce Thrust Measuring Rig
The Rolls-Royce Thrust Measuring Rig was a pioneering vertical take-off and landing aircraft developed by Rolls-Royce in the 1950s. The TMR used two Nene turbojet engines mounted back-to-back horizontally within a steel framework, raised upon four legs with castors for wheels...
(TMR), nicknamed the "Flying Bedstead
Flying bedstead
The Flying Bedstead was a nickname given to two completely different experimental vertical take-off and landing aircraft, both receiving the nickname because each comprised a skeletal platform raised on four legs that resembled a bedstead....
," which took its lift solely from engine thrust. An early dedicated lift jet was the Rolls-Royce RB108
Rolls-Royce RB108
|-See also:-Bibliography:*Gunston, Bill. World Encyclopedia of Aero Engines. Cambridge, England. Patrick Stephens Limited, 1989. ISBN 1-85260-163-9-External links:*****...
, first run in 1955.
In the early 1960s both the USSR and western nations considered the possibility of lift engines as a way of providing STOL
STOL
STOL is an acronym for short take-off and landing, a term used to describe aircraft with very short runway requirements.-Definitions:There is no one accepted definition of STOL and many different definitions have been used by different authorities and nations at various times and for a myriad of...
or even VTOL
VTOL
A vertical take-off and landing aircraft is one that can hover, take off and land vertically. This classification includes fixed-wing aircraft as well as helicopters and other aircraft with powered rotors, such as cyclogyros/cyclocopters and tiltrotors...
capability to combat aircraft. The Soviets did side-by-side testing of versions of combat aircraft using variable geometry wings
Swing-wing
A variable-sweep wing is an aeroplane wing that may be swept back and then returned to its original position during flight. It allows the aircraft's planform to be modified in flight, and is therefore an example of a variable-geometry aircraft....
and lift jets (the results became the Mikoyan MiG-23 and Sukhoi Su-24
Sukhoi Su-24
The Sukhoi Su-24 is a supersonic, all-weather attack aircraft developed in the Soviet Union. This variable-sweep wing, twin-engined two-seater carried the USSR's first integrated digital navigation/attack system...
). The problems with the auxiliary lift engine are high fuel consumption, extra weight (which is simply dead weight when the engines are not needed for lift), and taking up fuselage volume that could be used for fuel or other systems. The Soviets decided that variable-geometry wings provided comparable advantages in take-off performance without as many penalties.
Yakovlev Yak-38
The Yakovlev Yak-38 was Soviet Naval Aviation's first and only operational VTOL strike fighter aircraft, in addition to being its first operational carrier-based fixed-wing aircraft...
, a VTOL fighter used by the AVMF's small aircraft carrier
Aircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...
s, which were not large enough to support conventional fixed-wing aircraft. The Yak-38 proved disappointing in service for most of the above reasons, having limited payload and range as well as dismal handling.
A novel alternative to the lift jet for auxiliary vertical thrust is to be employed by the VTOL Lockheed F-35B
F-35 Lightning II
The Lockheed Martin F-35 Lightning II is a family of single-seat, single-engine, fifth generation multirole fighters under development to perform ground attack, reconnaissance, and air defense missions with stealth capability...
version of the U.S. Joint Strike Fighter program. This does not have a jet lift engine, but a central lift fan that is powered by the main engine via a driveshaft
Driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
. When not needed, it is declutched from the engine so that it does not consume power (analogous to the transfer case
Transfer case
A transfer case is a part of a four-wheel-drive system found in four-wheel-drive and all-wheel-drive vehicles. The transfer case is connected to the transmission and also to the front and rear axles by means of drive shafts...
of a four-wheel drive
Four-wheel drive
Four-wheel drive, 4WD, or 4×4 is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously...
vehicle). The operational viability of this system remains to be seen. It has, however, in pre-operational testing proven the ability to take off vertically and go supersonic in level flight in the same mission, a first for a combat aircraft.