Why do planes need a roll motion?

Question

Isn't only pitching up/down and yawing right/left enough for travelling from one location to another?

Answer 1

This answer is specifically for a fixed-wing airplane, not a helicopter or multicopter, although some aspects of it will apply to them too.

What is a turn? A turn is a curvature in the flight path. This means a turn is a form of acceleration. Specifically, a centripetal acceleration-- the flight path is being continually "bent" toward the center of the turn, creating a circular path.

Newton's second law is force = mass * acceleration, or acceleration = force / mass. You can't have an acceleration unless you are exerting a force to cause that acceleration. You can't have a centripetal acceleration unless you are exerting a centripetal force-- a force vector pointing toward the center of the turn.

What part of an aircraft is designed to produce force as efficiently as possible? The wing. If you roll the aircraft into a bank, you tilt the wing's lift force to one side, and you've now a created a centripetal force as efficiently as possible-- i.e. with the least drag possible.

Instead of banking, you can also turn by yawing the fuselage sideways relative to the flight path to expose the side of the fuselage to the airflow. Essentially this makes the fuselage act as a sideways wing, generating some sideways lift or "sideforce". This method also points the thrust vector from the engine or engines sideways relative to the flight path. So this method will create some centripetal force, but it is very inefficient-- drag will be very high. It will also be uncomfortable for anyone in the plane as they will tend to be thrown against the "upwind" side of the fuselage.

Note the difference between causing or allowing just enough yaw rotation to let the nose track around the horizon and "keep up with" the changing the direction of the flight path as the flight path curves due to the centripetal force generated by the banked wing, and forcing the nose to yaw to point in a different direction than the aircraft is actually travelling through the air at any given instant, as would be required if you were trying to turn without banking at all.

A car on flat ground can turn without banking fairly efficiently but that's only because the tires usually grip the road quite well, so it only takes a very small sideways angle between the alignment of each wheel, and the instantaneous path of travel of that part of the car, to generate a strong sideways force. So you don't scrub off a lot of energy due to excess drag. Very different from dragging a fuselage sideways through the air at a large sideslip angle.

A good source for starting to learn about the basic physics of flight is John S. Denker's "See How It Flies" website . On second thought, this website is not so much designed to introduce a complete novice to the basic principles of flight, as to help someone with some flying experience and training understand the "why" of it all on deeper lever than some pilots ever manage to reach in a whole lifetime of flying!

Answer 2

Think about the airplane as a body you want to control. That object has 6 degrees of freedom, including rolling. If you remove rolling you will not be able to control airplane's roll. Imagine a situation, when an small mass is over the wing and the airplane starts to roll and you are not able to compensate it.

Saying that, the way to control yaw is to use the vertical plane, and, the vertical plane will generate roll that you will need to compensate. So yaw and roll are coupled.

On the other side, the preferred way to perform a turn in the air is using roll, not using yaw. It is more efficient and faster (in space) than using yaw

Answer 3

In addition to the use in turning, you need to realize that the atmosphere is not static. It's in constant motion, both horizontally and vertically. The vertical currents can be strong enough, and localized enough, to tilt one wing or the other*, thus causing an involuntary roll. If you don't have some means to counteract this induced roll, you now have a plane that is highly unstable and likely to crash.

*Flying a sailplane or small power plane on a day with lots of thermal activity is a constant balancing act, using small control motions to keep the wings level (or at the bank angle you want) despite what the atmosphere wants to do to you. And let's not even get into the sort of vertical currents you can get from storms...

Answer 4

Here's simple way to conceive it. An airplane turns by tilting the lift force from straight up to angled off to one side, and to do this, it has to bank.

Imagine the SpaceX rocket hovering. If it wants to move sideways, it tilts its thrust line off vertical. With the thrust line now pointed at, say, 11 O'clock instead of 12, some of the thrust is now creating a lateral force and the rocket moves sideways. To keep from descending while moving sideways, the total thrust has to be increased to compensate for the portion of thrust now working to move the body sideways. But in any case, it's the tilting of the thrust line that makes it move sideways.

