While established in a turn must you maintain rudder
pressure to keep a turn coordinated?
I think this question goes to the fundamental issue of what happens when an aircraft "turns", and I'm always surprised that the process is not made more explicit in material dealing with aircraft stability and control. Of course that may be because I have it all wrong, but here we go...
Start with two preliminaries:
1) There's a difference between
turning and
yawing, which I'd like to make explicit for the purpose of this discussion. When a moving object
turns, its velocity vector (the direction in which the rigid body is moving) changes direction. When an aircraft
yaws, its heading (the direction in which the rigid body is pointing) changes direction. Because aircraft are generally very directionally stable, we tend to think of the two as a single action -- but they're not. In principle a body can yaw without turning (picture a spacecraft like the Apollo command module manoeuvring through 180 degrees to dock with something that was behind it while continuing at high velocity towards the moon), or turn without yawing (picture an object bouncing off a brick wall). The difference between the direction an aircraft is moving and the direction in which it is heading is the sideslip angle.
2) What does the fin do? It applies a yawing moment to the aircraft. When? Well two subtly different circumstances, in fact. a) If the aircraft is in a sideslip, the fin gets an angle of attack and applies a yawing moment. The size of the yawing moment is proportional to the sideslip angle. b) If the aircraft yaws, the fin also gets an angle of attack and applies a yawing moment. If you imagine the aircraft yawing left about its centre of mass, the fin moves right. That means that the airflow is no longer aligned with the fin, so there's an angle of attack. The yawing moment comes out as proportional to the rate of yaw, divided by the airspeed and multiplied by the distance of the fin from the centre of mass. The direction of the yawing moment is against the yaw that's causing it.
OK, now the process of turning. Let's go left, starting from a track of 180, heading of 180.
So we bank the wings to the left and tilt the lift vector. We now have an unbalanced force to the left, so the aircraft accelerates left. It
turns. But I haven't given it a reason to
yaw yet! It now has a track of, say, 175, and a heading still of 180. That means it's in a sideslip to the left. So the fin starts doing its job and applies a yawing moment to the left. And the aircraft starts to
yaw to the left. How quickly? Well remember b) above. The fin also applies a yawing moment to the right, because it now has a rate of yaw to the left. The steady state comes where the rate of turn is equal to the rate of yaw and the moments balance each other out. In an equation,
sideslip_angle = rate_of_yaw * fin_moment_arm / airspeed
So although the aircraft does yaw in the direction of its turn, the change in heading lags the change in the velocity vector slightly, by the sideslip_angle above. Instead of slipping, you can of course apply into-turn rudder to balance the right hand side.
How big is the effect? rate_of_yaw = 3 deg/s, fin_moment_arm = 10 m, airspeed = 100 m/s gives a sideslip angle of 0.3 degrees. Not a lot, but sufficient that if you don't add a little rudder, you will slip slightly.
Why do we notice different effects on different aircraft? As you can see from the above, a slow and long aircraft (e.g. a glider) that turns rapidly sees a much greater sideslip angle than a fast, short aircraft turning at even the same rate.
Hope that makes sense.