Aeronautics and astronautics – Aircraft – heavier-than-air – Airplane and fluid sustained
Reexamination Certificate
2002-04-01
2004-05-04
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Aircraft, heavier-than-air
Airplane and fluid sustained
C244S05300R, C244S055000, C244S02300R
Reexamination Certificate
active
06729575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of propulsion systems and, in particular, to a propulsion system for vertical and short takeoff and landing (V/STOL) aircraft.
2. Description of Related Art
The efficiency of a propulsion system for an aircraft increases as the exhaust velocity is reduced. Thus, during takeoff, landing and hovering, it is obvious that a helicopter, which provides a small incremental velocity to a large mass of air, is more efficient than a jet aircraft, which provides a large incremental velocity to a small mass of air. However, a helicopter, because of its very large diameter rotor, has a limited forward velocity, certainly not much over two hundred miles per hour. Thus, most V/STOL aircraft are compromises. For example, the AV-8A Harrier V/STOL aircraft utilizes a turbofan engine for both hover and cruise propulsion. As with a helicopter, the large fan provides significant thrust for vertical lift in hover, but its correspondingly large frontal area increases the drag of the aircraft and limits its maximum speed to subsonic speeds.
In U.S. Pat. No. 4,474,345 “Tandem Fan Series Flow VSTOL Propulsion System” by R. G. Musgrove, a jet engine with a small fan, which is capable of providing supersonic performance, is modified to provide vertical lift. The basic engine fan is split to provide fore and aft fans connected by means of a common drive shaft. The fans are centrally mounted in a duct located within the aircraft along its longitudinal axis. In normal wing borne flight (herein after referred to as normal flight), the fans operate in series with the fan exhaust mixing with the turbine exhaust and exiting through a nozzle located at the rear of the aircraft. In the vertical mode of operation, a diverter is positioned downstream of the forward fan and is movable to a position for diverting the exhaust from the forward fan downward relative to the longitudinal axis of the aircraft, while simultaneously opening an auxiliary inlet for permitting the introduction of air to the aft fan. An aft diverter is located in the nozzle which is also moveable to a position for diverting the exhaust from the aft fan and engine core downward. Thus, for vertical flight the diverters are actuated causing the exhaust from both fans and the core engine to be directed downward fore and aft of the center of gravity of the aircraft.
However, the tandem fan engine has less thrust in the vertical takeoff and landing mode of operation than it has in the normal flight mode of operation. The thrust is larger in cruise because airflow passes through both fans, and thus, the core is supplied with air that is raised to a higher-pressure level; whereas, in the vertical mode the core engine airflow passes through only the aft fan. Consequently, the tandem fan concept is not an efficient design for a V/STOL aircraft.
In U.S. Pat. No. 4,791,783 “Convertible Aircraft Engine” by R. E. Neitzel, a turbofan concept is disclosed for converting almost all the power used by the engine fan to shaft horsepower to drive a helicopter rotor. Guide vanes located on both sides of the outer portion of the engine fan can be actuated to block off airflow through the fan duct while still allowing airflow into the engine core. A gear mounted on the forward end of the fan shaft is coupled to a drive shaft, which in turn drives the rotor. Such a system provides maximum efficiency during takeoff and landing and also during normal flight. However, if high-speed flight is to be accomplished the rotor must be stopped (x-wing concept) or stopped and stored. The former concept severely limits the top speed of the aircraft, while the latter causes a severe weight penalty and requires a complex folding and storing system.
In U.S. Pat. No. 5,209,428 Propulsion System For A Vertical And short Takeoff And Landing Aircraft by P. M. Bevilaqua, et al. In detail, the invention comprises a turbofan engine mounted within the airframe having the fan face coupled to the inlet duct. The engine is a mixed flow type having fan, high-pressure compressor, combustion, turbine and exhaust nozzle sections. The turbine section includes a high-pressure turbine portion which drives the high-pressure compressor section and a low-pressure turbine portion which drives the fan section. The inner portion of the fan section is in front of the high-pressure compressor section and, thus, acts as a low-pressure compressor section. Since a first shaft to the low-pressure turbine portion connects the fan section and the high-pressure compressor is connected to the high-pressure turbine portion by a hollow shaft rotatably mounted about the first shaft, they are often referred to as spools. Thus, in the turbofan engine thus far described, it is referred to as a two-spool engine. Furthermore, the high-pressure compressor section, combustion section and high-pressure turbine portion are, collectively, referred to as the core or core engine.
In this type of engine, the turbine exhaust produces a significant portion of the total thrust thereof and, preferably, has a common fan and turbine exhaust nozzle section exiting (mixed flow) at the rear of the aircraft when operated in the normal flight mode. The exhaust nozzle section is designed to divert exhaust flow either horizontally for normal flight or vertically downward for takeoff and landing, and intermediate positions therebetween when transferring from vertical to horizontal flight and visa versa. A vertically mounted lift fan assembly, having a lift fan rotor, is positioned forward of the engine and connected by a drive shaft to the front of the engine fan. A clutch is mounted in the driveline between the lift fan assembly and engine for disconnecting the lift fan rotor from the engine. Power to drive the lift fan rotor is obtained by increasing the engine exhaust nozzle area (exhaust nozzle section exit cross-sectional area). This allows more power to be extracted from the turbine exhaust during V/STOL operation. The excess power is absorbed by the lift fan rotor, which is “clutched in” during takeoff and landing and the transition to and from normal flight. By doing so, the operating point of the engine is shifted so that more power is applied to the lift fan rotor, which is more efficient at these lower speeds. The lift fan exhaust duct assembly is equipped with a vectoring system to deflect the thrust from a vertical direction in vertical flight to an aft vectoring direction during transition to and from normal flight. After transitioning to horizontal flight, the operating point of the engine is returned to its normal cruise mode of operation, which is more efficient at higher speeds.
To control the power extracted from the low-pressure turbine section, a mechanism is provided for varying the exhaust nozzle exit cross-sectional area. Depending on the particular design of the turbofan, it may be desirable to add one or more additional turbines to the low-pressure turbine section in order to extract the additional power. It is important to note that only the low-pressure turbine section will sense the reduction in back pressure caused by an increase in nozzle exit cross-sectional area; thus, the high-pressure turbine portion driving the high-pressure compressor section will sense little or no decrease in back pressure.
If the engine is operated during normal flight as a mixed flow turbofan engine, it can also be operated as a separate flow engine in the vertical flight mode of operation. This accomplished by blocking off the fan duct with a plurality of doors, which divert the fan section exhaust to roll control nozzle assemblies. The roll control nozzle assemblies consist of a pair of ducts, which connect to the fan duct aft of the fan section and extend outward therefrom; terminating in downward directed variable cross-sectional area roll control nozzles. Valves located in the ducts, at the fan section duct wall, open to admit fan exhaust to the individual roll control nozzle assemblies, which are differentially controlled to develop roll control f
Barefoot Galen L.
Lockheed Martin Corporation
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