RETRO: Dan’s Magic Bullet… The Story of the Gurney Flap

It’s been 50 years since a moment of inspiration during a choppy test led to an aerodynamic breakthrough that still lives on, the Gurney Flap. In a story first published in RACER magazine in 2018, Marshall Pruett recounts how Dan Gurney’s eponymous invention left Eagle pilot Bobby Unser speechless (which is never easy to do…) and the rest of the 1972 USAC Champ Car field scrambling to replicate the simple, yet incredibly effective drag-reducing and downforce-increasing powers of the device. Half a century later, the Gurney Flap is still one of the go-to performance tools for race car designers and engineers.

For the birth of the Gurney Flap, we owe a debt of gratitude to restlessness and frustration.

If only there had been cameras on hand to capture the manic emotions at play in Phoenix in 1972 as development of All American Racers’ new Eagle 7200 Indy car was stuck in an unsatisfying rut around the rugged oval of a mile.

Bobby Unser, the Indy Car speed king, was also the long-established prince to test Dan Gurney’s patience. As the raw pace continued to elude AAR’s new freewheeling challenger ahead of the season opener in the desert, the first call in Uncle Bobby’s playbook was to harass the Big Eagle for a remedy.

Artwork by Ricardo Santos

“We were there for three days in Phoenix, and we weren’t doing competitive times,” Gurney said. (pictured, top, leaning down to talk to Unser in 1972) told Dave Despain in a 2014 interview. “Bobby walks up to me and says, ‘Boss, you’re supposed to be able to make things up all the time – you can’t make things up, for screaming out loud?'”

After a fiery exchange between two titans, Gurney thought back to the sports cars he had driven – Can-Am McLarens and Ford GT40s – which used vertical spoilers attached to the rear bodywork.

Before fenders entered motor racing, bolt-on elements, used to spoil the path of air as it left the car, were a crude but effective device for creating downforce. In the era of fenders, spoilers had been largely forgotten outside of NASCAR, where they are still in place today.

Rear spoilers on sports cars like Dan Gurney’s Ford MkIV, winner of the 1967 24 Hours of Le Mans, put the seed of an idea in Dan’s mind… Motorsports Images

“I was wondering if it would work on a wing – a spoiler on the wing, not on the body,” Gurney added. Nearby, Unser’s nerves waited to be appeased by Dan’s curiosity.

“I said, ‘I lost my speed,’ so Gurney comes up to me,” said three-time Indy 500 winner Unser. “I can tell he’s upset, but so am I. . It’s my test, but I can’t go as fast as I used to, and he says, “I have something I’d like to try. I say, ‘Whatever you want! How are we going to do it? When are we going to do it? What are we going to do?’ I get a little irritable. He said, “Well, keep doing what you’re doing,” and he left.

Still lapping as Gurney worked out his L-shaped experiment in his mind, Unser frothed in the setting sun.

“My head is spinning at millions of miles an hour,” Unser continues. “This fucking thing won’t go fast, and it’s not the engine. It doesn’t stick well. I run it and come back. We’re all trying to think things through, and pretty soon Dan comes in and says, “When would be a good time to try my offer?” I say, ‘Right now!’ and of course I become a tester.

“He’s straight to the trailer and he’s got vises and a hammer going over there. I don’t know what he’s doing. Soon he is back with a long, bent piece of aluminum. Nothing else, just a 90 degree aluminum strip, and I look at it and I’m about to lose it.

AAR Chief Mechanic Wayne Leary must have wondered if boxing gloves would be needed…

“We all start to get too argumentative, and [Gurney] said, ‘There you go. Put it on the rear wing,” Unser recalls. “I say, ‘Where?’ He says, ‘Clear in the back.’ I say, “That’s stupid,” and so I say to Wayne, “Pop rivet it on, whatever. Just put it on quickly. I have to do this and start over.

“Wayne and the guys put the fucking thing on. I took this son of a bitch out and did less than a lap, and I had just discovered the greatest thing ever seen in driving a race car.

With the turbocharged Offenhauser engine churning out obscene power, the straights at Phoenix went by in a blur. But during these formative stages of racing car aerodynamics, insufficient rear wing profile downforce meant that modest cornering speeds were the accepted norm. In the Gurney Flap, Unser had the solution that tied straights and turns together.

