Allburn had a more modest assessment: “Well, first – and we had confidence this could be done – the wing didn’t break off! Second, the computers were working and controlling the airplane so it didn’t go end over end. I guess it was the validation of some of the design activities.”
X-31
Dr. Michael Francis, the fourth program manager of the attention-getting X-31 Joint Enhanced Fighter Maneuverability (EFM) program, described the history-making performance of the X-31 at the Paris Air Show in 1995 as “our E.F. Hutton moment.” The E.F. Hutton financial firm was running a series of commercials at the time with the catchphrase: “When E.F. Hutton talks, people listen.”
“We’re the only X-plane that ever did an actual performance, an eight-minute routine that we did every day of the airshow,” Francis stressed. “Everybody stopped to watch it!”
The X-31 flight testing included an exploration of extremely short takeoff and landing (ESTOL) as part of the U.S. Navy Vector program. The X-31 was equipped with three paddle-like tail vanes that, when activated, redirected the engine’s exhaust plume in response to pilot input. This “thrust vectoring” provided control and lift at dramatically low speeds and high angles. U.S. Marine Corps photo by Maj. Cody Allee
What stunned show-goers was the X-31’s ability to maneuver at extremely high angles of attack in post-stall flight. The trick that gripped them most was called the Herbst Maneuver.
“It’s equivalent to a swimmer doing a flip-turn in a swimming pool,” Francis explains. “You go through zero forward flight speed. X-31 was capable of pointing independent of its flight path.” Amazingly, the airplane could fly at up to 70 degrees angle of attack (AoA).
“It would pop to very high angle of attack as a giant speed brake. As it would get there, it would start to do a velocity vector roll. And while it was doing that it would reverse its flight path in the opposite direction. The airplane would slow to between 40 and 50 knots as it’s turning around and pointing itself,” Francis said.
The X-31 would then accelerate “out the back end” employing its powerful General Electric F404-GE-400 engine. Thrust vectoring made the Herbst maneuver possible, allowing the X-31 to maneuver even as its aerodynamic control surfaces became ineffective beyond the stall at high angles of attack.
“Thrust vectoring is just another way to generate the forces and moments that you have to have to keep an airplane controllable and stable,” Francis said. “We used the canards on the front as an insurance policy if we lost control at high AoA. Those surfaces could articulate enough to bring the airplane back under control.” The successful performance of this unprecedented maneuver was the result of work carried out by a team including NASA, the U.S. Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany, and Daimler-Benz (formerly Messerschmitt-Bolkow-Blohm, MBB). DARPA program managers guided the effort, with Francis being the driver behind many X-31 program breakthroughs.
His four “maneuver milestones,” crafted in 1992 to accelerate what had been “a slow-moving program,” emphasized combat effectiveness and tactical utility.
“The first was steady flight at high angle of attack,” said Francis. “The second was rolling around the velocity vector in both directions through 360 degrees. The third was getting into post-stall conditions rapidly because the airplane had to be able to do a high-speed dynamic entry. The fourth was putting all the pieces together to do the Herbst Maneuver.”
In addition to proving thrust-vectoring supermaneuverability and “pointability” in a variety of post-stall regimes, the X-31 could fly at up to 70 degrees angle of attack (AoA), demonstrated extremely short take-off and landing capabilities, and employed helmet-mounted displays for pilots. NASA Dryden photo
Dr. Wolfgang Herbst, a noted engineer with MBB, originated the idea behind the X-31. While working on a fighter design in the 1970s that would become the Eurofighter Typhoon, he recognized the potential of extreme maneuvering to defeat short-range air-to-air missiles. Herbst theorized that an airplane capable of flying in the post-stall regime with full control at high angles of attack could gain a significant tactical advantage in close-in aerial combat.
The idea, dubbed “supermaneuverability,” made its way to the United States and to Rockwell’s Mike Robinson. Robinson began a collaboration with Herbst that led to the EFM program. In 1983, Allburn, already at work on the X-29 program, called then-U.S. Air Force Capt. Michael Francis to a meeting at DARPA.
“Jim Allburn asked if I would listen to a pitch on an interesting project with the name ‘SNAKE.’ That was the acronym for Super Normal Attitude Kinetic Enhancement,” Francis recalled.
Wolfgang Herbst and Robinson gave the presentation.
“I thought, ‘that was nice,’” Francis said. “I’ll read about it in the papers.”
Nearly a decade after Herbst first envisioned supermaneuverability, Allburn was looking for the right person to push the X-31 program forward. Greenlighted by Allburn in 1986, X-31 was the first international X-plane. Funding from the Nunn-Quayle amendment (an international defense cooperation initiative) provided the foundation for the American-German research program.
Initially, the Air Force had been sought as the program executive agent. But by the late 1980s, USAF interests had turned to the F-117 and the Advanced Tactical Fighter program – the effort that would yield the F-22 Raptor. Convinced that stealthy fighters with advanced sensor fusion would conduct aerial combat mostly from beyond visual range, the service had lost interest in the X-31, an airplane dedicated to research for close-range combat.