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03-28-09, 07:48 AM
Ride the Lightning: Testing the Marine Corps' latest fighter
March 27, 12:27 PM Add a Comment
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The F-35 Lightning II Joint Strike Fighter (JSF) is entering one of the most challenging phases of its development according to manufacturer Lockheed Martin. The ambitious 300 billion dollar project is intended to replace the aging arsenal of legacy fighters in service with the US Air Force (USAF), US Navy (USN), US Marine Corps (USMC) and a host of foreign allies.

Due to the disparate requirements of the three services, the aircraft will be built in three distinct variants. The first variant is the conventional take-off and landing (CTOL) F-35A for the USAF. The USN requires that their version of the airplane be capable of taking-off and landing on the Navy’s fleet of aircraft carriers. This naval version will be designated the F-35C or CV variant. The most challenging requirements however come from the USMC.

The Marine Corps version is known as the F-35B Short Take-Off and Vertical Landing (STOVL) variant. Due to the unforgiving demands placed on Marine Corps aviation, the F-35B has some unique features. The aircraft needs to be able to operate from the small decks of amphibious assault ships and short, unimproved airstrips at Forward Operating Bases (FOB).

The first among these unique features is a shaft driven lift fan mounted behind the cockpit of the STOVL aircraft, which provides vertical thrust during take-off and landing. The aircraft also has the capability to rotate the main engine exhaust nozzle downwards over 90 degrees in order to provide more thrust and additional control. Lastly, roll posts are embedded in the wings, which not only provide additional vertical thrust, but also act as control mechanisms. Combined, the system provides around 40 thousand pounds of vertical thrust.

Designing and testing the STOVL variant, which will also be used by the British, is one of the most challenging aspects of the Joint Strike Fighter project says Graham Tomlinson, Lead Test Pilot for the F-35B. Tomlinson, a former Royal Air Force (RAF) Harrier pilot and a graduate of the prestigious Empire Test Pilot School, has over thirty years of flight test experience with STOVL aircraft.

The F-35B is currently undergoing crucial tests in what is known as a hover pit. As Tomlinson explains, a hover pit is a specialized testing facility designed to measure the performance of a STOVL aircraft on the ground. “It’s not quite the same as flying, but almost. It’s like fooling the aircraft into thinking its flying. It’s as nearest to flying as we can make it.” Tomlinson said.

The hover pit, as the name implies, is a concrete pit. The pit is rigged with sensors to measure the thrust produced by the F-35B’s engines while also “getting rid of the hot gas”, Tomlinson explained. A metal grate covers the top of the pit. It is upon this metal grate that the aircraft sits during testing.

Hover-pit testing pushes the aircraft to its limits in a safe environment Tomlinson explained. The aircraft is tested at high power and at various stages of conversion from conventional mode to STOVL mode. For flight-testing purposes, the test pilots can go through the individual stages of conversion by inputting the necessary commands manually. The pilots “thrash it to death” inputting, “all sorts of silly commands which you would never do in real flight, like pitching the nose up sharply in STOVL mode” he explained.

The harsh test regimen is necessary to ensure that the computer model embedded into the aircraft’s flight control laws are robust, Tomlinson said, adding “we’re validating the model in a real plane.” Tomlinson emphasized that validating the aerodynamic model is especially crucial as “all that is part of the airplane and the simulators. These are really important steps. These are all part of the clearance process”. As hover-pit testing advances, the metal grate will be covered in order to simulate operating the F-35B on a hard surface such as an airfield or ship, Tomlinson explained.

For the operational Marine Corps pilots who will take the F-35B into battle, the brutal testing regime will provide tangible results, Tomlinson said. Compared to the legacy Harriers that Tomlinson flew during his RAF days and during most of his testing career, the F-35 is incredibly easy to fly. Converting the aircraft from conventional flight to jet borne hover is accomplished at the touch of a button. Every F-35 variant has an “identical cockpit. It’s just one switch that’s different”, Tomlinson explains, “On the USAF and CV versions that button says STOVL/HOOK and it drops the hook. In the STOVL version the same button initiates conversion.”

Because the vertical landing process for the STOVL variant has been so simplified, Tomlinson expects that the F-35B “will have a much lower training burden compared to the CV version.” Tomlinson explains that while the CV version will have a landing speed of around 145 knots, the STOVL will be able to slow down to much lower approach airspeeds and will be able to hover depending on its load. Lower airspeeds make the pilots’ job easier. While flying the STOVL will be easier, Tomlinson cautions, “ you still need the background knowledge.”

The flexibility in basing offered by the F-35B does come at a price however. The STOVL variant “loses a couple thousand pounds of fuel”, Tomlinson explained. The F-35B also has slightly shorter weapons bays, and is thus unable to carry the same 2000 lbs class weapons internally that the other two variants can. Tomlinson explains that the reduced capability is a trade-off for the flexibility required by the Marine Corps in order to base the aircraft on smaller ships and Forward Operating Bases. Tomlinson said that a fully combat loaded F-35B will take-off from a small unimproved airstrip in less than 1200ft.

The F-35B also loses the ability to pull some Gs compared to the conventional USAF variant. The STOVL can only pull 7G compared to 9G for the F-35A and 7.5G for the F-35C. Tomlinson explained that this is not a result of any trade-off made for improved short field performance. “There’s no reason we can’t make a 9G STOVL airplane”, he said.

Tomlinson explained, “Because of the stealth and sensors, the Marine Corps and Navy weren’t interested in more than 7G and 7.5G for their F-35 versions. The Marines and Navy have never been enthusiastic about a 9G capability. It’s not required for their mission. To get more G, you need to beef up that structure and that adds weight. The USAF made the trade-off for the 9G capability.”

Other than the reduced G-limit, in conventional flight the F-35B handles almost exactly like the F-35A, Tomlinson explained. The F-35B retains the same outstanding low-speed, high angle of attack handling qualities as well as the same incredible acceleration as the F-35A. “You struggle to tell the difference between the CTOL and the STOVL in the cockpit,” Tomlinson said, adding that test pilots are trained to notice even minute differences in aircraft handling qualities. Tomlinson noted that while the F-35B’s lift-fan causes a visible bump in the aircraft’s outer mold line, the only cue in the cockpit is a slightly different wind noise. “STOVL only applies below 10 thousand feet and below 250 knots,” Tomlinson notes.

Once BF-1, the first STOVL aircraft, completes the torturous hover-pit tests, Tomlinson said the aircraft would have to be thoroughly inspected and would subsequently be modified. The aircraft will then be flown conventionally in preparation for transfer to Patuxent River Naval Air Station where the first actual STOVL flight will take place later this summer.

However, prior to the cross-country trip to Maryland, the aircraft will have to be qualified for aerial refueling with a USMC C-130 tanker and undergo cross-country checks. The Navy and Marine Corps use a drogue aerial refueling system, which is different from the boom refueling method used by the USAF. Tomlinson said that the program would “build down” to the first STOVL flight starting with conversions at 200 knots from 5000 ft in the air- exactly like the hover-pit.