Dr. Iscold on defying gravity, time in the Red Bull Plane Swap

What can you do in 45 seconds? Could you skydive between two planes that are in a 140 mph vertical nose dive, regain control, and avoid the rather final hard stop at the end?

That is the premise of Plane Swap, the latest and possibly maddest world-first feat from the Red Bull Air Force aviation team, and 45 seconds really is the amount of time the pilots have to do it. It sounds impossible, so Digital Trends spoke to Dr. Paulo Iscold, the engineer in charge of modifying the aircraft that will be used in the endeavor, about making it a reality.

Slowing down, not speeding up

“It’s a pretty difficult challenge,” Dr. Iscold said, in what sounded like a serious understatement, before continuing with a laugh. “When Luke [Aikins, the Red Bull Air Force pilot who came up with the Plane Swap concept] put the problem in front of me, I was like, ‘What the heck are we doing here?’”

Dr. Paulo Iscold with a Red Bull Cessna 182

Iscold is exactly the type of person you want on this kind of project. Not only does he have a doctorate in mechanical engineering, but he has designed and built airplanes since 2001. His obvious expertise shone through during our conversation as did his enthusiasm for Plane Swap and aviation in general. However, this is very different from what he has done before.

“My background is in race airplanes and breaking records, but this is the opposite, it’s about how we slow down the airplane. From an aerodynamics perspective, that was a challenge. When you see the big picture, it’s two people swapping airplanes during flight and that’s very scary. But we don’t see that big picture, we see the small pieces that allow us to get there. That’s what this project is, it’s how you make this crazy thing not be crazy.”

There are two primary engineering challenges that stand out amongst all those small pieces: the development and fitment of a special speed brake, and a custom autopilot system. It was these aspects we explored during our conversation.

Making the speed brake

“When we first talked I thought the speed brake would be way smaller than what we have, and was thinking it would be on the wing like a glider,” Iscold explained, before smiling and adding, “That’s probably the reason I said let’s do it, because I thought it would be simple, then later on I found out it was not!”

Red Bull Cessna 182 with its speed brake deployed

The aircraft being used are two Cessna 182s, and the speed brake is essential for the planes to have a controlled nosedive, not just to maintain the 140 mph target speed, but also for stability. Despite speed and air brakes being commonly used in aviation, ranging from on aircraft that land on carriers to the side of a SpaceX rocket when it’s coming in to land, it’s uncharted territory here.

“It’s at least five times larger [than what I thought it would need to be],” he explained. “I thought it would be four feet by 12 inches over the wings, and now it is 6 feet by 5 feet and on the belly of the plane. It’s attached to the landing gear and another hard point in front of the fuselage, and it uses hydraulic actuators to work.”

Although it’s a large additional piece being added to the plane, it has been expertly integrated into the body. “It’s a very clean modification to the airplane, the landing gear works as normal and we don’t need to cut or drill any holes. It just clamps to it with one mounting point, and in 30 minutes the entire section could be removed and the airplane would be back to standard.”

F1 technology

Fitting a giant flat structure to the bottom of the aircraft created a few additional challenges. Iscold solved the problem of buffeting by adding holes to the speed brake, which lets air pass through it and break down the vortices that threaten stability, but an unexpected issue took a little more work. He explained the speed brake is actually made up of four pieces, and during the first flight tests no matter how many sections were used, the plane would not pass a 70-degree dive, and it needed to be 90 degrees.

Aerodynamic modifications made to the speed brake

“It took a while to figure out what was going on, even with more test flights and simulations,” Iscold said. The team eventually made a crucial discovery. “The speed brake has a low-pressure area right behind, and it makes the flow of air turn. The plane’s tail is in that flow, and that was forcing the airplane to pitch up. The two were fighting each other.”

The solution turned out to be simple (if you’re a mechanical engineer): “We created a gap between the fuselage and the speed brake, so the air flows through it, and that jet of air protects the tail from the flow of air created by the brake.”

Iscold compared this to how the Drag Reduction System (DRS) works on a modern Formula One car, where a section of the rear wing lifts to reduce drag. On an F1 car, it increases top speed, but on the Plane Swap planes, it means that a 90-degree nosedive can be achieved safely and reliably.

Autopilot from a rocket

The speed brake is just one part of what makes Plane Swap a challenge. Because each aircraft will be left unattended for a period of time, the autopilot needs to take over. Normally, the autopilot in an aircraft is concerned about keeping the plane level, but for Plane Swap, it has to do the opposite and maintain a vertical nosedive. Iscold explained that a normal autopilot isn’t suitable, as all its usual reference points become meaningless in that 90-degree dive. The solution? “We went to the same system that rockets use, as they operate at 90 degrees.”

Once the system was chosen, the tight tolerances and extreme precision needed for the plan to be successful had to be worked out, starting with the differences in speed and size of the objects involved. “The skydivers are falling vertically and can move forward and sideways a little, but not much. It’s about 10 miles an hour. They are also subject to the wind and will move around with it. However, in an airplane going straight down at 140 mph, if you change the angle by just four degrees that’s already 10mph on the horizontal. When the wind hits the skydiver the surface level is small, but when it hits the airplane’s wing it’s like a sail. It all means the autopilot needs to always be within three degrees pitch to make the trajectory of the airplane stable enough for the skydivers.”

At this point, it’s also important to remember there are two planes and two skydivers having to cope with all this. “We have a formation flight and both planes need to fly together, so you may think the natural solution would be to sync the two planes together,” Iscold told us. “We’re not doing that. They’re independent. We adjust them to behave in the same way, and when we’re doing the dive the autopilot is working to keep the pitch and heading correct. To stop them hitting each other they’re diving on a divergent path by a few degrees, but you’ll not see it with the naked eye.”

Unexpected complications

Because Plane Swap is a groundbreaking endeavor, there’s no blueprint for the plane design or established set of guidelines to follow, and that means there are always unexpected problems to solve. On the day we spoke to Dr. Iscold, the team had been battling with one plane behaving differently from the other. It was a surprise as both planes are essentially identical.

Red Bull Cessna 182 with its speed brake deployed

“The blue plane dives straight as a dart to the ground. It’s perfect. The silver airplane is a nightmare and never tracks correctly,” Iscold revealed, adding that both planes are exactly the same, apart from one slight difference on the tail.

“We tried to change some things to replicate the blue plane, but it didn’t help,” he continued. “The team changed the size of the speed brake, and we noticed if we made it a little smaller the plane became more stable. Unfortunately, this does make the plane go faster and it becomes harder for the skydivers.”

With further examination, Iscold found the problem. “We knew one plane had a slightly different center of gravity, and what happens is when you’re vertical the speed brake is like a parachute, and you want the center of gravity to be behind the parachute, if it’s above it’s not stable. So we’re playing around with this and it makes a difference. It’s obvious when I say it, but because the project is so big and complex, we lost track of it.”

The silver plane was the first one built, then the blue plane was developed to be identical. Problems like the one with the center of gravity are hard to pinpoint, especially when flight tests are logistically complex, as a large enough airfield is always required, along with the skydivers and testing equipment, and concern that should something go wrong, it may mean losing a plane. Solving problems takes time, a

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