Code One Magazine: Third Quarter 2001

These words, transmitted in full British accent from the cockpit of the X-35B, signal the beginning of the final portion of the concept demonstration phase of the Joint Strike Fighter program for Lockheed Martin. The pilot, Simon Hargreaves, an experienced Harrier test pilot from BAE Systems in the UK, glances at a lifeless windsock planted near the test ramp at Lockheed Martin facilities in Palmdale, California. He then scans the cockpit displays to check engine and aircraft systems. His voice is broadcast directly through a telemetry system to a data trailer where two dozen engineers scan detailed information on the X-35B and its unique vertical propulsion system.

The aircraft sits atop a thick metal grate that covers a large concrete pit. The hover pit, as it is called, is a scaled-up version of the one used for the very first vertical flights of the Harrier prototype, the Hawker P1127, performed in the fall of 1960 at Dunsfold in England. The pit collects exhaust thrust and directs it away from the aircraft to create a less turbulent and more controlled environment.

“Hatrick 3, everything looks good here,” radios Paul Bloxham from the data trailer. “You are cleared for takeoff.” The accent, again, is distinctly British as Bloxham, the flight test conductor, is another BAE employee recruited by the X-35 program for his Harrier experience.

“Power is coming up to eighty-five percent,” Hargreaves announces. The increased power pivots the aircraft on its main gear, gently lifting the nosewheel of the X-35B off the grate of the hover pit. The aircraft has gone from a level attitude to a slightly higher one of three-degrees nose high. One more scan of the engine instruments and Simon declares, “Powering up.” The Pratt & Whitney JSF119-611 engine roars intensely as 20,000 horsepower transfers from the engine’s core to the Rolls Royce lift fan just behind the cockpit. The aircraft rises gently in the air.

Maj. Art Tomassetti, a US Marine Corps test pilot stationed next to the pit, acts as a landing site supervisor, or LSS, for Hargreaves. When Tomassetti sees the main gear lift from the pit, he radios to the test team, “Lift Off!” Hargreaves eases the throttle back to idle. The main gear of the X-35B settle back onto the hover pit, followed by the nosewheel. The engine spools down to idle. This first unofficial flight of the X-35B attains a height of less than five feet. Though only a “hop,” the first vertical takeoff represents a huge leap for the X-35 team.

“The first hop was over in a few seconds,” notes Bloxham. “It was one of the shortest first flights in the history of aviation. Once we cleared the pilot to go, the test conductor in the trailer couldn’t do much else. The flight was over as soon as it started.”

“Everyone on the program breathed a sigh of relief after the first hop,” Hargreaves explains. “We knew in a heartbeat that the airplane and its flight controls work. Everything else from that point on is clearly challenging and must be completed in a short timeframe. But the first jetborne flight was a huge achievement.”

Within a few days after Hargreaves’ first hop, all three STOVL test pilots on the X-35 team hovered the aircraft. Tomassetti was the first military pilot to evaluate the X-35B in hover on 29 June. Squadron Leader Justin Paines, an experienced Harrier test pilot from the Royal Air Force, evaluated the X-35 for the UK military the next day. “The fact that other pilots hovered the aircraft so quickly in the test program is a testament not only to their abilities as test pilots but also to the design of the X-35,” Hargreaves says.

Converting From A To B

The X-35B airframe actually completed its first flight months earlier, on 24 October 2000 to be exact, as the X-35A, the conventional takeoff and landing, or CTOL, demonstrator. The X-35A went on to fly twenty-seven flights, expanding the flight envelope to 34,000 feet and supersonic speeds. It was then transformed into the X-35B STOVL demonstrator when a vertical lift system was installed in an empty circular bay behind the cockpit. (The upper and lower doors of this bay were fully operational during the X-35A flight tests.) The transformation continued when roll ducts and roll posts were installed in the wings, a straight duct was replaced with a three-bearing swivel duct on the aft end of the Pratt & Whitney JSF119-611 (the same engine used for the other X-35 variants), and the engine was connected by a drive shaft to a clutch and gearbox on the lift fan.

“The X-35A has undergone a metamorphosis of sorts, but almost nothing has changed on the surface of the aircraft or in the cockpit,” Hargreaves explains. “Exhausts for the roll posts are now visible under each wing surface. A thrust vector lever outboard of the throttle, present in the X-35A cockpit from the beginning, is now functional. The basic airframe has remained untouched with many subsystems and the CTOL flight control system remaining identical to the X-35A as well.”

The X-35A to B metamorphosis becomes more apparent in STOVL mode. The conversion from CTOL to STOVL begins when the pilot pulls back on the thrust vector lever. Doors open above and below the lift fan. Doors behind the lift fan intake open for an auxiliary engine intake. The engine nozzle twists and vectors downward. The clutch engages and transfers energy from the spinning shaft via the gearbox to the lift fan. Control valves open to divert bypass air from the engine to the roll posts. All of these various changes occur in seconds in precise, computer-controlled order.

