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It's floating on air!

By Jackie Valentine, Exhibits Experience Lead at the Fleet Science Center.

 

If you’ve ever been outside on a really windy day, you know that air can be pretty powerful when it is moving fast enough.  If you aren’t careful, a strong enough gust of wind can even knock you over.  If you were somehow able to create current with your body, you might be able to knock other people over, too (although that wouldn’t be very nice).  Or if your wind powers were strong enough and if you aimed them at the ground, you might be able to float up.  Since the Earth is too massive to be threatened by your puny mortal wind powers, you’d be the one moving this time.  A hovercraft is a vehicle that harnesses that power of air pressure to float the surface it wants to travel over.   
 
All around you, there is air.  Look to your left, air.  Look to your right, air.  Look under your bed, air!   
 
We call this wonderful air-filled layer on the surface of our planet our Atmosphere.  An easy way to imagine how it works is by thinking of it as an ocean of air of which we live at the bottom. When you swim to the bottom of a pool or maybe when you go snorkeling and swim down deep to the sand at the bottom, you’ll feel extra pressure as you go deeper and deeper. This pressure might make you feel like you’re getting squeezed tighter by the water or it might make your eyes or ears hurt a little bit. As you go down, there’s more and more water above you. Since water is heavy, this pile of water is pretty hard to miss as you dive deeper and deeper.  It only takes a few feet to notice a difference because of how much water weighs.  But at the bottom of our ocean of air, we feel fine for two big reasons.  
1. Air is about 104 times lighter than water.  
2. Our bodies are built for living down here.    
 
At sea level, the air pressure around us is about 14.7 Pounds-Per-Square-Inch or PSI (or roughly a heavy bowling ball’s weight on every inch of every surface, including your body, coming from every direction at once). This pressure is normal for us and we don’t notice anything weird because we are born down here and, unless we climb a high mountain like Mount Everest (where the air pressure is about a third of ours down here), we live our whole lives here never thinking anything of it. Most of the time, this air pressure is balanced all around.  But when a strong wind blows against you, one side of your body has more air pressure pushing on it than the other.  The lower air pressure loses and you can feel the higher pressure air trying to knock you over.   
 
A hovercraft uses an imbalance of air pressure to make itself float above a surface.  It takes air and shoots it under the bottom of itself, kind of like a windy day.  With this fast-moving air hitting the ground, it creates a more significant amount of pressure around it. The higher air pressure pushes down on the top of the hovercraft and up on the bottom part, making its way out from under the hovercraft as it escapes.  The result is that the hovercraft is floating on the space created by this high-pressure air cushion. 
 
Believe it or not, you can make a simple hovercraft of your own using some simple stuff like a balloon, a CD and a bottle cap. When you blow up a party balloon you put about 1 more pound of pressure per square inch inside the balloon than the regular air outside the balloon.  If you attach this balloon to the hole in the middle of a cd you’ll send that higher pressure air under it creating a nice cushion of air to glide on.   
 
If you want to get more into the math of it think of it like this: The surface area of a flat side of a cd is about 17 square inches. All over that cd, the air is pushing on all sides with 14.7 pounds of pressure. It’s pushing up from the bottom, down from the top and in from the sides all with the same 14.7 pounds. In this air pressure contest, the air from any direction is tied for the most pressure with all the other air from all the other directions.   But, if you created an area under the CD where every square inch had 15.6 pounds of pressure instead; then all at once, the entire 17 inches of the bottom side would be pushing up about 6% more pressure than the top would be pushing down. This doesn’t seem like a lot but that would be about 14 more pounds total of pressure going up instead of down.  
 
Now, remember these numbers are just for illustration and are leaving out how much pressure would be created by the pressure on the outside of the balloon and how much pressure would be made by gravity pulling down on the whole hovercraft. But you get the idea. 
 
If you want to try building this simple type of hovercraft, you can! Check out our video about it right here, and don’t forget to subscribe to be in the know about other great projects you can make from home.
 

 

Blog Type: 
Science With the Fleet