Think back to your first helicopter lesson. You might begin to sweat as you remember making your very first single control input, only to have the demon respond by moving in virtually all of the six possible degrees of freedom—simultaneously. The end of that lesson left you sure you’d never be able to master the motions created by such complicated mechanics. If you stuck with it and you’re grinning right now, I’m guessing you were one of the ones who did.
The helicopter has been called many things, some of which I can’t repeat. One way to describe it for certain is highly-coupled and asymmetric. It is the coupling that probably caught your attention and gave you the most trouble in your initial days of flying. It is coupling that can unmask a sloppy pilot from a smooth one, and it is coupling that can, on rare occasions, catch you off guard.
“Coupling” is the term used to describe the reaction of an aircraft along or about an axis that develops due to a disturbance along a different axis. It can arise from a control input, a wind gust, a change in G-load, a change in relative wind, etc. Because the source of lift comes from a rotating system, suffice to say coupling is ever-present in both hover and forward flight as aerodynamic and gyroscopic forces do the continual dance. Thankfully, a full understanding of the physics involved was not necessary between that humbling first flight and now. We’ve just learned to compensate for any reactions off the axis we want to manipulate by automatically making control inputs that cancel them. The smoothest pilots are the ones who can make these canceling inputs totally seamless. Kudos to human adaptability.
Maneuvering the helicopter involves tilting the rotor disk by unevenly distributing lift across it. This maneuvering can be thought of having three phases which happen in short order: 1) flapping the blades to a new position relative to the mast, 2) rolling or pitching the rotor/mast/fuselage together at a constant rate, and 3) the maintaining of a new attitude. Each of these can produce coupling for the pilot to deal with.
In looking at a right turn in a “conventional” single teetering rotor helicopter, right cyclic will decrease lift over the tail, and increase it over the nose, allowing gyroscopic precession to flap the disk approximately 90 degrees to the right. However, for rotors with flapping hinges offset from the center of the hub, or hinge-less rotors that flap by blade bending, the effect occurs in less than 90 degrees. There is an immediate moment about the hub due to blade centrifugal forces trying to pull the hub in line the instant they begin to flap. In a right roll the nose will pitch up due to this, and the pilot will have to counter this “acceleration cross-coupling” with some forward stick, or watch airspeed bleed off.
Once the hub aligns again with the rotor, that pitching moment is gone; but as the rotor/hub/fuselage as a whole continues to roll right, the downward rolling side sees an increase in angle of attack, while the upward rolling side sees a decrease, due to change in relative wind. This provides another difference in lift across the disk (due to roll rate) and again, the rotor precesses nose up, requiring a bit of forward cyclic to counter during this “rate cross-coupling.”
The pilot then pitches the aircraft around the turn. With aft cyclic, lift is made uneven across the disk’s left and right side, the rotor precesses nose up, and both pitch acceleration and pitch rate cross-coupling cause some left roll. However during heavy maneuvering, lots of aft cyclic and/or increased collective increases load factor and coning. As the aircraft pitches, excessive coning, aft disk tilt, and high load can combine to allow more air to push on the blades at the nose from underneath instead of from above, creating a large right rolling moment. Since more of the helicopter’s mass lies lengthwise than spanwise, its inertia (resistance to movement) is greater in pitch than it is in roll. For North American-spinning rotors with large hinge offset, especially the hinge-less variety, a large nose-up pitch input can create an even larger right roll couple—one that can exceed the ability of left cyclic to counter! It is advised in certain flight manuals to avoid slow, steep high-g right turns for just this reason.
The dynamics of coupling are just one aspect of helicopter flight that we should commend ourselves for conquering. Although we’ve developed the skills to make helicopters look easy to fly, we know the truth. Their unique and intricate dynamics are always in delicate balance. They take a long time to master, are amazing to see in action, and are certainly a force to respect.