Originally Posted by
RetroAcro
I like a good discussion on flight dynamics. And I would be interested in the physics explanation of how you can roll the airplane back and forth, in level flight, with the ball in the center - without s-turning to a degree. I'm still assuming that when folks say "nose on a point", they mean the airplane is not turning at all, and that the flight path remains perfectly straight. If not, please let me know as it will save much miscommunication. :-)
As long as you are banking the airplane under positive G with the ball in the center, the total lift vector of the airplane is produced solely by the wing, and will be perpendicular to the wing. This means that as soon as you start rolling off of a level attitude, that the airplane will start turning slightly, due to the developing lateral component of the lift vector due to the bank angle. If you are using shallow banks, and the airplane rolls fairly fast, and your airspeed is high, it could be that you are just not noticing the slight s-turning. But it would defy the laws of physics to fly the airplane at 1G, banking banking back and forth with the ball in the center, while keeping the airplane's heading and flight path constant. What your eyes interpret (or misinterpret) is different.
The reason I consider the nose-on-point Dutch Roll to require the ball to be off center, is that as you bank the airplane under positive G, the only way to keep the nose from beginning to turn off heading is to reduce AoA slightly, and to apply opposite rudder. This keeps the total lift vector of the airplane (which now includes fuselage lift in addition to wing lift) perpendicular to the ground, which prevents turning off heading. Opposite rudder introduces a fuselage lift vector that opposes the direction of the wing lift vector, which prevents turning.
I'll try go grab some video of these two different types of Dutch rolls in the Cub. Hopefully the visual difference will be clear. I've flown these two types and have definitely noticed s-turning while keeping the ball in the center, rolling the airplane back and forth.