The View Up Here

Random scribblings about kites, photography, machining, and anything else

Damped Pendulum KAP Suspension

Posted by Tom Benedict on 06/03/2013

In an earlier post I alluded to a pendulum test bed I wanted to build for experimenting with pendulum suspensions for kite aerial photography rigs. I haven’t cut metal yet, but I’m far enough along with the design to start sharing details.

Before I get into it, first a note about designing long spindly structures: Put simply, it’s a pain. The areas of interest wind up being separated by large swaths of open space that need to be navigated repeatedly in CAD. And visualizing the entire structure in one go becomes difficult at best. Shown below is a rendering of what the test bed is likely to look like when built. As you can see, it’s not fun to work with in CAD.

Damped KAP Pendulum - Uncompressed

Rather than put myself through this, I did the design work using 2″ long spars in place of the full-length spars I intend to install in the final hardware. Keep this in mind when reading the rest of this article. The design is compressed for the sake of making the CAD work easier. The final hardware will use full length spars! Shown below is the “compressed” version of the design using 2″ spars:

Damped KAP Pendulum - Compressed

Much easier to work with.

First and foremost, this is an experimental test bed, not a finished design. The whole idea is to build a kit of parts that can be used to test various ideas quickly and easily without having to make new mechanisms for each test, preferably just by swapping parts in the field. As much as possible, parts are interchangeable and should be easy to remove and install. Global geometries are defined by only two 2D parts to make it easy to test additional geometries without remaking every component of the design. And the dampers – one of the key elements under test – are tunable. Finally, the whole thing should be able to fold into a compact shape that will fit in a typical kite bag.

Moving from top to bottom, there is a stage at which the pendulum attaches to the kite line, a parallelogram stage, and a single pendulum stage. The entire design is modular, so stages can be re-arranged, or omitted altogether. This allows for experiments with a single pendulum, a double pendulum, a parallelogram pendulum, or a double pendulum in which the upper pendulum is a parallelogram pendulum.

The spar that the kite line attaches to is not completely drawn. The spar can be rotated into position against the locating pin, and is then locked into place using a 1/4″-20 screw (not shown). This is so the rig can be collapsed for transport. Also not shown at either end are the hook plates that keep the kite line from coming loose. I plan to use something very similar to the arrangement I had on an earlier gimballed suspension rig. In use the line is hooked under one end plate, and is then wound around the spar and under the other end plate. This arrangement worked remarkably well on the gimballed rig, though I plan to use a slightly different geometry for the end plates on this rig, and plan to add Velcro “keeper” straps.


Each pivot is made using a zero-play double row ball bearing cartridge, housed in a custom machined ball end. The stem on the ball end extends into the spar by at least one inch. This is deep enough that the clamp blocks never clamp on an empty spar – there is always a plug filling the spar to keep it from being crushed. The bearing cartridges are a slip fit inside the ball ends, and are fixtured using a screw that fits into a tapped hole in the side of the bearing cartridge.

Several of the pivots can be damped using stacks of slip plates, made from FR4 and packed with Nye Lubricants damping grease. The damping force of each damper can be tuned by adding or removing slip plate pairs, and by changing the grade of damping grease in the stacks. The stacks are fixed at either end. One end is fixed in the red bracket, and the other on an aluminum clamp block that is attached to the spar being pivoted. Any combination of dampers can be used, ranging from none installed to all installed.

By changing spar length and damping force at each stage, the oscillation characteristics of each stage can be individually tuned. For a given weight KAP rig, it should be possible to choose a primary frequency of oscillation, and to over-damp, under-damp, or critically damp each pivot.

So far all of these pivots damp motion in the pitch direction. Because of the bearings at each pivot, yaw should be quite rigidly constrained to track the kite line. But I am designing another damper to fit on the pan axis of the KAP rig to take out line wobble in yaw, should there be any. As with the rest of the design, this can be installed or removed for each test.

This leaves the roll axis. There is one additional experiment I’d like to try for damping oscillation in roll, but it can be attached to the hardware already shown using clamp blocks similar to the ones used on the dampers. It amounts to a damped pivot with two spars, each carrying a counterweight. This can be fixed to any of the spars shown, including the line suspension spar. It will use a custom mount block that allows the orientation of the pivot point to be changed anywhere between 90 degrees to the spar all the way to inline with the spar.

What I hope to gain out of this is a better understanding of the mechanism that causes rig shake, and the information I need to build something that’s easy to transport and use, and results in wobble-free KAP.

– Tom


4 Responses to “Damped Pendulum KAP Suspension”

  1. Charmian Wright DVM said

    Hi Tom, what’s the status with “Smilodon?” I scanned your posts but saw nothing new about repairs. As a P-Cat aficionado (I grew up on one, and recently purchased and have been sailing #111, very nicely restored), I have followed your P-Cat posts with great interest. Your photos and P-Cat stories have been excellent; I ho

    • Tom Benedict said

      Sorry, I haven’t been out on Smilodon much recently. The boat’s fine. It’s my poor tow vehicle!

      I’ve been itching to drive it over to Hilo to launch from the beach at Hilo Bay. (Hey, it’s the progenitor beach cat, after all!) But I’m not sure the Jeep Wrangler I’ve been using will make it there and back. I’ve been trying to fix it up and get it ready to sell so I can trade it in for something with a little more wheelbase, and just a bit more power. Then we’ll be back out on the water with more stories to come.

      The one change I’ve made since our last trip out was to pick up two paddles. It’s absolute hell maneuvering it out of the harbor where we normally put in. So far I’m the only one who can keep the boat from ramming up on the rocks as the sail goes up, so I haven’t been in a position to take my turn at the winch. (My position is to hold the front crossbeam and yell, “Get it up fast so we can get out of here!”) The plan for our next outing is to paddle out and raise sails in open water. WAY easier on the nerves.

      Of course all that will change when I finally get it over to Hilo side. I love launching off an honest to goodness beach that includes some form of sand.

      Glad you like the P-Cat posts! I can’t wait to get back out on the water. And congrats on your restoration of #111!


  2. Charmian Wright DVM said

    And I hope your adventures with “Smilodon” can continue!

  3. […] wanted to built a damped pendulum KAP suspension for a while. My earlier designs involved a parallelogram section with four pivots and a straight section with a single pivot to […]

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