Electric Jet - How the F-16 Became the World’s First Fly-By-Wire Combat Aircraft
IntroductionThe desert sun shined down on the glistening red-white-and-blue airplane, sending heat rippling and distorting off its surface as it sat on the runway at Edwards Air Force Base. General Dynamics test pilot Phil Oestricher sat in the cockpit, making last-minute checks.
The first test flight of any new plane is risky, full of variables that can’t be foreseen on engineering prints or out in the shop as it’s assembled. What’s more, this particular machine had more than the usual amount of such unknowns. The airplane Oestricher was strapped into was YF-16 number one, the prototype of General Dynamics’ entry into the U.S. Air Force’s 1974 competition to select a new lightweight fighter plane.
Filled with advanced systems and largely theoretical design concepts, the YF-16 couldn’t be evaluated on the ground, especially given the limited state of simulators and computer technology at the time. And so, despite having some of the most sophisticated equipment ever incorporated into an aircraft thus far, proving or disproving the merit of this radical design would have to be done the same dangerous way it had been done since Orville and Wilbur took to the air at Kittyhawk—with test pilots like Oestricher strapping in and betting their life on everything working.
This pioneering version of the YF-16 was to make its first flight today. But calling it a “flight” was perhaps being generous. “Hop” would be more accurate. Oestricher was to lift the plane only a few feet off the ground, for just a few seconds, then put it back down on the runway. And even this tentative, careful step toward a true first flight was to happen only after a series of conservative taxi tests, each one edging closer to flying speed.
The initial taxi runs had been completed without incident, and Oestricher was ready for the short hop to establish the design’s basic airworthiness. He pushed the throttle forward, and the plane began to roll, slowly at first, but steadily gaining speed at an increasing rate. Fifty, eighty, then past 100 knots. At approximately 130 knots, he lifted the nose of the aircraft to ten degrees, and then pushed the stick gently to the left and right, to get a feel for the control system. But the plane didn’t respond.
Figuring that the lack of control was due to the plane’s main wheels still being on the runway, he increased the angle of attack slightly, and the main gear rose off the ground. But as the plane began to fly, its left wing immediately dropped. Oestricher applied right stick to compensate, and the wing flopped the opposite way, dropping its right wing dangerously close to the runway.
The plane continued to accelerate. The oscillation worsened. Back and forth the wings rocked. As the situation continued to deteriorate, a Sidewinder missile on the left wingtip brushed the surface of the runway. The plane bounced on its main landing gear and the right horizontal stabilizer smacked the ground.
As if things weren’t going badly enough, the plane’s nose then swung toward the edge of the runway as the YF-16 continued to accelerate. The rough desert surface loomed as the plane careened wildly.
It wasn’t entirely surprising that this machine would have problems. The YF-16 was one of the first planes in the world have an all fly-by-wire control system. Unlike conventional controls, it didn’t have any direct mechanical connection between the stick and the control surfaces. The YF-16 instead used electronic sensors to read the pilot’s stick inputs and then transmitted that—with computer interpretation—to hydraulic actuators that moved the control surfaces the appropriate amount.
There were many advantages to such a system, foremost among them the fact that it would allow the plane to be highly unstable and yet easily controllable, the theoretical ideal for a fighter aircraft.
Generally speaking, the more unstable a plane is the more maneuverable it can be. Throughout most of aviation history, however, there had been a practical limit to this “relaxed static stability,” as engineers term it. If a plane was too instable, the pilot wouldn’t have been able to keep it under control, given the limitations of human reflexes.
Enter the computer age. With a fly-by-wire system, a computer is employed to constantly monitor the attitude of the airplane, compare that to what the control inputs dictate, and automatically make adjustments to the control surfaces to keep the plane flying the way the pilot intends. And it can do this many times per second. For the new generation of fighter plane the YF-16 represented, fly-by-wire could clearly pay tremendous dividends.
If only it could be made to work.
But at this point, the prototype YF-16 was quickly flailing toward becoming a ball of scrap metal. Then, with the plane heading off the runway and time running out, Oestricher pulled further back on the stick and flew the plane completely free of the runway, ignoring the original plan for the test. He climbed out slowly in a shallow left turn, setting up a wide pattern back around, finishing the incident with a long decelerating approach and a relatively uneventful landing.
