Thrust vectoring nozzles give pilots an edge.
During the Falklands War in 1982, subsonic British Harrier jets equipped with variable-thrust systems enabling vertical/short take-off and landing (V/STOL) operations were able to outmaneuver their Argentinian foes by using thrust vectoring in flight. While the U.S. Marines have practiced thrust vectoring with their own AV-8 Harriers for years, engine manufacturers in this country, the U.S. Air Force, and NASA are now working on thrust vectoring engine nozzles for supersonic fighters.
General Electric Aircraft Engines (Cincinnati) and the Pratt & Whitney Government Engines and Space Propulsion division (West Palm Beach, Fla.) have developed flightready thrust vectoring systems that are being tested in demonstrator aircraft. GE has completed ground tests of its Axisymmetric Vectoring Exhaust Nozzle (AVEN) on a GE F110 turbofan at Edwards Air Force Base in California and is scheduled to fly it on the Air Force's F-16 variablestability inflight simulator aircraft (VISTA) as early as May. Pratt & Whitney has completed ground tests of its Pitch/Yaw Balanced Beam Nozzle (P/Y BBN) for the P&W FIO0 line of turbofans at its West Palm Beach facility. A 2-D thrust vectoring nozzle developed by Pratt & Whitney has flown aboard the Air Force's F-15 STOL/Maneuver Technology Demonstrator (SMTD); Pratt & Whitney would like to fly the P/Y BBN on the aircraft as well. Meanwhile, NASA has been exploring the benefits of thrust vectoring using its F-18 high-alpha (high-angle-ofattack) research vehicle (HARV) and X-31 research aircraft at the NASA Dryden Flight Research Facility (Edwards, Calif.).
Controlling Thrust Direction
The nozzle is the part of the engine through which the thrustproducing exhaust gases exit. Nozzles of high-performance aircraft are equipped with mechanisms that increase or decrease the diameter of the aperture, called the throat, as appropriate to maintain the desired pressure ratio at different power levels. For example, when the afterburners are lit to generate more thrust, the throat is widened to accommodate the increased exhaust flow. The derivative nozzles used in experimental thrust vectoring have additional mechanisms to control thrust direction as well. "The nozzle acts as another control surface," explained Roger Bursey, P/Y BBN program manager. "The function is transparent to the pilot."
The P&W P/Y BBN is capable of altering the direction of engine thrust by 20 degrees from the centerline in a 360-degree radius of freedom. The variable-geometry nozzles move at a rate of 45 degrees per second, providing adequate response time for quick in-flight maneuvers. The P/Y BBN has three convergent and three divergent actuators controlling a series of jointed flaps around the diameter of the nozzle. In addition to directing thrust, the flaps are automatically adjusted to maintain a proper thrust balance in the engine. Pratt & Whitney is developing the thrust vectoring system for its F119 engine, which will be fitted on the F-22 Lightning II advanced tactical fighter.
The GE AVEN thrust vectoring system is similar in principle to the Pratt & Whitney design. Fly-by-wire commands control three actuators that vector thrust by a series of convergent/divergent flaps. Ground tests have demonstrated that the AVEN can vector thrust up to 17 degrees in any direction at rates exceeding 60 degrees per second. GE is expected to be the first manufacturer to get an axisymmetric nozzle off the ground when it commences the 60-flight test program on the F-16 VISTA, which is scheduled to begin in May.
The actuators of both thrust vectoring systems are designed to receive commands from a digital control system integrated with the pilot's flight controls. Thrust vectoring is directed by existing stick and rudder controls. A separate set of controls is not required for the variablegeometry nozzles. Flight software automatically adjusts the nozzles' vectors to the positions required to execute the pilot's commands.
By contrast, the thrust vectoring systems employed by NASA on its F18 HARV and X-31 research aircraft are comparatively crude devices. These systems have three metal and carbon paddle-like vanes oriented outside the conventional nozzle assembly of each engine. The vanes are actuated into the exhaust to alter its direction. They are positioned by a computer in response to normal stick and rudder controls.
According to Terry Putnam, program manager for NASA's HighPerformance Aircraft and Flight Projects division (Washington, D.C.), the purpose of NASA's thrust vectoring activities is to explore the utility of thrust vectoring, not to develop a production system. The F-18 HARV is a broad-based research project with the goals of understanding how thrust vectoring works and its effects on aircraft flying qualities. The X-31 program aims to investigate the military usefulness of thrust vectoring in high-alpha situations.
Angles of Attack
Flights in aircraft equipped with thrust vectoring nozzles have illustrated a number of benefits of the technology. The F-15 SMTD, for example-an F-15B equipped with the Pratt & Whitney 2-D thrust vectoring system and a set of movable canard wings that pitch in unison with the nozzles---can take off using 38 percent less runway and land using 60 percent less runway than a conventional F-15B. Some of these improvements can be attributed to thrustreversing vanes that are not part of current 3-D thrust vectoring systems. Thrust-reversing vanes channel the exhaust in a forward direction, slowing the aircraft. Flight tests also showed that thrust vectoring significantly improved aircraft performance at high angles of attack, when control surfaces tend to lose effectiveness.
