WINGLETS

While traveling via airplane or watching an airplane video you might have seen that airplane wing tips have different shapes. These are known as winglets. Their design differs from one aircraft to another and some aircraft might not have them all together. So, now the question that arises is why are they there?

credit: Julian Herzog

Well apart from their striking appearance on some aircraft such as the Airbus A350, these winglets serve a very specific and important function which saves airlines millions of dollars on fuel. To understand the purpose of winglets we first need to understand the concept of Induced Drag.

Induced Drag

To understand induced drag we first have to understand how the wings of an airplane produce lift. Wings generate lift by creating a low-pressure region above the surface of the wing and a high-pressure region below the surface of the wing. These high- and low-pressure airflow meets at the trailing end of the wing where the high-pressure airflow tries to meet with low-pressure air flow, creating vortices. These vortices are much stronger at the wing tip and their strength reduces towards the root of the wing.

credit: Real Engineering[YouTube]

In the right atmospheric conditions, you can see these vortex lines generating from wingtips. These vortices produce a downwash which reduces the effective angle of attack (α) of the wing. You can see in the image below how the effective angle of attack of the wing is reduced. Since lift acts perpendicular to the relative airflow, and because the relative airflow is at an angle, lift is also slightly tilted backward. We can now divide the lift into two components.

credit: Johnwalton

The vertical component has to balance the weight of the aircraft and a horizontal component which is our Induced drag. As we can see the direction of the induced drag is backward, it is resisting the forward motion of the airplane which results in more power required to move the aircraft forward. Since induced drag is produced because of lift it is also known as "drag due to lift". Hence if we want to increase the efficiency of our wing we have to reduce induced drag as much as possible.

Now let us see how the design of the wing affects induced drag. The coefficient of drag for induced drag is inversely proportional to the Aspect Ratio of the wing i.e. more the aspect ratio less the induced drag. Aspect ratio is the ratio of the square of wing span to its area. So, if we want to reduce the induced drag, we have to increase the wing span as much as possible.

The equation for induced drag is:

, Where

b = Wing span, and

S = Aerea of the wing.

By the above-mentioned formula, we can also conclude that as we increase the angle of attack the C_L, which is co-efficient of lift, will also increase and this will result in an increase in the induced drag. By this equation, we can conclude that if the lift increases the induced drag will also going to increase. This further concludes the fact that it is a lift-induced drag.

Wing Design

One solution to reduce induced drag is to increase the wing span as much as possible. But designers can only increase the span up to a certain point, after that, it will be impossible to accommodate the aircraft in the airport. So, designers have to find a different solution.

One such solution is to install wing-tip devices on the airplane wing. The earliest concept of such wing-tip devices is developed by an English engineer Frederick W. Lanchester. He patented the wing end-plate which was installed in the wing-tip and helps to reduce the wing-tip vortex.

Today most modern airplanes use a type of wing-tip device called “winglets”. These were designed by Richard T. Whitcomb in the 1970s at NASA LangleyResearch Center, What motivated the development of winglets was the rising oil prices in the 1970s which forced aircraft manufacturers to make aircraft more fuel efficient.

Effects of Winglet on Induced Drag

As we discussed how the induced drag is formed when the air from the high-pressure region (below the wing surface) tries to go to the low-pressure region (above the wing surface). To stop this movement of air winglets are used. As you can see in the image below how the wing without winglet airflow can easily travel from the high-pressure region to the low-pressure region creating a large vortex. While in the wing with the winglet, the vortex which is formed is much smaller.

credit:www.facc.com

As a result of this induced drag is decreased and lift distribution over the wing is more optimum. Aircraft manufacturers can achieve fuel efficiency of 4-6 percent by using winglets. Winglets also help to reduce the wake vortex produced by the wing which helps to increase the frequency of take-off and landing of aircrafts on airports.

As there is no single wiglet that fits on every aircraft, winglets come in a variety of shapes that differ from one aircraft to another.

Raked Wingtip

I talked about winglets up until now but if you have seen the latest Boeing jets like B777 and B787 you might have seen that they don’t have a winglet at their wingtip. Boeing uses something called a Raked Wingtip. These types of wings have larger sweeps at wingtip as compared to the rest of the wing.

credit: Yoshiharu Mohri

As you can see in the image the raked wingtips are much narrower than the rest of the wing. This results in decreased average chord length of the wing. This design helps to increase the wing span with a very small increment in the wing area. Due to this the aspect ratio of the wing increases and induced drag decreases.

Raked wingtips are more efficient than conventional wingtip devices. Recently Boeing has developed a new version of the B777 family called Boeing 777-X which has folding wingtips. The reason to have a folding wing structure is to increase wing span while flying at the same time not taking any more space in the airport than previous versions of B777 as wingtips can fold while in the airport terminal.