Back in its hangar, General Dynamics’ engineers swarmed over the plane, and found the problem quickly. Mostly it was the fault of an overly-sensitive control stick setup, which caused Oestricher to get a full-rate roll every time he tried to stop the oscillation of the wings.
By that evening, the team had come up with a solution, a manually operated switch in the cockpit to make the controls less-sensitive during taxiing, takeoff, and landing. But for now the YF-16 was grounded. Its stabilizer had been extensively damaged in its violent gyrations on the runway. It would need to be repaired before the next test flight.
Although the F-16’s control system may have at the time seemed a revolutionary development, it was actually the culmination of years of evolution, going back decades prior. One of the very first steps toward a full fly-by-wire system was created in the late 1940s, for use on the Northrop B-49. A “flying wing” design, the B-49 lacked conventional horizontal or vertical tail surfaces. Or a fuselage on which to mount them for that matter.
In test flights, the B-49 proved not as unstable as its radical configuration might imply, but neither was it as stable as it needed to be. To cure the problem, Northrop engineers designed an electronic stability augmenter that made the plane act more like those with normal tail surfaces.
In the years that followed, engineers began fitting high-performance aircraft with parallel control systems that assisted the usual hydro-mechanical arrangement. These new systems used analog computers to monitor and correct pilots’ control inputs on these unstable machines.
The culmination of this type of system came in the early 1960s, on the rocket-powered X-15. With the enormous range of speeds and altitudes the X-15 was capable of, it was apparent from the beginning of the program that ordinary control systems alone wouldn’t do. To that end, Honeywell developed what it referred to as an “adaptive control system,” to assist with the plane’s re-entry from outer space.
But despite the system’s promise, it proved trouble-prone and was said to malfunction on 25 percent of the plane’s free flights. It was partially blamed for the crash which destroyed the only X-15 the system was installed on.
It would be NASA and the space program that would ultimately sort out fly-by-wire’s inherent problem and prove its viability as a stand-alone control system. Most visible of NASA’s early forays into full fly-by-wire systems was the Gemini 2 spacecraft, which led to the final proof of fly-by-wire’s true promise, the Apollo Lunar Lander.
In connection with that program, one famous NASA employee proved particularly valuable in the evolution of fly-by-wire aircraft. After returning from the moon, Neil Armstrong accepted a position as NASA’s Deputy Associate Administrator for Aeronautics. He then used his pull as a former astronaut to secure the use of an Apollo Lunar-Lander computer, which had become available after the Apollo program’s premature cancellation.
A fly-by-wire system was built around this computer and fitted to a Navy F-8 Crusader, generally referred by its tail number, “NASA 802.” With test pilot Gary Krier at the controls, NASA 802 on May 25, 1972 became the first American airplane to fly without mechanical connection between pilot and control surfaces. The first being the Canadian built CF-105.
One of the people who was paying close attention to early flights of NASA’s fly-by-wire F-8 was Harry Hillaker. A longtime veteran with General Dynamics, he had begun his career with the company fresh out of college in the 1940s, when the company went by the name Consolidated Aircraft. He started his work there doing conceptual designs for the B-36 bomber, and over the next decades he would contribute to numerous high-profile military projects.
But during his tenure with the company, he had witnessed military aircraft designs almost universally become heavier, less maneuverable, and arguably less efficient at their intended missions. Dissatisfied with this trend, by 1965 he had begun to think about a lightweight, extremely maneuverable fighter plane. Assigned to work on General Dynamics’ troubled F-111 project by day, he began developing his dream fighter on his own time.
By the late 1960s he had teamed with civilian defense analyst Pierre Sprey and Colonel John R. Boyd, an outspoken, controversial Air Force fighter pilot who was similarly concerned about the state of military aircraft design. Together they began to push forward with Hillaker’s lightweight fighter concept.
As the 1970s began, the trio’s theoretical studies had captured the attention of the Pentagon. “We got our big break,” says Hillaker, “when David Packard, who was then Deputy Secretary of Defense, came up with his Technology Demonstration concept. He was concerned that we weren’t moving technology along fast enough and that we needed to do some demonstrators. So out of some hundred or so programs that the Air Force examined, they picked the Lightweight Fighter, and the program that the F-117 came out of.”
