I flew the Bell 47, which was the commercial result of the prototype Bell 30, designed by Art Young. It was Stanley Hiller that put paddles on it, aka the OH-23 Raven that flew from the 40's thru the Korean War. It became known as the Bell-Hiller mixing head, and is still used today in some designs like the Yamaha Rmax and Fazer.
Yes, with a flybar head you can set the static paddle pitch. Most set them at zero in relation to the flybar cage so the flybar is "neutral". But adjusting them to 1 or 1.5 degrees provides the same effect as more I-gain in a software rate controller. It causes a larger displacement of the flybar with swash input, and resists changes to the plane of rotation of the flybar when there is neutral swash input. The old timer who taught me to fly RC heli's and how to set up a flybar, showed me that you can set the paddle pitch to 5 degrees and put the helicopter in hover with a rope tied to a skid. It is so stable that you can pull on the rope and the helicopter will fight it and doesn't want to move. And this is just a straight flybar with no GPS or gyros.
It's not bypassed in the code. But it's turned off with the params with rate P, I and D all set to zero. I think this is the same setup that people that are using FBL controllers with Pixhawk are using. They set H_FLYBAR_MODE to 1, turn off the rate PID's and turn up the VFF. That lets the external FBL unit do it's job, just like a flybar.
I am going to try the same experiment I did this afternoon with the flybar, with the DFC heli and see what it does. The way I set those up before is to level the helicopter using my digital pitch gauge so the mainshaft is vertical. Then adjust the AHRS trims to get zero pitch and roll shown on the screen in the ground station. That method may not be quite accurate enough. I did definitely determine, at least with the flybar, that the discrepancy is related to those trim settings. I think there is some other dynamics there at play in flight, using the EKF state estimator for attitude. And if what the EKF state estimator thinks is desired, but actual is slightly different because of the trim value, then it shows up in the log.
That's my theory, anyway. I'm going to check it out with the DFC heli and see if it's the same result as I got with flybar. It seems that almost everybody that posts logs will have that discrepancy and problems with drifting. It would be kind of nice to figure it out, because it's not documented anywhere in helicopter tuning.