Tuesday, June 14, 2011

Pilot Recounts Taming World’s Fastest Helicopter

Pilot Recounts Taming World’s Fastest Helicopter: "

Kevin Bredenbeck is more comfortable in a flight suit than a tuxedo, more at ease in the cockpit than on stage. But there he was last month at a black-tie event in Washington D.C. with his colleagues from Sikorsky Aircraft to receive the Collier Trophy, the aerospace industry’s most prestigious award.

‘I wasn’t about to let this little helicopter do me in.’

Sikorsky was being honored for the X2, a record-setting helicopter that can fly at more than 300 mph. It is a remarkable, even revolutionary, machine, with two main rotors stacked one atop the other and a pusher propeller on the tail. The combination makes the X2 as fast as it is nimble. Bredenbeck is the pilot who explored the edges of the helicopter’s capabilities. And three years ago, he wasn’t sure he’d be able to land it.

It was late in the summer of 2008. Bredenbeck, dressed in a bright orange fireproof flight suit, was strapped into the cockpit of the radical new helicopter. He’d spent dozens of hours in the simulator, but this was the real thing. The first flight. He was about to see if the helicopter he and a team of Sikorsky’s best engineers had been pondering, designing, building and testing for five years would do what they expected it to.

For Bredenbeck, it was exactly where he wanted to be. For most of his adult life, he’d dreamed of being a test pilot flying on the bleeding edge of technology. The planning was done. The rotors were turning. The pre-flight checks on the flight test card were complete. Everything Sikorsky had invested in the X2 project was in his hands. His cool demeanor cracked, just a bit, and emotion flooded in.

“All of a sudden you get to the point on the card where it says, ‘pick it up’ ” Bredenbeck says, recalling the moments before the first flight. “I look up and there’s all these people standing on the berm, and that’s when I lost my composure a bit. This next point is going to bring it home or finish it.”

There was no time for sentimentality. There was a job to be done, a schedule to be met. Bredenbeck heard Dave Walsh, the director of the X2 flight-test program, through the radio.

“Next point,” Walsh said, referring to the test card where it says it is time time to take off.

Bredenbeck thanked the team and said, “I’ll see you in a bit.”

The X2 was airborne moments later.

Bredenbeck has flown helicopters most of his adult life. He started in the U.S. Army, flying a medical evacuation helicopter and working as a maintenance test pilot. He joined Sikorsky in 1994. With the support of the company’s chief test pilot at the time, Bredenbeck worked with the civilian National Test Pilot School in Mojave, California to develop a program based on the Navy’s helicopter flight test school at Patuxent River.

He was one of two Sikorsky pilots to graduate from the inaugural program and was a test pilot for many development programs within the company. It was a great job, but in the back of his mind, Bredenbeck knew he wanted to do more. He wanted to be on the leading edge, where the real innovations are made. He constantly asked his boss when Sikorsky might reprise the advancing blade concept it explored in the 1970s and finally caught his big break in 2003 when the company started discussing the X2.

Manufacturers were experimenting with different ideas to improve the top speed of the rotary wing aircraft. Several companies considered using jet thrust to push helicopters along at a faster speed and using small wings to provide additional lift. But the problem was always the main rotors developing asymmetric lift and instability because of retreating blade stall.

Retreating blade stall occurs at high speed in a helicopter when the rotor blade traveling backward cannot generate sufficient lift because its speed is too slow relative to the air. Compounding the problem, the added forward speed can cause the tips of the advancing rotor blade to reach supersonic speeds, creating shock waves and more vibration.

Sikorsky’s advancing blade concept, like those from the former Soviet Union, address the issue by using a a pair of coaxial main rotors so there are always advancing blades on each side of the helicopter. As the forward speed of the X2 increases, the rotational speed of the main rotors is reduced but the dual advancing blades ensure sufficient lift to keep the helicopter flying. With the slower rotor speed and sufficient lift, the slower relative airspeed of the retreating blades is not a problem.

The brass assembled the team that would develop the X2, naming Steve Weiner chief engineer and Bredenbeck the test pilot. The program was moving forward, and the first pieces of the X2 were being built by 2005. The helicopter was ready to fly by August, 2008.

Test pilot Kevin Bredebeck prepares for a flight in the X2.

But as soon as the wheels left the ground, Bredenbeck wasn’t sure the program would make it to a second flight, let alone the program goal of 250 knots [288 mph]. The X2 was “extremely responsive” to the slightest touch on the controls and trickier to fly than Bredenbeck expected. The second point on the test card was to land the craft, but with the handling much “sportier” than expected. Bredenbeck opted to keep flying and gather as much data as possible.

Landing might be a challenge, he thought, and it might get ugly. Better to stay airborne.

“It’s a hell of a thing to think about, but I wasn’t about to let this little airplane do me in or do in the company, it was just too hot,” he says.

In the end, he landed safely. The X2 flight test program was underway.

Once flight testing started, the program went better than anticipated and wrapped up earlier than expected.

“It was a 75 [flight] hour development program,” Bredebeck says. “We got 18.6 hours on the airplane and finished it.”

In that short timeframe, the Sikorsky X2 went from a shaky first flight to a top speed of more than 300 mph. But for every hour in the air, there were hundreds of hours spent on the ground, examining flight data, refining the flying characteristics in the simulator and fine-tuning the helicopter.

