Dynamic Flight Simulator Lets Swedish Pilots Pull Gs
By William B. Scott
January 4, 2004
The Swedish air force is "flying" a combined flight simulator/centrifuge that doubles as a JAS 39 Gripen fighter-pilot trainer and medical laboratory for researching the stresses of high-g combat. The system allows line pilots to pull up to 9g and command any pitch and roll attitude, ensuring their experience will differ little from actual flight.
Designed for exceptional flexibility and realism, Sweden's Dynamic Flight Simulator (DFS) in Linkoping, Sweden, is believed to be the first fourth-generation device--a ground-based system capable of pulling gs and replicating modern tactical-aircraft flight conditions--deployed by an air force.
Such simulators are a fairly recent advancement, although several companies have been working on centrifuge-based systems for years. A handful of fourth-generation units are being marketed globally, and by all accounts, competition is fierce, with myriad claims and counterclaims (AW&ST Dec. 1, 2003, p. 70). However, one Swedish test pilot, who has evaluated most of the systems now available, said realism and fidelity vary considerably.
I had an opportunity to "fly" the Swedish air force's DFS at the Defense Materiel Administration (FMV) test and evaluation center here, and was impressed by how closely the system approximates a high-performance aircraft. There are unavoidable physical limitations, of course, but it was definitely the most realistic flight simulation I'd ever experienced.
Basically a fighter cockpit mock-up mounted on the end of a centrifuge arm, the Swedish DFS represents a quantum leap in fighter pilot training and flight physiology research capabilities. It was designed and built by California-based Wyle Laboratories, and boasts a number of features critical to emulating high-g flight, such as:
* A high-torque direct-current (DC) motor that drives the 30-ft. centrifuge arm.
* A large spherical gondola, suspended by gimbals and configured with Gripen cockpit displays and controls, and a modest three-screen visual array.
* A control system based on the Gripen--or other aircraft--aerodynamic model that accurately simulates a fighter's dynamic-response characteristics.
* Perceptual algorithms in the control system that "trick" a person's physiological sensors, providing a degree of realism that "simply is not possible in conventional centrifuges," according to a Wyle official.
* A closed-loop control mode, which allows a pilot to fly the DFS and fully command its motion. Preprogrammed modes are also available.
* The Swedish air force/Wyle Labor-atories Dynamic Flight Simulator features a 30-ft. arm and fully gimbaled gondola. Minimal lag and delay times, ensuring the simulator's g-response closely approximates an actual aircraft.
Wyle engineers set out to develop a centrifuge-based simulator tailored for training and evaluating pilots in an elevated-g environment, yet flexible enough to also conduct aeromedical research. Consequently, they focused on features that would ensure rapid response times.
"We wanted to give pilots the ability to 'fly' and interact with the environment rather than just be a passive [centrifuge] rider," said Will Roberts, program manager for Wyle Laboratories' DFS programs. "We've come a long way in being able to translate the six degrees-of-freedom you get in an aircraft into the three degrees-of-freedom that we can control in a centrifuge. It's not perfect, but we think it's pretty good. There's room for more research to make it even better."
Swedish air force (SAF) officials were prompted to invest in a high-g simulator when they acquired the JAS 39 Gripen, a modern 9g fighter capable of rapid g-onset (going from 1g to high-g loads in a very short time) and sustaining those levels. They recognized that aircraft and pilots could be lost to the "G-Loc" (g-induced loss of consciousness) phenomenon, where humans can quickly pass out when subjected to high gs. Incidents early in the Gripen program led SAF leaders to conclude that not enough research had been done in the area of high-g physiology. Consequently, they decided to develop world-class expertise in this arena--while still training SAF pilots to properly handle g-caused stresses inflight, noted Kent Engstrom, FMV program manager for DFS.
Research is needed to better understand physiological phenomena "the moment before G-Loc and the moment after [recovering] from G-Loc--how you regain psychomotor and cognitive abilities with various delays," said Leif Pettersson, an FMV test center operation engineer. "These are possibilities for research . . . that I don't think anybody has done before."
The system I "flew" was "mechanically unique," Roberts said. "The arm's structure is like a wing--hollow aluminum with a stressed skin. Other centrifuges use a box-beam construction. The stressed-skin [design] gives us a much better natural frequency, which allows us to do a lot of things without being disturbed by structural effects."
