|
Print friendly version of this article (text only)
This article appeared in the Second Quarter 2001 issue of Code One Magazine.
 The X-35C banks left over a choppy Chesapeake Bay on approach to NAS Patuxent River, Maryland. Flaps hanging low, the aircraft straightens and glides over the water to the approach end of runway 32. At less than 135 knots airspeed in a brisk headwind, the jet’s relative ground speed is slightly more than DC commuters speeding to work on the freeways below. The roar of the Pratt & Whitney engine intensifies as the aircraft nears. Puffs of smoke signal the tires of the main gear have touched down. Suddenly, the pilot throttles to military power and the X-35C jumps back into the air for another circuit around the air station.
This completed landing, brief though it was, creates more data points for a database already filling with field carrier landing practices, or FCLPs. These landings and takeoffs represent a large portion of the testing to be done by the X-35C at the Naval Air Warfare Center’s Aircraft Division at Patuxent River. NAWCAD, as it is known, supports research, development, test, evaluation, engineering, and fleet support of Navy and Marine Corps aircraft. It is the steward of the ranges, test facilities, laboratories, and aircraft necessary to support the Navy’s acquisition requirements. It is also home to the United States Naval Test Pilot School. It is now the home of the X-35C, the Navy variant of Lockheed Martin’s JSF demonstrator aircraft.
 Cross-Country First
The X-35C arrived in Maryland on 10 February after a precedent-setting cross-country trip from Edwards AFB in California. The 2,500-mile journey, completed in two legs in two days, was the first transcontinental flight for any X-plane.
“We brought the X-35C to Pax River because it is the Navy’s premier test facility,” explains Joe Sweeney, Lockheed Martin test pilot for the first leg of the trip, from Edwards to Fort Worth, and the first pilot to fly the X-35C. “At Pax, Navy
personnel can see their airplane up close in the their own environment. Besides, NAVAIR headquarters is here, the JSF program office is here, and political decision makers are nearby. We can give more people a first-hand look at the airplane.”
“Flying any new single-seat, single-engine airplane across the United States is an accomplishment. Making that journey in an X airplane that has been flying for less than three months is a tremendous accomplishment,” says US Marine Corps Maj. Art Tomassetti, pilot for the second and final leg from Fort Worth to Patuxent River. “Pilots dream of becoming test pilots. Test pilots dream of flying X airplanes. So I guess X airplane pilots dream of doing something spectacular in an X airplane. Bringing the X-35 to my home base at Pax River after flying it cross country qualifies as something pretty spectacular in my book.”
Moving the X-35C from the high desert of California to the sea level environment at Pax has a technical rationale as well. “Edwards sits at 2,200 feet above sea level,” continues Sweeney, who is also lead Lockheed Martin test pilot for the X-35C. “Aircraft carriers sit at sea level. For a true one-to-one comparison and evaluation of the fidelity of our airplane and its capabilities in a carrier environment, Pax is the place to be.”
“Pax also has the tools and the flight test engineers who specialize in carrier suitability,” adds Lt. Cdr. Greg Fenton, the Navy’s newest test pilot for the X-35. “For the X-35C, carrier suitability involves testing how well we can fly the airplane on a landing path, how precisely we can fly it to a touchdown point, how quickly we can get it back in the air, and several other performance and handling characteristics.”
 FCLPs, Bolters, and Waveoffs
Carrier suitability testing for the X-35C encompasses three essential categories—FCLPs, bolters, and waveoffs. FCLP testing covers the flying qualities necessary to get the aircraft in a position to land on an aircraft carrier. In these tests, pilots progress from a nominal or ideal approach to less ideal approaches. This range of approaches is conducted at varying distances from the touchdown point and at varying high and low offsets at each distance. “The pilot intentionally deviates from an ideal landing path,” explains Glen Harbison, the lead flight test engineer for the X-35C, “so that we can evaluate how well the aircraft allows the pilot to compensate for these deviations. We also evaluate how quickly the airplane and pilot can recover from an offset approach and still land safely.”
A bolter occurs when the aircraft’s arresting hook fails to catch a wire. This may be caused by ship’s motion, a hook skip (where the hook bounces over a wire), wind gusts, or pilot technique. Bolter testing ensures the flying qualities and performance of the aircraft are sufficient enough to allow the aircraft to touchdown after the last arresting cable and to allow the aircraft to safely lift off again by the time it reaches the end of the landing area. “Because many factors can cause the aircraft not to catch a wire, Navy pilots always prepare for the possibility that they are going to miss the arresting cables on a carrier landing,” explains Tom Briggs, the carrier suitability flight test engineer, one of three Navy flight test engineers working on the X-35 program. “As soon as the wheels touch the deck, the pilot immediately throttles to full military power to prepare for another takeoff. If the pilot catches the wire, he decelerates. If he misses the wire, he takes off and tries to land again.”
Waveoff testing covers the performance of the aircraft during an aborted landing. “We try to land aircraft every forty-five seconds on a carrier,” continues Briggs. “If the crew has a hard time clearing an aircraft from the landing area, they have to make a ‘fouled deck’ waveoff.”
 Pilots rely on the landing safety officer on the deck to signal a waveoff with a Fresnel lens optical landing system. By pressing a button, the LSO activates a pattern of colored flashing lights on this system. (The landing system is normally used to give pilots a visual indication of their relative position with respect to a prescribed glideslope.) Waveoff commands are also issued over the radio.
