Control saturation in helicopters
 

I wish to share my views on the subject to invite comments on the mechanism of this phenomenon in helicopters.

Well... just to introduce what I'm talking about-

During sustained high 'g' turns (direction of turn will depend on the rotation of the blades) the lateral mechanical limit of the cyclic is reached-----The pilot may recover by reducing the 'g' loading or even by application of the 'Top' rudder. (This has been seen practically as well)----.

Here goes my explanation of the phenomenon--

As the helicopter banks, the blade at the nose of the helicopter experiences an airflow (the helicopter is also moving ahead !!) which may be from 'above' the disc or even 'below' the disc. In case, it is 'above' the disc it may result in reduction in the angle of attack due to increased 'induced flow'. For a clockwise turning rotor (when seen from above) mounted on a helicopter banking left, this may cause the disc to flap down to the retreating side (or to the right). However, if this airflow is from below - which is the case during high 'g' banked turns, this results in an increase of angle of attack resulting in the disc to flap 'up' as it retreats on the right side. If our helicopter is in a left banked turn this may cause it roll further left. However, if the pilot intends to maintain the original bank angle he/she will be required to move the cyclic further 'right' (outside the turn; against the resultant flapping). Ultimately, during a sustained high 'g' turn this may result in the pilot running out of the possible lateral cyclic mechanical movement to the right. We have reached now achieved CONTROL SATURATION.

This phenomenon is more likely to be seen in rigid rotors due to associated high control moments.

I consider this as a 'Control Margin' issue. There are other explanations of this phenomenon using retreating blade stall. I somehow need more convincing on this explanation since the retreating blade stall in the above example will cause the helicopter to roll right and not further to the left.

Well ...... any rotary wing crew for some added explanation and comments !!!

If this is happening on an older model of Puma, it's actually a lateral directional problem not a control saturation problem. It sounds suspiciously like a Puma issue - never heard of it happening on any other helicopter, but would love more details!

This phenomenon is presents itself in the 'Advanced light helicopter' developed by Hindustan Aeronautics Limited. This is primarily noticed during high 'g' turns to the left. I've also heard that this also shows up in BK 117 (MBB).

The plot thickens.
There was an issue with the BO-105 that was similar, but in turns to the right. The crash that killed the chief test pilot of MBB is a case in point.
Not sure what it should be called - the term 'control saturation' is not one I've ever heard before (and I've been flight testing helicopters for 30 years now).

I'll see what else I can dig up on this.

Sounds like "jack stall" or "servo transparency".

Rather than repeating the description, just search the forum. It's been covered many times.

Well, terms used may be different but it sure shows up: for the BO 105 does the rotor turns anticlockwise when viewed from above?- if this is the case then it seems to be the similar phenomenon. I had considered the contributing factors in this phase of flight as the rigid rotors with low inertia blades at 2 'g' turns to the left.
It just two factors I could think of......

I may not go with the jack stall or servo transparency though.

Could I know the forums that this has been discussed?

I remember having this phenomenon demonstrated to me during slalom flying in the RAE Bedford Puma many years ago - full lateral cyclic to one side to hold a turn in the opposite direction. I further recollect that the Bo105 was mentioned at the time as having a similar characteristic, but reversed in terms of turn direction.

I think it's an Lv/Nv issue ie aerodynamic, and I think it's mentioned in Gareth Padfield's book.