Journalists on the scene for the recent preseason MotoGP test at Malaysia's Sepang International Circuit have called the downforce devices on the latest Yamaha fairing "split wings." Looking from one side, you see a leading downforce wing with a similar second one behind and slightly above it but set at a higher angle of attack, separated from the leading wing by a visible gap—the "split."

If you flip this over to produce lift instead of downforce, what you see resembles the slotted flap systems that allow modern aircraft to develop low-speed lift that enables them to take off and land. Without that added lift, they would have to reach a much higher speed to be able to take off, which would require longer runways and impose dangerously high landing speeds.

You may ask the reasonable question, “But why the split? Why not join the two airfoils together with the same overall curvature? Wouldn’t that work better than having a ‘leak’ between two wings?”

To get the most lift out of low-speed airflow, as when an airplane is taking off or landing, it is essential to turn the flow through as large an angle as possible. Lift, after all, is just the reaction to changing the direction of motion of a mass of flowing air. In the case of a wing, deflect the airflow downward and you get a reaction force upward—lift. But there are limits to how much a simple wing can turn a flow, which is why modern airliners are given as many as three narrow subwings carried within the main wing during cruise and extended rearward and downward to increase wing area and curvature during takeoff and landing. The main wing begins the process of turning the flow downward and each successive trailing flap element, set at greater and greater angles, turns the flow more, thereby getting maximum lift from it.

If we keep on increasing the angle of attack of a simple wing, the flow will separate from the low-pressure surface (on airplanes, this is the upper surface, but on these downforce wings, it is the lower surface). Once flow separates, lift decreases, so it’s better to turn the flow conservatively with the leading wing, then start the process over again with a subwing behind it, and so on. During landing approach, it is typical for the flap system to be deflected 45 degrees!

Valentino Rossi
Valentino Rossi (46) and factory Yamaha teammate Maverick Viñales tested different aerodynamic packages during the MotoGP preseason test held earlier this month at the Sepang International Circuit in Malaysia.Gold & Goose

Flow separation occurs because of the appearance of reverse flow in the boundary layer. The boundary layer is the thin layer of sluggishly moving air that is closest to the vehicle. It becomes sluggish because its molecules make myriad collisions with the moving surface, losing much of their original free-stream velocity. The farther the flow travels over a surface, the thicker the boundary layer becomes. (This is why the forward parts of the British Spitfire fighter were smoothly flush-riveted, but surfaces farther back were more cheaply and quickly riveted with button-head rivets. By the time the flow reached those button heads, the boundary layer had grown thick enough that the rivet heads could no longer produce drag by protruding into the free stream.)

slide-into-place method
Just as airlines provide guidelines for carry-on luggage, this template represents a simple, slide-into-place method for the factories to confirm their fairings meet the 2019 MotoGP dimension limits announced eight months ago.Gold & Goose

On Yamaha’s new fairing, the leading wing turns the flow upward as much as it reliably can, and then its growing boundary layer is dumped into the gap between wings. Higher-energy air from the front rushes through the gap onto the second wing, helping to postpone formation of a thick boundary layer there as it turns the flow even more upward.

Flow separation occurs because, as the boundary layer naturally flows toward regions of low pressure, it may become thick enough to develop instability that pops the flow loose (this is air getting into a “speed wobble”).

Ducati Sepang
Yamaha was not alone testing aerodynamic options. Ducati evaluated a three-tier fairing configuration (also note carbon-fiber shrouds protecting carbon Öhlins fork stanchions). KTM and Suzuki also unveiled new designs.Gold & Goose

In the past, MotoGP teams have used cascades of as many as three stacked airfoils—one above the other, rather than one behind the other as in Yamaha’s new fairing—rather like three slats from a venetian blind or the World War I triplanes from Fokker or Sopwith.

The idea of employing a slot between tandem airfoils dates back to German aeronautical engineer Gustav Lachmann, who had plenty of time to think while in a hospital following a flying crash as a result of wing stall. A slot, by feeding high-energy air onto the leading edge of the second airfoil, would scour away enough boundary layer to postpone flow separation and stall. Lachmann’s first patent application was rejected because the examiners denied it could work. To them, such a gap didn’t make intuitive sense. Englishman Frederick Handley Page had better luck with his patent office so such gaps are now called Handley Page slots.