Friday, February 24, 2012

Ground Resonance

I am a pilot so nearly all things aviation interest me to some degree. I also have a degree in Physics, so the physical aspect of nature is a constant source of curiosity for me. One of the most interesting aspects of nature is Harmonic Resonance.

First things first, we need to look a a wave:

The important part of this wave is the amplitude. The amplitude where the wave stores its energy. Think of a wave in the ocean, breaking on the shore. A small wave doesn't have much power so it breaks quietly and isn't very exciting. Then you head out to the North Shore of Oahu in Hawaii and you get HUGE waves that crash with great intensity. What is the difference between the waves? They might be the same in every way possible, but the North Shore waves have greater amplitude, they are bigger, and thus have more energy.
Another example that we will keep going to will be sound, or more specifically music. A sound can be soft or loud. It may be the exact same sound. Think of the volume on your radio. If you want the sound to be louder, you turn up the volume. That feeds more energy to the speakers, and thus increases the amplitude of the sound waves that are generated from the speaker.

Get it? Bigger amplitudes mean more power. Enter in Resonance.
Resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies). At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.

Thank you Wikipedia. Anyway, what that means is that there are certain frequencies, different for all things, that can cause the vibrations to get bigger and bigger. Think of blowing air over an open bottle. Unless you get the direction just right, nothing happens. But, if you hit that one special area, the air will vibrate the bottle just so that a sound is formed. The construction of the bottle, and how full the bottle is will determine the pitch and tone of the sound, but what you are doing is hitting that special frequency where the bottle vibrates and the waves are able to hit that harmonic resonance. This is not always a fun thing.

We have all heard the story of an opera singer breaking a crystal glass with only her voice. This is actually true. They crystal glass is called "crystal" because the material in the glass has bonded in a very special uniform way. Because it is uniform, it has a common harmonic resonance frequency. Now, the crystal will vibrate along with whatever sound wave is near it. If the wave is not the harmonic frequency, nothing really happens. However, when the singer hits that very special frequency, the sound waves cause the crystal to vibrate at the same frequency, and, because at this special frequency, the energy of the wave is stored in the glass, the amplitude of the wave gets bigger and bigger until BANG, the glass shatters. It is very cool to see.

It turns out that these resonance frequencies are also found in... well everything. It is just a matter of finding the frequency and keeping it going until the wave has time enough to gain enough energy to make whatever it is that is vibrating shake apart. A perfect example of this is "Galloping Gertie."

Opened in July of 1940, the Tacoma Narrows Bridge was to be a great economic boon to the Tacoma area. However, as soon as the bridge opened up, people stared noticing that when the wind blew, the bridge had a tendency to rock back and forth. Then, on November 7th 1940, the wind hit caused the bridge to sway at a very special frequency. The bridge's harmonic frequency was hit, and the wave began to grow and grow in amplitude until it tore itself apart. You can watch it happen:

Coming back to aviation. In primary flight training, every pilot learns about a neat phenomena called "Ground Effect." When an aircraft is close to the ground the air can literally "bounce" in between the ground and the aircraft's wings. This adds lift, and can be really fun to play with if you are in a fixed wing aircraft (air plane). However if you are in a rotating wing aircraft (helicopter), it opens up an entirely new problem. You see, the helicopter has harmonic frequencies, just like anything else in nature. If that frequency is reached, such as in ground effect, the aircraft can literally be torn apart. Fortunately there is an easy solution, simply increase power and lift off. Once the frequency is changed the destructive wave dies out and all is good with the world. But what happens if the pilot freezes? This:

Here the pilot makes the wrong choice and his machine is forfeit. Luckily everyone survived. So, let's see an example of a good pilot making the correct choice. For that, as for all good things in life, we turn to MacGyver!

This pilot, realizing what was going on, doesn't bother with closing his door or anything else, he gets the hell out of Dodge, and saves his machine, his life, and the lives of the actors and crew on the ground. This pilot gets an A+ for being awesome!!

Resonance is a fascinating topic, and I have been interested in it ever since I first learned about it. To put your mind at ease, while your body DOES have a resonant frequency, your body is such a complex system that is is very unlikely that you could shake apart from the wind blowing or from some opera singer hitting the proper note. Which is why we don't hear stories of people walking outside during a windy day and exploding. That might be cool to see though...

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