Wingtip vortices

Wingtip vortices stream from an F-15 as it disengages from a KC-10 Extender following midair refueling.

Wingtip vortices are vortices that develop at the edge 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.

As a wing flies through the air, it generates a low pressure zone on top of the wing through the Bernoulli effect. Fluids naturally flow from high to low pressure and the relatively high pressure air below the wing has a natural tendancy to flow to the top of the wing. The air naturally cannot flow around the leading or trailing edge of the wing due to airspeed, but it can flow around the end. Consequently, air flows from below the wing, out around the edge to the top of the wing in a circular fashion. This raises the pressure on top of the wing and lowers the overall lift that the wing can produce.

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 wide high aspect ratio. This is evident in the design of long-range airliners and gliders, where efficiency is of critical importance. However, increasing the wingspan reduces the manoueverability of the aircraft, which is why combat and aerobatic planes usually feature short, stubby wings despite the efficiency losses.

Another method of reducing wingtip vortices is winglets, as seen on a number of modern airliners such as the Airbus A340. Winglets work by interfering with the formation of the vortex, thereby effectively increasing the aspect ratio of the wing. 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 vortices allowing wintip votices to be seen. The is most common on fighter aircraft when they are pulling high G manouevres

Wingtip vortices can also pose a severe hazard to light aircraft, especially during the landing and takeoff phases of flight. Since the size and power of wingtip vortices are directly proportional to the size and speed of an aircraft, large jet aircraft can generate vortices which are larger than an entire small plane. If such 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 slamming the airplane into the ground earlier and harder than the pilot expected. 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.