Variable Cycle Engine
Encyclopedia
A variable cycle engine (VCE) is an engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic.
The next generation of Supersonic transport
(SST) may require some form of VCE. SST engines require a high Specific Thrust
(net thrust/airflow) at supersonic
cruise to keep the cross-sectional area of the powerplant to a minimum, so as to reduce aircraft drag
. Unfortunately, this implies a high jet velocity not only at supersonic cruise, but at take-off, which makes the aircraft noisy.
A high specific thrust engine has a high jet velocity by definition, as the following approximate equation for net thrust implies :
where:
intake mass flow rate
fully expanded jet velocity (in the exhaust plume)
aircraft flight velocity
Rearranging the above equation, specific thrust is given by:
So for zero flight velocity, specific thrust is directly proportional to jet velocity.
The Olympus 593 engines in Concorde
had a high specific thrust in supersonic cruise and at dry take-off power. This alone would have made the engines noisy, but the problem was compounded by the need for a modest amount of afterburning
(reheat) at take-off (and Transonic Acceleration). An SST VCE would have to increase the engine airflow substantially at take-off, to reduce the jet velocity at a given thrust (i.e. a lower specific thrust)
modified a Pratt & Whitney
JT8D to a Tandem Fan configuration and successfully demonstrated the switch from series to parallel operation (and vice-versa) with the engine running, albeit at part power.
In the Mid Tandem Fan concept, a high specific flow single stage fan is located between the high pressure (HP) and low pressure (LP) compressors of a turbojet core. Only bypass air is allowed to pass through the fan, the LP compressor exit flow passing through special passages within the fan disc, directly underneath the fan rotor blades. Some of the bypass air enters the engine via an auxiliary intake. During take-off and approach the engine behaves much like a normal civil turbofan, with an acceptable jet noise level (i.e., low specific thrust). However, for supersonic cruise
, the fan variable inlet guide vanes and auxiliary intake close-off to minimize bypass flow and increase specific thrust. In this mode the engine acts more like a 'leaky' turbojet (e.g. the F404).
In the Mixed-Flow Turbofan with Ejector concept, a low bypass ratio engine is mounted in front of a long tube, called an ejector. This silencer device is deployed during take-off and approach. Turbofan exhaust gases induce additional air into the ejector via an auxiliary air intake, thereby reducing the specific thrust/mean jet velocity of the final exhaust. The mixed-flow design does not have the advantages of the mid-tandem fan design in terms of low-speed efficiency, but is considerably simpler.
The opposite is true for a low specific thrust engine- i.e. poor reheat SFC, good dry and throttled SFC, good reheat thrust boost and, by definition, low dry and reheated thrust.
A high specific thrust engine would favour an aircraft requiring good duration in reheated combat, but it would be penalised on the range available in dry power.
On the other hand, a low specific thrust engine, would favour an aircraft with the need for long range in dry power, but compromise the time spent in reheated combat.
Thus engine designers often have to make a compromise on engine specific thrust.
However, the ideal Combat VCE would have the high reheat thrust/good reheat SFC associated with a high specific thrust engine, but would have the low SFC of a low specific thrust engine in dry power and throttled back. Devising such an engine is difficult. However, General Electric
did develop a Variable Cycle Engine, known as the GE37 or General Electric YF120
, for the YF22/YF23 fighter aircraft competition, back in the late 80's. GE used a Double Bypass/Hybrid Fan arrangement, but to date has never disclosed precisely how they exploited the concept. Although the YF120 was a good (possibly better) engine in the fly-off, the USAF erred on the side of caution and selected the more conventional Pratt & Whitney F119
as the powerplant for the production F22.
The next generation of Supersonic transport
Supersonic transport
A supersonic transport is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. The only SSTs to see regular service to date have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 with its last ever...
(SST) may require some form of VCE. SST engines require a high Specific Thrust
Specific thrust
Specific thrust is a term used in gas turbine engineering to show the relative bulk of a jet engine and is defined as the ratio: net thrust/total intake airflow.-Low specific thrust engines:...
(net thrust/airflow) at supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
cruise to keep the cross-sectional area of the powerplant to a minimum, so as to reduce aircraft drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
. Unfortunately, this implies a high jet velocity not only at supersonic cruise, but at take-off, which makes the aircraft noisy.
A high specific thrust engine has a high jet velocity by definition, as the following approximate equation for net thrust implies :
where:
intake mass flow rate fully expanded jet velocity (in the exhaust plume) aircraft flight velocityRearranging the above equation, specific thrust is given by:
So for zero flight velocity, specific thrust is directly proportional to jet velocity.