Now with government support, Hillaker’s dream plane came together into a functioning prototype, by this time referred to as the YF-16. Originally intended strictly to satisfy Packard’s demand for experimentation with advanced technologies, the program nonetheless steadily moved toward an actual production airplane. It would compete against Northrop’s somewhat more conventional YF-17 design.
For the YF-16, Hillaker sought maneuverability equal or superior to that of any fighter plane thus far. He had noted that NASA’s fly-by-wire F-8 test aircraft had about two-and-a-half times the response of one equipped with a hydro-mechanical control system. Impressed by such an advantage, Hillaker saw to it that a similar fly-by-wire system was incorporated into the F-16.
However, while the NASA F-8 had a digital system, the F-16’s system was analog—General Dynamics engineers felt that digital systems weren’t yet reliable enough for a production aircraft.
Along with incorporating the fly-by-wire control system, Hillaker and his team moved the control stick from its traditional position between the pilot’s legs to along the right side of the cockpit—a rather unusual location at the time. While this offered some bio-mechanical advantages for the pilot, the unconventional stick position actually came about to solve a more fundamental problem. “One of the reasons we wanted the sidestick controller was because the airplane was so small,” says Hillaker. “If we’d have had the stick in the middle, that would have obscured some of the instrument panel. It made more of the panel available for decent displays.”
However, on a pioneering fly-by-wire aircraft it wasn’t as simple as just moving the stick to the side a dozen or so inches. The feel was entirely foreign to pilots, regardless of position. “So far as the flight control system goes, the biggest thing we felt we had to overcome was the sidestick controller,” Hillaker says. “Not only its location, but the fact that it initially was operated totally by the pilot’s force. We subsequently then, after pilot comments, put a little bit of motion in it so they had some sense that something was happening.”
Although not without problems early on, as Oestricher unwittingly discovered, both the fly-by-wire system and its radical sidestick controller were quickly refined into effective, reliable features of the new aircraft. With the help of these systems, the plane proved itself superior in most respects to Northrop’s YF-17, the YF-16’s primary rival in the Air Force’s lightweight fighter competition.
But in some ways, it was surprising that the YF-16 even made it to the competition at all. From the beginning, Hillaker’s dream plane had a number of factors working against it. Foremost among these were initial lack of interest, an Air Force steeped in opposing philosophy, and Pentagon brass antagonized by the unyielding personality of Boyd.
And these hurdles were, of course, in addition to the shock value of the basic F-16 design itself, which did nothing to hide its radical design philosophies and somewhat unproven technologies. The plane seemed like a big gamble to many within the Air Force.
Nonetheless, the F-16 design had passed its first milestone, beating its more conventional Northrop competitor in a series of extensive tests. The F-16 was steadily gaining momentum and talk of a production version began.
But the F-16 wasn’t in clear sky just yet. Far from it. The very idea of such a plane still had some stubborn obstacles to overcome.
Foremost among them was a gigantic Eagle.
The climate in the Air Force at the time wasn’t the best for a plane like the F-16. The Air Force hadn’t had its own original-design air-superiority fighter since the 1950s. The F-4 Phantom, while highly effective and well-liked in Air Force service, was a Navy design that had been adopted at the last minute. And the more recent F-111 program had strayed so far from its original intent that the plane’s only resemblance to a fighter was in the almost ironic leftover “F” in its official alpha-numeric designation.
Now, in the early 1970s, the Air Force was finally getting the all new, exclusive air-superiority fighter it had been begging for—the McDonnell Douglas F-15 Eagle.
With the chance to get a new fighter having been such a rare opportunity, the Air Force didn’t want to risk blowing the deal by filling the plane with revolutionary ideas and unproven technologies. Thus, the “new” F-15 was in fact a relatively straightforward plane, built along the thinking of its F-4 Phantom predecessor. (Hillaker would later remark that the F-15 wasn’t even as sophisticated as the F-4 in many respects.)
