What’s A Gurney Flap, and Why is the GT350 Getting One?

The Gurney Flap. It’s not a legendary confrontation at a race track infield, or a lesser-known Robert Ludlum novel, or even some sort of aid to help keep patients attached to a stretcher. What it is doing is making the upcoming Mustang Shelby GT350 go faster and turn more quickly around a road course. It’s a simple invention that makes a big difference. Here’s how it works.

The flap was invented during a 1971 test session. Dan Gurney, one of the greatest American drivers and race car builders, was testing one of his USAC Eagle open-wheel cars, intended for the Indy 500. Bobby Unser was behind the wheel of the car, which was sporting a new wing design. The story goes that the car was slower than it should be, so Unser was itching for a solution.

Gurney had seen other teams in other series trying out spoilers, so, as the story goes, he wondered what would happen if he was to put a tiny spoiler on the rear edge of the big new rear wing. So in an hour or so, Gurney built and attached the piece.

It turns out that the car was no quicker than before. But not because the spoiled wing was holding it back. No, it was because there was too much downforce at the rear. Unser said the front end was pushing so badly that the car was slow. Add more downforce at the front and they were off to the races.

In 1972 the wing made it to their race car, and the Gurney Flap took off from there. Cars, planes, and even helicopters now use the device.

Flap on Porsche 962

So what is it? It’s just a small piece that gets mounted to the trailing edge of a wing or spoiler. It sticks out perpendicular to the chord of the wing. That’s the imaginary line that you get if you draw a line from the front edge to the rear edge of the wing or spoiler. In length, it only needs to be 1.5 percent of the length of that chord line. That’s pretty short for a big difference.

That little piece increases the coefficient of lift, decreases the angle of attack to get the same amount of lift, and it doesn’t increase the coefficient of drag. On a car, it means more downforce (which is just lift when the wing is upside down), with less wing angle needed, which means less drag. So faster cornering and more top speed.

The Flap with Airflow (allamericanracers.com)

Take a look at the technical description, and it won’t make much sense. Unless you’re an aerodynamicist. I’ve taken this from Wikipedia: “The Gurney flap increases lift by altering the Kutta condition at the trailing edge. The wake behind the flap is a pair of counter-rotating vortices that are alternately shed in a von Kármán vortex street. In addition to these spanwise vortices shed behind the flap, chordwise vortices shed from in front of the flap become important at high angles of attack.”

Simple as pie, right?

Let’s break it down. The Kutta condition is a fancy way of saying that when air is split by a wing with a sharp trailing edge, the two airflows (one over and one under the wing) will come back together just behind the trailing edge of the wing. The condition is the point the two airflows meet. The flap changes that spot, because it’s no longer a sharp trailing edge. A von Kármán vortex street is the swirl of air as the two flows join back together.

Adding the flap makes a vortex in front of the area of the flap. That’s where air moves in a circle and stays in the same spot, while the main airflow moves around it. The area behind the flap gets two vortices of air, one above the other. The two vortices behind the flap (the von Kármán vortex street) move the Kutta condition to an area further behind the end of the wing.

Moving that point, the wing acts like it’s longer than it really is. Like having a bigger wing without actually fitting a bigger wing. And because there isn’t any more physical wing, the airflow can’t separate after it passes over the actual wing. That lets you put it at a more aggressive angle. Or, take advantage of the extra downforce and use a less aggressive angle. Then you get less drag and more speed.

Dan Gurney – 1967

It doesn’t work on every shape of wing, and some rulebooks don’t allow it. So you won’t see it on every race car. But you will see it on some race cars. And some street cars. It’s impressive for a man who wasn’t an engineer, but not his most important contribution to motorsports. His most important contribution to motorsports was the champagne spray. He and A.J. Foyt did that for the first time after winning Le Mans in 1967.

So now you know how the Gurney Flap works. Or at least why that little piece is sticking off of the spoiler of the new GT350.

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