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Common Tread

What's a NACA duct? Why do some motorcycles have them?

Apr 22, 2020

A year into World War I — but two years before the United States entered — a new federal agency was created to advance the newfangled field of flight. The National Advisory Committee for Aeronautics, or NACA, interacted with everyone from Orville Wright to Chuck Yeager until it was disbanded in 1958.

You’re probably familiar with NACA’s replacement agency, NASA. I wonder if they bought a new sign, or just one letter?

NACA Mustang
NACA's innovations could be found on some of the most iconic airplanes of the era, including the P-51 Mustang. Hey man, you need a tie clip in the wind tunnel! NASA photo.

NACA’s greatest accomplishment was arguably its support of Allied aircraft technology during World War II. Eisenhower believed that D-Day wouldn’t have been possible without air superiority, and he personally thanked the agency for its contributions. NACA made all kinds of discoveries in forced induction, airfoil design, and other aspects of aviation, including a development in low-drag ducting called, appropriately, the NACA duct, which appeared in 1945. You might see it called a NACA scoop or a NACA inlet.

YF93 Saber NACA ducts
Here's a YF-93 "Sabre" with some monstrous NACA ducts, not long after the war. Other planes put ducts at the nose or at the base of the wings, but the NACA design had some advantages. National Museum of the USAF photo.

Imagine a giant hood scoop on an old muscle car. That’s a basic duct. It directs air from one place to another. The scoop looks cool, and it does move air, but it also introduces drag. That drag isn’t a big deal on a street car, but it’s definitely something to think about as you’re cresting 700 mph in an experimental fighter!  Drag, after all, is proportional to velocity squared. So how do you get air into the vehicle without introducing too much unwanted drag? 

You do it very carefully, that’s how. A NACA duct sits flush with the fuselage, fairing, hood, or whatever surface is moving though the air, rather than rising above the surface like a scoop. The duct’s gentle ramp (typically around seven degrees) angles towards the interior, and it widens as it deepens. Air passes through the duct, and drag remains manageable, even at high speeds. Incoming air can then be directed with hoses, if needed. And as some NASCAR teams learned before the practice was banned, pointing the duct backwards makes it work like an extractor. Useful and fairly efficient, the NACA duct’s shortcoming is that it doesn’t move as much air as high-drag designs, which “ram” the air into the duct with accompanying drag and turbulence. The NACA design is just one more tool in the toolbox for vehicle designers and engineers.

Ferrari F40
The Ferrari F40 isn't shy about its use of ducting. Ferrari photo.

Suzuki GSXR1100
Early full-fairing production bikes like this Suzuki GSX-R1100 made use of aerodynamic tricks. Suzuki photo.
High speed, low drag, performance, aerodynamics… it’s no surprise that the car world adopted NACA ducts for use on the ground. Ferrari’si F40 and Lamborghini’s Countach are probably the two best-known examples of NACA ducts on cars. Some Shelby GT350s and GT500s had them, along with the second-gen Dodge Viper and many other vehicles from around the world.

As fairings became more common, the distinctive ducts started showing up on motorcycles, too. The best example that comes to my mind is the Honda VFR750, for obvious reasons.

VFR750 NACA duct
If there's a prize for "Best Moto NACA Duct," I nominate this VFR. Honda photo.

As a general rule, one big NACA duct is better than a bunch of little ones. You'll spot them on all kinds of early sportbikes, and even modern MotoGP bikes. Like airplanes and cars, motorcycles need to move air to hot components. It's fascinating to me that we're using a solution from the 1940s to get that air where it's needed.

Honda RC213V NACA duct
This RC213V has a textbook NACA duct on its lower fairing. Honda photo.

There’s one more thing the NACA duct does. It implies that a vehicle is fast, and, by extension, cool. I don’t know of many boring vehicles that came with NACA ducts from the factory. If you want to add your own, you can always buy premade ducts and cut some holes in your ride. How much effort you put into their placement is up to you.

Indian Challenger NACA duct
What, you thought this was only a sport bike thing? The Indian Challenger wears two NACA ducts up front. You'll also see the distinctive wedge shape on windshields sometimes so that just a little air can get through. Indian photo.

Will modern aero-modeling software kill off the NACA duct? I doubt it. Whether your vehicle is wearing them for actual cooling or just cool points, I think there will be a place for these funny little indentations for as long as there’s a need to duct air at speed.