I recently watched Neil deGrasse Tyson explains modern airplanes doing turns and how the fluid level wouldn’t change inside the cabin when turning.
Video: https://m.youtube.com/watch?v=3SCxKTnC5YE
I’ve tried observing this myself and it didn’t always work as described. Reminder: you should still hang on to your drink just in case.
The comment section below the video mentions something called a “coordinated turn”.
What is it? When do pilots perform coordinated turns? Presumably a set velocity is required to achieve this for a given banking angle?
In: 2
The main way to turn an airplane left or right is by banking the aircraft. You use the ailerons, which are the devices that move up and down near the outsides of the wings, to cause one wing to produce more lift and one wing to produce less lift. This change causes the aircraft to rotate around its longitudinal axis, i.e. the direction from the tail to the nose.
This changes the direction in which the aircraft moves forward, either towards the left or the right, because the lift produced by the wings, which was formerly pointing entirely up, now points either towards the left or the right as well. In order to avoid accelerating downwards, you also have to increase the angle of attack of the wings by pulling the nose up slightly (or otherwise generate more lift). If you were only generating the same amount of lift, when you roll the plane to get it to begin turning left or right, there’s no longer enough force in the up direction to support the weight of the plane.
If all you do to turn the plane is bank using the ailerons and pull the nose up slightly (corresponding to the amount the plane is banking), you will successfully turn. The acceleration vectors that are in the up down direction and left right direction will be balanced. In the vertical direction, the force of gravity is balanced by the vertical component of lift. In the horizontal direction, the horizontal component of the lift vector (plus or minus any forces generated by the aircraft not quite traveling along its longitudinal axis, see my next few sentences) is balanced by the acceleration of the plane to follow the curved trajectory towards the left or the right. However, this doesn’t quite balance all of the forces on the plane. The side of the aircraft with the aileron pointed down, which is the side that will rotate upwards, generates more drag than the opposite side. If you imagine the plane turning to the left, the plane will be experiencing more drag on the right hand side, because in order to turn to the left the aileron on the left wing will be pointing up and the aileron on the right wing will be pointing down. This increases both the lift and the drag on the right wing. In addition, as the plane turns, it is exposing one side of the fuselage and the vertical stabilizer to a higher velocity of air than the other side. For small angles of misalignment, the difference in drag on the wings is bigger than the difference in force on the fuselage and vertical stabilizer, which tends to work in the opposite direction. On balance, rolling without rudder input causes an acceleration of the plane in the yaw axis, around the up down direction (that is, the plane tries to turn around the approximate center of its fuselage), in the opposite direction of the turn. So if you are turning to the left, the natural tendency of the plane is to yaw to the right. In order to counteract this tendency, you need to apply a rudder input to increase the force turning the plane to the left. That is, you “step on the rudder” in order to make a coordinated turn. You apply force to the rudder pedal that is on the same side of the plane as the direction you are turning.
If you do all of these things properly, you will be performing a coordinated turn. A coordinated turn is a trajectory of the aircraft where it’s turning to the left or right, but all of the forces are balanced. When this is the case, from the point of view of someone sitting in the aircraft, the force they experience is directly down into the seat. And if the pilot is managing to perform the entire maneuver in a coordinated turn, it will feel no different to the passengers, the pilots, and everything else in the plane, as straight and level flight. The water in your glass won’t tilt.
With respect to your question about whether a specific velocity is needed for a specific bank angle, the answer is mostly yes but sort of no. For a real aircraft, in order to perform a coordinated turn, you do have to be traveling at a specific speed. This is because, as I mentioned, if the airplane is banking to the left or right, the lift produced by the wings is no longer pointing straight upwards, but the plane still has to support the same amount of weight. As a result, the plane needs to produce more lift in order to accomplish the turn at a particular bank angle. It does that by increasing the angle of attack, pulling the nose up. But airplanes can only pull the nose up so far before pulling the nose back even further no longer produces additional lift. This is called stall. When the aircraft is just below its stall angle, it is producing the maximum lift it can at a given speed. If you want to produce more lift, you would have to be moving more quickly. So, in order to bank very sharply and turn rapidly, the aircraft has to be traveling at a certain minimum speed. The reason the answer is sort of no is that the aircraft can be traveling faster than this speed and still perform a coordinated turn with the same bank angle. It just doesn’t pull the nose up as much.
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