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Balancing act

BALANCE IS A term used repeatedly by the F-22's designers when describing what they have achieved, and balancing lethality, survivability, supportability and affordability - and several other 'ilities' - has proved a formidable task.

Driving the design was the requirement to combine stealth with speed and agility, in an aircraft that was at least twice as reliable, maintainable and supportable as the fighter it would replace. Some requirements were easier to balance than others: carrying all fuel and weapons internally to minimise radar cross-section also reduced supersonic drag and improved supercruise performance; while the high unreheated thrust required for supercruise minimised afterburner use and reduced infra-red signature. The need to combine low observability with high manoeuvrability was more problematic.

 

APPROACH REVEALED

When the Skunk Works rolled out the first YF-22 in August 1990, Lockheed's approach to balancing those disparate requirements became apparent. The aircraft was shaped for stealth, with all exterior angles aligned with the wing leading or trailing edges, and equipped for agility, with four tail surfaces, thrust-vectoring and a prominent bubble canopy.

Sherm Mullin, who led the dem/val effort later became president of the Skunk Works, subsequently revealed that the YF-22 was the result of an intensive three-month redesign in 1987, after the team decided that Lockheed's original configuration was "technically and competitively unacceptable". Externally, the F-22 differs only in detail from the prototype.

Key design features are the integration of aerodynamics and propulsion for supercruise and manoeuvrability; the blended wing-body configuration for structural efficiency and fuel capacity; the shaping for reduced radar cross-section; and the inlet design for simplicity, performance and stealth.

A modified-diamond wing planform was chosen for its structural efficiency and large fuel volume, while its relatively large area increases manoeuvrability and reduces take-off and landing distances. The wing has leading-edge flaps and trailing-edge ailerons and flaperons.

Four tail surfaces provide the control power and margins required for unrestricted manoeuvrability, while the thrust-vectoring nozzles act, in effect, as extra horizontal tails. According to Mullin, thrust-vectoring added 15-25kg to the nozzle weight, compared to the 180kg penalty for a larger horizontal tail. The nozzles operate in pitch only, and always symmetrically. At high angles of attack, they are used for pitch control, allowing the horizontal tail to be used for roll.

F-22 chief test pilot Paul Metz says thrust vectoring amplifies the tail's effectiveness. "By design, the aircraft can fly to very high angles of attack on aerodynamic control only-but thrust-vectoring gets there quicker," he says. Vectoring is active at all times, but its use is restricted to lower speeds and higher angles of attack - and not always in combat: "We use thrust-vectoring to rotate the aircraft for take-off, and to offload the nosegear when carrying tanks," Metz says.

Angled doors above the inlets, for the low-observable boundary-layer bleed system, are also used as pitch-control surfaces. The wedge inlets have no moving ramps, although the F-22's maximum speed exceeds Mach 2. The lip is angled to reduce radar signature, and the duct is shaped to block totally the radar line-of-sight to the engine.

 

DESIGN CHANGED

When the F-22 external design was frozen in 1992, several changes were made. Wing span was increased to improve manoeuvrability and reduce drag; leading-edge sweep was reduced, again to enhance manoeuvrability; root thickness was decreased to reduce drag; camber and twist were modified to improve supersonic manoeuvrability; and the tips were "scarfed" to improve the look angle of antennas in the edges.

Shortening the fuselage reduced weight, while moving the inlets aft improved controllability and enhanced visibility from the cockpit, which was reshaped and moved forward, also to improve the pilot's downward vision. Redesign of the forward fuselage also improved radar performance while reducing radar cross-section.

The vertical stabilisers were reduced in area by 20%, after flight testing revealed that the YF-22's fins were bigger than required. Asymmetric rudder deflection replaced the YF-22's dorsal speedbrake. The horizontal stabilisers were reshaped to align their trailing edges for reduced radar cross-section, but their size was not changed.

According to programme general-manager Tom Burbage, the resulting aircraft "-is about the size of an F-15, yet carries all its weapons internally, plus more fuel and all its support equipment on board-and it weighs not much more than the F-15." The F-22 "-is 10,000lb [4,500kg] lighter than the YF-22," he says. Weight is not a design requirement, but minimum weight is the overriding design driver, Burbage says, both to meet the performance specification and to reduce cost.

The F-22 is currently over its target weight, but the aircraft is meeting all of its performance specifications, and weight-reduction efforts have been shelved in favour of pursuing producibility enhancements in a bid to reduce cost. Weight is steady, "-and will not come down without additional funding," says Burbage.

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