Does the rudder need to be held throughout a coordinated turn?

Question

It's widely known that to perform a coordinated turn, rudder must be turned as well as ailerons. My question is what to do after the desired bank angle is achieved?

In the Airplane Flying Handbook we read:

After the bank has been established in a medium banked turn, all pressure applied to the aileron may be relaxed. The airplane will remain at the selected bank with no further tendency to yaw since there is no longer a deflection of the ailerons. As a result, pres- sure may also be relaxed on the rudder pedals, and the rudder allowed to streamline itself with the direction of the slipstream.

So does that mean that the pedals will remain in "shifted" position as a result of the rudder remaining slightly turned?

I also wonder how this is imitated in simulators (I'm using FlightGear) as all (even expensive) simulator pedals return to "zero" position themselves?

Answer 1

When you deflect the ailerons, you increase lift on one side of the wing and decrease it on the other. This causes a parallel increase of the local induced drag where lift is increased and vice versa. The consequence of that is a yawing moment which needs to be corrected with rudder deflection.

Once your ailerons are neutral, this yawing moment goes away, but now your airplane is in a constant yawing motion which increases airspeed on the outer wing and swings the tail around, so your vertical tail sees some sideslip even if your ball is centered. The airspeed differential between inner and outer wing causes some friction drag differential which tries to pull the aircraft out of the yawing motion. Depending on the tendency of the rudder to follow a sideslip, you will need some pressure to keep the yawing motion going.

It is hard to be more specific, because details depend on the particular airplane.

In most of the airplanes I have flown, you also need to keep some aileron deflection to keep your bank angle constant. Here are two effects which go against each other, so again the details depend on the particular aircraft. One effect is caused by the airspeed difference which causes a lift difference between inner and outer wing. To correct this, you need to deflect the aileron against the bank. The other effect is caused by the different centrifugal forces of the masses of the inner and outer wing. The whole wing is moving at the same angular velocity, but centrifugal acceleration is the product of the square of the angular velocity and the radius of your motion. Since the outer wing flies at a larger radius than the inner wing, the masses of the wing will try to pull the aircraft level. This needs aileron deflection into the bank. If you are lucky, both effects cancel each other out, but normally they don't.

Answer 2

As an example, consider the Automatic Flight Control System (AFCS) of the A7-E. The A7-E is a Vietnam era light-attack jet that was deployed until being replaced by the F-18. I suspect that the system used to maintain coordinated flight hasn't changed much since then.

The AFCS is a 3-axis autopilot with control augmentation. It includes automatic pitch trim, failure monitoring, with automatic disconnect and warning. There are 8 operational modes:

1. Yaw stabilization
2. Control augmentation
3. Attitude hold
4. Heading hold
5. Heading select
6. Altitude hold
7. Automatic carrier landing system
8. Ground controlled bombing

The operation of the AFCS required both electrical and hydraulic power.

The yaw stabilization mode provided coordinated turns, with an aileron-rudder interconnect, and provided up to 5 degrees rudder trim in either direction. The coordinated turn was automatic, without pilot input. The yaw computer got its signal from a lateral accelerometer, and movement of the rudder pedals placed this signal on hold. As previously noted, once airborne you kept your feet off the pedals. A pilot could engage and disengage the yaw stabilization mode.

I went to maintenance to read the maintenance log book on the aircraft. I had a launch, and was on my way out to the flight deck. This is from my memory of over 30 years ago, but nonetheless it is fairly accurate. There were pink slips and yellow slips. Pink slips were maintenance problems that had downed the aircraft at some point. I would read these to get an overall idea of the problems, recurring and otherwise. Yellow slips were, "Hey, keep an eye on this. We haven't downed the aircraft, but ..."

The mission included unusual attitudes, and low speed flight, common in air-combat maneuvering. I looked over the log book and noticed the AFCS had a yellow sheet, and stated that the system had initiated abrupt rudder control inputs at a point in a previous flight. It was yellow because maintenance was not able to reproduce the problem. I didn't connect the dots, although you might have.

I didn't think too much of the yellow sheet because I never had problems with the AFCS system. Additionally, it was automatic and so I rarely interacted with it except for hitting the switch that engaged it.

In the dog fight I ended up on my back with 0 airspeed at around 20,000 feet. The AFCS inadvertently commanded an abrupt rudder input. A departed aircraft, with a yaw input, are the conditions necessary for a spin.

And spin I did.

The AFCS yaw stabilization is great for allowing the pilot to concentrate on other tasks other than looking at the turn needle, like looking for the enemy at your 6. One must always remember, though, that automatic systems are often clever by one half.

Answer 3

How much rudder input you need during a turn depends on the aircraft. As long as the ball is out of center, there is lateral force on the aircraft because it isn't going in the direction it is pointed and there is a lateral component to the relative wind acting on the fuselage. For some aircraft, a bit of rudder will be needed throughout a turn; for others, rudder is needed as the turn is initiated but the rudder input can be relaxed until you roll out of the turn when you may need a bit of opposite rudder.

As far as centering behavior of the rudder pedals, I think the pedals on most aircraft will tend to center themselves due to aerodynamic forces on the rudder. When the aircraft is sitting stationary on the ground, the pedals may stay where they are left. Many aircraft use the rudder pedals to steer the nosewheel (which is typically set up to caster), making pedal feel somewhat variable, depending on whether the aircraft is on the ground or in the air, and how fast it's taxiing or flying.

Answer 4

Old question, but relevant to new readers. This is a question you ask someone who has actually flown the type of aircraft you are interested in. There is no general rule because of differences in wing and tail design.

A high wing dihedralled aircraft will need significant rudder and aileron inputs while turning, but a low wing "warbird" type designed to fly right side up, inverted, on its side etc. has much less built in "righting" properties and there for will much more readily hold a bank when rolled, hence the term "bank and yank" (elevator).

Rudder does generally help coordinate the turn as the faster outside wing has more drag. But again design comes into play. Vertical tail area will help yaw the plane into the turn with or without additional rudder. So it is possible to "slip turn" the plane, as airliners do.

As far as rudders "remaining in the shifted position" believe me, you will move them if the plane isn't doing what you want it to.

So the rudder, as with all controls, is there if you need it. So are qualified instructors. Take a lesson in your type, it will be well worth the investment.