To many non-aviation types, one helicopter looks just like any other. Usually differences in design can be most obvious to those who spend their lives in and around helicopters. Yet some features and their purpose might be more subtle.
“Hinge offset” is one of those features that might not be very notable to most. But it turns out to be one of the more important design considerations, having multiple effects on configuration, stability and control of the helicopter. The simple act of moving the flapping hinge out from the center of the hub is what is meant by hinge offset, and it creates very different control characteristics.
Two-bladed teetering (semi-rigid) rotor systems have their flapping hinge at the center of the mast. Simple in design, these are the rotor systems on helicopters such as the Bell UH-1, Bell 206 and Robinson R22. Control of these helicopters is accomplished by tilting the thrust vector through cyclic pitch change, essentially having the blades flap or “fly” themselves to a new roll or pitch condition. Since the hinge is right at the hub center, none of the rotor’s rolling or pitching moments are transferred to the mast in this way. But once the thrust vector tilts, it is no longer in line with the aircraft center of gravity (CG) and this creates a pitch or roll moment about it. The fuselage will then follow, as it acts like a pendulum, working to align the CG with the thrust line once again. The trouble with this method of control is that low thrust equates to less control moment available for maneuvering, as history has unfortunately shown.
Bell’s UH-1 Huey and AH-1 Cobra models saved countless lives during the Vietnam War, but also brought to light the phenomenon of “mast-bumping,” which often resulted in fatality. Aircraft wreckage was occasionally found on the backside of a hill, with the rotor system detached from the fuselage. It was discovered that while flying low level, pilots would execute a pull-up, then push-over, to match the contour of a hillside. As load factor dropped to zero during the push-over, the thrust produced by the rotor also dropped to zero. The push-over revealed the physical limitation of most teetering rotors: As thrust approaches zero, as in a low-g condition, cyclic control becomes ineffective. As the fuselage gyrated under various other aerodynamic forces and moments, the pilots had no ability to change the flight path. Typically at that point the thrust line of the tail rotor would get out of line with the CG and cause a roll. As cyclic was applied to attempt recovery, the mast was “bumped” by the under-slung rotor hub making detrimental contact with it, causing the rotor system to depart the aircraft.
Fully-articulated and rigid rotors take advantage of offset hinges to better affect aircraft control. While control is still accomplished by cyclic pitch change, a rotor system with an offset hinge has the added benefit of being able to impart a rolling or pitching moment directly to the hub, even if it is producing zero thrust. This happens because as soon as the blades flap up or down, the centrifugal forces which are slinging the blades outward no longer act through the hub center. This creates a “force couple,” or moment about the hub which acts to realign it with the plane of the rotor at its new roll or pitch condition.
The large centrifugal forces acting on the hub typically provide a crisp, powerful additional control moment that quickens aircraft response. Rigid rotors, which do not have any flapping hinges, are considered to have an “effective hinge offset,” through blade bending at some calculated distance from the hub center, and tend to provide the most benefit.
Offset hinges have other effects as well. The increased control power they provide allows for a wider aircraft center of gravity range. The mast can be designed to be shorter in length, since control does not depend on rotor thrust vector distance from the CG alone.
Not all effects are good, however. Offset hinges can have some destabilizing effects, as they introduce some cross-coupling that the pilot must cancel (see “A Couple of Things,” Rotor & Wing June 2012). So a rolling moment (pilot or wind induced) will cause some pitch, and a pitching moment will cause some roll. This is due to the flapping frequency not being purely 90 degrees, as it is in teetering heads. Higher structural loads are also a trait of rotors with hinge offset, as is the possibility of flipping the aircraft over while on the ground at flat pitch, from the hub moment alone!
All in all, the offset hinge has proven to be an invaluable evolution of helicopter design. It’s amazing what a difference a few inches can make.