How Do Winglets Work?

Disclaimer: 

I do not own any of the images used in this article. They are solely being used for your education purposes and not for advertisement or to gain revenue of any sort.

Good that's out of the way.

Welcome to my article on; 

Winglets

What Do Winglets Do?

    So to understand what winglets do, we first have to understand what happens to a wing during 'normal flight'. That is not whilst stalled or at a high angle of attack for example. In my pervious article I discussed how aircraft fly and if you have read that you should have a good knowledge that above the wing there is a low pressure area and below the wing there is a high pressure area as shown in the image below.

     Yes this is good for making our aircraft fly but it does have disadvantages. At the outside edges of wings the high pressure from beneath the wing forces it's way around the edge into the low pressure above the wing. The slightly over complicated image below shows the arrows of high pressure air coming around the edge to the side with the lower pressure. In doing so, spinning whirls of air known as voritces form and reduce the efficiency of the aircraft. The image below also shows in which direction the vortices spin in which is always from the high pressure to the lower pressure area.

Come On Already Tell Me What Winglets Do!

     Whoa, whoa, slow down! Now that we understand what's happening at a wing edge we can understand what winglets do. Quite simply one could explain a winglet's use to be 'reducing wing vortices' although it's not as simple as that.

     Winglets come in a few shapes, sizes and configurations. The majority of these being;

Standard Winglets - Your average fin stuck onto the wing at 90°.

Blended Winglets - A curved section of wing normally at 70° to the main wing.

Spiroid Winglets - A  blended winglet that is curved completely over and reconnected to the wing. 

And Split Scimitar Winglets - A blended winglet that has an upside down winglet to encourge smooth flow.

     Each of these winglet configurations have specific characteristics but all generally do the same job. We'll use the blended winglet as our example because it is the simplest of all of them. Below you can see an image of a winglet blocking the airflow and eliminationg the large vortices discussed earlier. 

     As I said before winglets only reduce the effects of vortices; they do not get rid of them. It works like this. Winglets block the path of the higher pressure air and stop it from getting to the lower pressure therefore stopping all forming of large vortices discussed earlier. However winglets themselves make tiny vortices because the higher pressure area is now on the outside of the winglet and the low pressure area on the inside. Do you see where this is going? The difference in pressure causes the higher pressure area to move towards the lower pressure are thus making a vortex.

    Yes the vortices cause an inefficiency however they are far more efficient than not having winglets; up to as much as 6% more efficient. These vortices also cause a small amount of inefficiency because they 'use up' less energy which will be explored in the next paragraph.

    Furthermore, winglets produce a forward thrust vector by being rotated toe-out slightly therefore 'stealing' the energy from the vortices and turning it into a 'thrust' allowing the engines to run at a lower rpm to achieve the same airspeed. The forward thrust vector is formed because the higher pressure on the outside pushes the winglet in and forward due to the toe-out attitude.

Why Do We 'Blend' The Winglets?

    On standard winglets we get something called interference drag. This is caused when air flows over sharp angles or curves. Imagine now cutting a knife through butter and how little force is required. This is because a knife is streamlined like the shape in the image below. If you read my previous article 'How Do Aircraft Fly?' you'll understand how shape affects drag.

     The small blue arrow to the rear shows how much drag is being made; very little therefore this shape is efficient. Now imagine dragging a shovel through butter with the face towards yourself. An abstract idea I know but the proof is in the pudding. The large surface area creates alot of drag because it has to move the butter quite far out of the way for the shovel to pass through; expending energy in doing so.

     To quote Sir Isaac Newton "Energy is not created or destroyed, it is merely transferred." For this reason the energy being used up to move the butter out of the way means that the shovel is harder to pull through than a knife because more energy is needed to pull it through at the same speed. This is shown by the large arrow to the rear of the image above signifying the drag of the shape.

