Do modern aircraft require rudder input in order to perform a coordinated turn?

Question

With today's state-of-the-art electronic systems in modern aircraft, do aircraft equipped with autopilot systems, still require pilot rudder input to keep a turn coordinated?

For the context, I'm assuming the pilot is hand-flying the aircraft so the autopilot must be at least partially disengaged to let the pilot manually initiate and perform the turn via the yoke/stick, but still make the rudder use unnecessary or optional.

For example, would an F-16 pilot still need to apply the rudder during a turn? If the military case is not generic enough, how about an airliner pilot? Or a GA aircraft like the Cirrus SR22?

Answer 1

Most transport aircraft use yaw damper systems to take care of minor rudder inputs. Most autopilot systems are actually only 2-axis - pitch and roll since the rudder's job is only to keep the tail lined up behind the nose. The yaw damper is a separate "autopilot" system and has limited authority, sufficient to deal with minor yaw disturbances, dampen dutch roll, and counter adverse yaw from the ailerons and is active all the time whether the autopilot is on or not.

With a Y/D system the only time a pilot really needs to make a rudder input in flight is if an engine quits, because the yaw damper's authority is not sufficient to counter asymmetric thrust. Generally in a jet, once through the departure profile, feet are on the floor even when hand flying. They are only on the pedals to steer the nosewheel during the takeoff and be ready in case an engine quits.

On a swept wing aircraft the yaw damper is essential; if the yaw damper is off and yaw disturbances occur, dutch roll motions can start and if the pilot tries to respond with rudder inputs he almost always can't stay in phase and things get exciting. Because of this criticality the Y/D is usually dual channel. One of the tests done on production aircraft is to induce a large yaw movement with both YD channels off to get a dutch roll going, then engage each YD channel and make sure it stops the dutch roll.

Answer 2

In my Cessna Cardinal, the rudder is cross coupled to the yoke through some kind of spring/bungey cord system (I don't have the manual handy), so I can fly with my feet off the pedals most of the time. That's from a 1968 design, keeping turns coordinated with no autopilot needed.

Answer 3

The need for rudder to coordinate a turn is directly dependent on Angle of Attack (AOA). At positive angles of attack, the down aileron is more in the relative wind than the Up aileron (due to blanking by the wing in front of the aileron). An aircraft that is at zero AOA, (like a fighter unloaded, in a zero-G ballistic arc) requires no rudder. To avoid this problem, in modern aircraft, like the F-15, for example, differential stabilator is used to mitigate this issue. The stick is mechanized so that the further aft it is, the more any lateral motion is directed to generating asymmetrical stabilator deflection, rather than aileron deflection. SO, at high AOA (assuming that stick position is an accurate indicator of AOA), when the pilot moves the stick to the side, the ailerons deflect very little or not at all, but the stabilators at the tail deflect asymmetrically.

The F-16 flight control surfaces are completely computer controlled, so pilot inputs are interpreted as commands for aircraft movement. The computer then determines, based on all known factors (AOA, Airspeed, etc.) what to do with all control surfaces (including the leading and trailing edge flaps) to get the airframe to move in the manner commanded by the flight control inputs.

I never flew the F-16, (perhaps someone who did can clarify), but my guess is that there is no need to depress the rudder pedal in the F-16 to coordinate a turn - that the computer automatically determines how much rudder, or differential stabilator, should be deflected to coordinate the requested roll rate.

Answer 4

I once spent a couple hours flying a full-motion 737 simulator with an instructor for that type. My previous flight experience comprised entirely one hour at the controls of a Cessna 180 (cruise only) and hundreds of hours of Microsoft Flight Simulator with just a joystick, so I was pretty excited to show off that I knew, in theory, what a coordinated turn is. The very first flight set us up in a situation necessitating a decisive turn to enter the traffic pattern correctly. Being somewhat overwhelmed (hand-flying a 737 in a real sim is a fairly big leap from on a desktop computer), I of course completely forgot I had rudder pedals until halfway through the turn. The moment I even brushed my foot onto one, the instructor said, and I remember this exactly, "woah there; we only use those if we want to make all the passengers sick!" We continued the session hand-flying approaches in all kinds of weather in all kinds of places. I never once needed to touch the rudder pedals again.

The modern Boeing jets all have the ability to use their yaw-damper systems to coordinate turns (although turn coordinating is physically not the same as yaw damping). The details of the implementation depend on the model. For example, 747s coordinate turns only with the flaps down, while 777s and C-17s always do.

Boeing also has a design philosophy known as the "tameness criterion", which dates back to the 707 program. This rule necessitates an aerodynamic design that allows control to be maintained with only the ailerons in asymmetric-thrust conditions, up to and including one engine out. (There are more specific restrictions that vary among models.) The rudder cannot be used at all to meet the requirement, even by the autopilot. While a turn in this condition may not strictly be "coordinated", the design of the big jets takes into account a successful turn without rudder.