Fun Science: Airplanes!

I’ve spent a good amount of my summer on airplanes, so I thought I should learn a bit more about them. Also, I’ve been letting my brain go to mush with movies and video games and I need to do something educational.

Note: Wikipedia is actually a good source on science and math, so go check it out! Also, if I wrote something which is incorrect, make a note of it in the comments.

The Basics:

How do airplanes fly in the first place? Well, if you’ve taken eighth grade physics, you know that it boils down to Bernoulli’s Principle. While the teacher probably didn’t phrase it like this: ” Bernoulli’s principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy” (Wikipedia) you probably got the basic idea that the faster a fluid moves, the less static pressure it exerts. In the case of an airplane, the air moving over the airfoil (the wing) moves faster than the air below it, and you get a lift force. “But why?” asks the 2-year-old.

That is a very good question. While Bernoulli’s Principle does a good job of provided the math to describe lift, it doesn’t really explain why the air over the wing travels faster in the first place. So we have to look elsewhere for an explanation. The second most common explanation for how airplanes fly can be attributed to the genius of the master physicist Isaac Newton. As you probably also learned in eighth grade physics, Newton’s Third Law states that “When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body” (Wikipedia). In the case of the airplane, the air is forced downward by the airfoil, and it exerts an equal and opposite force on the wing, creating lift. The “angle of attack” determines the amount of lift, but interestingly enough, so does he curvature of the top of the wing.

To explain this happens, we have to go back to the Wikipedia again. The Coanda effect “is the tendency of a fluid jet to be attracted to a nearby surface” (Wikipedia). The air flowing over the top of the wing is attracted to its surface, and is deflected downwards instead of shooting straight out behind the wing. So both the top and the bottom of the airfoil create the forces that lead to lift.

These are the two most commonly stated explanations of lift when applied to aircraft, but there also three more: “the circulation-based explanation, the flow-turning or streamline-curvature explanation, and the 3D vortex-shedding explanation” (amasci). I included a bit of the flow-turning explanation when I talked about the Coanda effect, but the other two are a bit more complicated. To avoid boring people who might not be so interested in fluid dynamics, I won’t go into detail. However, if you are a budding engineer or physicist, you can take a look at the articles here:

Circulation (scroll down to the “Potential flow, the Kutta condition, and circulation” subheading)

3D vortex-shedding

And one completely wrong explanation from XKCD:

Inside the airplane:

There are a lot of interesting things going on with the airplane during flight, but I decided to focus on two things: how cabins are pressurized, how they keep the air inside the cabin clean.

Engineers are smart people, so they decided that they would use the engines that help the airplane take off to help you avoid asphyxiation. According to an excellent Air & Space article, the air which is compressed by the engines gets sent to the cabin itself. To avoid frying the passengers, planes are designed so that the air goes through a bunch of cooling machines and then a manifold, which mixes the air currently in the cabin with the new air from the engines. While the new air is coming in, an outflow valve lets air out and prevents the air pressure from getting too high.

Around five years ago, a new device was invented by BAE Systems and Quest International to replace the High-Efficiency Particulate Arrest filters most airplanes used. Called the AirManager, it uses a “non-thermal plasma (sometimes called a “cold plasma”) using a high voltage to strip electrons from some of the molecules in a gas. The plasma is confined using an electric field and the cabin air is passed through it. The free electrons disrupt the molecular bonds of any particles in the air, causing them to break up into electrically charged pieces. An electrically charged filter then traps the bits like fly paper” (Economist). Air sent through the filter once loses 99.999% of pathogens it once contained, meaning you can rest a bit easier even with that guy coughing away in the back row.

Outside the airplane:

“The Captain has turned on the fasten seatbelt sign. Please return to your seat and fasten your seatbelt until the sign has been turned off. Thank you.” Turbulence. For a lot of people, it’s the least pleasant part of flying in an airplane. Most turbulence is weather related. This may seem obvious to you if you’ve ridden through a thunderstorm, but there are lots of more minor meteorological events that can cause turbulence. One of the most simple is Clear-air turbulence. Jet streams cause a change in air speed, and as we’ve seen above, a change in air speed leads to a change in lift (thank you Bernoulli). All of the other forms of turbulence are the result of a change in air speed; what differentiates them is what causes the change. It could be the wind coming of a mountain, or even another plane (PBS).

Or it could be these guys:

The Far Side

I want to talk about something that occurs when you’re way outside of the plane. I was walking around one day and happened to look up and see a commercial airplane flying above my head. Suddenly, I was struck by that fact that I could hear it’s engines even though it was flying several thousand feet away. The noise I was hearing was generated by two main sources. First is engine noise. The compressed air coming out of the engines creates a longitudinal wave that we hear as sound. Interestingly, there is also aerodynamic noise, which is caused by the “airflow around the aircraft fuselage and control surfaces” (Wikipedia). Ultimately, this adds up to the 65 dB that I heard on my walk.

I hope you learned something about planes, and I’ll leave you with one more cartoon:

This post inspired by Charlie McDonnell’s Fun Science videos.

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