BASICS / CAMERA CRADLES /
Picavet Principles (Optional Information) Page 2
DAVID HUNT AUGUST 9, 2002
THE FUNDAMENTAL PRINCIPAL
EQUAL ANGLES PRINCIPLE
The operation of the picavet is based on one fundamental principle which will be referred to as the "Equal Angles Principle" (EAP).
A sagging line whose two ends are fixed, and a weight hung from the line such that it is free to move along the length of the line, will settle into a configuration where an imaginary vertical line running through the lowest point of the line will form equal angles with the line on each side.This is illustrated by the 2 adjacent drawings.
THE EAP APPLIED TO THE PICAVET
Figure 1
Figure 2
Figure 3
Figure 4
For purposes of this discussion, references to the various arms of the picavet cross will use the labels in Figure 1. Arms A & B are the arms of the picavet cross that line up with the kite line. Lines L1 & L2 are the lateral arms.
When we look at the Classic Picavet from the side, in a stable and level configuration, we notice that arms L1 & L2 are in the equilibrium of the EAP
(Red Lines of Figure 2).
The vertical white lines are added for reference.
Arms A & B are not in the EAP equilibrium (Blue Lines Figure 3). The outer angles are smaller than the angles toward the center of the cross.
This means that arm A will exert a force trying to move to the left, and arm B will exert a force trying to move to the right. If the picavet was constructed properly and A & B are equidistant from the center then the two opposing forces will be equal and cancel out (Figure 4).
One of the purposes of the small ring, where the picavet lines cross, should now be evident. Arms A & B will have a tendency to try and rotate to the positions occupied by L1 & L2, which are in EAP equilibrium. Without the ring, if the cross begins to turn, the two lengths of line which normally touch begin to separate. By adding the small ring this tendency to rotate can't get started.
Notice what would happen if you did not use a ring. If all arms of the picavet cross are of equal length then arms A & B would only rotate half the distance before arms L1 & L2, now out of EAP equilibrium, would exert an equal and opposite force stopping the rotation in what might be called an "X" configuration. This is actually the same as the Rendsburg configuration described by Beutnagel, Bieck, and Böhnke in the Aerial Eye (1:4 fall 1995). They use an H-shaped "cross", but a standard cross would serve the same purpose.
WHY YOU DON'T WANT TO USE PULLEYS AT THE ATTACHMENT POINTS
As was mentioned on page 1, the only time that the picavet line would travel through pulleys at the attachment points is when the "level" of the cross is first adjusted as the rig is place on the kite line. The line does not move through the attachment point pulleys when the kite angle changes and the picavet re-levels. It could be detrimental to the continued levelness of the rig if the ability of the line to slip through the attachment points is enhanced by the use of pulleys. Any movement of the picavet line through the attachment points after the rig is in the air would only mean that the cross and rig are no longer level. It makes more sense to prohibit the line from slipping through the attachment points once the rig has been initially leveled.
Cord Locks
Brooks Leffler wrote about this in the Aerial Eye (4:1). The problem became evident when he designed and built a stereo rig for John Carlson. The picavet cross for this rig was 30" across (76.2cm) and most of the weight was 27" (68cm) away from the center point of the cross. This produced a rig with large torque moments, which immediately began producing problems for Brooks as he tried to keep the rig level in flight.
Unlike the stereo rig that Brooks built, most KAP rigs are compact with their weight more centrally located and have much smaller torque moments. For this reason the problem is not as easily manifested. Many KAPers continued to use pulleys at the attachment points without ill effects other than the added expense and labor.
Brooks encountered another problem with this large stereo rig. The rig's large wind surface caused it to be forced back and thus out of level. Brooks solution was to pre-load the rig to be out of level in the opposite direction. When the wind forced the rig back it would them be level. Brooks found that he could "lock" the picavet suspension into the out of level position by using a cord lock to keep the picavet lines from moving through the attachment points. This implies that the picavet can be locked into perfect level when using rigs of a more typical design.
The bottom line:
You do not want the picavet line to slide through the attachment points once you have the rig level. In most cases you can get by without using a cord lock, but for an extra measure of safety use a cord lock and lock in your level setting.