Tech Theory

Swash Bucklers

1. Non-rotating outer ring (blue) 2. Turning inner ring (silver) 3. Ball joint 4. Control (pitch) preventing turning of outer ring 5. Control (roll) 6. Linkages (silver) to the rotor blade #. Linkages (black) that make the inner ring turn

There is a saying in the aviation community that helicopters are a thousand moving parts going in generally the same direction. There is some truth to this. How does the helicopter change direction? Black magic? Pure, unadulterated evil? A mechanical linkage that changes the pitch and path of the blades, creating a higher lift region opposed by a lower lift region allowing the aircraft to move towards the lower lift region? Most likely, it’s a combination of all three.

The swashplate is what makes it all possible. It consists of a fixed outer ring where inputs from the pilot are received and a rotating inner ring that transmits those inputs to the rotor blades. As the pilot moves the stick, he actuates the pitch and roll linkages causing the ‘copter to move in the direction of the lowered actuator. As the pilot moves the collective, he raises and lowers the swashplate which changes the angle of the blade and increases or decreases how much lift they generate.

Here’s an excellent video demonstrating how this devilish system works.


[Ed. Reader TheTokenGreek mentioned this in a comment a while back, and The Professor brought it to my attention. Why am I telling you this? I’m not sure, but I thought they deserved some recognition.]

[Image Credit: Guido Brüscher]

  • The Professor

    Well done, nice article. I wonder how long that it took a team of engineers to come up with a working swashplate? It looks like one of those ideas that would give you a headache from trying to sort it out in your head.

    • Thank you very much, good Professor.

      The idea of a swashplate goes back to probably the late 1800s. I know some positive displacement pumps have used a swashplate to convert the rotary motion of the motor to a "back and forth" motion of the pistons.

      It is an incredible device, and when you see one in motion you can't help but think how elegant it is.

  • highmileage_v1

    If you really want to twist your brain look into helicopter aerodynamics. Any flying machine that includes the terms "blade flapping", "retreating blade stall", "dis-symmetry of lift", etc, is not to be trusted. Check this out:

  • theTokenGreek

    thanks for the nod! The swashplate setup is astonishingly complex (for how robust it has to be)… since both swashplates need to rotate around a fixed axis while still allowing for them to be tilted (for control input) they end up being these two nested sockets around a single pivot – the uniball. The upper (rotating) swashplate actually sits mostly inside the stationary swashplate, which receives the inputs from the pilot. It's a little tough to visualize, since power via the driveshaft is being transmitted through the uniball the entire time.

    Since we're talking about how the control inputs of the pilot make their way to the rotors, I want to point out that the effect of pitch change is not immediate. One of the major problems with helicopters at their inception was that cyclic input didn't correspond naturally to direction of movement. Left cyclic input would send the helicopter skidding aft… increasing the pitch of the rotor blade on the right (advancing) caused it to actually lift higher (an action called flapping that needed to be addressed/allowed for as well, eventually with hinges) than the retreating blade on the left, and they would reach their peak/trough at the fore/aft position, where they then would trade places. As a result, the rotor disk (or tip-path-plane) was essentially tilted aft, and the total lift vector went with it.

    To correct this, the control rods are positioned 90 degrees forward of their respective blades' positions, so that the inputs of the pilot affect the attitude of the whole rotor system in a logical manner.

    A good analogy for the swashplate system is actually a camshaft… the stationary swashplate is essentially an adjustable cam, and the rotating swashplate acts as a pushrod, following the rise and fall of the cam, and constantly adjusting the pitch of the blades along with.

    There's really no adequate way to portray all this simply through text, so I hope you guys can get something from all that. Either way, good to see some rotor-head love here on AT!

  • highmileage_v1

    OK, I have a pic of a swash plate and a failure but it is a JPG that isn't on a server. Anybody have a hint as to how to upload it?

    • As far as I can tell you'll need to post it on a hosting site, such a flickr, then link to that here.

      • highmileage_v1

        Thanks. Ah well, a shame. It's an interesting shot. The pitch links are wrapped around the mast, most ugly.