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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?

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    $\begingroup$ Step on the ball! (Go up and try the maneuver, paying attention to the ball on your turn coordinator. If the ball remains centered, leave the rudder wherever it is. If the ball is to the right or left, apply right or left rudder accordingly. Only do this from a safe altitude and after clearing the area, and don't keep your head inside the cockpit too long without doing a proper visual scan outside! $\endgroup$ – mah Jun 11 '14 at 11:09
  • $\begingroup$ Here's an interesting read (but far beyond the scope and level of the question ) --aviation.stackexchange.com/questions/31440/… $\endgroup$ – quiet flyer Jun 25 at 12:06
  • $\begingroup$ Feet aren't that sensitive to position. To a first approximation, having the rudder pedals center themselves with springs is fine for the simulator. The rudder would only tend to be very slightly offset during a feet-off-rudder turn, since the vertical fin tends to streamline itself to the airflow. $\endgroup$ – quiet flyer Jun 25 at 12:09
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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.

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  • $\begingroup$ Originally my question was about some light aircraft (e.g. Cessna 172). It's quite clear for this plane how airflow is able to "shift" pedals during the already established turn via strings going to rudder itself. But what about say B777? Is airflow strong enough to override hydraulics that is holding the rudder in "straight" position after the pilot returns pedals to neutral position and releases any pressure on them? $\endgroup$ – HUB Jun 11 '14 at 13:49
  • $\begingroup$ @HUB: No, on an airplane with hydraulic actuation the actuators are by design strong enough to hold the rudder at any position within the envelope (full deflection up to vA, 1/3 deflection at vD). $\endgroup$ – Peter Kämpf Jun 11 '14 at 18:37
  • $\begingroup$ @PeterKämpf: Don't the pedals have feedback? I believe they do. I don't know how the yaw damper command is added though. $\endgroup$ – Jan Hudec Jun 11 '14 at 20:00
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    $\begingroup$ @PeterKämpf: Also note, that there are no FBW designs where force would be generated purely by springs and position of the control element corresponded to position of the control surface. Airbus side-stick is spring-loaded, but it's displacement corresponds to the pitch and roll rate of the aircraft, not position of the surfaces. And Boeing FBW simulates the mechanical link. $\endgroup$ – Jan Hudec Jun 12 '14 at 4:51
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    $\begingroup$ For what it may be worth, in 747-100 and -200 aircraft, operationally with all engines operating, you take your feet off the rudder pedals after liftoff and don't put them back on until short final. The yaw damper handles (by way of the hydraulics of course) all adverse yaw including any in a sustained bank. $\endgroup$ – Terry Feb 23 '15 at 4:44
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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.

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  • $\begingroup$ Nice reading! Thanks for sharing.. $\endgroup$ – gusto2 May 11 '17 at 7:26
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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.

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