Barriers are Meant to be Broken

In some ways, my travel through Saudi Arabia brings me shockingly back to a completely unrelated experience that I had many years ago in the states. Here is my journal entry from that day, so long ago.

This weekend I had the opportunity to go to a military air show. My friend, Jane, invited me to watch her husband participating as a pilot. We anxiously sat, finding ourselves seated next to a friendly retired air force pilot and his wife, Rob and Karen. I quickly learned of Rob's love for airplanes as he wanted to share his vast knowledge of the science and history behind the planes and how they were able to break the sound barrier.

Rob and Karen

Rob said, “In 1935, due to many difficulties of flying at supersonic speeds the phrase, “sound barrier”, was born because pilots and scientists thought of it as a barrier that could not be overcome (Taylor Redd, 2012).”

He continued, “The speed that sound travels is called the sound barrier. Why would speed be called a barrier? It is because as the plane travels faster than the speed of sound it produces noise that emanates from all parts of the plane. Due to a Doppler Effect, as the plane travels, the waves in front of the plane are crowded together”(Taylor Redd, 2012).  The illustration below demonstrates this principle.

The sound barrier is an unseen barrier caused by air pressure

I have summarized the rest of what he explained:

As the plane reaches the speed of sound the waves will begin to pile up on each other causing the air to be compressed. This air compression creates a force on the plane and impedes it. The air pressure is a greater force on the plane than the force caused by the plane, thus, there is what is called an “aerodynamic drag” on the plane which causes it to break through the sound barrier. When objects break through this sound barrier it is called supersonic (as seen in the figures above).

The breaking of the sound barrier is very similar to that of a speed boat passing through water. As it travels through the water, the boat pushes the water out of the way and a wave crest is formed and stretches throughout the lake as it continues to travel, leaving a cone shape behind it. The wave front caused by the boat is similar to the sonic boom caused by an airplane breaking the sound barrier. Just as the boat creates a conic shape as it travels, an object flying in the air will create the same conic shape stretching out starting from the nose of the plane towards the back, as seen in the pictures below (2001).

During the air show I witnessed airplanes traveling at extreme speeds. These speeds build up waves causing a large amount of pressure on the wave front, which is known as a shock wave. As the wave front passes the plane it causes a dramatic change in pressure and produces what is known as a “sonic boom”, which is audible to hear (2001).

Something called a mach number is how to measure how fast an airplane is traveling regarding the speed of sound. Simply put, the mach number is the airplane’s velocity divided by the speed of sound at a certain altitude. The ratio is:

When an airplane is traveling slower than the speed of sound it is called subsonic (M<1). When the airplane is traveling faster than the speed of sound it is flying at supersonic speeds (M>1). In order to break the sound barrier a plane has to travel at speed greater than one mach (Alexander, 2003). 

Breaking the sound barrier can be damaging to the plane because of the intense, pressure change caused by the sonic boom. As the plane reaches the sonic boom, violent turbulence occurs.

Rob informed me that in the early 1930’s when first attempting to break the sound barrier, planes had great difficulty because of the turbulence that preceded the sonic boom, causing propeller problems with the plane. However, modern technology and high speed aircraft have allowed planes to travel through this time of turbulence and break the sound barrier. Below is a video of what I saw today (Alexander, 2003).

Did you know that airplanes aren’t the only things that can travel faster than the speed of sound? The crack you hear from a bullet, a cannon, or the crack of the tip of a bull whip do as well (2001)!

Go see what you can find breaking the sound barrier!

That does it for today! 

-Tanya