      On a standard winglet air flowing over each of the two aerofoils mingle in the right angle formed at the base of the winglet. here small whirls of wind (vortices) are made that reduce efficiency due to the interference drag. The left image  below is similar to a standard winglet.

     The images are computer generated models of winglets the left being less blended than the one on the right. Pay key attention to the swirls in the bend on the left image and the efficiency data at the bottom of the images. The more blended winglet (above right) is 0.0067% more efficient than the less blended winglet (above left). Compared to a standard winglet this efficiency difference would be even bigger. 

     Blended winglets work at higher angles of attack (AOA), remeber AOA being the angle between the wing chord and apparent wind [see 'How Do Aircraft Fly?]'. This is because as AOA increases so does lift. This is caused by the increasing relative pressure difference between above and below the aerofoil (see image below).

     The increasing difference then makes larger and larger vortices like wind gets stronger when more particles want to transfer (below). This is what winglets of any sort are trying to stop from happening because the creation of vortices is incredibly inefficient.

     In some cases the difference in pressure can be so great it will make votices off of the fuselage like on this F/A-18 Superhornet at a high AOA.

What Do Spiroid Winglets Do?

     First used in 1992 on the Gulfstream II, spiroid winglets are like blended winglets and standard winglets but “The Spiroid eliminates concentrated wingtip vortices, which represent nearly half the induced drag generated during cruise.” - FlightBlogger. This is because rather than the pressure coming around the side and trying to get over the tip; it cannot get over because there is no tip of a winglet. For this reason the air flows over like the sphere in a fairing (far above).

     Research done by Tung Wan and Kuei-Wen Lien found that spiroid winglets can save as much as 5% fuel burn however their efficiency decreases with AOA; level flight being their most efficient. Simply, again, spiroid winglets are not going to feature massively on small aircraft because they dont cover the distance or burn enough fuel per year to benefit from the efficiency of winglets.

What Do Split Scimitar Winglets Do?

     Split scimitar winglets may look cool but they give more efficiency than blended winglets, thier predecessor. They take the technology from upward wingtips and lowered wingtips and combines them. Back in the day on the MD-11 they found that lowering the winglet and giving them a toe-in attitude, the opposite of a standard winglet, also gave a forward 'thrust component'. So by combinig these two methods an efficiency saving of 6% can be achieved.

    Also, like blended winglets, they make smaller vortices by making the air travel further around, therby stealing it's energy and because the difference in pressure on each side of both winglets is reduced the passing over of air is slower and weaker.

     We can relate this to our diffusion model above the F/A-18 Superhornet and to wind. On windier days more particle have to diffuse which gives the appearence of wind. on less windier days less particles need to move. In comparison to a wing a windy day would be without winglets, a light breeze would be with blended winglets and a very light breeze would be with split scimitars. You get the jist. Below is the same image with particles for visualisation if you are struggling.

Why Doesn't My Cessna 172 Have Winglets?

    Whilst on your average Sunday flight you might save a few dimes from the aid of winglets; 172s or any small aircraft of any kind simply dont fly fast enough or fly far enough per year to benefit from the tiny gains that winglets provide. Whereas, large airliners can save thousands per year.

     Yes, for one airliner blended winglets save 54 million gallons of fuel. Thats $96,660,000 USD in today's market (£62,776,424 GBP). It not only means that we get cheaper flights but also less CO2 is made; slowing global warming. Furthermore, split scimitar winglets are now the new hype and in the next few years with them being more and more widely accepted we can save much, much more.

To Conclude

     In this article we began by discussing how vortices form on wings. We then talked about the types of winglets including Standard, Blended, Spiroid and Split Scimitar. We then discussed how winglets produce a forward thrust, reduce the effects of vortices and how interference drag is such a nuisance. I then moved on to talk about how Spiroid and Split Scimitar winglets work and we touched base on simple diffusion knowledge. To finish with we asked why small aircraft don't have winglets. Either way one looks at it; having winglets is far more efficient and fuel saving than without them.

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Safe Flying!