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."