Graham Warwick/NAS PATUXENT RIVER
Development flight testing of the Boeing F/A-18E/F Super Hornet will not end as planned in November, but will be extended into next year. The continuation is necessary because of the effort that was required to resolve the manoeuvre wing-drop issue, but will be covered by available programme budget reserves.
Finding a solution to the wing drop problem required 315 test flights totalling 512h, in a development programme that was originally budgeted for a total of 2,500 flights and 4,000h, says Capt Jeff Wieringa, co-lead of the F/A-18E/F integrated product team. "The windtunnel was not a good predictor of the problem," he says, and extensive flight testing of several potential solutions was required before the US Navy and Boeing selected a porous wing fold fairing earlier this year.
Although manoeuvre wing drop has been eliminated, "-there is still a degree of buffet", Wieringa says. "Tweaking" of the fairings continues, "-but it's now a quality issue", he says, noting that some pilots have not noticed the buffet until it was pointed out.
STILL ON TRACK
Development flight testing, now at almost 1,900 flights and over 2,800h, is expected to continue into March next year, but the programme remains on track to meet the original May 1999 date for the start of operational evaluation, Wieringa says, explaining: "We built in a buffer between November and May."
Seven development aircraft are in flight test, six of them at NASPatuxent River, Maryland. One is based at NAS China Lake, California, for weapon system testing. "The aircraft is performing extremely well," says Wieringa, noting that the F/A-18E/F has been flown to 35° angle of attack "with no problems".
The Boeing /Navy integrated test team has been criticised for not recognising sooner that manoeuvre wing drop, first observed in March 1996, was a serious problem. Wieringa says the team had other, higher, priorities at the time, including stores separation challenges and wing drop in the power approach configuration.
Two other factors influenced the test team's tackling of the manoeuvre wing drop issue, he says. First, the extent of the phenomenon was not appreciated until the flight envelope was opened up "-and we found it was a wider problem". Then, the relative ease with which earlier aerodynamic issues with the F/A-18E/F were overcome led testers to believe that the problem would be equally tractable.
Instead, understanding and eliminating the wing drop proved frustratingly difficult . "We still do not know the root cause," Wieringa says. The phenomenon occurred in the heart of the manoeuvring envelope, at between 7° and 12° angle of attack (AoA) and Mach 0.7-0.95, producing sudden, uncommanded, bank angles of up to 22°. It was not repeatable, however. "You could take the same aircraft up and get wing drop in the morning and none in the afternoon," he says.
Wing drop in the power approach configuration, a major issue because aircraft carrier suitability is a fundamental requirement for the F/A-18E/F, had been cured by retracting vents in the wing leading-edge extensions - "a simple logic change" in the flight control system, Wieringa says. These vents, intended to increase tail control power, have been found to be unnecessary and will be deleted from production aircraft.
Another early aerodynamic problem, roll-off at 25° AoA, was traced to asymmetry of the filler used to fair the flight test air data probe into the radome. "The F-18 is extremely sensitive to nose irregularities.A 2 milliradian asymmetry can cause nose slice," he says.
PROBLEMATIC PHENOMENON
Manoeuvre wing drop proved more problematic, as the aerodynamic phenomenon involved is "so transient", Wieringa says. As AoA increases, airflow separation moves rapidly forward from the trailing edge. This may be a result of the wing's lack of twist, for low-observable reasons, and its flat upper surface. "We notice wing drop if there is a really small timing difference between left and right," he says.
The porous fairing eliminates wing drop by re-energising the separated airflow. Ram air enters through a gap between the leading-edge flaps and exits through holes drilled in the fairing over the wing fold hinge. The new "variable porous" fairing is in three sections, with hole size increasing towards the aft end. The final design is still being optimised in an effort to minimise buffet.
Increasing the F/A-18's range is a key goal for the E/F, and some of the wing drop solutions tested would have had a significant impact on combat radius, but the final "range hit" with the porous fairing is just 7.5km (4nm), "-and may be going down" as optimisation continues, Wieringa says. Fighter escort combat radius for the single-seat E is now 803km, compared with the specification 758km.
Flight testing has begun with a centreline buddy-refuelling store - "-something I thought I'd never see on an F-18", says Wieringa. The aircraft, carrying five 1,800litre (480USgal) external fuel tanks, will have a similar fuel offload capability to that of the Grumman A-6, he says.The test aircraft has been dubbed the "KF/A-18E/F".
CLEARING WEAPONS
Much of the testing remaining concerns weapons clearance. The aim is to have 29 weapons configurations cleared by the end of development - compared to just two for the original F/A-18A/B programme - and 59 by the first operational cruise in 2002. These will include the Joint Stand-Off Weapon (JSOW), and an F/A-18D has been modified with the E/F's cockpit displays to begin JSOW integration.
Stores separation emerged as an early challenge. "We have seen bomb-to-bomb collisions, as we did on the A-6, AV-8B and F-14," Wieringa says. Bomb tailfins have hit the fuselage after release from the inboard stores stations, so the underwing pylons have been toed out by 4° to align them with the airflow around the E/F's wider fuselage. Bombs now drop straight, without yawing, he says. Bomb-to-bomb collisions are being tackled by optimising the release sequencing.
An "interesting challenge", Wieringa says, has been the ALE-50 towed decoy. This is deployed from under the centreline of the aircraft and the engine exhaust has burned through the tow cable in flight. The solution is a small bracket which holds the cable 150mm away from the fuselage. The ALE-50 is intended as an interim solution until the E/F's integrated defensive electronic countermeasures system, with its fibre-optic towed decoy, is ready.
Survivability was balanced with affordability in design of the F/A-18E/F, says Wieringa, and involved a combination of reduced signatures and vulnerable areas and improved situational awareness and weapons. Low observability (LO) features include "common sense stuff" such as edge alignment, he says. Less obvious is an inlet device which prevents radar energy reaching the engine fan face. In-flight measurement of radar cross-section has been performed "-and the aircraft is doing well", he says, adding: "It takes two flights to totally measure the aircraft, and no preparation is required."
The General Electric F414 engine is also performing well, Wieringa says. The aircraft were grounded briefly in November 1996 when a compressor stator failed in flight. The cause was traced to a design change which offered a small efficiency improvement, and all test engines were retrofitted with the original design of stator. "Cold rotor" stalls observed during warm up are being tackled with changes to the full authority digital engine control system software.
The next phase of independent operational testing will begin in June. Operational fleet pilots have been involved in the development test programme from the outset, Wieringa says. "You get better, safer, operational testing if the pilots have experience of the aircraft," he believes, noting that an aircraft was lost during F/A-18A/B development while being flown by an operational test pilot.
"There is also the low observability aspect [of operating the aircraft], which is often counter-intuitive. Pilots can't just walk into it," Wieringa says. "No-LO to an LO aircraft is a hard transition to make. We are working on LO tactics, which are the key to success."
The next major milestone is sea trials, planned for October-November. Initial sea trials were accomplished "flawlessly" last year on the USS Stennis, he says, using aircraft F2.
Wieringa says that the integrated contractor/ government test team has eliminated a lot of duplication by combining Boeing's development testing with the Navy's technical evaluation. The overall F/A-18E/F "-is still on cost. on schedule and on performance", he says. "Flight test is a bit behind schedule because of the wing drop challenge, but we have the ability to meet all of our targets."
Source: Flight International