The Olympus 593 engines in Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
had a high specific thrust in supersonic cruise and at dry take-off power. This alone would have made the engines noisy, but the problem was compounded by the need for a modest amount of afterburning
AfterBurner
The AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs.Originally, portablemonopoly.net was a website created to petition Nintendo to put some kind of light in their Game Boy Advance system...
(reheat) at take-off (and Transonic Acceleration). An SST VCE would have to increase the engine airflow substantially at take-off, to reduce the jet velocity at a given thrust (i.e. a lower specific thrust)
Examples
One SST VCE concept is the Tandem Fan engine. The engine has two fans, both mounted on the low-pressure shaft, with a significant axial gap between the units. In normal flight, the engine is in the series mode, with the flow leaving the front fan passing directly into the second fan, the engine behaving much like a normal turbofan. However, for take-off, climb-out, final-descent and approach, the front fan is allowed to discharge directly through an auxiliary nozzle on the underside of the powerplant nacelle. Auxiliary intakes are opened on each side of the powerplant, allowing air to enter the rear fan and progress through the rest of the engine. Operating the fans in this parallel mode substantially increases the total airflow of the engine at a thrust, resulting in a lower jet velocity and a quieter engine. Back in the 1970s, BoeingBoeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...
modified a Pratt & Whitney
Pratt & Whitney
Pratt & Whitney is a U.S.-based aerospace manufacturer with global service operations. It is a subsidiary of United Technologies Corporation . Pratt & Whitney's aircraft engines are widely used in both civil aviation and military aviation. Its headquarters are in East Hartford, Connecticut, USA...
JT8D to a Tandem Fan configuration and successfully demonstrated the switch from series to parallel operation (and vice-versa) with the engine running, albeit at part power.
In the Mid Tandem Fan concept, a high specific flow single stage fan is located between the high pressure (HP) and low pressure (LP) compressors of a turbojet core. Only bypass air is allowed to pass through the fan, the LP compressor exit flow passing through special passages within the fan disc, directly underneath the fan rotor blades. Some of the bypass air enters the engine via an auxiliary intake. During take-off and approach the engine behaves much like a normal civil turbofan, with an acceptable jet noise level (i.e., low specific thrust). However, for supersonic cruise
Supercruise
Supercruise is sustained supersonic flight of an aircraft with a useful cargo, passenger, or weapons load performed efficiently and without the use of afterburners ....
, the fan variable inlet guide vanes and auxiliary intake close-off to minimize bypass flow and increase specific thrust. In this mode the engine acts more like a 'leaky' turbojet (e.g. the F404).
In the Mixed-Flow Turbofan with Ejector concept, a low bypass ratio engine is mounted in front of a long tube, called an ejector. This silencer device is deployed during take-off and approach. Turbofan exhaust gases induce additional air into the ejector via an auxiliary air intake, thereby reducing the specific thrust/mean jet velocity of the final exhaust. The mixed-flow design does not have the advantages of the mid-tandem fan design in terms of low-speed efficiency, but is considerably simpler.
Other Applications
Another application that could benefit from the VCE approach is combat aircraft. Designers normally have to make a compromise on specific thrust of the engine. If they choose a high specific thrust, the reheat specific fuel consumption (SFC) will be very good, but the dry SFC poor. A high specific thrust implies a high fan pressure ratio, which infers a high nozzle temperature in dry power. Consequently the thrust boost in reheat is relatively low. By definition, both the dry and reheat thrust levels are good.The opposite is true for a low specific thrust engine- i.e. poor reheat SFC, good dry and throttled SFC, good reheat thrust boost and, by definition, low dry and reheated thrust.
A high specific thrust engine would favour an aircraft requiring good duration in reheated combat, but it would be penalised on the range available in dry power.
On the other hand, a low specific thrust engine, would favour an aircraft with the need for long range in dry power, but compromise the time spent in reheated combat.
Thus engine designers often have to make a compromise on engine specific thrust.
However, the ideal Combat VCE would have the high reheat thrust/good reheat SFC associated with a high specific thrust engine, but would have the low SFC of a low specific thrust engine in dry power and throttled back. Devising such an engine is difficult. However, General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
did develop a Variable Cycle Engine, known as the GE37 or General Electric YF120
General Electric YF120
|-See also:-References:* Technology News Flight International 23/09/03 .-External links:* . Press release GE Aviation .*...
, for the YF22/YF23 fighter aircraft competition, back in the late 80's. GE used a Double Bypass/Hybrid Fan arrangement, but to date has never disclosed precisely how they exploited the concept. Although the YF120 was a good (possibly better) engine in the fly-off, the USAF erred on the side of caution and selected the more conventional Pratt & Whitney F119
Pratt & Whitney F119
|-See also:-External links:* *...
as the powerplant for the production F22.