The lighter, stronger arm simplifies controlling g-onset rates, and keeps start/stop stresses lower, which enables the use of a smaller drive motor. Wyle chose a 1,900-kw. Westinghouse DC motor originally designed for steel mill applications, connected directly to the centrifuge-arm drive system. It delivers about 7 megawatts of peak power in approximately 100 millisec. when a pilot pulls sharply on the simulator's cockpit control stick, demanding a high g-onset rate.
"WE CAN GO FROM [ZERO] to maximum g-onset in about 120 millisec. with this motor," Roberts said. But because the high power demand is very brief, the motor doesn't overheat.
Before my DFS mission, a flight surgeon attached electrocardiogram sensors to my chest, allowing real-time monitoring of heart parameters. He assured me that was standard procedure. I also was fitted with a Swedish-type g-suit, then strapped into a Gripen ejection seat mounted in the DFS' gondola--a ball-type structure suspended at the end of the centrifuge arm.
The Swedish AF simulator/centrifuge gondola emulates a Gripen fighter cockpit with a three-screen out-the-window visual display. Pilots are routinely "wired" with sensors that monitor heart functions.
Up to 400 electrical power, video, computer, medical and other signals are transferred from the cockpit to a control room via numerous slip rings that also allow unrestricted rotation of the gondola and arm. Five channels of gases--such as oxygen--also are passed through a sophisticated slip-ring system.
Like all Swedish pilots who go through training in the DFS, I did not wear a helmet or mask, because controllers need to monitor facial expressions during high-g maneuvers. After a thorough briefing and cockpit preflight by Capt. Christopher Gruv, I was closed into the gondola, and controllers prepared for the "flight." Maj. P.A. Klingstrom, an aerospace physiologist pilot and training specialist, served as the mission controller, talking to me via intercom.
Initially, the centrifuge arm turned at a slow, steady speed, producing a 1.5g "steady state" or baseline. Inertial restrictions require that the arm be in motion before a pilot starts pulling high-onset gs. Rapid g-onset would demand almost infinite power to go from a dead stop to a 10g/sec. rate the system is specified to deliver. Roberts said the DFS has demonstrated a 14.5g/sec. rate with a full load, plus an additional 100 kg. (220 lb.) of weight.
I had been told to fly the simulator however I wanted, but with a couple of cautions--no rapid negative-g pushovers, and don't be too aggressive when rolling immediately from a right-bank high-g condition to a left-bank high-g maneuver. The system would respond, but these inputs would cause unneeded stress on its components.
I made a few gentle turns to get a feel for the Gripen flight controls, noting that the three-screen visual scene produced a comfortable perception that one was flying contact. Then I rolled into a steep turn and pulled on the stick, advancing the throttle a bit. The DFS responded instantly, smashing me into the seat. Over the next few minutes, I maneuvered in the low-g arena, eventually pulling up to 4.7g. I've pulled many more in actual aircraft, but never particularly enjoyed what some call the "practice bleeding" of high-g flight.
I was quickly convinced the DFS would respond rapidly to control inputs, and could easily deliver the requisite normal acceleration to produce tunnel vision and make me gray-out. I had no desire--or need--to demonstrate my limited "g-tolerance."
Flying daily in a tactical aircraft, a crewmember develops a capacity to handle high-g conditions. He instinctively tightens his legs and abdominal muscles, assisting the anaconda-like g-suit's inflation, which squeezes legs and abdomen. Pilots are also trained to perform a specific anti-g "straining maneuver" that involves short-pulse breathing, while keeping the lower body muscles tensed.
All this effort is essential to keep one's blood flowing to the head as g-loads increase. If blood drains from the cranium while pulling high gs, vision seems to close in from the edges, until it appears you're looking through a small tube. That's called tunnel vision. Higher gs--or relaxing at the wrong time--can close that tunnel completely, and the eyes-wide-open pilot sees nothing but a gray curtain. Objects are no longer visible, but he can still hear the radio and engine, and is still flying the aircraft. He's functioning, but just can't see anything.
