Plane Sailing

The festive Christmas period is only just easing to an end and you’ve already begun teasing the thought of flying away to a warm summer destination for your next vacation. Perhaps Dubai, or maybe even Hawaii? I can see the drool starting to gather at the corners of your mouth. I’m drooling too, but maybe not for the same reason…

If you’re planning on going anywhere remotely exciting, odds are that you’ll probably have to take some sort of express transportation vehicle, such as an airplane. These extraordinary machines have been around since the Wright brothers invented them in 1903, yet most people still do not understand how they work. We take for granted the fact that we are able to travel from one side of the world to the other in just a matter of hours.

Fundamentally, the plane is quite simple. For it to stay flying at a constant speed in the air, all it needs is for the net force on it to be zero. This means that whatever horizontal or vertical forces it encounters, it needs to balance in some sort of way. Now what might these pesky forces be? Ah, none other than the usual suspects – air resistance and gravity respectively.

The air resistance, or drag, can be countered trivially just as a car does, by using an engine which exerts a forward driving force, known as thrust. However, this part isn’t really that important. In fact, it’s possible to have a plane without engines. Just take a look outside and see if you can spot any small creatures in the air.

The hard part, rather than keeping the plane moving forward, is to keep it elevated. For this, the weight of the plane has to be countered by an upwards lift force. Considering a plane weighs several hundred tonnes, not to mention the additional weight of passengers, food and luggage, an extremely large force must be overcome. No, I’m not calling you fat.

The wings do this by essentially changing the pressure and direction of the air as it crashes into them. There seems to be a lot of debate around how this works exactly. The general misconception goes something like this: as the air rushes past the top curved surface of the plane wing, it has a further distance to travel, and therefore travels at a higher speed so that it can cover move distance in the same amount of time. Then according to Bernoulli’s principle, the faster-moving air is at a lower pressure than the air beneath the wing, creating a pressure difference above and below, which produces the required lift.

You will often see this idea being thrown around on websites and even textbooks, which causes a lot of confusion. Known as the ‘equal transit theory’, its error lies in the idea that the two paths of air above and below the wing must keep ‘in step’. There’s no reason why our air molecules must arrive at the tail of the wing at the exact same time.

So what’s the correct explanation?

airfoil
Image courtesy of imgur.com

As air flows above the wing, the curvature will cause the air to be dragged down towards it. Since this air now occupies a larger volume, the pressure decreases. The opposite happens on the other side of the wing, so now we have a pressure difference which provides the lift. Using Bernoulli’s principle again, we can see that the speed of air above the wing is faster than the air below. This faster air accelerates downwards from the tail of the wing, which provides another important form of lift. Clearly there are more factors to a plane’s’ flight than the aforementioned, but that is fundamentally and most basically how a plane operates.

So next time you’re bathing in the sun at some tropical resort and you spot a plane pass overhead… nah, screw it. Just relax and have a good time.

Harvey

 

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