As far as I know none of the aircraft I have flown have this feature. (You can turn it on in Flight Sim though!)
According to the book it's sound aerodynamics. The rudder is only needed when banking not when banked (if you know what I mean). It makes sense if you think about steep turns - in fact the author says it might be a good idea to teach steep turns before shallow ones! It also goes on to say that turn coordination is primarily with aerelon and elevator back pressure.
David,
You may be surprised...rudder influence is brought to bear in various light airplanes through interconnect mechanisms of different types. You can usually see it by sitting on the ground and moving the ailerons...you should see the rudder move a little. Conversly, push the rudder a little without touching the stick or yoke, and on airplanes which have some interconnect, you'll often see the stick or yoke moving a little, too. This doesn't mean the airplane automatically "coordinates" for you...just that it's designed to give you a little help. Not all airplanes incorporate this in their design.
The amount of rudder needed in a turn depends on the nature of the turn, as well as the type of airplane. A steep turn has different rudder needs than a shallow turn. One type of airplane may require considerable rudder into or against the turn, whereas another may require little or none.
I'm flying a type right now that requires nearly no rudder input. Feet remain on the floor most of the time; the airplane does it all.
When I was younger and had just started doing ag work (crop dusting), found myself really fighting the airplane in the turns. We would pull up out of a field to clear power lines and trees, and make a steep turn right to the stall buffet, at 75 to 150' above the ground. I found that even though the airplane would be vibrating or buffeting, others could turn considerably tighter than me. I was white-knuckled on the stick, afraid that if I pulled it in any tighter, I'd stall and roll over into a spin. I was holding considerable bottom rudder because that's what I thought the airplane needed. It didn't feel right without it.
After a quick analysis by my boss, a very experienced ag aviator, he succinctly summed up my problem. "You're being stupid."
A more detailed analysis showed that if I used top rudder, as required, in the turn, it made all the difference. Once I tried it and got better at feeling my way around the turn, I discovered that I'd been slipping and skidding through turns. Suddenly I could go a lot tighter, reduce my turn times, and didn't get the rough stall burbles and snatches like I had. We still rode the airplane around the turn close to the stall, as it's part of that kind of a turn in that kind of work, but suddenly I was much safer, no longer putting myself close to a spin-under situation.
As Chuck noted, for a normal turn, keeping the inclinometer, or skid ball in the center of your instrument, is all you're really interested in doing. Whether it takes no rudder input, top rudder, bottom rudder...is really irrelevant. Do whatever is necessary with the control to make the airplane do what you need it to do. Your drag is reduced, your climb performance enhanced, your turn made more comfortable and smoother, and it's safer if you're slow (avoid getting slow).
A good exercise for developing a feel for coordinating your rudder input against other control inputs such as roll, is to practice making roll entries. Roll left to a bank of about 30 degrees, then roll right back to the right 30 degrees. Keep a constant roll rate going back and forth, and concentrate on keeping your skid ball in the center the whole time. Once you get comfortable with that, increase the bank to 45 degrees each way, keeping a constant roll going back and forth. You'll find that after a little practice you can vary the rate of roll and change the rate and amount by which you use the rudder to keep that ball centered. Once you've got that mastered, you'll understand normal use of the rudder, and can move on.
Remember that's normal use of the rudder. The rudder has many other uses too, aside from "coordinated" flight...but for now, and for the purposes of this discussion, keeping the ball in the center of your instrument means using whatever rudder input is necessary. That could be top rudder, bottom rudder, or no rudder at all. Typically because flights don't take place on perfectly smooth days and typically because none of us are perfect, keeping the ball in the center may mean small, smooth corrections involving top, bottom, and center rudder...much of the time. Use whatever is required to make the airplane do what you want it to do.