PRUETT: Why Helio Castroneves' car flipped

PRUETT: Why Helio Castroneves' car flipped


PRUETT: Why Helio Castroneves' car flipped


Questions regarding the cause of the crash that sent Team Penske’s Helio Castroneves flipping backwards through the air during practice for the Indy 500 have circulated since the No. 3 Chevy took flight at approximately 12:45 p.m. ET., and while the Brazilian’s airborne maneuver was definitely spectacular to watch, the reason for the backflip is rather simple.

To start, the crash has nothing to do with aero kits. Rear blowovers like the one experienced by Castroneves have been happening for decades by cars with large underwings. It happened for the first time in the 1980s when underwings were introduced, and they will continue to happen in any form of motorsports where they’re used in high-speed vehicles.

And the aerodynamic safety changes incorporated by IndyCar for 2015 – the holes in the floor to reduce downforce, and the brand-new wicker that runs down the center of the tub – aren’t designed to prevent an Indy car from taking off by lifting at the rear.

The new-for-Indy center wicker.

The reason for the No. 3 lifting and flipping was due to the car’s aerodynamics working in the opposite direction they were designed to function. Castroneves entered Turn 1 on his first flying lap and lost control of the rear of the car. The three-time Indy 500 winner attempted to catch the car by turning into the slide–which initially worked – but it wiggled a second time and quickly swapped ends. The total time from pointing straight prior to the second wiggle and being completely backwards took just 1.08 seconds.

While accelerating forward, an Indy car generates significant amounts of downforce, and in Indy 500 trim, most of it comes from the wing-shaped floor. Chevy (and Honda) use relatively small front wings and tiny rear wings. The long, wide underwing is where most of the car’s downforce is generated, and with air flowing in from the front and out through the back, it functions like an inverted airplane wing and sucks the car to the ground

Turn an Indy car around at 200mph and feed that air backward through the underwing, and it will behave like a normal airplane wing and generate lift. With enough air speed, and pressure build-up in the  tunnels–which Castroneves obviously achieved, lift turns into liftoff.

Seen from below, the “holes” at the front of the floor and the curved underwing profiles.

ABOVE: The curved Dallara DW12 floor, shown here in yellow, makes downforce when pointed into the wind, and lift when turned 180 degrees.

IndyCar made a change to its spec floor this year to cut downforce by reducing the size of the floor’s total area, and the 18-inch-long holes are also designed to help bleed downforce from the floor in the event of a forward takeoff.

“The holes are for a nose-up condition,” said one IndyCar veteran. “The hole in the floor is all about the front going up first.”

With the rear of Castroneves’ car lifting, the floor holes toward the front of the car – a full six feet from the rear of the underwing – were never going to help keep the No. 3 on the ground.

The center wicker on the tub will help to trip the air and reduce the likelihood of a tipover in a progressive slide – in a state of yaw, but with an almost instantaneous spin, the front-mounted wicker was never going to keep Castroneves pointing forward.

“Yaw, backwards, is different than yaw frontwards,” the IndyCar veteran added. “Obviously, the wicker is at the front. It’s all about the incident, the angle you’re at, the speed you’re at. There’s no single component you can put on the car and fix everything. There’s no doubt the center worker helps in certain situations.”

The underwing tunnels were fed high-speed air from behind, which lifted the No. 3 off the ground.

If there’s one thing for IndyCar to investigate for the future, it would be the possible introduction of NASCAR-style flaps on the exposed underwing diffusers. With the lower rear suspension a-arms and mounting rods that hold the floor to the drivetrain, the tops of the underwing diffusers (BELOW) are covered with plenty of items that would make incorporating flaps a significant obstacle.

Provided IndyCar could find a method to vent the underwing and divert the air upward through the floor in a situation like the one witnessed today, it could be possible to reduce the chances of backflips taking place. Until the series tests a few solutions in a virtual capacity through computational fluid dynamics (CFD) software, we won’t know whether such a change would be worthwhile.

2015Indy500 MarshallPruett Mon511 744

RACER did learn the series tested a flap-style system at the front of the floor in CFD to see if it would prevent a nose-first blowover, and the results showed the flip happened faster than was optimal – the flaps did not have enough time to open and reduce downforce. It’s unknown whether the same problem would happen with a rear-first blowover, but IndyCar is expected to review all of the data from Castroneves’ car and to work with Chevy and Honda to work on CFD models to replicate and learn from the crash.

Thanks to a reminder from a friend who worked on the original DeltaWing program, IndyCar could consider another solution to help prevent underwing-based blowovers. The 2012 version of Ben Bowlby’s DeltaWing, which was designed to compete at Le Mans, made almost all of its downforce from an underwing, and with the car’s aerodynamic performance coming from the bottom of the car, he was concerned about liftoff in a high-speed rotation on the Mulsanne straights.

Bowlby and his design team came up with a smart blocking system (BELOW) at the back of the DeltaWing’s tunnels that were hinged at the top, held in place by magnets, and deployed–allowed to drop down and close the entrance to the back of the underwing–once the on-board data system saw the criteria for a spin had been met.

BWillow DeltaWing Test Pruett 3112 066 a

Granted, the rear tunnel blocking system would only work effectively if the DeltaWing was engaged in a flat spin; the blockers would drop and hit the ground if the rear tires were in contact with the tarmac, but if the back of the car was elevated in any way during its 180 degree rotation, air could find its way into the tunnel and create lift.

There’s nothing close to an all-purpose fix to underwing-based flips, but there are options for IndyCar to explore as possible improvements to make for future superspeedway visits.