The lift system has proven to be dependable. “One of the myths we have to overcome is that a shaft-driven lift fan is somehow too complex to achieve an acceptable level of reliability,” notes Hargreaves. “Well, since February 2001, when the lift fan was coupled to the engine in the aircraft for the first time, we have completed a further forty engagements at all the power settings we will ever require in flight. We have not had a single problem. The system is mature, reliable, and ready for in-air conversions.”

“Every step we take from the pit tests in the first months of 2001 to the ground runs in the spring have instilled more confidence in our system,” adds Bloxham. “The airplane hasn’t missed a beat mechanically from the day we built it.”

Hargreaves’ and Bloxham’s confidence and the confidence of the entire X-35B flight test team was reinforced by the success of the ground test program of the vertical lift system. In these tests, the STOVL propulsion system was subjected to more than twice the operating time and more than twice the operating events expected during the X-35B STOVL flight demonstration program. The system was put through about 250 total accumulated engine cycles in these accelerated mission tests, which also included more than 110 hours of total STOVL lift system operating time and 171 dynamic clutch engagements of the lift fan. The STOVL propulsion system was cycled through the equivalent of 132 flight test missions. The operations were identical to those expected during subsonic and supersonic flight testing, ground testing, and STOVL operations.

Kathy Zapka
Propulsion Integrated Product Team Lead

“These tests were crucial in establishing the reliability of our propulsion system,” notes Kathy Zapka, the propulsion integrated product team lead for X-35. “Thrust is crucial to safety in STOVL modes since thrust acts as a control surface. Defining our expected performance is important because it relates to safety. Generally, thrust is not crucial to safety for CTOL aircraft. Thrust is more closely associated with maximum performance.”

Vertical Lift Contributors

The counter-rotating blades of the lift fan provide about 18,000 pounds of lifting power at the front of the aircraft. Lift fan thrust is controlled by a combination of engine rpm and fan inlet guide vane angle. The thrust from the lift fan can be vectored from thirty-four to ninety-five degrees (as measured from the horizontal line from the nose to the end of the fuselage). Each roll post provides about 1,500 pounds of lifting thrust on each side of the aircraft. Adjusting the thrust split to the two posts provides lateral control. The engine, which provides about 26,000 pounds of nonafterburner thrust in CTOL mode, contributes about 18,000 pounds of downward thrust in STOVL mode. Nozzle exit area and engine rpm are used to control the thrust from the aft end of the aircraft. The engine angles downward thrust through a three-bearing duct that can swivel ninety-five degrees from the horizontal and sweep plus or minus twelve degrees from left and right.

Simon Hargraves
BAE Test Pilot

The lift fan approach was chosen for its many attributes. It extracts power from the engine, thus reducing exhaust temperatures from the engine by about 200 degrees compared to exhaust temperatures of direct-lift systems. It significantly reduces exhaust velocity as well. Engine exhaust air combines with the low-temperature and low-velocity air from the lift fan to produce a more benign ground environment. Cool exhaust air from the fan prevents hotter exhaust from engine from being reingested into the intakes. Hot gas reingestion, a common problem on legacy Harrier-type approaches, causes compressor stalls and other severe engine performance degradations. Most importantly, the lift fan system was chosen because it does not detract from the up-and-away performance of the JSF119-611 engine.

“The ground environment and the growth potential convince me that this is the right concept,” Hargreaves explains. “From 1960, the Harrier grew fifty pounds per year on average. No reason prevents this airplane from doing the same. The shaft-driven lift fan gives us huge amounts of available thrust in the ground environment with little to no weight penalty. A STOVL aircraft must be tolerant of weight growth. An augmented system lends itself to weight growth.”

Flight Test Envelope

The flight test envelope for STOVL operations is divided into four basic sections—ground, jetborne, conversion, and semi-jetborne. Ground testing includes taxi tests and restrained tests. Jetborne testing covers vertical takeoff, vertical landings, and hover operations at air speeds under forty knots (speeds at which the normal CTOL control surfaces are not involved and at which the aircraft mass is not supported by lift generated from the wing). Semi-jetborne testing includes STOVL flight at speeds greater than forty knots (speeds at which the wing is generating lift). Semi-jetborne encompasses flight at greater than forty knots in STOVL mode, including short takeoffs and short landings. Conversion testing covers the transition from CTOL to STOVL modes in flight.

Conversion and transition carry specific meanings in STOVL testing. Conversion refers to a mechanical process, the act of engaging the clutch to spin up the lift fan. Transition relates to a change in the aerodynamic state of the aircraft from solely wingborne or jetborne flight modes to a semi-jetborne flight mode. The aircraft has to be converted to start a transition. Conversion takes place from about 160 knots to 200 knots, speeds well above the stall speed of the aircraft. The minimum speed is dictated by the amount of aerodynamic tail plane power needed to control the pitch moment caused by the initial thrust of the lift fan. The maximum speed is determined by the aerodynamics of the lift fan intake.

