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In a related answer, I stated

"The tail rotor of a helicopter also generates an aerodynamic sideforce. Again, just as with the twin-engine airplane with one failed engine, when the fuselage is streamlined to the airflow in forward flight, the slip-skid ball will be slightly off-center, deflected opposite to the direction of the tail rotor's sideforce-- at least in cases where the body of the helicopter tends to tilt along with the disk of the main rotor. If the disk of the main rotor can be tilted to counter the tail rotor's sideforce while the fuselage or body stays completely vertical, then the slip-skid ball can stay centered even when the fuselage or body is completely streamlined to the airflow."

I now ask:

In cruising flight in a helicopter with a tail rotor, is it ever the case that the rotor disk may be tilted to counter the tail rotor's sideforce while the fuselage or body stays completely vertical, thus allowing the slip-skid ball to stay exactly centered at the same time that the yaw string is centered, i.e. the fuselage is fully streamlined to the airflow?

Basically this is a question about how the main rotor disk system is attached to a helicopter and whether there is some way to keep the main rotor disk constantly tilted while the body stays exactly upright, in the context of a helicopter with a tail rotor.

Is this something that is possible in some helicopters with tail rotors but not others? If so, what are some examples of helicopters where this is possible, and what is the mechanism or other design feature that makes it possible? Can the pilot somehow adjust the "tilt" (in the left and right sense) of the fuselage or body relative to the main rotor disk at will, and maintain any desired "tilt" angle for as long as he or she desires, without altering the physical relationship between the orientation of the main rotor disk and the horizon or earth?

I suppose another possibility is that the main rotor disk system is permanently attached to the fuselage at an angle that is offset (tilted to the left or right) from the fuselage to allow the fuselage to be closer to upright in cruising flight. Is this ever done? And is the offset ever enough to allow the slip-skid ball to be fully centered, or just closer to centered?

I realize that some (nearly all?) helicopters have either flapping or teetering blades, so maybe the angle between the disk of the main rotor system and the fuselage should not be imagined to be a definable design feature. Still, my impression is that there is more going on than the fuselage just "hanging" freely from the main rotor-- see for example concepts in this related answer. So it seems in general, in cruising flight, if the disk of the main rotor blade system is tilted to one side relative to the horizon in the left-right sense, the fuselage generally is too, with the direction of tilt being away from the lowered side of the rotor disk. In other words it seems the fuselage tends to tilt relative to the horizon in the direction that reduces the amount of tilt between the fuselage and the main rotor disk, in the left-right sense, just as generally happens in the fore-and-aft-sense. I guess I'm looking for exceptions to this rule, especially in the context of cruising flight, understanding that the disk of the main rotor must be tilted to offset the sideforce from the tail rotor.

An additional point is that even if the fuselage could be envisioned to be simply hanging freely below the disk of the main rotor blade, the sidethrust from the tail rotor would still have some tendency to tilt the fuselage relative to the main rotor disk, if the tail rotor thrust is acting significantly above or below the top of the rotor mast. In cruising flight, it appears that the fuselage would tend to be tilted toward the low side of the main rotor disk if the tail rotor was above the top of the main rotor mast, and the fuselage would tend to be tilted toward the high side of the main rotor disk if the tail rotor was below the top of the main rotor mast.

Perhaps if the tail rotor were high enough above the top of the main rotor mast, everything could balance out and the helicopter could cruise with the fuselage perfectly upright relative to the horizon in the left-right sense, even as the main rotor disk was tilted to one side to balance out the sideforce from the tail rotor?

Restating the question once more, "Are there helicopters with tail rotors that can cruise with the fuselage perfectly level in the left-right sense at the same time that the yaw string is fully centered, and if so, what specific design features allow this?"

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  • $\begingroup$ This answer deals with the dynamics of helicopter blade systems but not in a way that answers my specific question-- aviation.stackexchange.com/a/38904/34686 $\endgroup$ – quiet flyer Apr 16 at 19:51
  • $\begingroup$ Yes the tail rotor's lateral thrust is offset by an opposing tilt in the main rotor that the pilot applies without realizing it, so if the tail rotor is pushing right, the main rotor will be tilted slightly left. The machine just hangs below at whatever angle gets its C of G under the mast laterally. $\endgroup$ – John K Apr 16 at 20:01
  • $\begingroup$ @JohnK -- third-to-last and second-to-last paragraphs now may better inform the question in light of your comment. $\endgroup$ – quiet flyer Apr 16 at 20:30
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    $\begingroup$ The tail rotor also imparts a rolling torque to the airframe because on most machines the thrust line is above the body center of mass. If you do a "bunt" in a helicopter, a zero G pushover, the tail rotor will try to roll the body under the rotor once the pendulum effect is gone.Google "mast bumping" to learn how this causes lots of grief in turbulence with teetering rotor helicopters. Articulated rotor machines just hit the blade flap stops and it's not so bad. There are so many different forces and moments going on, with this big pendulous hunk of metal below it, I think the answer is no. $\endgroup$ – John K Apr 16 at 20:54
  • $\begingroup$ Interesting video on mast bumping - youtube.com/watch?v=_QkOpH2e6tM $\endgroup$ – quiet flyer Apr 16 at 21:17
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In steady flight (no acceleration) the slip ball position is offset by roll only. So this question boils down to this: can a helicopter fly with 0 sideslip (streamlined) and 0 roll (wings level)?

As we know the tail rotor provides a lateral thrust force that must be countered. The main rotor is often tilted slightly laterally to counter tail rotor thrust. However, this will NOT result in trimmed flight with precisely 0 roll AND 0 sideslip at all speeds. One problem is that the amount of tail rotor thrust differs with airspeed, and hence the amount of main rotor tilt + flapping would need to differ as well. To fly (streamlined) with 0 sideslip some amount of roll will typically be required, depending on the airspeed. That roll will offset the slip ball, if only by a small amount (pilot's may not even notice the difference between wings level / centered slip ball and 0 sideslip).

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