Notes on Kite Aerial Photography: Equipment
BETWEEN the CRADLE and the KITE LINE
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My first Picavet
The Picavet in flight
My second Picavet
Picavet No. 3
Attaching the Picavet to the kite line
KAP Rig No. 1 hanging from its Picavet cross in my living room. March
1995 (28K jpg)
Over a century ago M. A. Batut's kite aerial photography apparatus involved a camera attached directly to the kite. By 1906 George Lawrence was suspending the camera from his kite line (cable) well below his train of kites. Separating the camera from the kite reduces camera motion and thus lessens blur in the image. By 1912 Pierre L. Picavet was describing a cross-shaped suspension we now call the Picavet. It involved a rigid cross suspended below the kite line with each of the cross' four ends connected to two attachments on the kite line. The line providing these connections is a continuous loop. Its attachment to the kiteline and Picavet cross sometimes involves pulleys. The result is a nominally self-leveling platform that resists a turning moment (as in the camera cradle rotating below it.)
Picavet diagram (from the Aerial Eye, Fall 1995)
This diagram of the Picavet shows the systems major
landmarks. The cross dimensions seem to vary from 20 inches across to as little as 3
inches. The loop of line that threads between the cross and the kite line attachment
points is often 50 feet in length. A small split ring (R) is used to constrain the two
innermost lines as they cross. Using the labels on the diagram as a key, a suggested
threading sequence is
A1 - 1 - B1 - R - 4 - A2 - R - 2 - B2 - 3 - A1.
Through much of this century the Picavet suspension was a lost idea and most KAPers used a pendulum-based suspension (see Kite Aerial Photography by Mark Cottrell, 1987). In 1988 the Picavet method was rescued from obscurity by Michel Dusariez in an article published by the Kite Aerial Photographers Worldwide Association (KAPWA). More recently the Picavet was described in an Aerial Eye article by Ralf Beutnagel, Wolfgang Bieck, and Otto Bohnke (Picavet - Past and Present, Aerial Eye, Vol. 1, No. 4, Fall 1995). The Picavet is now a popular means for suspending camera cradles from the kite line.
Vertical
axis attachment to Picavet cross (38K jpg)
Since Anne Rock used a Picavet suspension I figured I should too. I've not regretted the
decision. My first Picavet cross is made from aluminum u-channel with an outside dimension
of 1/2" square. The two 12" long legs of the cross are half-lapped at the center
and the result is a light, stiff cross. A hole is drilled at the center of the cross and
the camera cradle's vertical axis is attached to this hole using a wingnut. At the ends of
my first Picavet cross I used four aluminum keyrings to hold the line running back and
forth to the kiteline. At the kiteline, small loops of line attached by Prusik knots hold
a swivel that in turn holds a split ring. The Picavet line runs through these split rings.
The cross' keyrings do not let the line slide as well as pulleys might so the rig will
hold a tilt if one is present. To avoid this you level the cross by manually aligning it
with the horizon before sending the rig upward.
(left) Image
(albeit fuzzy) showing kite, kite line and attached Picavet rig with camera cradle, April
1995 (29K jpg)
(right) Picavet rig showing keyrings used for line attachment, April
1995 (36K jpg)
These images show the camera rig and Picavet suspension as
they appear immediately after launch of the camera. You can see how the suspension keeps
the camera near level with the horizon. If you think about the lefthand image it also
makes sense that the rig resists rotation. For the cross to turn it would have to rise, a
movement resisted by the attached camera's weight.
The
Picavet rig flying above the Berkeley Waterfront
October 1995 (46K jpg)
The image shows a "Picavet eye" view of the camera rig about 50 feet above the ground. This was taken by a disposable camera rig attached to the kiteline just above the radio-controlled rig. The sharp-eyed among you might notice that the Picavet cross in this image is much smaller than my original. It is a setup based on Wolfgang Bieck's rig and uses miniature pulleys from radio-controlled sailboats.
