YAW - AND A SHORT PIECE OF STRING
This is all going to be old hat for other pilots. So, they have some options. (1) not read it all, and go do something useful. (2) read it and offer corrections or supplementary information. (3) read it and chuckle at what a simple mind thinks is worthwhile doing while penned up in isolation.
Once a plane gets off the ground, it’s in a three dimensional world. To help visualize that 3 dimension vehicle, get a ball point pen and draw two, little pictures on your palm. Or, on paper if you're a neatnik. Your first little picture can be the outline of the left side of an airplane, with the nose pointing to your left, to be conventional. Next to that, a little picture of looking straight down on an airplane from directly overhead, again with the nose pointing to the left. On the first picture, draw a thin, horizontal, straight line through the length of the fuselage from nose to tail, extend out the ends a bit beyond the nose and tail. Nice! That line is the longitudinal axis of the aircraft. The plane rolls left and right around that axis like a wheel and axle. Now draw a similar vertical line, straight up and down at right angeles to that longitudinal axis. Place it where the wings would be. Extend it just a bit above and below the silhouette, so the ends stick out a little bit. That is the vertical axis, and the aircraft yaws (turns) left and right around that axis. Again, like a wheel and axle.
Time to move on to the second picture. Again, draw that longitudinal axis the length of the fuselage, plus a little bit beyond both ends. And, draw a line across from wingtip to wingtip, extending the ends a little bit. That line through the wings is the horizontal axis. The plane pitches nose up and nose down around that axis.
When a plane flies, it rotates around all three axes. Sometimes around only one at a time, sometimes around two, and sometimes all three at the same time. Mankind developed and evolved on solid ground, operating in only two dimensions. Flying in three dimensions takes us out of our natural element, into territory that can be uncomfortable. Man was never intended to fly. Sorry bout that, but it’s a fundamental, biological truth. Maybe that’s why having the ability to fly has been a dream of mankind that is older than dirt? Could man ever evolve into naturally being a three dimensional critter? Probably no time soon. Only about one in 300 people are pilots, and that’s not enough to sway much geneology.
Almost all of the time, it’s best to have the longitudinal axis lined up pointing straight ahead, in the direction you want to go. If the plane is aimed at, say, to the left or right of straight ahead, it is “out of trim”, and doesn’t feel right inside the pilot’s head. Our ears are telling us that we’re heading in a different direction that what our eyes are telling us. That’s confusing. Part of learning to fly is learning to recognize and handle such confusing signals. The more aerobatically you fly, the more of those confusing signals you deal with. I try to not press the edges of the flight envelope. Instead, I tend to fly that wrinkly, narrow crease right down the middle of the envelope. I also try to stay near the middle of the sky, avoiding the edges where there are obstacles like tall buildings, towers, trees, rocks, towers, etc.
So, when the plane is out of trim relative to the vertical axis, yawing out of line with the flight path, and your head and the seat of your pants tell you that something is wrong, how does the pilot figure out (1) what’s wrong, and (2) what should I do about it? About here, OldBuffPilot Andy is jumping up and down, waving his hand in the air, and hollering, “I know! I know! “. It’s such an elementary situation to an experienced pilot that he does it automatically, without even thinking about it. “Step on the ball!”
And, of course, he would be right - if the plane was equipped with a slip indicator. A slip indicator is kind of like a carpenters’ level, only different. The glass tube in a carpenters’ level is straight. The glass tube in a slip indicator is an arc, cruving upwards on the ends, and the center is the low spot. And, instead of a bubble, the slip indicator has a steel ball. Different, but similar.
If the plane is yawed to the right, and out of line with the flight path, the ball will be to the left of center. “Stepping on the ball” means to step on the rudder pedal on the side where the ball slid to. Yawed to the right, ball slid to the left, corrective action is to step on the left rudder pedal. Simple, every pilot knows it. And almost every pilot does it.
Almost every pilot; not all. There’s an even more simple slip indicator than that tube and ball - a piece of string. And Andy knows about it. We also have (at least one) a Certified Flight Instructor on here, and he’s familar with a yaw string too.
Most single engined aircraft have the engine up in front, and the prop is pulling the aircraft through the air. It’s called a tractor configuration. The propellor is producing a strong air current called prop wash. On a single engined plane, that prop wash flows straight back over the nose cowling. BUT, the Wabbit Hunter is a pusher. The engine is behind the cockpit, turned around with the prop on the back of the engine, blowing the prop wash back. The only airflow over the nose is the slipstream. Air flows over the nose naturally - not forced. So if the plane yaws, say, to the right, the slipstream will continue to flow straight back, but the plane isn’t pointing straight into the slipstream. It’s at an angle to the right.
If you take a short piece of string, tape it to the nose of the plane where it’s easy for the pilot to see, we now have what is known as a yaw string. It will follow that off center flow of air when the plane is yawed. When flying straight, not yawed, the string flows straight back, centered. With no prop wash to overpower the slipstream, the yaw string is deadly accurate. So, on the windscreen of the Wabbit Hunter will be - - - a simple, little piece of string, a yaw string. It will be one of the primary flight instruments.
As an aside, a yaw string indicates in the reverse of the steel ball in a slip indicator. When the yaw string is off center, say, to the right, you don’t step on the string. The proper corrective action with a yaw string is to “step on the blank side”.