WITH DELIVERY OF the first McDonnell Douglas (MDC) F-18E/F Super Hornet to the US Navy's Patuxent River, Maryland, test centre on 15 February, testing is poised to move into high gear. A second F-18E/F is expected at Pax River, by the end of February and all seven development aircraft, are scheduled to be in flight testing, by the end of 1996.
Some 2,000 flights are planned during the three-year test programme. So far, the first two aircraft, designated E-1 and E-2 and both single-seaters, have logged some ten sorties only since the maiden E/F flight on 29 November 1995, but already MDC and the Navy are optimistic that the upgraded F-18 will deliver the hoped-for performance improvements.
The E/F is a structural upgrade of the F-18, designed to increase range and weapons-carrying capability, improve survivability and provide growth capability. The aircraft has a stretched fuselage, bigger wing, increased fuel-capacity and up-rated General Electric F414 engines (Flight International, 16-22 February, 1994).
An early flight-test goal was to check whether the range projections for the E/F would be met. According to Jim Sandberg, E/F project pilot for principal subcontractor Northrop Grumman, this cruise-performance test involved trimming the aircraft and setting the throttles to maintain constant speed and altitude, then measuring fuel flow.
Engineers had predicted that the cruise fuel-consumption would have to lie within a certain band to achieve the E/F's range targets. Sandberg says that initial results indicate that the fuel flow "...is within the band of uncertainty, on the good side of the line". Navy F-18 programme director Capt Joe Dyer echoes Sandberg's optimism, saying that the initial results "look good".
Equally important to the Navy is whether MDC has met the challenge of equaling, or improving on, the present F-18C/D's performance and handling while substantially increasing the aircraft's size. Sandberg, for one, believes that the E/F's handling qualities are "far superior" to the C/D's.
INERTIA OVERCOME
The F-18E/F is some 25% larger than the C/D. "Photographic enlargement" of the present aircraft would result in a 12% reduction in agility, Sandberg says, because of the larger aircraft's increased inertia. To overcome this, the E/F control surfaces have been enlarged, their deflections increased and actuator power and rates stepped up.
A wing leading-edge dogtooth, or snag, was introduced to enhance roll control-power by creating a vortex, which increases the energy of the airflow over the aileron. The snag was a feature of the original F-18A/B design, which was deleted when the wing proved too flexible. The E/F wing is stiffer and thicker, allowing the dogtooth to be re-instated.
Another noticeable change from the C/D to the E/F is the substantial increase in size of the distinctive wing leading-edge extensions (LEX). These were redesigned early in the E/F programme, and the area increased by 34%, to improve pitch control at high angle-of-attack (AoA) and so maintain C/D manoeuvrability.
The larger LEXs led to other design changes. To allow high-energy air to flow over the wing-fuselage blend at high AoA, "vents" were added at the junctions of the LEX and the wing leading-edge flaps. The LEX vents have only two positions, open or closed, Sandberg says, and are deflected automatically as a function of the AoA.
A spoiler on the upper surface of each LEX serves several functions, but primarily acts as a speed brake, generating additional drag without appreciable pitching moment, he says. As a weight-saving measure, the E/F lacks the C/D's upper-fuselage airbrake, relying instead on rudder and flap deflection for aerodynamic braking on landing.
The spoilers also control the LEX vortices, which sweep back over the wing and tail at high AoA. In-service problems with fin cracking caused by vortex-induced vibration led to the installation of LEX fences on F-18s. On the E/F, in a bid to reduce drag and radar cross-section, these bolt-on plates, are replaced by the deployable spoilers.
Other LEX-spoiler functions include the venting of intake boundary-layer air at high Mach numbers, Sandberg says. The E/F uses suction through perforated panels in the intake walls to remove boundary-layer air. This reduces radar signature compared with the present system of diverter ramps and LEX slots.
MDC says that the LEX spoilers also increase nose-down pitch-control authority at high AoA, although Sandberg says that their effect has proved hard to discern in simulator tests. The speed brake effect of the two-position (open or closed) "bang-bang" spoilers is noticeable, he says, and the lack of transient pitch effects was confirmed in early flights.
BACK-UP REMOVED
The E/F is a totally fly-by-wire aircraft, the present F-18's mechanical back-up flight-control system having been removed to reduce weight, complexity and cost. Four digital flight-control computers provide redundancy, while a direct electrical link provides a reversionary, "get-home", capability.
To allow the mechanical back-up controls to be removed, redundancy in electrical generation was increased substantially. Whereas the C/D has three independent power sources, the E/F has nine, Sandberg says.
