As a baby learns how to walk, he or she concentrates intensely on maintaining balance. As we get older, we can walk and run without thinking much about our balance. However, if you put on a pair of roller skates or jump on a skateboard, you quickly remember Newton’s laws and the effect of our center of gravity on balance.
In this picture of Jake, he has plenty of static stability while seated. If he were to stand, he would not be able to balance as well, especially at this early age.
The center of gravity, or CG, is a theoretical point in the “middle” of an airplane, or a human body, upon which we could balance the entire thing on the tip of a pencil (or any other pointy thing that could support its weight).
Here are two experiments you can try to explore static stability.
1. Standing straight up
Stand up. Your CG is directly over your feet. Have someone gently push your shoulders (not so hard that you have to move your feet). You will likely sway back a little bit and then return to your initial stance.
2. Leaning forward
Now lean forward just a little bit–perhaps you can enlist some help and get someone to take a picture of you from the side. That way, you can see how far forward you are leaning. You should lean far enough forward that you cannot lean forward anymore without falling over or losing your balance–in other words, if you lean any farther forward, you will have to move your feet to keep from teetering over. Notice how your weight shifts–before it was evenly distributed on your whole foot, but now, your weight is on the balls of your feet.
What would happen if someone nudged you, gently pushed you in the back? You would fall over. This is an example of negative static stability.
Static stability is the initial tendency to return to the original state, and the illustration below shows the stability of a ball on a smooth surface.
In the first case, the ball is at rest in the bottom of the bowl. If you nudge the ball, it rolls back towards the bottom of the bowl and will eventually come to a stop–this is an example of positive static stability. In the second case, the ball is on a flat table. If you move it, it will not move back to its original spot. The third case corresponds with when you were leaning forward. If you nudge the ball, it will roll off the top of the bowl, just as you would have fallen over if someone nudged you.
The Porch Swing
One last thought experiment for you to understand the effect of center of gravity. Find a porch swing that you can sit on all by yourself. Sit on the far left or right of the swing (not the middle) and push yourself really good. You will notice that the swing oscillates, especially when it gets to the top of its swing arc. Now redo the experiment but sit in the middle. The oscillation is gone. When you don’t sit in the middle of the swing, the CG is not in the center of the swing, and this is what causes the slight oscillation at the top of the swing arc.
All of these phenomena occur in airplanes as well, but there are more degrees of freedom, more axes of motion, which adds layers of complexity. Read more about longitudinal static stability on wikipedia, though a google image search might prove more helpful.
For More Exploration
Additional ways to explore stability and CG include scooters, skateboards, bicycles, etc. Be sure to do everything with caution and the proper safety equipment though.
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Don’t just imagine your dreams–explore them, because we need you. The aerospace industry needs innovators. The flight test community is looking for the next Neil Armstrong, and that’s what this column is about, helping you take that next small step.
Thanks for reading Launch Your Flight Test Career #17. Send a message to @FlightTestFact on Twitter to ask questions about launching your flight test career.
