1-Line KAP Rig Suspensions


Traditionally, KAP rigs have been one of two kinds: Picavet or Pendulum.

Picavet Suspension

Pendulum Suspension

Picavet suspensions are smaller, but pendulums may have a slight advantage in terms of stability, especially when the kite line is steep. Both have some shortcomings, however:

  • They use a two-point connection to the kite line for stability in the Pan direction, but this makes them susceptible to swinging back and forth at right angles to the kite line. This shows up as either a tilting or a rolling motion from the camera's perspective (or both).
  • The Pan stability depends on the horizontal component of the kite line vector. When the line is close to vertical, there is little or no horizontal component of the line vector and this makes the rigs unstable in Pan as the line becomes more vertical.
  • The previous issue cannot be completely controlled or anticipated because the angle that a kite flies at will depend on many factors that can change from day to day or minute to minute.
  • These suspensions tend to be short and typically hang three to five feet from the line. This makes them easy to carry (in the case of the pendulum), less prone to tangling (in the case of the Picavet) and easier to launch. It also has the side effect of being more sensitive to movement of the kite and movement of the kite line. Shorter suspensions also swing back and forth faster, which is more troublesome for photography.
  • Picavet suspensions tend to twist back and forth when the kite line is steep.
  • Pendulum suspensions also twist back and forth. This twisting is less severe than with a Picavet, IMO, but the pendulum's twisting will depend on the tension on the kite line (another factor that is difficult to control and predict).

KAP rigs rotate in one or more of three directions. Here is a video showing those three kinds of rotation.

Kinds of KAP rig rotation from Scott Armitage on Vimeo.

Here is an example of what a camera sees from a KAP rig without any attempt to stabilize it. You can notice motions in all three rotating directions: Pan (left/right), Tilt (up/down), and Roll (clockwise/counterclockwise).

Unstabilized video from a KAP rig from Scott Armitage on Vimeo.

The Single-Line Suspension

A solution I have created to deal with this problem is what I call the Single-Line Suspension. This suspension is a single vertical line that is attached to the kite line at a single point. Typically the Single-Line Suspension is longer than a Picavet or Pendulum because it can make use of the improved performance of longer suspensions without adding bulk or complexity.

Single-Line Suspension

The longer suspension makes it more tolerant of kite line movement and makes any swinging motions more gentle.

It has one major problem, however. It has no stability in the Pan direction.

The Moving Vane Rig

To deal with this problem, we must add features to the rig to compensate for the lack of Pan stability. One way to do this is to use the wind as a directional reference.

Here is a rig I made that uses a vane attached to the rig to stabilize the rig in the Pan direction. The vane always goes downwind and as long as the wind is not turbulent, the rig keeps pointing in the same direction. On this rig, I attached the vane to a servo. This allows the vane and therefore the rig to turn over a 180 degree range. In addition, this rig has a gyro for the Pan axis that corrects for changes in the wind direction. The disadvantages of this approach are that the panning range is limited and it does not work well near the ground or in otherwise turbulent conditions.

Demonstration of a Moving-Vane KAP rig from Scott Armitage on Vimeo.

The UFO Rig

Another approach to adding Pan stability is to make the rig push against the wind to hold its position or to move to a new position. I experimented with fan blowers (not enough force) and propellers (too much power required to pull the rig into a strong wind). In the end, the best configuration was a set a pusher paddles that rotate about a vertical axis. These paddles use drag rather than use lift like a propeller. The paddles make it possible to move the rig in Pan, but do not provide automatic stabilization. For my design, I added gyro stabilization to the paddle controller to keep the rig pointed in the same direction.

In this type of rig, the rig is turned sideways so that the main frame of the rig is horizontal and the camera hangs off one end. This gives room under the rig for the paddle assembly. A larger and somewhat more complex version could place the paddle on top of the rig and suspend the camera from the underside. The rig is suspended from the single support line by a tripod of three lines, which keep the rig frame horizontal.

The Tuned Mass Damper

All single-line suspensions tend to rock back and forth about its center of mass when it is disturbed. Fortunately, this rocking is of a predictable frequency, so it responds well to a technique used in other fields: the Tuned Mass Damper (TMD).

The TMD is a way of converting mechanical motion into heat, effectively dissipating this undesired motion. A TMD consists of a counterweight, a spring and a friction device to convert motion into heat. To be an effective damper, the TMD must be adjusted so that its counterweight oscillates back and forth at the same rate as the rig that you are attempting to stabilize. In addition, the TMD mass must be large enough to quickly remove the rocking motion. In my experiments, a counterweight of about 20% the mass of the rig does a good job.

In my UFO rig, I attached the three support lines to a platform that amplified any rocking movement and then placed a relatively small TMD at the point where the lines come together. The counterweights are 10 gram bolts, and the spring and friction device are both implemented by a single sheet of rubber.

Effect of a Tuned Mass Damper on a KAP rig from Scott Armitage on Vimeo.

One could also add gyro stabilization for tilt and roll, but this is probably unnecessary, since a single-line suspension with a TMD is already very stable.

Here is a video showing the UFO rig in action. It shows how the rig corrects for spinning, how it turns in pan and how it is launched.

KAP Rig with a Single-Line Suspension from Scott Armitage on Vimeo.

Another advantage of single-line suspensions is that the rig can be near the ground but not near the operator. This gives the rig more "reach" to position it over areas that cannot be walked on.

Super Simple Stabilized System (S4 Rig)

In addition to move complex rigs requiring gyro-controlled paddles or vanes, it is possible to use a single-line suspension with very simple rigs.

The "S4" rig is intended to always point in a single direction, due to its vane always being downwind from the camera. It works well for rigs where the camera always points in the same direction, such as straight down. In spite of its simplicity and low cost, its stability outperforms many conventional KAP rigs. Like all single-line suspensions, it tend to rock about its center of mass, so it benefits from a TMD, This rig rotates slowly as the line swings back and forth, so it's probably better for still pictures than for movies.