I've never understood the purpose of yawing, i.e., rotating around the vertical axis by activating the rudder. Provided that airplanes have ailerons that allow banking (used for turning), why do they actually need to yaw? Furthermore, I've read that yawing induces an undesired banking because of the different lifts generated by each wing, due to the fact that one wing "feels" a higher airspeed than the other (a similar effect as a car's wheel spinning faster than the other during a turn). So, if yawing is so "useless" or bothering, why do airplanes have rudders? Moreover, could an airplane takeoff, fly and land without rudder?
From a totally practical standpoint, let's say you're in a small tricycle gear aircraft with a freely castoring nose wheel (or a tail dragger) and you're starting your takeoff roll in a calm wind. When you bring the power up, the aircraft is going to want to turn left (why is another question), in other words to yaw about its vertical axis in a counter-clockwise direction, and head for the grass off the left side of the runway. You counteract that adverse yaw by using right rudder to keep the airplane straight. Initially you might have to use some right brake as well until there's sufficient air moving past the rudder. If you had a steerable nose gear, you wouldn't need the right brake.
You reach flying speed, apply up elevator, and lift off. As the airplane rotates about its lateral axis, there'll be some additional adverse yaw as a result of that rotation (again another question). Even if you had a steerable nose gear, that would now be useless as the nose gear is no longer on the ground. Your means of continuing to fly straight ahead is to counter the adverse yaw with rudder, and you would continue to use right rudder as you climb out. If you don't, you'll start turning left.
You reach the altitude you want, level off, and trim for level flight straight ahead. Then you decide you want to turn left. You start the turn briskly with left aileron, but you forgot to use any rudder. If you were looking at the ball on the bottom of the turn and bank indicator, you would see it shoot left. If you had used left rudder while initiating the turn, that wouldn't have happened. You would have started a coordinated turn.
Now you're coming back to the airport, and there's a crosswind from the left. You decide you want to counter that crosswind by slipping into it. In other words, you want the airplane to travel sideways at a speed that matches the crosswind while still having your nose pointing straight down the runway. So, you lower the left wing and to keep the airplane from turning, you use right rudder to keep the airplane aligned with the runway. You match the crosswind by adding or decreasing your bank.
There are other uses of rudder, and especially in aerobatics.
I once flew a few loads of skydivers. The jumpmaster sat behind me, and to help him in positioning for the drop zone, he wanted flat turns. In other words, he wanted the airplane turned while keeping the wings level. So, if he wanted a left turn, that meant left rudder and right aileron. Uncomfortable from a pilot's standpoint.
Finally, back in the 1960s when I was working on my instrument rating, I had an instructor who taught that the best way to track a localizer in close was to use rudder only for small corrections. Personally, I don't like that method, but it's one way of doing things.
A useful thing, the rudder. You need to be able to directly control movement about all three aircraft axes.
The purpose of yawing is, as you note, to provide a way to rotate the airplane around a vertical axis. Ailerons can roll the airplane around a longitudinal axis and produce a slipping turn, and turning with the rudder alone produces a skidding turn, but proper turning is a combination of both aileron and rudder inputs and produces a coordinated turn.
Turning with aileron alone creates adverse yaw when the rising wing provides more lift and more drag than the lowering wing, causing a yawing moment away from the intended direction of turning. Additionally, if you want to maintain effective gravity downward in your reference frame, you need a yaw moment and rudder is what provides that.
There are many other situations where rudder is essential to flight
- Correcting for p-factor turning tendencies
- Takeoff and landing in crosswinds
- Spin recovery
- Controllability in stalls to prevent spin entry
- Engine failure in multi-engine airplanes
If rudder were useless, we wouldn't have them. There are airplanes without rudder pedals, e.g. the ercoupe but this plane still has rudders that act in accordance with the ailerons through turning the yoke to provide the necessary forces for coordinated turns.
The primary purpose of yawing is to stop yawing.
Airplanes with long wings have a tendency called adverse yaw. Adverse yaw causes the plane to yaw away from the direction of turn when attempting to turn using ailerons and elevator. The more you try to turn left, the more the nose point right.
The big problem with adverse yaw (apart from it being potentially confusing to pilots) is that while the lift from the wings held at a bank angle is trying to turn the plane in one direction, the nose is going in the wrong direction eventually causing the plane to stall. This can make turning impossible.
Early aerodynamicists didn't fully appreciate adverse yaw because experiments with model planes tended to have the plane fly straight. It's not until you attempt a turn that you'd encounter adverse yaw. Even early glider pioneers didn't fully appreciate it much because a lot of the early hill hopping glider flights were also in straight lines.
Among the first people to encounter and attempt to solve the problem of adverse yaw was the Wright brothers. Taking inspiration from other early glider pioneers and large kite fliers their early gliders didn't have a rudder. They found that it was unstable when the wings tip to either side (bank). What they experienced was adverse yaw. Their solution was to add a vertical stabilizer and it solved the problem for the most part. Later they made the rudder movable to allow the pilot to further correct for adverse yaw manually. We still teach this "correction" maneuver - it's called the coordinated turn.
A turn performed with just the ailerons and elevator will not be "coordinated". This is to say, the apparent G force will not go straight "down" through the aircraft's floor, which is desirable for various reasons. Among them is passenger comfort (being pulled down is less unpleasant than being pulled sideways), but there are other advantages too.
In addition to making coordinated turns, the rudder is useful to correct yaw induced by gusts of wind, to set up and maintain a wing down attitude in a sideslip crosswind landing, to quickly lose altitude without picking up lots of speed (somewhat notably used by the Gimli Glider), and for various aerobatic maneuvers.
To my knowledge, at least during combat/ww2 Using the rudder to yaw in that way was often applied by pilots who were weary of being attacked from behind. Adding a small amount of rudder/aileron to yaw, meant that instead of flying straight and level, the aircraft would sideslip through the air, therefore throwing off any enemy fighters aim/Bullets. Any combat airman will tell you, never ever fly straight and level in a combat zone...
There are two main reasons: control on the ground and efficiency in flight.
When an aircraft is rolling on the ground, the only way to control its direction is with the rudder. When a pilot both takes off and lands an aircraft they use the rudder to steer. If the aircraft had no rudder, it would likely roll off the runway.
In flight, the rudder is used to keep the fuselage parallel to the line of travel. For example, if the pilot makes a turn without using the rudder air will hit the side of the aircraft and the drag on the plane will be significantly higher.
Another cause of inefficiency in propeller aircraft is what is known as P-factor. This is a yawing of the aircraft due to asymmetrical force in the propeller causing drag. The pilot must hold down the rudder constantly to prevent this. It can become quite tiring to do so in long flights.