Paul Bloxham
BAE Flight Test Conductor

“We have to use a different approach for expanding the flight envelope of STOVL than the approach for expanding the envelope of a conventional aircraft,” Bloxham explains. “For example, we wouldn’t consider testing at 100 feet off the ground as an envelope expansion point for a CTOL aircraft. While we use the same flight envelope graphics of altitude plotted against speed for both CTOL and STOVL, speeds over 250 knots are not meaningful to the STOVL environment. We concentrate on nozzle angle, power, and attitudes as a better description of the limits of the flight envelope.”

“The STOVL airplane can do everything the CTOL airplane can do when the STOVL airplane is in the CTOL mode,” adds Tomassetti. “STOVL takes the aircraft into a realm of flight that involves more than just aerodynamic surfaces. The engine and the lift system fly the airplane. The transition from one type of flight to the other is a major portion of the testing. We also test the jetborne side of the equation, which is completely different from CTOL testing.”

Ground Testing

After the X-35A was fitted with the vertical lift system in January 2001 and transformed into the X-35B, it was fixed to the hover pit with a special landing gear that allowed load cells to measure STOVL lift forces and moments directly and that prevented the airplane from lifting into hover at higher power settings. More than 100 of these restrained tests were conducted with all control functions fully under pilot command. X-35B test pilots performed full rehearsals of vertical flight missions, including conversions from conventional takeoff and landing modes to vertical modes. These tests were conducted in several configurations, including open-grate conditions on the hover pit to represent out-of-ground-effect flight and closed-grate conditions to produce the ground effects encountered during vertical landings and takeoffs.

“Results from those tests were extremely positive,” notes Zapka. “Our measurements showed no thermal distress from either hot-gas reingestion or flow-field effects on the aircraft surfaces. We also recorded favorable thermal conditions at ground level near the airplane.”

The ground tests included twenty-six CTOL to STOVL clutch engagements of the lift fan at high engine rpm. The X-35B operated at maximum STOVL thrust levels for periods of up to ninety seconds. Individual test series were run with a full aircraft fuel load for as much as one hour. “All twenty-six of these conversions worked exactly as expected,” notes Hargreaves, who was in the cockpit for many of them. “Noise level and vibration in the cockpit were virtually unchanged at idle compared to CTOL levels. The propulsion system responded predictably to pilot inputs. Thrust and thrust-vector commands were crisp. Noise and vibration at full power with the thrust vector at the hover setting were comfortable.”

Before the X-35B flew, it was put through a series of taxi tests in STOVL mode. “The taxi tests determined how we slow the aircraft from higher taxi speeds,” Bloxham explains. “To stop from a taxi speed of 100 knots, for example, we position the nozzle all the way down and forward a little bit and use the thrust of the propulsion system to slow down. We don’t use forward-directed thrust all the way to a standstill because hot exhaust gas can be thrown forward of the airplane and reingested in the intakes, which is not good. We wet the runway in these tests to determine visually at what speed the gases start moving forward. At that speed, the nozzles go back to about forty degrees and the airplane uses its brakes to come to a complete stop.”

Jetborne Testing

Before the first hop, the X-35B completed vertical takeoffs without leaving the ground in a test called a no-go VTO. “We overfilled the airplane with fuel so it was too heavy to hover,” Hargreaves explains. “That required full fuel, about 37,000 pounds full-up weight. As we pushed the power up, the landing gear extended. When the main gear got to within one inch of their total travel, the weight-on-gear switch told the airplane it was flying. That closed the loop on the flight control system, which meant that a lot of the clever STOVL functions came into play. The airplane automatically trimmed itself to a hover attitude, which is three degrees nose up, so the nosewheel came off the ground a couple of feet. We then went to full power. For all intents and purposes, we were flying. I made pitch inputs by moving the sidestick controller fore and aft. I made yaw inputs by moving the rudder pedals. I also made nozzle nudge inputs to the thrust vector lever to move the aircraft forward and aft. We collected a lot of data during this test.”

After its first hop and one or two more hops at different fuel loads, the X-35B completed its “official” first flight — a sustained and controlled hover at about twenty feet above the ground on 24 June. “The flight was a stunning success,” recalls Hargreaves.“The aircraft was easy to control within the confines of the hover pit. Handling was extremely precise.”

Other jetborne testing included vertical landings after completing transition from semi-jetborne to jetborne flight. Completing these hover tests in the high desert in California, about 2,500 feet above sea level during the heat of the summer, highlights the thrust margin of the X-35B and its lift fan system. “A jet engine loses about one pound of thrust for every foot over sea level,” Hargreaves explains. “Going to sea level conditions would give us the equivalent of 2,500 pounds of extra thrust. But thanks to the efficiency of our vertical lift system, we don’t need it. We plan to complete all the required testing in the high desert.”