"Expanding the flight envelope, particularly in high-alpha situations, is what axisymmetric nozzle vectoring is all about," said Pratt & Whitney's Bursey. "It permits pilots to do maneuvers they previously were not capable of doing."
The angle of attack is the difference between an aircraft's attitude and the direction in which it is traveling. At high angles of attack, airflow over the aircraft's control surfaces can be interrupted, resulting in loss of control. Pilots consider the ability to maneuver at high angles of attack to be an important advantage in combat. (Western pilots were surprised when the former Soviet Union demonstrated the high-alpha capabilities of the SU-27 fighter-bomber at the Paris Air Show in 1989. In the celebrated Cobra maneuver, the SU27 muscled its way through an extreme high-alpha situation by the weight of its engine thrust.)
By vectoring engine thrust in a more refined manner using fly-bywire controls, U.S. aircraft engine makers want to impart high-alpha flight characteristics to American fighter-bombers. In post-stall situations, where the lack of airflow makes conventional control surfaces all but useless, directed engine thrust could allow pilots to maintain control of their aircraft. Combat applications include improved air-to-air capabilities and better survivability in air-toground engagements. In the latter role, thrust vectoring could be used by an attacking plane, after delivering its ordnance, to confound enemy antiaircraft artillery and missiles by moving in an unexpected direction. "Thrust vectoring improves controllability in situations where an airplane is normally less controllable," said Rodget Mishler, manager of GE's AVEN program. "It can help improve exchange ratios in the air and avoid counterfire from the ground."
Mishler said that it might be possible to integrate thrust vectoring as a flight-critical system, versus its being simply an enhancement for STOL and high-alpha performance. If thrust vectoring takes on a flight-critical role, an aircraft would be unable to fly without it. Some in the industry have proposed "tailless" stealthy fighters without radar-reflecting rudders, stabilizers, or flaps that rely on thrust vectoring to perform the functions of the control surfaces. Less exotic applications include real-time trim control in level flight. Mischler noted, however, that such thrust vectoring control systems would have to withstand rigorous testing.
Both GE and Pratt & Whitney are pursuing thrust vectoring research largely with their own funds. The engine makers are in fierce competition to win expected Department of Defense contracts to retrofit existing aircraft with their thrust vectoring systems. Nearly all of the front-line fighters and fighter-bombers in the Air Force and U.S. Navy have variants that use some model of the GE F110 and P&W F100 engine families, including the F-14 Tomcat, F-15 Eagle, and F-16 Fighting Falcon. The F18 Hornet is equipped with the GE F404 engine and GE is scaling AVEN to fit that aircraft as well.
In order to retrofit existing aircraft, GE and Pratt & Whitney had to design their thrust vectoring mechanisms to fit into available space. Both the P/Y BBN and AVEN systems were designed to require the same amount of space as the nozzle assemblies they will replace. Both vectoring systems add about 300 pounds to the nozzle's weight. Engineers also had to ensure that the engine and aircraft could handle the different thrust load distribution created by thrust vectoring.
While the baseline nozzle has been strengthened in the past to accommodate more powerful F-i00 variants, the P/Y BBN represents the first major redesign of the baseline F-100 nozzle kinematics since the engine was introduced in the 1970s, said Kevin Barcza, a Pratt & Whitney mechanical design engineer. "Pressure loads incurred by the baseline nozzle are well documented," he said. "However, most of the load calculations required by the new nozzle had to be performed by hand."
A series of free-body diagrams were generated for many different flight conditions with altitude, power level, Mach number, and afterburning engines as variables. Each freebody diagram described an individual part isolated according to where it formed links with other parts of the mechanism. A team of designers, with each member working on a different part, looked at how loads were applied to links during the specified flight conditions. This way, a matrix of loads describing the P/Y BBN as a whole was constructed. The designers used the data to modify existing parts and design new ones as required. "Calculating the free-body diagrams by hand is a couple of days' worth of work," Barcza said. "We needed to do calculations for each part covering the entire flight envelope."
To expedite the process, the Pratt & Whitney nozzle group has installed Applied Motion software from Rasna Corp. (San Jose, Calif.) on its Sparcstations. Designers built a kinematic analysis model by importing CAD files drawn using Unigraphics from Electronic Data Systems (Maryland Heights, Md.) into Applied Motion via IGES. The model will be used to determine the loads for individual parts throughout the flight envelope and to perform static analysis at any nozzle position or dynamic analysis over a range of motion. The results will be used to verify the earlier work performed by hand.
Advocates of thrust vectoring systems view the technology in revolutionary terms. Pratt & Whitney compares its significance to that of stealth and the conversion from propellers to jets. Enthusiasm is based on the promise of more effective combat aircraft and is fueled by the success of ground and flight tests conducted thus far.
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|Date:||Mar 1, 1993|
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