Dutch Rolls

[WeaverJ3Cub]

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.

I was just out in the Cub, and this was my experience.....

If I tried to keep the nose on a point (on a fixed point on the horizon) then it was not coordinated (ball-in-the-center). There's no turn coordinator in the plane, but it didn't feel coordinated to me. If I just tried to "keep the ball in the center" then yes, I made small s-turns as I went left and right. Seemed that the shallower the maximum bank, the less the nose moved, which I guess is no surprise. So....since seeing is believing....then I believe!

[somorris]

I haven't tried it again yet, but I did read a couple of noted authors on the subject yesterday. The late Bill Kershner said the ball would not be in the center in doing Dutch rolls. Bill Thomas said the Dutch roll is a coordination exercise and the airplane would stay on point and the ball centered. I think it has to get back to what RetroAcro said about the two different types of Dutch rolls. By the way, not in discussing Dutch rolls but in discussing turns, Rich Stowell in one of his books said to practice rolling into a turn with rudder and aileron with the nose on a point (he didn't say coordinated, but I interpreted it and practice it that way), then turn using the elevator to "pull the airplane around." I haven't checked, but I think that is what "Stick and Rudder" said also. It is still a little foggy!

[WLIU]

For what its worth, a competition turn, as defined in the IAC judging criteria, requires that the airplane roll to at least 60 degrees of bank without changing heading, and once the bank has been established, the heading change is done. The heading change is stopped first and then the bank is rapidly brought to zero. Coordinating the entry of exit to the turn is a points deduction.

A typical general aviation turn coordinates rudder and aileron on both turn entry and turn exit.

Fly safe,

Wes
N78PS

[WLIU]

So I went out this afternoon and flew "Dutch Rolls" trying to keep my rear end centered in the seat since my Pitts does not have a ball. Was surprised that I had to work at it, but I got the airplane to swing gently from side to side as I went from bank to bank, with the nose aimed at a far away point. Was harder than I expected and I see why you have a student do it. I had to apply the right amount of rudder, but not too much, to match the aileron as I rolled, to keep the nose pointed where I wanted. I have no doubt that my flight path was not a straight line, but a gentle "S".

Interesting change of pace from +6G -3G. Give it a fly.

Regards,

Wes
N78PS

[somorris]

Thanks Wes. That confirms my understanding that the nose does move slightly, although not a lot. It is harder to see when your flying is not as precise as a competition pilot has to be. Good information in this post and once again thank you for your (and others) help.