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?
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basically a coordinated turn is one where the pilot balances the various forces on the plane (bank angle, rate of turn, etc) such that the overall forces on the plane balance out to the point it doesnt really *feel* like your turning, at least until you look out the window and see the turn happening. your inner ear isn’t picking up the turn because theirs no change in the apparent motion of your body.
as you say, they dont always achieve it, but generally, its what thier supposed to aim for.
Do you remember the term centrifugal force? We can have a whole discussion about why it’s called a fictitious force later if you’d like, but essentially, things accelerating (in physics, a change of direction is still considered acceleration even if you’re not changing speed. We can talk about that too if you’d like) feel a new force
No idea which came first, the centrifuge or the force with its name, but you can imagine a centrifuge which spins reeeeeally fast to push everything outside. Just like when you take a sharp corner at speed, you feel a force pulling you to the outside of the turn. Turn left, you get pulled right. Turn right, you get pulled left. It’s your body trying to stay on the corse it’s already on, but the car is moving underneath you… more or less.
We (by we, I mean all things that have mass) have no detector that can pull out the force of gravity (sigh, this is going to take all day if I keep offering to talk about things I’m more depth… gravity isn’t really a force either, just ask if you’re curious) from any other forces. As far as our mass is concerned, the force that’s pushing on us is just an average of all the forces. If gravity is pulling you down and centrifugal force is pushing you to the right, then the average force is diagonal down and to the right.
But what is down anyway? We have a pretty good idea, or so we think, but we live on a sphere (ok, of for some strange reason, you don’t believe that, let’s have another discussion. But just pretend we do). Down changes depending where on Earth you are. So what is a physicist’s definition of down? Is it the direction of local gravity. Gravity always points to the center of earth (mass center, not geometric center…. whatever, just ask about anything you like at this point. I’m ready for it.) But remember, our mass doesn’t know gravity from any other force. So if you’re inside a plane without a reference of where ground is (maybe your window shade has been closed, or you’re an unconscious body of liquid fuel who doesn’t even know what the ground is), then your body can only tell you the direction of the average force. That’s down for all you know.
There’s a video I’ve seen. I’ll try and find it and link it so come back in a bit if it’s not here. A guy is inside one of those spinning carnival rides. You’re supposed to lean against the wall to stay safe, but he actually stands. And he’s standing at a super extreme angle because his ‘down’ is different than your down. When spinning a drink, the liquid always wants to go ‘down’ but down changes while it’s spinning because the majority of force comes from the spin vs gravity like normal. (Edit: (https://youtube.com/shorts/SUzeCGTSJIg?si=YF1ub0-MIarpQEHB) just FYI. It doesn’t end well, so don’t try this one at home. Edit 2: [maybe a better video](https://youtu.be/RpQx204BJ7A?si=NvYUkoVpvxniRxjf) there’s a point in here where he actually dangles by the rafters. As you might be familiar, if you hang from something, it’s really hard for you legs to go anywhere but ‘down.’ That’s why Olympic gymnasts on the hanging rings have to be in such phenomenal shape to do the things they do)
When making a turn, the plane experiences some centrifugal force which changes the direction of down. The plane tilts in order to point the bottom in the new ‘down’ direction. The fuel inside doesn’t know the difference and remains perfectly level. Ta-Da!
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.
> I’ve tried observing this myself and it didn’t always work as described.
Where did you try this? On a plane? Because early student pilots are stereotypically not so great at maintaining coordination. 😀
Starting from straight-and-level flight, if you just use the ailerons to bank the airplane, the ailerons deflecting up and down on opposite wings cause more drag on the aileron down, because it’s producing more lift and thus more drag. This is known as **adverse yaw** and tends to pull the nose in the opposite direction. To maintain coordination, the pilot (and/or the airplane systems) need to apply rudder to keep the yawing in the right way.
Uncoordinated flight in a turn is either **skidding** or **slipping**. In this case the airplane is moving somewhat left or right, presenting the side of the airplane to the airflow, resulting in a lot more drag.
Among the flight instruments, there is a **slip-skid indicator** that measures relative G-forces going left or right. In a coordinated, banked turn, this is zeroed out with the rudder.
https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/airplane_handbook/04_afh_ch3.pdf I think is the most direct, but the Pilot’s Handbook of Aeronautical Knowledge and the Airplane Flying Handbook available from the FAA have the basics of aerodynamics too: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation
A coordinated turn is a little tricky in a real plane when you’re learning. It gets much harder when you can’t experience the flight forces on your body and even harder when you have no turn coordinator (a special ball level inside the cockpit). I have no experience flying RC but I suggest just adding a little rudder into your turns, you’ll need less rudder with more bank angle.
The answers already given seem are mostly good, but I’m going to try to give a somewhat simple answer.
The main way an airplane turns is by rolling or banking (leaning to one side like a bicycle or motorcycle). Velocity is one component of the required bank angle. But how sharp the airplane is turning is important as well. A sharp turn at a lower speed can have the same bank angle as a wider turn at a higher speed.
A coordinated turn is a turn where the airplane is banked just the right amount so that the force of gravity plus the force from the acceleration of the turn add up to act straight “down” — toward the floor of the airplane. Put another way, if you were standing up in the airplane, in a coordinated turn you would feel like the floor of the airplane was level side-to-side even when the airplane was banking.
The ailerons near the wing tips are used to roll the airplane. The rudder on the tail of the airplane is used to balance the airplane to keep the turn coordinated. On most (probably all) modern airliners, the airplane will automatically adjust the rudder to keep the turn coordinated. But, the pilot can override this by pushing on the rudder pedals with their feet. There are some times (such as a side slip to loose altitude faster) where an uncoordinated condition is desirable.
Almost any time you turn you’re going to do a coordinated turn. Your bank angle is going to be dependent on the speed you’re cruising at, how quickly you need to make the turn and how much your aircraft is able to safely bank.
When flying a hang glider there is only a two axis control. When we talk of a making a coordinated turn, the more the wing will stay on a flat plane and the least altitude is lost. In an uncoordinated turn the wing wants to slide or slip sidewise losing altitude quickly. Sometimes even more so than in a dive. This can be intentional or dangerous if you’re close to the ground.
You can do this with a bucket and water. Hold it and start spinning around so where the bucket is slightly lifting up. The water will flat in the bucket. It’s centripetal force.
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