peeush
16th Apr 2011, 14:37
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. "
Hi Frank,
It is just one of those things that derailed my understanding of the phenomenon when I looked at it again.
As in the example, for a counter clockwise turning rotor, considering the direction of the reaction forces the disc must flap up to the left, thus resulting in a roll to the left.
This part of the explanation would thus ease out the control saturation rather than aggravating it. At least this is what I could make out.
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. "
Hi Frank,
It is just one of those things that derailed my understanding of the phenomenon when I looked at it again.
As in the example, for a counter clockwise turning rotor, considering the direction of the reaction forces the disc must flap up to the left, thus resulting in a roll to the left.
This part of the explanation would thus ease out the control saturation rather than aggravating it. At least this is what I could make out.