Conversions

After the first official flight, the X-35B completed a functional check flight, which was essentially a CTOL flight. “This flight proved that the airplane has the same performance of the X-35A CTOL version,” notes Bloxham. “We flew a variety of points in the envelope and verified that they match the performance of the X-35A. We also opened the STOVL doors in flight for the first time, the first step in the conversion process from CTOL to STOVL. We observed loads and flying qualities as we cycled the doors.”

The first conversion occured about 10,000 feet and 180 knots. After that first conversion, the X-35B completed several more conversions and flew in STOVL mode at slower and slower speeds by vectoring the nozzles at greater downward angles. Subsequent conversions from CTOL to STOVL and back occur at a maximum altitude of 10,000 feet and a minimum altitude of 2,500 feet.

Semi-Jetborne Testing

The aircraft entered the semi-jetborne state in up-and-away flight the first time it was converted. “After the initial conversion, we continued to operate in STOVL mode and assessed the handling of the airplane,” Bloxham explains. “We work down to the hover point we established on the first official flight over the pit. Eventually, we hover and land on a solid pad at Edwards AFB.”

After the initial conversions at 10,000 feet, the aircraft performed conversions at similar speeds at 5,000 feet. In STOVL mode, the aircraft starts slowing down to speeds of about 100 knots. “We don’t want to go any slower than 100 knots at 5,000 feet, because the slower the aircraft gets, the more ground references a pilot needs,” Bloxham says. “After establishing 100 knots at 5,000 feet, we drop down to about 150 feet above the ground for slower speed testing.”

The 150-foot maximum altitude for hovering is dictated by visual references. “We decelerate into a hover at about 150 feet above the runway,” Hargreaves says. “This height provides visual references in terms of peripheral vision, sideways movement, and general awareness of the energy state without looking down at flight instruments. Hovering is like driving a car: With experience, drivers know about how fast they are going without looking down at their speedometers.”

The first STOVL takeoffs and landings were conducted at fixed nozzle angles of twenty-two degrees and then at thirty-four degrees. These initial flights were followed by takeoffs and landings at slower and slower speeds, down to minimum speed of about eighty knots. From here, semi-jetborne testing advanced toward jetborne conditions from slower and slower air speeds at 150 feet above the runway.

“The final decelerations to purely jetborne flight, around eighty knots or so, were the most challenging tests,” Bloxham says. “We knew we have a good end point, but we had to go through lots of unknowns to get there. The computer modeling looked good. But the jet effects and interactions make the region difficult to predict. The area is also unforgiving because the aircraft is so close to the ground.” Tomassetti agrees with the assessment. “The first vertical landing in ground effect to a solid surface was the biggest challenge for this phase of the program,” he says. Adds Hargreaves: “When we started from 150 knots and built down in speed, we were going to a known and good endpoint. We didn’t want to tiptoe into the unknown. That’s why our first flight was a vertical takeoff. We removed a lot of risk by hovering before we decelerated into a hover condition.”

Mission X And Beyond

Maj. Art Tomassetti
USMC Test Pilot

Mission X, flown by Tomassetti on 20 July, provided a taste of the operational utility of the STOVL version of the JSF by combining all of the various aspects of STOVL flight into a single mission. The Marine Corps pilot performed a short takeoff, converted from STOVL to CTOL at 5,000 feet, climbed to 25,000 feet, accelerated to supersonic speeds, converted back to STOVL, decelerated to hover, and then performed a vertical landing in ground effect at Edwards.

The first vertical flight of the X-35 and its subsequent testing opens a new era for the US Marines and for the UK Royal Navy and Royal Air Force. “STOVL gives us capability we don’t have with helicopters,” explains Tomassetti, who has over 1,500 hours of Harrier flight experience in the US Marines as well as combat time in the Harrier in Desert Storm. “We deploy expeditionary units that fit onto amphibious ships that require STOVL-capable aircraft. Fixed-wing aircraft offer these units huge tactical advantages over helicopters. Fighters typically fly at least 600 knots faster than helicopters can fly. They give commanders an ability to project power much farther and faster than typical rotary wing aircraft. They give commanders a much larger sphere of influence for reconnaissance and attack.

“The JSF improves on the Harrier, our current STOVL fighter,” Tomassetti continues. “The Harrier, with very little computer augmentation to reduce pilot workload, is difficult for pilots to learn to fly and to stay proficient. The JSF, with computers to reduce pilot workload, will be easier for pilots to learn how to fly and to stay proficient. In addition, the performance will be better than the Harrier, the range longer, the payload larger, and the shape much stealthier. I’m looking forward to getting this airplane into the fleet.”