Lesson 2: Rotor Systems

As briefly mentioned in the lesson 1, there are three fundamental types of helicopter rotor systems: rigid, semi-rigid (or teetering), and fully articulated. These are discussed below, along with descriptions and operating principles of other important rotor components. To a large extent, the information is applicable to both main and tail rotor systems. Of course, the tail rotor does not have cyclic control, but its operation is similar to collective control on the main rotor, even though it provides the yaw reaction to main rotor torque on the airframe. Its operation can also be likened to that of a variable pitch propeller.

Fully Articulated Rotors

Fully articulated rotor systems allow each blade to feather (rotate about the pitch axis to change lift), lead and lag (move back and forth in-plane), and flap (move up and down about an inboard mounted hinge) independent of the other blades. As we will discuss, each of these blade motions is related to the others. Fully articulated rotor systems are found on rotor systems with more than two blades.

As the rotor spins, each blade responds to inputs from the control system to enable aircraft control. The center of lift on the whole rotor system moves in response to these inputs to effect pitch, roll, and upward motion. The magnitude of this lift force is based on the collective input, which changes pitch on all blades in the same direction at the same time. The location of this lift force is based on the pitch and roll inputs from the pilot. Therefore, the feathering angle of each blade (proportional to its own lifting force) changes as it rotates with the rotor, hence the name ‘cyclic control’.

As the lift on a given blade increases, it will want to flap upwards. The flapping hinge for the blade permits this motion, and is balanced by the centrifugal force of the weight of the blade, which tries to keep it in the horizontal plane. Either way, some motion must be accommodated. The centrifugal force is nominally constant, however the flapping force will be affected by the severity of the maneuver (rate of climb, forward speed, aircraft gross weight). If you ever get the chance to watch a helicopter hovering from the side (particularly a heavy helicopter), you can see all the blades ‘cone’. Appropriately, this is called ‘coning’. Some rotor systems have a ‘pre-cone’ but that is not important to discuss here.

As the blade flaps, its center of gravity changes. This changes the local moment of inertia of the blade with respect to the rotor system and it will want to speed up or slow down with respect to the rest of the blades and the whole rotor system. This is accommodated by the lead-lag hinge, and is easier to visualize with the classical ‘ice skater doing a spin’ image. As the skater moves her arms in, she will spin faster because her inertia changes but her total energy remains constant (neglect friction for purposes of this explanation). Conversely, as her arms extend, her spin will slow. An in-plane damper typically moderates lead-lag motion.

So, following a single blade through a single rotation beginning at some neutral position, as load increases from increased feathering, it will flap up and lead forward. As it continues around, it will flap down and lag backward. At the lowest point of load, it will be at its lowest flap angle and also at is most ‘rearward’ lag position.

Because the rotor is a large, rotating mass, it will behave somewhat like a gyroscope. The effect of this is that a control input will usually be realized on the attached body at a position 90 degrees behind the control input. This is accounted for by the designers through placement of the control input to the rotor system so that a forward input of the cyclic control stick will result in a nominally forward motion of the aircraft. The effect is made transparent to the pilot.

There are a few other considerations to the placement of control inputs also transparent to the pilot, but still interesting to discuss. Location of the input links to the rotor blades is related to the phasing of the rotating and stationary controls and also to the amount of blade input rotation required. Because the lead-lag hinge and the flapping hinge are not necessarily coincident, the location of the input may be located such that as the blade flaps or lead-lags, there may be a change in blade pitch input as flapping or lead-lag occurs (or both). This is a little difficult to visualize, but imagine that the input link is located at the same distance from the center of the rotor hub as the flapping hinge. As the blade flaps, there will be no effect on pitch because the pivots are along the same line. If the input link is inboard or outboard of the hinge, some coupling (or change in blade angle as a result of an input from another control axis) will result. If an increase in blade angle results because of an increase to blade pitch, the situation will compound. This situation is nominally unstable, but depending on the rotor system, is not necessarily bad. This can similarly occur in lead-lag.

