The wings of airplanes were initially inspired by the anatomy of birds. Although their function has not changed since the day the Wright brothers took flight for the first time, it’s true that its shape and design have improved a lot over the years. A relatively recent breakthrough, that certainly does not go unnoticed, is the introduction of wingtip devices. Although the so-called winglets were already present much earlier in combat aircraft, it was not until the 1980s that they started to appear in commercial airplanes. But what caused the introduction of winglets in commercial aviation? And also, what is exactly their purpose?
As we saw in previous videos, the primary function of wings is to generate lift, by creating a pressure difference between the upper and lower surface. However, producing this lifting force also comes at a price. Near the wingtip, the lower flow, which is at a higher pressure than the one on top, tends to sneak onto the top of the wing. This produces turbulent wakes with the shape of a vortex that are called wingtip vortices.
Wake turbulence is especially dangerous in the region behind an aircraft during the takeoff or landing phases of flight, when the airplane operates at high angles of attack. This flight attitude maximizes the formation of strong vortices, which can remain over the runway for at least 3 minutes. Wingtip vortices also reduce the performance of an aircraft, producing what is known as induced drag, an inevitable consequence due to the creation of lift in three dimensions.
You see, induced drag is simply one of the many types of drag that there exists. It receives this name because it is created as a penalty when producing lift and, in fact, here is how to calculate it: the square of the lift coefficient divided by pi and the aspect ratio. And at the same time, the aspect ratio of a wing is simply the ratio of the wingspan and the mean chord. Although it’s impossible to get rid of induced drag, there are many ways to reduce it. The first is by designing longer wings, as this reduces the size of the vortex at the wingtip. This method is very common in gliders, which have very high wingspans to stay afloat for a longer period of time.
During the golden age of aviation, aircraft with even longer wings were manufactured, such as the first Boeing 747, with an aspect ratio of 6.96 and a wingspan of 195 ft. This new design lowered the operating costs of commercial aircraft, increasing airlines' profits. The wingspan of aircraft was pushed to the limit, not only structural, but also operational, due to the space available between boarding gates.
But everything changed during 1973 oil crisis. In response to the sharp increase in fuel prices, Richard Whitcomb, a NASA engineer, began developing the winglets, hoping to increase aircraft aerodynamic performance. And the truth is that he was on the right track. After many years of studies and testing, Whitcomb proved that a carefully designed winglet could reduce induced drag by about 20% due to the decrease on size of wingtip vortices.
However, and despite the widespread belief, winglets are not always advantageous, and can even decrease the performance of an aircraft. You see, by adding a winglet you are also increasing the wing area of an airplane, and therefore increasing drag. This type of drag is called parasitic drag and is caused by the friction of air. If the increase in parasitic drag is greater than the reduction in induced drag, adding a winglet will increase total drag, and thereby, decrease performance. Let's look at this concept in more detail with a graph. In the horizontal axis we place the airspeed, and in the vertical axis the drag force.
Parasitic drag increases with airspeed and, on the other hand, induced drag decreases with airspeed. The sum of both is then the total drag of our plane. Now let’s imagine that by adding a winglet we decrease the induced drag by this much, and we increase the parasitic drag by this much. The total drag of our plane then varies as follows. As you can see, depending on the cruising speed of the aircraft, it’s advantageous or counterproductive to add a winglet. Another metric that affects a winglet’s performance is altitude. Since air becomes much thicker the lower you go, adding a winglet to airplanes with low cruising altitudes would increase parasitic drag even more.
Today, winglets continue to evolve and come in a wide variety of shapes. The Airbus A320 originally featured a wingtip fence that is being replaced by a much more efficient sharklet that reduces fuel consumption by 3%. In the U.S., the new models of the Boeing 737 have winglets that also emerge towards the bottom of the wing, which are called split-tip winglets. In most modern aircraft, we find very innovative designs that feature wing tip devices, once again mimicking the anatomy of birds. While it’s true that we still have a lot to learn from nature, all we have to do is take a look.
