RETRO: Dan’s magic bullet... The story of the Gurney Flap

William Murenbeeld/Motorsport Images

RETRO: Dan’s magic bullet... The story of the Gurney Flap

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RETRO: Dan’s magic bullet... The story of the Gurney Flap

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It’s 50 years since a moment of inspiration at a fractious test led to an aerodynamic breakthrough that still endures, the Gurney Flap. In a story first published in RACER magazine in 2018, Marshall Pruett recounts how Dan Gurney’s eponymous invention left Eagle driver Bobby Unser speechless (never an easy thing to do…) and the rest of the 1972 USAC Champ Car field scrambling to replicate the simple, but incredibly effective device’s drag-reducing, downforce-increasing powers. A half 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 agitation and frustration.

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

Bobby Unser, Indy car’s speed king, was also the long-established prince of testing Dan Gurney’s patience. As raw pace continued to elude AAR’s new open-wheel challenger ahead of the season opener in the desert, the first call in Uncle Bobby’s playbook was to badger the Big Eagle for a cure.

Illustration by Ricardo Santos

“We’d been there driving for three days in Phoenix, and we were not doing competitive times,” Gurney (pictured, top, leaning in to talk to Unser in ’72) told Dave Despain in a 2014 interview. “Bobby comes up to me and says, ‘Boss, you’re supposed to be able come up with things all the time — can’t you come up with anything, for crying out loud?’”

Post-feisty exchange between two titans, Gurney thought back to sports cars he’d raced — Can-Am McLarens and Ford GT40s — that used vertical spoilers attached to the rear bodywork.

Before wings entered motor racing, the bolt-on items, used to spoil the air’s path as it departed the car, were a crude but effective device to make downforce. In the age of wings, spoilers had been largely forgotten outside of NASCAR, where they still remain in place today.

Rear spoilers on sports cars like Dan Gurney’s 1967 Le Mans-winning Ford MkIV put the germ of an idea in Dan’s mind… Motorsports Images

“I wondered if that would work on a wing — a spoiler on the wing, not on the body,” Gurney added. Nearby, Unser’s bundle of nerves was waiting to be soothed by Dan’s curiosity.

“I said, ‘I’ve lost my speed,’ and so Gurney comes over to me,” three-time Indy 500 winner Unser says. “I can tell he’s upset, but I am too. It’s my test, but I can’t go as fast as I’d been going, and he says, ‘I’ve got something I’d like to try.’ I say, ‘Anything you want! How are we going to make it? When are we going to make it? What are we going to do?’ I’m getting a bit testy. He says, ‘Well, you just carry on with what you’re doing,’ and he took off.”

Still out lapping while Gurney drew up his L-shaped experiment in his mind, Unser worked himself into a lather under the low evening sun.

“My head’s going a million miles an hour,” Unser continues. “That friggin’ thing just won’t go fast, and it’s not the engine. It’s just not sticking good. I run it and I come back in. We’re all trying to think of things, and pretty soon Dan comes over and says, ‘When would be a good time to try my deal?’ I say, ‘Right now!’ and of course I’m getting testier.

“He’s straight over to the trailer and he’s got some vice grips and a hammer going over there. I don’t know what the hell he’s doing. Pretty soon he’s back with a long, bent piece of aluminum. Nothing else, just a 90-degree strip of aluminum, and I just 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’re all starting to get too argumentative, and [Gurney] says, ‘Here it is. Put it on the back wing,’” Unser recalls. “I say, ‘Where?’ He says, ‘Clear in the back.’ I say, ‘That’s stupid,’ and so I tell Wayne, ‘Pop rivet it on, whatever. Just get it on quick. I’ve got to get this done and over.’

“Wayne and the guys put the friggin’ thing on. I took that son of a bitch out and made less than one lap, and I had just discovered the biggest thing ever in race car handling.”

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

With a big secret to guard, frustration was replaced by something approaching paranoia.

“I came back in and Dan says, ‘Well, what happened?’ I says, ‘You ain’t going to believe this. That’s the biggest discovery I’ve ever seen in my life for a race car,’” Uncle Bobby explains. “He says, ‘Well, why didn’t you make more laps?’ He’s getting a little bit angry. I go, ‘Hey, just slow down.’ I didn’t even get out of the car. And I say, ‘You got any more of that aluminum?’ He says, ‘For what?’ I go, ‘I want a couple for the front [wings] real quick.’

“Then I’m looking around the grandstands. Where’s Al [Unser]? Where’s Parnelli’s people? Where’s all these people? I don’t even give them a full lap, see? When he puts them on the front, I go out and I think, ‘I don’t believe this. It’s a different world.’ But I won’t do a lap, because I know even the firemen are enemies…

“Dan’s still upset I’m not running laps, so I tell him, ‘I’m going to break every record there is at this racetrack, and I’m going to do it anytime you want.’ Well, I was really getting mad. I say, ‘You can’t believe what I just discovered. You can’t believe what you’ve done.’ He says, ‘Really?’ I say, ‘You know I don’t lie. You know if I tell you I’ve got speed, man, I’ve got it.’

“Back there for the race, I smoked their asses, won, broke the records. I say, ‘Now the problem isn’t finding speed; It’s hiding this friggin’ secret. You guys just don’t know how big this is.’”

Having trounced the field at Phoenix, March 18, Unser and that L-shaped marvel would go on to top 1971’s fastest lap at Indianapolis Motor Speedway by a full 17mph 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 stopped in Unser’s pit by mistake for his final stop on lap 188.

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

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

“You know, everybody in the press said, ‘Oh, that was Bobby’s idea,’” says Unser of the simple, but incredibly effective device. “My ass it was. That was Dan’s idea.

“But no more arguments [afterward] between Dan and I. Everything’s cool because I can run so hard, you can’t believe it. Turn it, push it, shove it, anything you wanted, that thing would stay stuck, and nobody knew why we were doing it, but that’s Dan. His friggin’ head used to go like that all the time.”

HOW THE GURNEY FLAP WORKS

If air has one big character flaw, it’s that it’s kind of needy. It hates to be split, separated, or diverted from the larger group.

And whether it’s traveling over and under a car or enveloping a wing, rushing to regain its unbroken form is all air wants to accomplish. It makes the creation of the Gurney Flap and its attachment to the trailing edge of race car wings one of the finer inventions the sport has known.

Illustration by Ricardo Santos

Flowing at a shallow angle, air will do a fine job of staying attached to the top and bottom side of a wing before peacefully rejoining after leaving the trailing edge. Start to crank some angle into the wing to generate more downforce, and air attachment to the bottom of the profile starts to suffer. That separation, which creates drag, is where aero efficiency is lost.

With the Gurney Flap installed on top of the wing’s trailing edge, the air passing under the wing is drawn upward as it reaches the end of its journey. Drag is greatly reduced, downforce is increased, and overall wing efficiency is improved.

In the case of All American Racers’ fateful 1972 test at Phoenix, airflow separation beneath the thick wing profile was largely cured with the introduction of a Gurney Flap as it restored harmony between the top and bottom airstreams.

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

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