Some of the concepts that we have to grasp in aviation are fairly straightforward. If you nose the plane over towards the ground, you tend to speed up. Wind blows you sideways, so you have to correct for that drift by adjusting your heading into the wind. These principles are fairly predictable and easy to visualize. When I first learned about gyroscopic precession, however, there was nothing natural about visualizing it. I read about relating it to playing with a gyroscope as a kid, but I never had a gyroscope as a kid, so that didn’t help much. Here’s the definition that didn’t click, from The Pilot’s Handbook of Aeronautical Knowledge:
“Precession is the resultant action, or deflection, of a spinning rotor when a deflecting force is applied to its rim. When a force is applied, the resulting force takes effect 90 degrees ahead of and in the direction of rotation.”
It’s ok if that didn’t resonate with you. The two big areas that this understanding comes in handy with aviation are (1) the forces acting on the propeller and (2) understanding the gyroscopic instruments themselves in the airplane. I managed the book knowledge of it, but I’ve since found a few good examples of gyroscopic precession that I thought I’d share with you. So spin up, turn 90 degrees and get your applied forces ready.
If you were like me and didn’t have a gyroscope as a kid, you can go grab one at Hobby Lobby. I think it’s considered a vintage toy. It’s basically a disc that spins on a mount in a round metal cage. By winding a string around the mount and pulling quickly, the disc spins rapidly. There are two remarkable features about what happens next. The first isn’t so surprising, but it’s called “rigidity in space.” Basically, it’ll stay upright like a spinning top or coin. Not surprising and fairly easy to visualize, right? The next part happens when you tilt the gyroscope in different directions. In a very unexpected fashion, the spinning creates pulling forces in seemingly unnatural directions. You tip it forward and it pulls sideways. You tip it sideways and it pulls forwards. Magic? Pretty much. It’s precession. Let’s keep exploring.
The Bicycle Tire
Ok, so with the gyroscope, one might just reason that “it’s built to do that.” Maybe it doesn’t make a ton of sense, but you can write it off to it being designed that way – a clever gadget. So let’s take an everyday item like a bike tire. The bike tire is tied by one side of its axle to a rope hanging from the ceiling. Sounds like a regular Friday night to me. When it’s still, the wheel just hangs there horizontally, as if it were laying on the ground. Spinning the wheel up, however, activates the magic of science, and the wheel spins upright as if the bike were standing up. Rigidity in space keeps the wheel upright and precession slowly rotates the wheel. Don’t believe it? Watch the video here to see it in action.
The Fidget Spinner
Fidget spinners are the slap bracelets of today; every kid has to have one. They seem endlessly entertaining to do nothing but spin. In a modern world, it seems like a throwback to my grandpa telling me that all he had to play with as a kid was a stick and a tire. Sounds like some precession in the making to me. My eight year old obtained one of these fidget spinners, so I had to see what it was all about. I grabbed the center and spun it up. Yep, it spins. But when I went to put it down, I noticed that it pulled in a funny direction. I tilted it forward and it pulled to the side. I tilted it to the side and it pulled forward. It’s as if it were 90 degrees ahead of and in the direction of rotation. That sneaky precession. This thing’s got the magic, too. A few days later, I interacted with three kids that all had one. I found myself telling them about precession and how if they ever get their pilot’s license they’ll learn about it more. They seemed genuinely amazed by it, but I’m sure I looked like the goofy dad blabbing on about how cool science is.
So what does all this have to do with the airplane? Well, by understanding these real world examples (meaning go snatch your kid’s fidget spinner), you can better understand the forces that happen to a spinning propeller on the front of an airplane. The propeller is basically like a fidget spinner, or a bike tire, or a gyroscope. It’s a lot sharper and more aerodynamically efficient, of course. But when it’s spinning, the movement of the airplane will cause the propeller to precess, creating a force that you might not have expected. When you pull back to enter a climb, or nose over to begin a descent, that precession force will occur, pulling you right or left. Yawing the nose left or right will similarly cause up and down pressures that must be considered the stronger they get. As an example, consider a tailwheel airplane on its takeoff roll. At a point the tail raises up. This is like a force being applied at the top of the spinning propeller, in the direction of the airplane. Given that the propeller rotates clockwise as the pilot sees it, the precession force is felt 90 degrees ahead of the force, so it comes across as a yawing force to the left. A little right rudder will counteract that force. So grab your fidget spinner and make a little airplane out of your hand. Spin it up and watch that precession magic happen.
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