The airplane's wings are making thrust (by redirecting a large package of air above and below downward) as it moves along. If you tilt the wings, it's like the rocket tilting. The thrust being created by the wings is tilted by the amount of the bank angle, and part of the wings' thrust or lift is now creating a lateral force, making the airplane move sideways.

But the airplane isn't hovering, it's moving forward while this is going on. Because it's moving forward, the sideways movement created by banking the wings results in it moving in a horizontal arc. That's a turn. Like the earlier rocket analogy, to keep from descending the airplane has to increase its total lift thrust to compensate for the thrust lost by using it to move laterally. So you have to pitch up a bit to make more total thrust (lift) from the wings as you turn or you'll descend.

Helicopters are the same. If it's hovering stationary, and you want it to move sideways, you tilt the rotor disc to angle the thrust line of the rotor. If the helicopter is flying forward while this is being done, it moves sideways and forward at the same time, creating a horizontal arc, or a turn, just like the airplane.

Turning by skidding is possible also, but is also very inefficient. Hovercraft, which are really just ground effect airplane/helicopters, turn this way because they have no choice; they simply can't bank when they need to stay so close to the surface just to stay clear of it. A hovercraft takes forever to turn because it has to skid sideways and use the lateral force of it's engine thrust to make the change in direction and this really limits their maneuverability.

However, if you drive your hovercraft on a bicycle race track with banked corners, at the right speed appropriate to the bank angle, you just have to keep driving it "straight", that is, just keeping the tail end lined up with the nose end, and the hovercraft is changing direction without having to skid. In an airplane, you bank the lifting force instead of the supporting surface and just work on keeping the tail lined up behind the nose (ball centered), and around you go. Not a perfect analogy by any means but sufficient to be able to visualize the forces at work.

There ARE times in flying where you use skidding to make changes in direction - mainly formation flying, which includes glider towing. When close to another airplane, it's not a good idea to move closer by banking toward the airplane you're formating on. You maneuver by skidding, taking advantage of the inefficiency of skidding maneuvers you might say, to able to make subtle position changes will less risk of running into your formation object. Glider pilots also learn to adjust lateral position by skids and minimal amounts of bank where needed.

Answer 5

Pitching up/down and yawing left/right would be adequate for a high wing dihedral design, were it not for the need for cross wind control. A high wing dihedral will make beautiful banked turns simply by pushing the rudder. The dihedral banks the plane away from the rudder induced slip. Great for indoors models.

Unfortunately, in a full scale outdoors (pilot in the plane) cross wind landing situation, this design would be very difficult to control without ailerons to bank the plane into the wind (in order to prevent lateral drift), while the rudder maintains ground heading.

Without ailerons, this type of plane would be easily rolled and blown sideways out of control, unless it was landed directly into the wind.

Answer 6

If a plane is moving e.g. north and one wants it to be moving northeast, one must apply an eastward force to the airplane. A plane has four kinds of forces acting upon it--thrust, drag (force parallel to motion), "lift" (force perpendicular to motion), and gravity. The direction of gravity can't be changed, and drag will be parallel to the direction the plane is moving, rather than the direction one wants it to go. Thus, the only two forces that can be used to change the direction the plane is moving (as opposed to pointing) are thrust and lift.

If one didn't turn very fast, one could change an airplane's direction of motion while keeping the wings "level" side-to-side, but one's ability to do this in most planes would be very limited because thrust is small relative to lift, and wings are designed to produce lift from relative air motion in a fore-to-aft direction, rather than sideways. Consequently, it wouldn't be possible to change the direction of thrust very much relative to the direction of motion without rendering the wings ineffective. Banking makes it possible to change a plane's direction much more effectively because rather than using a tiny portion of the relatively small thrust force for that purpose, it uses a much greater portion of a much larger lift force.

As mentioned by Quiet Flier, the side profile of the fuselage may act something like a wing with regard to air which is moving past the plane at a sideways angle, and thus provide some sideways acceleration, but since most planes' bodies aren't designed to be used in this fashion, they will be far more effective at producing force perpendicular to air movement than would be wings which are actually designed for that purpose.