At this point, a pilot or backseater is only a g or so away from having his world go completely dark, a condition known as "blackout." Then, he's no longer able to help himself or fly his aircraft. Recovery requires reducing g-loads on the body, but after blacking out, human beings are typically slow to respond. Many seconds or even a minute can pass before a pilot is really capable of controlling his aircraft properly.
The DFS is an effective means of training pilots to resist graying- or blacking out by properly performing the anti-g straining maneuver under controlled conditions. Some catch on faster than others, but all can be trained, according to Klingstrom, who has flown hundreds of high-g sorties in centrifuges. He's known here as "The G-Monster."
As mission controller, he next suggested I fly a tail-chase profile, presenting another aircraft in my simulated HUD. I was at the aircraft's 6 o'clock position, and my job was to stay on his tail as he maneuvered. I noted that the simulator's mechanization was so good that I felt as if I really were flying, pulling whatever gs were necessary to stay with the target as it accelerated, climbed, dived and turned sharply. The profile was an actual mission Klingstrom had flown in the DFS, where data were captured and "canned" as a target for training missions.
As long as I kept my head motionless, pressed against the seat headrest, the simulator closely replicated tactical flight, I felt. Experimenting a bit, I turned my head slightly to the left and right, and tilted it up and down. Immediately, my vestibular system sensed I was rotating. However, pilots are told that will happen--so don't do it.
However, rolling the "aircraft" and pitching up and down produced very realistic feelings of flight. The fully gimbaled gondola is unrestricted in pitch and roll, but sophisticated, well-tuned control algorithms are required to produce such realism.
"WE HAVE TO DO some strange things to . . . fool your vestibular system," Roberts said. For example, "when the arm speeds up, we compensate for the increased gs by pitching the seat forward, because the [human] vestibular system is sensitive to rotational acceleration. You stay aligned with the force vector.
"How fast we accelerate the arm, then accelerate the pitch and roll axes [simultaneously] is a big factor in the design," he added. "This is a simulator mounted on a centrifuge, not a centrifuge that happened to have a simulator in it, so that [required] a different approach. Getting the simulation right was [a priority]."
Maj. Richard Ljungberg, a Swedish test pilot, has flown many tedious hours in the DFS, trying to tweak and optimize those control algorithms to ensure the simulator closely approximates actual Gripen flight. "I've changed a lot of numbers [related to] movement in roll, pitch and delays. The biggest thing is g-onset response time, and the ability to control the centrifuge from low gs to high gs. If you have a long delay [between] pulling the stick and getting the gs, you learn the [wrong] response coordination. If you're at high gs, and you feel like you're starting to gray-out, and you unload [relax stick pressure], you want the machine to respond. If you unload and nothing happens, it's ugly." That can lead to negative training, doing more harm than good.
My flight was only about 30 min., but long enough to convince me that the DFS emulated high-g tactical flight quite well. Through clever algorithms and persistent refinement, the Wyle engineers and Swedish pilots assigned here have done a good job of simulating six-degrees-of-freedom flight.
Probably the most telling validation of DFS fidelity comes from SAF pilots who go through training here. After flying a tail-chase scenario and trying to shoot down the simulated target aircraft, pilots are informed their time is up. They often respond, "Just one more, OK?"
Normally, pilots hate simulator training. "I think we've come a long way, creating a centrifuge/simulator that pilots can actually enjoy flying," Ljungberg said. "Normally, when you go to a centrifuge, you're like a dog--just sitting there, trying to strain and survive the g-forces. But in this one, you're in control. Here, you pull gs the same way [you do] when you go out and fly. It's a good training tool."
The DFS also ensures pilots develop desirable habits in coordinating stick movement with anti-g straining maneuvers. About 60 pilots have been trained in the DFS, but the FMV/Wyle team here is continuing to refine both the initial and refresher programs.
However, they also see tremendous potential for serious physiological research. Ljungberg mentioned experiments to determine how a pilot's perception of color and his hearing degrade at high-g loads, for example. Combat scenarios involving target acquisition and tracking, missile-firing and other tasks can be explored to determine how pilots perceive an objective's location under high-g loads. Similarly, the position of cockpit controls, or even the color of head-up and head-down display symbology, might be altered as a result of research findings.
"The sky's the limit," Roberts said. "We're just barely getting started.
