Engineering news
The swirling, invisible trails of artificial turbulence can flip small planes and force flight pattern separation, but research from the University of Illinois could help lessen the risks.
Led by aerospace engineer Phillip Ansell, the research simulated air flow around three wing designs – elliptic, and designs from classic studies by R T Jones and Ludwig Prandtl. The work studied the probable formation of “destabilising flight hazards” behind the wings.
The risk to small aircraft is clear, said Tim Robinson, editor in chief of the Royal Aeronautical Society’s Aerospace magazine: “Don’t get into the wake vortex.”
“If you are a smaller aircraft and you get into the vortex of a bigger aircraft like an A380, it can flip you over,” he told Professional Engineering. “It is not just Cessnas, there have been cases when business jets come across a wake in the cruise and experienced upset, tipped the autopilot and things like that.”
Although most wing shapes used today create the turbulent vortices, the Illinois study demonstrated different geometrics to reduce or eliminate them almost entirely.
Assistant professor Ansell said: “The elliptic wing configuration has been used as the gold standard of aerodynamic efficiency for the better part of a century. We teach our students that it has the optimal loading characteristics [ratio of lift to weight] and that it's often used when looking at wing efficiency for, say, minimising drag.
“Previous academic studies have shown that, theoretically, there are other designs that actually provide lower drag of a planar wing for a fixed amount of lift generation. But what has been missing is an actual apples-to-apples experiment to prove it.”
Ansell and his graduate student Prateek Ranjan used data from previous research to analyse the elliptic, Jones and Prandtl designs. The pair saw “significant differences” in how the wings’ wakes developed, with no vortices behind the Jones and Prandtl configurations.
“They had a much more gradual bulk deformation of the whole wake structure, rather than an immediate coherent roll-up,” said Ansell. “We now know that we can delay the formation of wake vortex structures, and increase the distance it takes a trailing wake vortex to roll up by about 12 times, making it weaker and less of a hazard to the aircraft entering its wake.”
‘It is a big issue’
Reducing vortices could allow closer flight patterns or help develop ideal lift configurations for take-offs and landings.
“Wake vortices and the weight of aircraft is what drives separation, both at airports and at cruise,” said Robinson. “It is a big issue, and for busy airports it is staggering arrivals. If the wake vortex dissipates, you can squeeze more planes in.”
Despite finding Jones or Prandtl wings would have less turbulent air in their wake, Ansell said they are not always the right option for new aircraft.
“One of the things that first drew me to the topic of aerodynamics is that the right answer always depends on what your constraints are,” he said.
“If you're building a tiny unmanned vehicle that will fly at a low speed, you'll get a different solution for design needs than if you're building an aircraft that will carry people at high altitudes and high speeds. So technically, you could argue that all three wing types are the best solution. The question is, what are your driving constraints, such as wingspan and weight, behind selecting one of them?”
The research was published in the Journal of Aircraft.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.