# Helicopter Swashplate

The swashplate, located in the rotor hub, is a key component in controlling a helicopter.  Pilot control inputs tilt and raise / lower the swashplate which effectively feathers the rotor blades as described below.

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.  Control rods attached to the lower swashplate move according to collective, longitudinal cyclic and lateral cyclic flight controls.  These rods raise/lower and tilt the swashplate (three DOF).  Pulling the collective control up raises the swashplate (pushing it down lowers the swashplate).  Pushing the longitudinal cyclic forward typically tilts the swashplate down in the front / up in the back.  Pushing the lateral cyclic right tilts the right side of the swashplate down / left side up.  There may be phase offsets in some rotors.  E.g. forward cyclic may tilt the swashplate down at some angle offset from the forward edge.

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 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.

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, just an azimuth offset.)  If the pilot increases collective, all control rods extend 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 shorten while others lengthen.  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.  This imparts a periodic motion to the pitch links.  Each revolution of the rotor the bottom of a pitch link travels from a maximum vertical position at the aft edge of the swashplate $$\psi =0^o$$ to a minimum position at the front edge $$\psi = 180^o$$.  Because of the 90-degree offset explained above, this imparts a periodic feathering to the blades with maximum at $$\psi=270^o$$ and minimum at $$\psi=90^o$$.  This, in turn, causes the rotor to flap longitudinally with blades at a minimum flapping over the nose of the aircraft, at $$\psi=180^o$$.  The fact that flapping is offset 90 degrees from feathering is not intuitive, but is a consequence of rotor dynamics explained in this article.