rad wrote:Dear Hari Nair
Please explain what is control saturation , how and in what mode of flight can it occur and what we should do to avoid
getting into that situation . I have 6 hrs on Robinson R-22 but that that was nearly 20 years back , cheers
Well – Rad, you just talked yourself into one torturous session of helicopter aerodynamics! I’ll keep it as short as possible and if you have the patience to read it till the end and still make some sense of it
…. so here goes-
The term 'Control Saturation' is totally inaccurate - actually nothing (neither the control nor the rotor gets 'saturated'). The helicopter is very much controllable.
Due to the fact that a helicopter’s main rotor is going ‘sideways’ through the air, it experiences dissymmetry of lift. That is – the advancing rotor blade going into the direction of flight experiences a faster relative airflow (due to the rotation of the rotor) and the retreating blade experiences a slower relative airflow. A perfectly rigid rotor would thus be unable to keep going and would ‘flip over’ due to the resulting imbalance of lift between its advancing and retreating sides.
To alleviate the lift imbalances, rotor blades are flexibly attached to the rotor hub, allowing the rotor blade to ‘flap’ up and down. Earlier technology required a flapping hinge to be built into the hub at the point of attachment of the rotor blade. Typical examples being the Russian Mi-8/17/24 series of helicopters. A variation of that theme was two diametrically opposed rotor blades attached with a see-saw type gymbal joint at the hub – The AH-1 Huey Cobra and the UH-1 are examples. Then there are newer ‘hinge-less’ rotors which have flexible ‘quasi-hinges’.
Whilst manoeuvring a helicopter, the pilot using the controls, tilts the main rotor into the desired direction. Thereby, a portion of the total rotor thrust is vectored into that direction. The helicopter’s body suspended pendulously below, responds with some delay to the changed direction of rotor thrust.
Now, if the flapping hinges (or quasi-hinges) are placed some distance away from the centre of the rotor hub, there is an additional force couple available to tilt the helicopter. The further out these flapping hinges are, the more powerful the force couple and the more instantaneous the helicopter’s response.
The advantages of high-offset flapping hinges are that even below zero ‘G’ flight condition, the pilot retains control over the rotor and can manoeuvre the helicopter to extreme flight attitudes. Now in certain conditions of extreme manoeuvres due to the fundamental issue of dissymmetry of lift of a single main rotor and certain other aerodynamic effects that also then kick in, the pilot may experience the cyclic control column at an extreme limit with the helicopter still in the manoeuvre. This depends on direction of rotation of the rotor - for a rotor turning to the left, the limit is reached during extreme manoeuvres to the right and vice-versa for a rotor turning to the right.
Recovery from this condition can be immediately affected by application of another control. However, the point is during hard manoeuvring in low level flight, the combination of factors may be such that the recovery control application is a tad too late or the height above ground is inadequate and the helicopter then impacts the ground. If adequate clearance is available above ground, recovery can be affected.
Therefore to answer your question, since the R-22 has a teetering head (no flapping hinge offset at all) and if you attempted some seriously severe manoeuvres, you would hit other rotor limits first (such as ‘mast bumping’) with perhaps more disastrous consequences, even before reaching cyclic stick limit. Which means that even if adequate clearance was available above terrain, you may not be able to recover! The R-22 being a trainer machine and having a teetering rotor has a modest flight envelope. That's defined in your Flight Manual - stick within those limits and you are safe.
So given the choices available with today’s level of technologies, I’d rather throw my hat in with the more manoeuvrable high-offset flapping (or quasi-flap) hinge rotor.