Helicopters: Control saturation

[peeush]

Hi People,
I had floated a query about control saturation in helicopters some time back.

I'm still looking for some unaddressed aspects of the same.

Here's the situation as I could state as explicitly-
'During sustained high 'g' banked 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 understanding 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 think 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 ...... that's all I guess-

Can any of readers help me make more knowledgeable on the subject.

Thanks anyway for the reading..

[nodrama]

I can't give you a more in depth explanation, but if I mention it, it might entice someone to help...

The Bo105 can suffer from this , in lateral. I've come across this where, in certain wind conditions, a pilot has run out of LH lateral control.

[peeush]

Well Thanks-

I hope that would bring some more views on the subject

[Brian Abraham]

peeush, see here http://www.pprune.org/rotorheads/441054-servo-control-reversibility-g-limits.html Nick Lappos at post #3 is an ex Sikorsky test pilot, so knows what he is talking about. Also take note of the link in post #2, and particularly the references on the last page of the linked item.

[peeush]

Thanks Brian,

Well, the issue being discussed in the above references made, point to the retreating blade stall(or even Mach Tuck) as the start point;that may be the case in those examples. The core issue I had tried to place my understanding around is ' Control Margin'. The case I've quoted has the cyclic lateral movement achieving 100% of its possible travel. BO-105 issue as stated above may be a similar case- though I don't have sufficient information of the accident.
Thanks again anyway.

[rotorwash4944]

***Control Margin and Control Saturation***
When a flight control is deflected to the point that achieves the highest control moment (i.e. steady state roll, pitch, yaw rate) attainable by the aircraft, it is said to be "saturated." The difference between the control moment required to accomplish a task and the maximum control moment available is defined as Control Margin.

Therefore, most aircraft exhibit more than adequate control margin to accomplish normal flying tasks. But in the scenario described in the original post, a fully saturated control system (system giving maximum available control moment) may still not provide enough control to counter the uncommanded roll.


The condition spoke of when a pilot may run out of left cyclic in counterclockwise-from-above rotor systems while in a right, high G turn is due to gyroscopic forces. A test pilot would call this cross-coupling. An engineer describing it in the equations of motion would call it "rolling moment due to pitch rate."

Think of a toy gyroscope. Hold it by its center (yaw) axis while its spinning, and try to pitch it. The gyro will react by rolling either left or right about the roll axis (depending on how you spun it), 90 degrees to the direction of pitch.

We tilt the helicopter rotor by "flying" the individual blades to new positions, compensating for the gyroscopic precessive forces with controls rigged to apply forces 90 degrees prior to where we'd like them to act.

But then we must also think of the spinning rotor disk as a whole. As the whole disk pitches or rolls about the hub, gyroscopic forces cause reactions (i.e. forces/moments) 90 degrees later. The cyclic is essentially a rate controller. Small cyclic movements command small pitch and roll rates, and cause small easily-compensated-for gyroscopic reactions.

But with the case in point, during aggressive maneuvering, large forces at the front and rear of the disk are required to command large (high-G) pitch rates. These large forces create equally large gyroscopic reaction forces, which will cause large rolling moments about the hub (in this case, to the right). So, in a right high-G turn, these rolling moments can be so great as to cause the pilot to reach the limit of left lateral cyclic in attempt to control the right roll. The condition is aggravated with an increase in collective, since the rotor disk is unstable with an increase in angle of attack ( the coned rotor will exhibit increased blow back, which in turn will add to the gyro-induced right roll).

Rigid rotors are most prone to this for the same reason that they are the most responsive to control. Their stiff hub and lack of flapping hinges create an "effective hinge offset" which provides extremely large hub moments.


I hope this helped.
Frank Lombardi

[peeush]

Frank, Thanks a TON for this explanation- I think I am fairly convinced. I therefore understand that in this case there would be no role played by 'Retreating Blade Stall' or 'Servo transparency'. The answer thus lies in 'Cross coupling'.


Thanks again for the insight.

[rotorwash4944]

Glad to be of help. :ok: