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Wingtip vortices are regions of high vorticity which develop at the tip of a wing as it flies through the air (or potentially another fluid). Wingtip vortices dramatically reduce the lift generated by the wing, and are therefore critically important in aerospace engineering. Wingtip vortices also form the major compononent of Wake turbulence, or "Jet-wash."
Cause and effects
As a wing flies through the air, it generates a low pressure zone on top of the wing through Bernoulli's principle. Fluids are forced to flow from high to low pressure and the relatively high pressure air below the wing tends to escape to the top of the wing. The air does not escape around the leading or trailing edge of the wing due to airspeed, but it can flow around the tip. Consequently, air flows from below the wing and out around the tip to the top of the wing in a circular fashion. This leakage will raise the pressure on top of the wing and lower the overall lift that the wing can produce. It also produces an emergent flow pattern with low pressure in the center surrounded by fast moving air with curved streamlines.
Luckily, wingtip vortices only affect the portion of the wing closest to the end. Thus, the longer a wing is, the smaller the affected fraction of it will be. As well, the shorter the chord of the wing, the less opportunity air will have to form vortices. This means that for an airplane to be most efficient, it should have a very high aspect ratio. This is evident in the design of long-range airliners and gliders, where fuel efficiency is of critical importance. However, increasing the wingspan reduces the maneuverability of the aircraft, which is why combat and aerobatic planes usually feature short, stubby wings despite the efficiency losses.
Another method of reducing fuel consumption is use of winglets, as seen on a number of modern airliners such as the Airbus A340. Winglets work by forcing the vortex to move to the very tip of the wing and allowing the entire span to produce lift, thereby effectively increasing the aspect ratio of the wing. Winglets also change the pattern of vorticity in the core of the vortex pattern; spreading it out and reducing the kinetic energy in the circular air flow, which reduces the amount fuel expended to perform work by the wing upon the spinning air. Winglets can yield very worthwhile economy improvements on long distance flights.
Since vortices cause a low-pressure area at their centre, sometimes water precipitates out to form clouds in the vortex cores, allowing wingtip vortices to be seen. This is most common on aircraft flying at high angles of attack, such as fighter aircraft pulling high G manoeuvres, or airliners landing.
Hazards
Wingtip vortices can also pose a severe hazard to light aircraft, especially during the landing and take off phases of flight. The intensity or strength of the vortex is a function of aircraft size, speed, and configuration (flap setting, etc.). The strongest vortices are produced by heavy aircraft, flying slowly, in a clean configuration; large jet aircraft can generate vortices which are larger than an entire small plane. These vortices can persist for several minutes, drifting with the prevailing wind. If a small plane is immediately preceded by a large aircraft on the runway, there is a high risk that the winds in a vortex will cause uncontrollable and sudden variations in altitude, possibly violently slamming the airplane into the ground without warning. Worse, the circular nature of vortices can flip a small plane upside down. At the low altitudes involved with landing and takeoff, this is completely unrecoverable. The hazardous aspects of wingtip vortices are most often discussed in the context of wake turbulence.
External links
- Video from NASA's Dryden Flight Research Center tests on wingtip vortices:
- Wind prediction for analysis of vortex drift
"Wingtip vortices"