Normal tendency of helicopter with CCW rotating main rotors (top view) with force trim off and leaving controls?

[Shehryar]

Is it normal for the helicopter with CCW rotating main rotors (2 bladed), to pitch down and roll left in forward flight, when force trim is turned off and hands off the cyclic? Moreover, when we put the helicopter in right bank, would it tend to return to straight & level / left bank after a few seconds when hands off the cyclic (force trim off)?

By forward flight I mean 80 to 100 kts

[Eric Hunt]

Are you talking about an aircraft with hydraulic controls? Friction off and force trim off?

With force trim off, the stick would tend to flop in whatever direction it was in when the FT was turned off, so if the stick was forward of centre (you expect it to be there if doing 80-100 kt) then the stick would tend to continue moving forward.

In a right turn, if steady, then the cyclic would be almost central (laterally) so might not go anywhere in particular. Most likely not return to level attitude or left turn, as it is dynamically unstable.

However, if talking about a non-hydraulic aircraft, like an R-22, where you are holding against control forces all the time, no telling what it would want to do. Very foolish to let it go.

 

[Shehryar]

I am talking about the hydraulic controls.

What if the cyclic is moved right without pedal input; would it then tend to return to a level flt?

 Any role of dissemetry of lift in this case?

[Wally]

4 hours ago, Shehryar said:

I am talking about the hydraulic controls.

What if the cyclic is moved right without pedal input; would it then tend to return to a level flt?

 Any role of dissemetry of lift in this case?

NO

[Eric Hunt]

Only if a spring-assisted force trim was on, and even then the FT would only be returning the stick to central, it will not restore the attitude to level flight, so you are in a turn somewhere and about to depart from your desired path.

But with no force trim, and hydraulics preventing any feedback, the aircraft will continue to roll, the nose will drop, the blades cut the tail boom off, and it is a short ride to your final destination.

Edited October 24, 2020 by Eric Hunt

[Shehryar]

The case stated above is of a helicopter whose rigging has been done according to the manual but has fol entries:

1. Tends to pitch down and roll left with FT off and hands off the cyclic

2. Tends to return to the level flight once helicopter  put in right bank of 20 degrees and then hands off cyclic, FT already off. (Doesn't seem to do it once put in left bank)

Any suggestions?

[SBuzzkill]

Tell them to buy an airplane.

Sorry, that's not helpful.  What is the helicopter these write-ups are for?  I've never flown one that will stay level without the aid of force trim.  Even with force trim most of them fall off in some direction due to the center of gravity, or gust of wind, or previous control force, etc.

Edited October 25, 2020 by SBuzzkill

[Wally]

On 10/24/2020 at 11:52 PM, Shehryar said:

The case stated above is of a helicopter whose rigging has been done according to the manual but has fol entries:

1. Tends to pitch down and roll left with FT off and hands off the cyclic

2. Tends to return to the level flight once helicopter  put in right bank of 20 degrees and then hands off cyclic, FT already off. (Doesn't seem to do it once put in left bank)

Any suggestions?

 A couple decades ago I had a lot of loitering in an AS355 (whatever they're called these days) so I experimented a bit with cross controlling. That a/c had no force trim, only friction.

A couple fundamentals I believe are true of helicopters: In forward flight, they have some yaw stability according to power, airspeed, the tail will try to stay behind the nose; Next they have some pitch stability, and will return to the same disc angle of attack, more or less; And almost no roll stability.,

Established at a cruise, friction the cyclic and collective, hold pedals. The 355 usually gradually rolled off the right and pitch down, oscillating nose up and down, but increasing the bank.

If I induced a roll to the left, with pedals or a power reduction, my memory is that the aircraft would start the same process, increasing roll, pitching down but would eventually roll level, pitch up and then start the start the right roll, nose down process in the paragraph above.

Never tried this experiment in a 2-blade Bell and never flown a Robbie, but Bells seem to have more airspeed/pitch stability than the 350/355. Their rotors turn in opposite directions, so your right bank is equivalent to the left in the 350/355, which did return to a wings level but nose up, before they diverged to the right. I think that trend is part of the trim stability of each design, and the Bell/Robbie is more stable in the airspeed pitch trim and will diverge over a longer period, to the left. 

[helonorth]

I’m with Wally. No.

[Shehryar]

Thanks for sharing your experience.

I agree with you on this. Unless helicopter has a 3 or 4 axis autopilot, it cannot maintain a level flight with hands-off controls.

It tends to roll in the direction of rotation of mainrotors and the reason of this seems to be something to do with dissymetry of lift.