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Is it normal for the helicopter with CCW rotating main rotors 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

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  • $\begingroup$ Your title states "counter rotating" while your question states "counterclockwise rotating main rotors". Do you mean a single, CCW rotating main rotor, or do you mean dual counter-rotating rotors (plural)? Please edit your post to clarify. $\endgroup$ – FreeMan Oct 23 '20 at 17:18
  • $\begingroup$ I meant CCW rotating rotor system (2 bladed to be specific) $\endgroup$ – Sierra Pak Oct 23 '20 at 19:58
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With a American made helicopter equipped with a CCW rotating rotor, the helicopter will, as it accelerates past 10-15 KIAS, begin to pitch nose up as well as roll to the right. This is a direct result of Effective Translational Lift (ETL) as well as Transverse Flow Effect. Airflow through the main rotor disk past these speeds begins to take a more horizontal flow through the rotor disk, increasing lift on the advancing side of the rotor disk resulting in this. European made helicopters with a CW turning main rotor will have a tendency to still pitch up as ETL becomes effective but will roll left due to gyroscopic precession from a greater lift imbalance on the front of the rotor disk being applied to the right side of the disk.

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Both of the previous answers are really good, but if you're after a practical answer to your question I thought I'd see what happens in an EC135 (CCW rotor). With the Autostabilisation and back-up SAS disengaged flying straight & level at 90kts the aircraft is quite unstable. As such, whatever slight attitude the aircraft is in prior to loosing the controls, it continues in that direction. This departure is very rapid but controllable, there is no bias in any particular direction. This also applies in turns, the attitude very quickly continues in the direction of the control input. Hope that helps.

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The answer depends on what disturbs the helicopter (with no disturbance it has no tendency). For example, let's say there's a gust of headwind, blowing into the nose of the aircraft, increasing the airspeed. This adds more lift to the advancing side of the main rotor (pilot's right side for CCW), which has the effect of flapping the rotor up 90 degrees later over the nose and pitching the helicopter nose up. This end effect (pitch up) is the same regardless of the direction of rotor rotation.

Depending on the rotor design, the largest increase in flapping may be less than 90 degrees, which would add a (very secondary) roll effect. Here the direction of rotation matters. For a CCW main rotor this would cause a "left wing down" roll moment.

For more about this 90 degree (or less for some rotor designs) flapping offset see this: Basic Helicopter Flap Dynamics.

Another effect of the increased airspeed is that it focuses more of the wake in the rear of the rotor, decreasing lift there relative to the front of the rotor. This causes the rotor to flap down 90 degrees later on the pilot's right side (CCW), which adds a "right wing down" roll moment.

Analogous comments apply to the right wing down bank turn. Let's say the pilot hadn't trimmed the climb rate yet, he entered the turn but is now accelerating down due to the main rotor thrust being tilted further from vertical. Since the rotor is tilted right side down, the increase in air flow up (due to the helicopter falling down faster) adds velocity and lift to the rear of the rotor (CCW) in the same way increased airspeed added lift to the right side of the rotor. This lift will cause the right side of the rotor to flap up, pushing back towards a wings-level condition.

As you can see, in both cases, the primary response is stabilizing. This is described better here: Helicopter Stability

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