Removing the mechanical control-linkages enabled designers to reduce E/F longitudinal static-stability, to improve manoeuvrability. Whereas the present F-18 had to be naturally stable to be flyable with mechanical controls, the fly-by-wire E/F can by operated closer to the neutral-stability point. "Some E/F configurations can be unstable," he explains.
Although the E/F flight-control software is based on that of the C/D, improvements have been made. The most important of these, Sandberg says, is the introduction of "beta-dot" feedback to increase resistance to departure from controlled flight at high AoA. The term beta-dot refers to the rate of change of sideslip (beta), he explains.
At AoAs above 25°, the aircraft's laser-gyro inertial-navigation system (INS) is used to sense sideslip. The INS provides a more-sensitive measurement of nose movement than that of the roll/yaw accelerometers used in the C/D, says Sandberg.
As there is only one INS in the aircraft, there were reliability and redundancy concerns about using it in the safety-critical flight-control system, he admits. These were overcome be ensuring that the E/F has the same departure resistance, without beta-dot feedback, as that of the C/D. The feedback is only used at high AoA and, if the INS fails, "...the E/F is as good as the C/D", according to Sandberg.
EFFORT EXPENDED
Considerable effort has been expended to improve the E/F's handling at high AoA, and to eliminate departure modes experienced with the present F-18. One of these is described by E/F project pilot Fred Madenwald as the "falling-leaf" departure, a semi-oscillating tail slide which results from a lack of nose-down control at aft centre-of-gravity positions.
Madenwald says that the C/D can be manoeuvred at up to 35° AoA, is controllable up to 45° and can make brief excursions to 50°. The E/F, with its larger control surfaces increasing roll power and redesigned LEX increasing pitch authority, should be manoeuvrable up to 40°, he says.
One aim of improving the E/F's high-AoA behaviour was to lower its approach speed, to reduce the kinetic energy associated with aircraft-carrier landings and so save on structural weight. The E/F is designed to land 10kt (18km/h) slower than the C/D, at 125kt instead of 135kt, despite its 2,700kg higher landing weight.
This was demonstrated, inadvertently, on the first flight of aircraft E-1. The flight was cut short after just 20min, when a warning light illuminated and Madenwald landed the aircraft with 4,500kg of fuel remaining, "more than a C/D takes off with," he emphasises and touched down at 132kt whereas, the F-18 chase aircraft with 1,800kg less fuel on board, touched down at 146kt.
Handling on the approach is crucial because one of the goals of the E/F upgrade is to improve the F-18's carrier suitability at higher "bring-back" weights. This will enable the aircraft to be returned to the carrier without first having to jettison unused high-value weapons such as stand-off missiles. Weight growth will have reduced the present F-18's bring-back payload to the equivalent of two AIM-9 and two AIM-120 air-to-air missiles by the year 2000, MDC estimates.
Sandberg says that the E/F appears to be less "nervous" on the approach than the C/D, which he describes as an "excellent" carrier aircraft. He says that the C/D's leading- and trailing edge flaps are continuously in motion during the approach, whereas the E/F's larger, higher-rate surfaces do not appear to be as active.
"The aircraft is more steady," he says, adding that the E/F is more like the "solid" MDC F-4 than the more-active F-18. "[The aircraft] feels like it is on rails, yet you still have the control power to change [the flight path]," he says. Madenwald says that E/F longitudinal control feels "very C/D-like", but lateral control is better.
SUITABILITY TESTED
The next E/F to fly, aircraft F-1 - the first two-seater - will put the design's carrier-suitability to the test. Extensive land-based carrier landings will be conducted, beginning in mid-1996, leading up to the first sea trials, scheduled for January 1997. The trials will be flown by Lt Cdr Tom Gurney, the US Navy test pilot who ferried the E-1 to Patuxent.
Gurney, Madenwald and Sandberg are part of the E/F integrated test team (ITT), which includes MDC, Northrop Grumman and Government staff. In a change from the C/D programme, MDC is in charge of E/F development testing, with the Navy acting as co-director of the ITT. Some 60 Navy personnel will be assigned to the 300-strong ITT as "employees".
A year has been removed from the test schedule, Madenwald says, by integrating Navy development testing and early operational-assessment of the E/F into the contractor-run test programme. Two early operational-assessments are planned, one in 1997 lasting one week and involving aircraft E-2, and a two-week effort in 1998 involving aircraft E-5 and F-2 - the first aircraft with full mission-capability.
The first batch of production E/Fs will be used for operational testing and weapons clearance, while the second batch will be used for training, leading to an initial operational capability early in the next century. Madenwald says that 30 different weapons configurations will be have cleared by the time the E/F enters service, a task which was not performed with the F-18A/B until after the development phase had been completed.
Source: Flight International