Wake turbulence may have played a role in the American Airlines Flight 587 crash that killed 265 people on November 12, according to crash investigators.
The tail fin of the Airbus A300 jet sheared off after the pilots struggled against the wake turbulence, or wake vortices, left by a Boeing 747 that had taken off less than two minutes earlier.
But adding triangular flaps to the design of aircraft wings dramatically cuts the strength of turbulence generated in a plane’s wake, according to research at the University of California, Berkeley.
‘The wing we designed could make substantial differences in flight safety and airport capacity,’ said Omer Savas, professor of mechanical engineering at UC Berkeley. Savas and former UC Berkeley graduate students Jason Ortega and Robert Bristol experimented with wing designs that would quickly render wake turbulence harmless after takeoffs and landings.
Regulations in the US require two flights to be spaced far enough apart during takeoff and landing to avoid the potential hazards caused by wake turbulence. While wake turbulence alone would not likely have caused the crash of Flight 587 in New York, ‘turbulence in combination with a possible structural problem in the tail fin could be devastating,’ said Savas.
Savas has been testing a design with triangular extensions jutting behind each wing. He has found that with the design, the wake vortices generated dissipate two to three times faster compared with traditional wing designs.
A wake vortex results from the mismatch in speed, direction and pressure of air moving above and below a plane’s wing. These differences govern the lift generated during flight and planes that are large, heavy and moving slowly create stronger wake vortices.
Depending upon weather conditions and the plane’s speed and size, the wake vortices generated are relatively stable and can stretch a distance of hundreds of wingspans, or three to five miles for a commercial aircraft, said Savas.
For decades, engineers have sought ways to disrupt the stability of wake vortices in efforts to transform the forceful swirls into benign puffs of air. Wing designs have included small pulsing jets mounted at the wing tips, spars and oscillating spoilers. Most of the designs have been ineffective or impractical; some involve moving parts that require greater maintenance.
In tests, Savas’ design created instability in the vortices without generating too much additional drag. The design also has the benefit of involving no actively moving parts.
In an earlier experiment Savas towed tapered sheet metal wings – one a traditional rectangular design, the other with the triangular flaps – in a 70-metre-long water tank at speeds of 1.6 metres per second. Fluorescent dyes marked the vortex wake generated by the 40-centimetre-wide wings.
Savas found that the traditional wing created two stable, counter-rotating swirls from the outside tips. In comparison, the wing fitted with triangular flaps created four vortices, two from the wing tips and two from the flaps. When the flap and tip swirls – each rotating in opposite directions – ran into each other, they quickly became unstable.
Savas has since conducted several other experiments in which he refined the geometry of the triangular flaps. In one test where the flaps span up to half the length of a wing, the wake turbulence began to dissipate four to eight times faster than the wake vortices created by the traditional wing.
Savas said the design would entail some increased drag, but it could be easily compensated for with extra engine thrust during takeoff. After takeoff, the triangular extensions could be retracted so that drag would not be a factor.
Savas is currently working on a pilot program with scientists at NASA Ames Research Centre to incorporate the triangular-flapped wings in aircraft designs.