This post is one in a series on the development of the F-35. Click here for a Flash graphic of the F-35B with systems cutaways. For more details and updates on the program check out next week’s special coverage in the Dec 12 edition of AW&ST.
Some 15 months ago I blogged that the boss of JSF testing at Edwards AFB, U.S. Air Force test pilot Lt. Col. Hank "Hog" Griffiths, had flown faster in an F-35 than anyone else.
At that time, Griffith had taken one of the initial F-35A test aircraft to 583 KCAS (exceeding Mach 1.2). Now, as the pace of testing continues to accelerate despite earlier delays caused by an inflight dual generator failure, and problems with the integrated power package (IPP), the jet has been flown to Mach 1.61.
Griffiths, who is 461st Flight Test Squadron Commander and F-35 Integrated Test Force (ITF) director at Edwards AFB, was the pilot of one of the two F-35As flown to the desert base in May 2010 for the start of developmental test and evaluation (DT&E). However, unlike 2010, when the slower pace of testing meant Griffiths’ record stood for a while, his latest moment of supersonic glory was eclipsed barely a day later when another test pilot took an F-35A to 666 KCAS.
From now on, with the exception of going for higher speed test points in the 660-700 KCAS range, the bulk of envelope expansion is focused on the “the lower right, higher dynamic pressure, corner” says Griffiths. “At least we’ve shown we can go to design limits.”
Six F-35As are currently in test at Edwards AFB. (Lockheed Martin)
Testing at Edwards continues to remain ahead of plan and for the year to-date, Griffiths says “we’ve achieved 430 flights out of 390 planned, so clearly we’re ahead and we’ll make our year’s goal for CTOL.” By the start of December, the CTOL fleet (not including the pre-production test aircraft) has amassed more than 625 flights and around 1,190 flight hours. The test rate is 10% above the planned rate despite a “lot of significant modifications and a lot of smaller road blocks,” says Griffiths.
The aircraft has also been flown to 9.9g – which is 0.9g beyond the operational limits. Testing has also been focused on reducing the onset of uncommanded roll encountered during a turn. Dubbed the transonic wing roll-off, the phenomenon is common in fighters, and has been countered on the F-35A by using roll-rate feedback to tailor the scheduling of leading- and trailing-edge flaps.
The non-linear aerodynamic response “was expected,” says Griffiths. “We’ve had to try and predict vortex shedding off the wings, and the loss of lift it causes.” The goal was to use existing flight surfaces to cure the problem. “I’m still within the design space and didn’t have to add hardware like fences or anything that would impact the low observable characteristics,” he adds. The cure was fast tracked using a flight controls system parameter change device that enabled multiple control law solutions to be evaluated in a single flight.
The Edwards test fleet includes three flight sciences aircraft and three missions systems aircraft. Two of the six are the first LRIP 1 standard airframes, diverted to bolster the program earlier this year. (Lockheed Martin)
Griffiths has also been in the driving seat, literally, for the start of full-up mission system testing in the F-35A. Initial sensor fusion between the EW systems and radar “went pretty well,” says Griffiths. “We have had the DAS (distributed aperture system) running on the aircraft for the first time, providing 360 deg. coverage. You can see right through the aircraft which is wild,” he comments. The DAS is an internally mounted, multi-functional sensor for air-to-air and air-to-surface targeting capability. “It’s pretty cool and sort of feels like Wonder Woman’s invisible jet. The DAS is working well and enables you to pick up things you wouldn’t normally be able to see because the system’s apertures work at different wavelengths to the human eyeball. It can see details that with your eyes you cannot see, for example on overwater flights looking along the coastline you can pick out details of buildings much more clearly,” he adds.
DAS feels like flying Wonder Woman's invisible jet says Griffiths. (engadget.com)
Tests of another key sensor, the nose-mounted Electro-Optical Targeting System (EOTS), are also getting underway with the first firing of the laser range finder in November from AF-3 equipped with Block 1B software. “We have also have conducted CNI (communications, navigation and integration) sweeps and tested the anti-jam and secure voice systems. We’ve also completed signature testing using AF-3, 6 and 7,” adds Griffiths. The aircraft “is meeting or exceeding the low observable requirements, so we know we have a stealthy aircraft which is fantastic.”
From a flying qualities perspective, Griffiths remarks the F-35 is a real pilot’s aircraft and a “dream to fly.” As a single-seater, this meets the goal of making the aircraft as easy as possible to fly – thereby giving the pilot time to focus on the complex mission tasks. “Flying ought to be the easiest part. I want to be able to put it somewhere so that it won’t move. It trims perfectly to the angle of attack you need,” he adds. The F-35A is also “the easiest I’ve ever landed. It flies like a big aircraft when landing because of the relatively large wings and flaps. When I first landed I didn’t even know I’d touched down.”
(U.S. Air Force)