Helicopters: Control saturation

[rotorwash4944]

Control Margin vs Control Saturation

***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