The NACA duct

Penetrating the air efficiently while also ingesting a little of it has been a concern of aerodynamicists since the early years of the science, and intermittently of racing car designers for almost as long. With the notable exception of rockets, most powered vehicles — earthbound or flying — generate their motive force by burning gas or liquid fuel, and that requires an air supply for combustion and, usually, cooling. The trick is to tap this from the passing airstream with maximum efficacy but minimal drag.

Even in the era of propeller-driven aircraft this was an important issue, but with the advent of the jet engine it became a major source of concern. A jet lives on airflow; restrict it in any way and performance falls off in proportion.

So it was that once the US had been passed details of Whittle’s invention across a Whitehall office table in July 1941 — as part of the wartime free trade in secrets, a process which also saw the handing over of the cavity magnetron, the key to centimetric radar — American researchers began a thorough investigation of the aerodynamics of jet intakes. Much of this work was carried out by the National Advisory Committee for Aeronautics, NACA, which in later years metamorphosed into the National Aeronautics and Space Administration, NASA. NACA had already distinguished itself by designing improved engine cowlings for radial-engined prop planes; now it set about the jet intake problem.

And problem there was. One vital measure of an air intake’s efficiency is its pressure recovery. Air has mass and therefore momentum, so a moving airflow exerts a force (pressure) if slowed. In fact the expression describing that pressure is closely allied to the familiar F = 1/2rnv^2 taught in school physics lessons as the equation for kinetic energy. A perfectly efficient inlet would extract all this energy from the impinging air and have a pressure recovery of 100 per cent. What the NACA researchers, working at the Ames Aeronautical Laboratory at Moffett Field, California, soon discovered was that very few existing jet inlets achieved better than 65 per cent pressure recovery — a startling inefficiency that took a large toll on aircraft performance potential.

The solution they developed, first described in an Advanced Confidential Report dated October 1945, was a form of submerged inlet that was later to become universally known as the NACA duct. Unlike the gaping jet intakes that were to become the norm, the NACA duct comprised a carefully formed indentation in the fuselage, which started as a narrow, shallowly inclined inward ramp and widened with reflexly curved sides to a squared off end.Trailing vortices created along the flared edges of the duct deflected airflow into the gradually deepening inlet with remarkable efficiency, so that even in early form the NACA design offered pressure recovery of over 90 per cent under optimal conditions and incurred very little drag penalty.

As events would develop, the NACA duct was not to see much use as a jet intake but for other air inlet purposes it rapidly became standard. In 1951 the original NACA report on it was declassified, and the design of the duct became public information, available for use by anyone. Racing cars were an obvious application, but it wasn’t until 1956 and the emergence of Frank Costin’s redesigned Vanwall that someone had the nous to exploit it.

Costin, still working at that time for De Havilland, was an aircraft aerodynamicist by training. Racing car design was a relatively new diversion for him and it doesn’t take much imagination to appreciate the horror with which he viewed contemporary aerodynamic practice on the track. For all their semblance of streamlining, race cars of the period were mostly a triumph of hope over understanding.

It was Costin who famously, if vainly, was to offer up the prayer, “Save me, oh Lord, from the statement: ‘What looks right is right.'” He had a rigorous, mathematical approach to aerodynamics, and when invited by Tony Vandervell to recast the Vanwall’s bodywork he set about the task with a meticulousness foreign to most racing teams, with the possible exception of Mercedes. One feature of the old Vanwall to be binned was its prominent bluff induction air intake — an ugly carbuncle on the right side of the bonnet which Costin replaced with a discreet, efficient NACA duct

Other F1 teams hardly fell over themselves to copy Costin’s example, but aerodynamics was, in time, to become a major design issue. Science gradually supplanted guesswork, and in the process the NACA duct became a common feature on racing cars of all persuasions. Or rather, simulacrums of it did, as many designers were lax in appreciating certain subtleties of the duct which were essential to its performance: the need to place it in an area of laminar airflow with a thin boundary layer, the need to align it accurately with the airstream, the need to restrict the ramp angle to a maximum of 10 degrees, the need to have sharp edges to ensure energetic vortex generation, and the need to form the terminating edge carefully.

One or more of these important features was often missing, but the NACA duct had by now taken on an almost mystical quality, becoming itself another victim of Costin’s pet hate: the triumph of aesthetics over functionality. Brake ducts, engine bay cooling, oil and water heat exchangers, cabin ventilation — NACA ducts of varying quality came to be widely deployed in all these applications, as both inlets and outlets, and still are. In pretty short order, due to Costin’s close association with Lotus, the distinctive NACA air inlet even made its first, albeit fleeting, appearance on a road car. Asked to modify a number of racing Elites, Costin turned once again to a bonnet-mounted NACA duct for conveying cool inlet air to the carburettors. It was so successful that when the final Super 95 version of the road-going car was launched it was incorporated as a standard feature — making this last Elite variant the first road car to boast what has since flowered into one of the more unlikely motorsport icons.