By Stuart Grant
Wings, scoops, splitters and ducts are the must-have go-faster goodies on any boy racer’s car, plane or rocket. But this is not necessarily the best option and often causes more drag than good. However there’s a carefully-calculated solution to this age-old problem of channelling that much-needed air into your ride – and it’s nothing really that new. Enter the NACA.
The National Advisory Committee for Aeronautics (NACA) was a US federal agency founded in March 1915 with the intention of undertaking, promoting and institutionalising aeronautical research. Its duty was to direct the study of the problems of flight with a view to their practical solution. In addition to formal assignments, employees were encouraged to pursue unauthorised research, provided that it was not too exotic. The result was a long string of breakthroughs, including the NACA engine cowl of the 1930s, the 1940s’ NACA airfoil series and the area rule for supersonic aircraft in the 1950s. Come 1958 and the agency was dissolved, with assets and personnel transferred to the newly-created National Aeronautics and Space Administration, which we all know as NASA.
The name NACA remains familiar to us car lovers though, in the form of the NACA duct – a low-drag air intake. If you think drag on your car is a problem, try it on an aeroplane: every bit of drag on an plane means more power, and therefore more fuel, is needed to keep airborne. And it kills the top speed too.
The NACA duct brings air into a vehicle, while not disturbing the boundary layer, and offering minimal increase in drag or flow disturbance. Consisting of a shallow ramp with curved walls recessed into the exposed surface, it was originally called a ‘submerged inlet’. When the cross-sectional flow area of the duct is increased, you decrease the static pressure and make the duct effectively operate as naturally powered vacuum cleaner, but without the drag effects of a more traditional snorkel-like scoop. The reason the duct is narrow and then widens in an arc is to increase the cross-sectional area slowly, so as not to separate the airflow which would cause turbulence and therefore drag.
NACA ducts are useful when air needs to pulled into an area which isn’t exposed to the direct air flow. It does this by taking advantage of the boundary layer – a layer of slow-moving air that ‘clings’ to the bodywork of the car – like the roof and side panels. The longer the panel, the thicker the boundary layer becomes. This design is believed to work because the combination of the gentle ramp angle and the curvature profile of the walls creates counter-rotating vortices, which deflect the boundary layer away from the inlet and draw in the faster moving air, while avoiding the form drag and flow separation that can occur with protruding scoop designs.
By design NACA ducts don’t achieve the larger ram pressure, like those monster intakes above an F1 driver’s head, so are better as cooling air-supply vents than they are performance enhancers. This means applications like brake, engine and driver cooling are most common.
If you plan on fitting a NACA duct it’s best to follow these
guidelines to prevent becoming the laughingstock at the track:
1. Design is very important and must have the correct sharp wall angles, base slope, and width-to-depth ratio in relation to speed. If the edges on the slopes are not sharp enough the flow separates.
2. Installation needs to be done in an area of high pressure. Place it in a region with a positive pressure gradient, where the body is increasing in size and not constant or decreasing. If you put it in an area of negative pressure, other than looking cool, it will not do the job at all.
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