With a big secret to keep, the frustration was replaced with something akin to paranoia.

“I came back and Dan said, ‘Well, what happened?’ I say, ‘You’re not going to believe this. It’s the biggest discovery I’ve ever seen in my life for a racing car,” says Uncle Bobby. “He said, ‘Well, why didn’t you do more laps?’ He gets a little angry. I go, ‘Hey, just slow down.’ I didn’t even get out of the car. And I said, ‘Do you still have that aluminum?’ He says, ‘For what?’ I go, ‘I want a couple for the front [wings] really fast.’

“Then I look around the stands. Where is Al [Unser]? Where are Parnelli’s men? Where are all these people? I don’t even give them a full turn, see? When he puts them on the front, I walk out and think, ‘I can’t believe it. It’s a different world. But I won’t take a ride, because I know that even firefighters are enemies…

“Dan is always upset that I don’t do laps, so I tell him, ‘I’m going to break every record that’s on this circuit, and I’m going to do it whenever you want.’ Well, I was getting really upset. I said, ‘You can’t believe what I just found out. You can’t believe what you did. He said, ‘Really?’ I say, ‘You know I’m not lying. You know if I tell you I got speed, man, I got it.

“There for the race, I smoked their asses, won, broke records. I say, ‘Now the problem isn’t finding speed; He’s hiding this fucking secret. You don’t know everything just not how important it is.

After beating the pack at Phoenix on March 18, Unser and this L-shaped wonder would go on to complete the 1971 Indianapolis Motor Speedway fastest lap of 17 mph on the way to pole for the 56th Indianapolis 500.

In the race, Bobby led the first 30 laps with ease, but retired with a broken ignition rotor. His teammate, Jerry Grant in the purple Mystery Eagle, took the lead on lap 176, but lost any chance of victory when he pitted Unser by mistake for his final stop on lap 188.

Pole sitter Bobby Unser led the opening laps of the 1972 Indy 500, but it was teammate Jerry Grant (above) who came closest to victory. Image by William Murenbeeld/Motorsport Images

Dan would have to wait until 1975 for his first victory as an owner (with that Unser man, of course), but his Eagles would be the cars to beat for much of the 1970s, and the Gurney Flap continues to play a role important in racing car aerodynamics.

“You know, everyone in the press said, ‘Oh, that was Bobby’s idea,'” Unser says of the simple yet incredibly effective device. “It was my ass. It was Dan’s idea.

“But no more arguing [afterward] between Dan and me. It’s all cool because I can run so hard, you can’t believe it. Turn it, push it, push it, whatever you wanted, this thing would get stuck, and no one knew why we were doing it, but it’s Dan. His fucking head used to go like this all the time.

HOW THE GURNEY FLAP WORKS

If the air has a big character flaw, it’s that it’s a little needy. He hates being divided, separated or turned away from the larger group.

And whether it’s traveling above and below a car or wrapping around a fender, rushing back to its unbroken shape is all the air wants to accomplish. This makes the creation of the Gurney Flap and its attachment to the trailing edge of racing car fenders one of the finest inventions the sport has ever seen.

Artwork by Ricardo Santos

Flowing at a shallow angle, the air will do a great job of staying attached to the top and bottom of a wing before rejoining peacefully after leaving the trailing edge. Start pushing a little angle into the wing to generate more downforce, and the air binding at the bottom of the profile starts to suffer. This separation, which creates drag, is where aerodynamic efficiency is lost.

With the Gurney Flap installed above the trailing edge of the wing, air passing under the wing is drawn upwards as it reaches the end of its travel. Drag is significantly reduced, downforce is increased and the overall efficiency of the wing is improved.

In the case of the fateful 1972 All American Racers test at Phoenix, the airflow separation under the thick wing profile was largely cured with the introduction of a Gurney flap as it restored the harmony between upper and lower airflow.

And as race teams would soon learn, Gurney Flaps could be used to increase downforce without resorting to the same steep wing angles that previously ruined fuel mileage and created significant turbulence. Small, light and extremely efficient, it’s only fitting that the game-changing device bears Dan’s name.

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