Older hinge designs relied on conventional metal bearings. By basic geometry, this precludes a coincident flapping and lead-lag hinge and is cause for recurring maintenance. Newer rotor systems use elastomeric bearings, arrangements of rubber and steel that can permit motion in two axes. Besides solving some of the above-mentioned kinematic issues, these bearings are usually in compression, can be readily inspected, and eliminate the maintenance associated with metallic bearings.

Semi-rigid (teetering) Rotors

Semi-rigid rotors are found on aircraft with two rotor blades, such as Robinson, Hiller, and many Bell products. The blades are connected such that as one blade flaps up, the opposite blade will flap down. Allowing the rotor system to ‘teeter’ at the top of the rotor mast accommodates this. The Robinson system, although basically teetering, permits some independent flapping of each blade and operates in a similar fashion. The Hiller design uses the large main blades for lifting, but relies on two smaller blades 90 degrees to these for cyclic control.

Because the rotors are tied together rigidly in-plane, there is no lead-lag between them. The rotor does not necessarily ‘cone’ but rather will tilt up on the side with more lift and tilt down on the other. Flapping is therefore self-balancing. Issues of phasing, gyroscopic precession, and flap coupling are still present, but easier for the designer to deal with.

Rigid Rotors

Rigid rotors want to behave similarly to fully articulated rotors, but do not provide flapping or lead-lag hinges. The blade roots are rigidly attached to the rotor hub. Instead, the blades accommodate these motions by bending. Because the kinematic loads are not resolved by actual blade motion (or blade reaction to load may be different from that desired), high vibration may result. Rigid rotor systems are rare, but may become more common as improvements in material properties and vibration control evolve. They are fundamentally easier to design and potentially offer the best properties of both teetering and fully articulated systems.

Other Components

Any discussion of helicopter rotor systems would be incomplete without mentioning the swashplate, or star assembly. This mechanism provides for the transmission of control from the stationary, aircraft system (where the pilot is) to the rotating system (where the blades are). Swashplates come in an assortment of sizes, from six inches to six feet. The typical arrangement is a bearing (or two…one to take the up load and the other to take the down load) mounted in the horizontal plane below the rotor head (OK, Enstrom mounts theirs below the transmission and Hiller has some really bizarre flight control arrangement that I just can’t figure out). The bearing acts in thrust, which means that loads work in its axial (as opposed to radial) direction.

The output side of the swashplate (the side that connects to the blades) has attachments for a link to each rotor blade. These will be evenly spaced around the swashplate, coinciding with the blade positions on the rotor head. The input side has attachments for three control inputs (more on this in a moment). These are typically placed 90 degrees apart (although fly-by-wire aircraft can have them spaced 120 degrees apart…this will be discussed further when we get to controls). The center of the swashplate usually consists of a large ball or a gimbal. This permits the stationary side to tilt in response to cyclic control inputs. This ball is also typically free to slide up and down to allow response to collective control inputs.

Also attached to the swashplate are two special mechanisms called scissors. The upper scissors forces the swashplate to turn with the rotor head, but can also hinge to accommodate control motions. Sometimes there are two rotating scissors to share the load, usually placed 180 degrees apart. Since the scissors is a rotating mass which must be balanced, this is a somewhat useful application of weight that would otherwise be required. The lower scissors keeps the stationary side of the swashplate in the proper orientation with respect to the aircraft and the controls, and usually occupies the position of the ‘missing’ control input.

If you can remember spatial geometry (promise…this will be simple), you will recall that it takes three points in space to define a plane. Demonstrate this to yourself by selecting three fingers (they don’t even have to be from the same hand) and placing a flat object on them (your empty X-Plane CD jewel box is ideal for this). No matter how you move your fingers up or down (or the attached hand round the room), the three fingers will always be supporting the box, the bottom of which represents a plane in space, or for that matter, a swashplate.

Since the rotor head is fixed vertically at the top of the rotor mast (held in place by what is often called the Jesus nut…because if it fails the pilot’s last words will be ‘Oh Jesus’), as the swashplate tilts it will come closer to the rotor head in one area and farther away in another. Because of the relatively simple kinematics of their linkage to the swashplate, the blades will be forced to change pitch as they rotate.

People often mistake helicopter control as tilting the rotor disc. While this may make for simple explanation and visualization, it is more accurate to think of helicopter control as the tilting and lifting of the swashplate.