“Waveoff testing determines how much altitude the aircraft loses and how much time it requires to go from a stabilized rate of decent to a positive rate of climb,” explains Briggs. “In other words, we want to determine how far down the glideslope the pilot can initiate a waveoff and not touch down or catch a wire when the hook is extended. The LSOs will use this information in determining how to safely bring the aircraft aboard the carrier.”
“Landing on a carrier has to become second-nature to every Navy pilot,” adds Harbison. “This testing ensures that we have designed an airplane that makes carrier landings second nature.”
The X-35C is designed to survive carrier landings, which tend to be more severe than typical Air Force landings. In numerical terms, when a pilot flies the X-35C on a carrier landing, the rate at which the aircraft hits the runway, called the sink rate, is approximately eleven feet per second. The aircraft is designed to withstand a sink rate of almost eighteen feet per second. By comparison, the typical sink rate for an X-35A landing, the conventional takeoff and landing variant of the JSF demonstrator, was around two feet per second.
 X-35A/X-35C Differences
“As for performing the mission, the distinction between what makes a good Navy fighter and what makes a good Air Force fighter is fairly insignificant,” notes Fenton, who comes to the X-35 program from a recent fleet tour in F-18s on the USS Enterprise. “In both services, the airplane must have a decent amount of time on station and provide an advantage against any current or projected foes. The big difference between the two services involves carrier suitability. A Navy fighter has to take off and land from an aircraft carrier, which requires some structural considerations and flying qualities.”
Those structural considerations and flying qualities explain most of the differences between the X-35A and the X-35C. Internally, the X-35C is a little beefier to handle the harder landings. Externally, the wing area and control surfaces are larger to improve low-speed handling characteristics essential for carrier landings. The X-35C also has two extra control surfaces in the form of two ailerons outboard of the flaperons.
“The difference in performance between the X-35C and X-35A at landing speeds is very noticeable,” explains Sweeney, who flew the X-35C on its first flight and has flown the X-35A as well. “The X-35C can fly about 130 to 135 knots on the landing approach, about twenty-five knots slower than the X-35A.
 “The landing approach control laws and flying qualities for the X-35 were designed primarily for the Navy environment,” Sweeney continues. “Most of those control laws were used in the X-35A to keep the variants’ flying qualities common. Commonality also reduced cost. Everyone who flew the X-35A— including test pilots with Navy, Marine, and Air Force backgrounds—was very pleased with the way it landed: It was very easy to land. Both variants are also very similar in up-and-away flight. Handling characteristics between the two aircraft are not noticeably different. Performance differences are more noticeable, for example how fast the aircraft accelerates and climbs and how well it maintains energy in turns. The control laws on the X-35C have a couple of extra features that take advantage of the extra control surfaces. These features give the pilot more precise control of the glide path.”
“On every approach, we look for good glideslope control,” adds Fenton. “We have to be able to put the airplane down with pinpoint accuracy to hit one of the four wires on a carrier deck, within plus or minus forty-five feet or so. The airplane has to track glideslope precisely and make small glideslope changes on short order.”
A carrier box painted on the runway at Pax River indicates the amount of deck area the aircraft has to land and then takeoff again. “During bolters, I could tell from the cockpit that the aircraft was well off the ground before the edge of the carrier deck,” reported Fenton in one of his many flight debriefs. “Engine and aircraft response was positive and quick.”
 Autothrottles
If landing a 30,000-pound aircraft on a forty-five-foot postage stamp in a rolling sea sounds simple, you’ve been playing too many video games. But technology has made the job easier with an autothrottle system called an approach power compensator, or APC. Almost every Navy airplane since the F-8 has had one. “The autothrottle takes the pilot’s left hand (throttle) out of glideslope control,” explains Harbison, “leaving him only the control stick to maintain glideslopeand lineup.”
“The APC mode can greatly reduce the pilot’s workload during a carrier approach. The APC on the X-35C has been very smooth and although we have yet to finish all the testing, it looks like the guys in Fort Worth did a great job. The developmental work in the simulator has led to a final configuration that has worked nicely. The capability is useful for conventional landings as well. Air Force test pilot T.P. Smith noted that the APC is a must for the Air Force variant.”
“In the X-35C, the APC is a mode in the flight controls,” explains Briggs. “In other aircraft, the APC is often a separate component, like a radio. Regardless of its system location, the APC works like the cruise control of a car. The pilot can
deselect it or move the throttle to override it.”
 The Bottom Line
While carrier suitability is a critical factor in evaluating the Joint Strike Fighter for the Navy, it isn’t the only one. Test pilots at Pax will be expanding the flight envelope of the X-35C and evaluating the up-and-away performance of the aircraft as well. “Like any other fighter pilot, a Navy fighter pilot will probably go straight for the max performance numbers—Mach, g, range,” Briggs says. “But the Navy guys are also going to ask, ‘How does it fly on the ball?’ That question gets away from raw performance figures and into handling qualities, which we are evaluating closely here at Pax.”
“And that’s the primary reason we are here—to prove that the Lockheed Martin JSF design meets the Navy’s carrier suitability requirements,” says Sweeney. “Being at Patuxent this early should instill confidence in our program and in our approach. In the next phase of the program, we get into the detailed evaluations necessary to make this a fleet-capable airplane. Since the design we’re proposing for the next phase is so similar to the X-35C, everything we are doing here will support the transition to an operational aircraft.”
Eric Hehs is the editor of Code One.
|
