Helicopter Main Rotor Swashplate

A helicopter swashplate is a disk that transfers control inputs from the fixed system to the rotating blades. The swashplate is located in the rotor hub, and is a key component in controlling a helicopter. Pilot control inputs tilt and shift the swashplate, which effectively feathers the rotor blades as described below.

Swashplate components

As shown above, a swashplate consists of a lower, nonrotating swashplate and an upper swashplate that rotates with the rotor. Bearings allow the upper swashplate to slide on top of the lower swashplate. If the rotor spins at 300 RPM, you'll see the upper swashplate spinning at 300 RPM while the lower remains mostly fixed with the fuselage.

Control rods attach to the lower swashplate move according to the pilot's collective, longitudinal cyclic and lateral cyclic flight controls. Pitch links connect the upper swashplate to the blades and hence rotate about the mast with the rotor. Swashplate movement causes pitch link movement which changes the feathering of the blades.

Behavior

The control rods may raise/lower the swashplate. This occurs when all rods move up/down by the same amount, and is a response to changes in the pilot's collective control. This feathers (pitches) all blades by the same amount, e.g. raising the swashplate 1" may increase the pitch angle of all blades by 1 degree (in response to the pilot raising the collective lever). More on that later.

Additionally, if the rods are moved by different amounts, the swashplate will be tilted. The swashplate may be tilted in different directions, say longitudinally and laterally, depending on which rods are raised and which are lowered. This is done in response to the pilot's cyclic controls.

Notice the rods are attached to the lower swashplate. This means they do not rotate with the rotor. If a rod on the left side of the hub is raised (and right lowered), the swashplate tilts right side down. The upper swashspins with the rotor and hence an atom on the upper swashplate moves up and down with each turn of the rotor.

Pitch links

Pitch links connect the upper (rotating) swashplate to the pitch horns on the rotor blades, outboard of the feathering hinge. This is shown in a figure above. Pitch horns typically extend from the leading edge of the blade and hence raising them feathers (pitches) the blade, leading edge up. Lowering them decreases feathering.

There is typically a 90-degree azimuth offset between the pitch link attachment point on the swashplate and the blade it’s connected to. The vertical position of the swashplate at \(\psi =90^o\) feathers the blade at \(\psi = 0^o\). The convention for azimuth angles \(\psi\) is shown in the diagram below.

Full system

Let’s put all this together and see if it makes sense. We’ll start with the simpler case of collective and then try longitudinal cyclic. (Lateral cyclic is the same concept as longitudinal with an azimuth offset.)

If the pilot increases collective, all control rods rise and hence the lower and upper swashplates rise. This does not change the tilt of the swashplate. This causes all pitch links to raise all pitch horns by the same amount, which increases the feathering angle of all blades uniformly. This increases each blade’s angle of attack and therefore lift. This provides more thrust, which primarily accelerates the helicopter up, just as the collective is intended to do.

What about longitudinal cyclic? When a pilot pushes the longitudinal cyclic forward some control rods move down and others move up. These motions are rigged to tilt the swashplate front edge down / aft edge up. There's no change to the average vertical position of the swashplate.

The tilted swashplate imparts a periodic up/down motion to the pitch links. The pitch links reach a max height where the swashplate is highest—aft at \(\psi \approx 0^o\). As mentioned in the pitch link section, this pushes pitch horns up the most at a \(90^o\) offset location: \(\psi \approx 270^o\).

The raised pitch horn at \(\psi \approx 270^o\) means the blades will reach a peak pitch angle and hence have a higher angle of attack there. Each turn of the rotor all blades will pitch to maximum feathering angle around \(270^o\) and minimum around \(90^o\).

The periodic aerodynamic forces will accelerate blades up the most around \(270^o\). While not obvious, this turns out to cause the blades to flap upward to a peak position around \(\psi=0^o\), and a minimum position around \(\psi=180^o\). The fact that flapping is offset 90 degrees from feathering is not intuitive, but a consequence of rotor dynamics explained in this article.

This forward flapping of the rotor pitches the helicopter nose down and increases forward airspeed. This is indeed what the pilot expects when pressing the cyclic forward.