Even the F-15’s physical size was comparable to the F-4 Phantom, having been created with the same “brute force” thinking, according to Hillaker—to gain more speed, add bigger engines. More range? Bigger fuel tanks. And so on down the line. The result was a rather large aircraft that represented no dramatic breakthroughs in design or technology.
But that was just what the Air Force wanted. They were reluctant to embrace anything that might jeopardize the F-15 project. And that made them especially wary of an upstart little single-engine fighter, controlled by a high-tech electronic system that had never even flown in a production airplane, much less endured the rigors of combat.
Many within the Air Force brass viewed the F-16 project as an unwelcome interloper, a threat to their juiciest project. “The F-15 had more emotional backing than any program I know,” says Hillaker. “It got to the point where anytime anybody said ‘F-15’ I cringed.”
Some of the Air Force’s bias against the F-16 was understandable; the Blue Suiters had been stung years earlier by a similar type of plane, the Lockheed F-104. Designed in the early 1950s to satisfy pilots’ endless wishes for higher speed and greater maneuverability, the F-104 proved to be useful for little else than going fast and jockeying around the sky. It had very limited range and payload capacity, and thus the Air Force only ordered a few hundred.
At least on the surface, the F-16 was a lot like the F-104—compact and single-engine, with a clear emphasis on all-out performance. None of these similarities were lost on the more conservative element of the Air Force establishment. The F-16 to them looked like a rerun of a bad situation.
But while Hillaker and his team couldn’t do much initially to overcome the “bigger is better” contingent of the Air Force, they left themselves an out with the most unique of the F-16’s high-tech features. They designed the plane so it could be fitted with conventional hydro-mechanical controls if the fly-by-wire system couldn’t be made to work acceptably. “We spaced the bulkheads so that we could move the wing back and have a statically-stable airplane,” says Hillaker. “We were just giving ourselves some insurance. The wing would have had to have been moved back eighteen inches. All we had to do was make the two bulkheads have the same load capacity. One of them that we would’ve moved the wing to was higher than it needed to be, unless you moved the wing back.”
The structural differences carried over to the production F-16 design, but fortunately were never needed. The fly-by-wire system’s control problems were quickly ironed out and the plane, even with all its advanced features, steadily gained support both in the U.S. and abroad.
At about this time, the F-16 project got one of its bigger breaks, according to Hillaker. “[U.S. Defense Secretary James] Schlesinger had been trying for some time to get some commonality with the Europeans and the U.S. on fighters. He decided to go with the Lightweight Fighter as the uniform airplane for the U.S. and Europe. He had us down, I think, for 300 planes and the Europeans, I think, for 350. But that grew like mad.”
And even some of the most stubborn F-15 proponents within the Air Force eventually warmed to the F-16. After seeing its true merits, they realized that there was a place in the inventory for a smaller sibling to their beloved Eagle. With the Air Force’s official approval of the F-16 came an increased acceptance of fly-by-wire technologies in general.
In the years that followed, the F-16 perhaps more than any other design has proven the viability of fly-by-wire technology for production aircraft. In 1978, the McDonnell Douglas F/A-18 Hornet became the first production plane to use a digital instead of analog fly-by-wire system. (The F-16 was eventually upgraded to a digital system in the late 1980s.)
In the years since the F-16’s introduction, fly-by-wire systems have become increasingly the norm for high-performance aircraft. They’ve been incorporated into planes ranging from stealthy weapons such as the B-2 and the F-22, to prosaic civilian haulers such as the Airbus A320 and the Boeing 777.
Nearly two decades after its adrenalin-soaked first flight, YF-16 number one sat forlornly, withering away in a storage hangar at General Dynamics’ Fort Worth division. Suffering scars from a number of harsh experimental programs over the years, the red-white-and-blue machine hardly hinted at the triumph of its earlier days.
As the hangar-doors rolled open, Oestricher and a group of fellow YF-16 test-pilots respectfully surveyed the ragged plane, walking along it and gently touching parts every so often. After a few minutes, Oestricher turned to a reporter from Lockheed Martin’s Code One magazine and said, “What a wonderful airplane.”
Legions of pilots around the world surely agree.
The quotes are original from former General Dynamics engineer and F-16 developer Harry Hillaker, in a phone interviews in about 2007 - 2008.
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