Why do planes have winglets?

What’s the point of the little flip at the end of the wings?

Apr 14, 2021

Planes have been pretty much the same for a while. The engines all look the same, the shape of the fuselage looks the same, and the pilots’ outfits haven’t changed in over 40 years. If it ain’t broke… These days most, if not all, changes to airplanes are to improve efficiency and at this point the changes are pretty minuscule. The change to a larger engine that caused the issues with the Boeing 737 MAX? That was to increase the fuel efficiency of the plane by ~8%. This doesn’t sound like a lot, but in aviation that’s HUGE. Engine and aircraft manufacturers alike will do a whole lot for just 1% better. Why? Because that’s 1% better than their competition, and almost a guarantee of their customer’s business. Also, over the course of a single flight, that 1% improvement equates to over 200 gallons of fuel saved, or, to the airline, over $400. Add this up over the aircraft’s typical 20,000 cycle life (20,000 flights) and you’re looking at $8 million in savings per aircraft. You can see how this can grow extremely quickly. The little wingtips are no different. They provide a tiny bit of benefit, but that benefit adds up.

To start we have to talk about vortices. [1]

The air passing above and below the wings is traveling at a different speed. This is designed so the wings can generate lift; however, that air doesn’t just travel in one direction. It also “bleeds” off of the ends of the wings, which is what causes these vortices. [2]

For the most lift, we want all of the high-pressure air to remain under the wings, to push them up. When high-pressure air leaks over in the form of a vortex, it pushes down on the wing. This obviously is bad, but can’t we just use slightly larger wings to account for this? Yes! But like most things in this newsletter, that’s not the whole story.

In comes induced drag. This is a pretty confusing topic even for Aerospace Engineers so bear with me. Remember how we said this air leaking over pushes down on the wing and reduces lift? Well, that concept is called downwash. The reduction in lift caused by downwash can be overcome while flying but at a cost. Take a second and think about how planes can generate more lift. They can speed up, to get more air passing over the wings, or they can use flaps, which effectively make the wings a little bit larger, but neither of these things are very effective when the plane is cruising. It’s already at its top speed and putting down the flaps at 500 mph would result in a pretty wild ride. The other way to increase your lift is to angle the nose of the plane up. This increases what’s known as your “angle of attack”. Imagine sticking your hand out of a car window. If it’s parallel to the ground nothing happens, but if you tilt it up, air will rapidly push your hand up. This represents an increase in lift. Unfortunately, when you expose the wings a bit more to the air, it also increases drag. Recall the equations for lift and drag:

These two equations are very similar, and a quick glance will show you that any increase in area to improve lift will also increase drag the same amount. Sadly, however, this is something we’ll just have to deal with, otherwise, our airplane won’t be able to generate enough lift to stay in the sky. Yeah—I’m not a fan of that alternative either.

This increased drag caused by our downwash vortices is what is known as induced drag. Congratulations! You did it!

This drag reduces our fuel efficiency because we must burn more fuel to overcome it. Think back to rolling your windows down while driving—you’re increasing the drag that your car feels, and while it may not seem like much in the moment, it has a significant effect on your car’s fuel efficiency. To counteract this drag, we either have to use more gas or roll the windows up. That’s exactly what these wingtips are doing: [3]

They’re putting a wall in the way of the high-pressure air. Sure, some still leaks over, but much less than what we had before. The result? Less downwash—which means we don’t have to point the nose of the aircraft as far up—which means we experience less drag—which means we get better fuel efficiency. All things considered, these wingtips provide a 5% reduction in fuel consumption. By the same logic that we discussed before, that equates to, on average, $2,000 saved every flight. Multiply that by the roughly 45,000 flights overseen by the FAA every day [4] and we see a savings of $90 million every single day. This assumes, however, that every plane has these wingtips. Virtually all new ones do, but many airlines operate planes that are older than these wingtips and don’t get the same fuel savings. Regardless, these little curly bois save the airline industry billions of dollars every year, and all they have to do is, well, nothing. That sounds like a pretty nice life don’t you think?

Thank you as always for reading, I appreciate your continued support and will see you all next week! Don’t forget to subscribe!

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