Early on, one of the first steps was to develop the fly-by-wire system to control the new helicopter. To do this, Sikorsky turned to the most basic helicopter in builds, the S-333.

“We flew a surrogate 333 with our makeshift fly-by-wire system to look at some of the control laws and decide what worked well,” Bredebeck says. “Just put the makeshift controls in [on one side of the cockpit], it was pretty hilarious, but it worked out great.”

In addition to using the small, four seat S-333 to develop the fly-by-wire system, Bredenbeck says spent more than 100 hours flying the X2 in the simulator. The simulation team spent even more time than he did.

Typically a fly-by-wire system uses an augmentation system to help the pilot control the aircraft. But Bredenbeck says the first few flights were flown without any augmentation or rate dampening on the controls to help mellow out the flight characteristics. This nearly direct digital control is what led to the extreme handling characteristics on the first flight. But after every flight, the digital controls were tuned.

“Every flight we would go fly the simulator after the flight” Bredenbeck says. “I would have to tune the simulator to what I thought the airplane was actually doing. Then they would look at the actual data, look at how I tuned the simulator and then they would tune the model.”

By the third flight, Bredenbeck says, the X2 was flying “pretty good. We were adjusting proportional gains that feed from the stick to the servo assemblies to the head,” he says. On the fourth flight he engaged the X2’s pusher propeller for the first time.

The first four flights of the X2 were made in New York, and then the helicopter was shipped to Sikorsky’s flight test facility in Florida. There, the team began expanding the flight envelope. Slowly the speed increased as Bredenbeck used the pusher propeller more and more. To really get the X2 moving, the pilot transfers much of the engine power from the twin main rotors to the pusher propeller at the back.

The business end of the X2 helicopter.

“You feel it, every time you hit the prop, you feel like you’re going into a new gear in a Chevelle SS at the race track,” Bredenbeck says.

The pusher prop gives the X2 other capabilities not found in conventional helicopters. From a hover the pilot does not need to tilt the main rotors — and the entire helicopter — to move forward or backward. From a level attitude, the pilot can simply engage the propeller and accelerate or decelerate without changing the angle of the main rotors. The helicopter can also hover with the fuselage at an angle by using some thrust from the propeller.

“I can hang from the prop or I can stand on the prop,” Bredenbeck says.

But the main purpose of the propeller is to push the X2 along at ever faster speed. But as the X2 approached 180 knots [207 mph] during testing, it began to show longitudinal instability. Bredenbeck says at this point he wasn’t sure they would meet their goal of 250 knots.

“I was just thinking, ‘Here’s all this power and performance and we’re never going to get there.’ I was trying to get as much data with pulses and frequency sweeps so they can at least look at the stability of the airplane and do something,” he says. “That was a time that I went, ‘we might not get there, we’re so close.’”

The engineers changed the stability augmentation system in the fly-by-wire controls and added a horizontal tail surface to aid stability. Six weeks later the X2 was back in the air.

The added horizontal surface can be seen on the tail of the X2.

Eventually it was time to try for the goal the X2 team had been working towards, a flight of 250 knots. It ended up happening much sooner than anyone planned. In fact, it was almost uneventful.

“When I hit 250, 263 knots, it still didn’t hit me, they have it on the tape, I said, ‘Wow, that was nice.’” Bredenbeck says. “I could have gone faster on that day, we were only at 70 percent power. But the card was ‘Hit 250, take data and then drop the nose’ and when you drop the nose, it wasn’t in a dive. I cruise at about three degrees nose up. I drop the nose to one degree nose up and the aircraft went to over 260 which went to our next endpoint just in a slight dive so we could get data.”

Like any engineer who wants to get maximum performance, Bredenbeck excitedly describes the details about how the X2 could have flown faster even without using more power.

“I want to go right back up to the power setting we set for that day and bring the rotor back because we were over our critical mach speed, we were at .92, I should have been at .87, .89 on my advancing tip mach,” he says. “Because if I’m over that now I’m creating drag and little burbles on the tips. If you look at the video you can see that my head’s bobbling a little bit. That’s not vibration, that’s vibration roll oscillation caused by those two advancing tips just hitting the critical mach and releasing. If I brought the rotor speed back and transitioned that power back to the prop, we would have went even faster.”

Kevin Bredenbeck stands next to a plaque listing the names of all of the Sikorsky test pilots, beginning with the company founder.

It’s unlikely Bredenbeck will get another shot at flying faster. The X2 program achieved its goals and there is only one more flight planned. Bredenbeck won’t be worrying about eking out a few more knots by reducing the critical mach number on the rotors. The last flight, slated later this summer, is purely for show, a demonstration flight in front of potential customers.

The military, of course, is the focus of the sales pitch and Sikorksy already has plans for a larger helicopter using X2 technology. The S-97 Raider is being developed with the Army in mind, but Sikorsky sees the technology being used by search and rescue teams and even private firms.

None of that is Bredenbeck’s concern. He’s a test pilot, not a salesman. But for all of his enthusiasm about flying the X2, he’s quick to praise the engineers behind the project. He won’t go so far as to make the old joke about his job being so easy a monkey could do it, but he says all the credit goes to the guys who never fly.

“I just got to sit at the pointy end, point it in the right direction and get a banana at the end of every flight,” he says.

Top photos: Sikorsky, bottom photo: Jason Paur/Wired.com


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