Picavet No. 2
(after Wolfgang Bieck)
October 1995 (top image,33K jpg - bottom image, 28K jpg)
My second Picavet cross is patterned after Wolfgang Bieck's miniature Picavet design. My sense is
that the trend in Picavet rigs has been to smaller and smaller sizes. A small Picavet is
easy to transport (shirtpocket), lightweight, less prone to flex. For the cross I
laminated a sheet of aluminum and model aircraft plywood then cut the cross shape (about
4" wide) out with a band saw.
I've
followed Wolfgang's lead and used small, precision pulleys (or blocks) from
radio-controlled sailboat suppliers. The small blocks are beautiful and perform extremely
well. My source for the German Pekabe-brand hardware was R/C Model Yachts and Accessories
(see this site's catalog page). In fact, one thing I
noticed since switching to this Picavet is that my rig's balance was not very precise.
With so little friction in the suspension a poorly-balanced camera cradle will tilt and
images will not be square with the horizon. At the time of this writing I've only taken
two rolls of film with the small Picavet but these indicate the need for even more
balancing.
I've also noticed that the smaller Picavet resists rotation less effectively than my larger cross. It seems spongy when rotating the camera in plan where the larger cross seemed more crisp. I guess this makes sense if you think about it. I'm now thinking about scaling the cross up to about the width of the camera cradle.
In an admittedly lame maneuver, I left my Picavet No. 2 on the grass one day and neglected to pack it up. By the next day it was gone for good. So I set about making a replacement.
Picavet
No. 3 uses parts scavenged from an old 10 mB disk drive. It too follows the Bieckian
credo of small is good. May 1997 (top image,36K jpg - bottom image, 40K jpg)
This image shows my third Picavet cross as a work in
progress. Son Charlie Benton and I have been taking things apart lately and our
approach this time was inspired by the lightness of some "swinging head arms" we
removed from a 10mB hard drive. These arms (one is lying on the sketch in the photo
opposite) appear to be made from a very light aluminum alloy. I cut four of the arms
down and epoxied them around a central brass tube. The structure was then reinforced
with a set of four wood corner blocks and bound by a single compression ring.
The
6/32 bolt of the camera cradle (not shown) enters the brass tube from the bottom and is
captured by a threaded aluminum insert that screws in from the top. The threaded
insert has a knurled plastic knob attached to its upper end. The camera cradle has a
small wire bracket that keeps the entire cradle from unwinding out of the insert (the
dreaded "bombs away" scenario.) Otherwise a sharp turn of the rig in the
unwinding direction might rotate the cradle by momentum to the point of separation.
Like its predecessor, the new Picavet uses Pekabe ball-bearing blocks to guide the
suspension's line. Each block is attached to the Picavet cross by a small swivel
bracket, also a Pekabe product.
(return to top)
Kite
line clip (after Brooks Leffler)
October 1995 (top, 19K jpg - bottom, 29K jpg)
In the fourth issue of the Aerial Eye, Brooks Leffler
described a simple device he called Brooxes Hangup™. The Hangup is "a universal
hanger which provides a secure, quick connection with little sacrifice in line
strength" and in Brooks' version was made of a single UHMW polyethylene grooved on a
table saw. Prior to seeing this article I attached my Picavet to the kite line by tying
short loops of line around the kite line using a Prusik knot. While this worked reasonably
well I didn't like two aspects of the arrangement: the knots would slip down the kite line
and they were a pain to untie as you brought the rig down. Brooks' approach seemed a nice
alternative.
I've
made several pair of hangups using Brooks' geometry but different materials. As shown in
the images, my hangups are made using an aluminum backplane and model plywood front plane.
The two planes are connected by small cap screws tapped into the aluminum. Nylon spacers
separate the two planes and provide a smooth bearing surface for the kiteline. The kite
line makes a complete loop around the upper two posts in the Hangup. At this writing I've
been flying with them for a month and they seem to be working very well - easy on and easy
off.
Postscipt: I've now made a dozen or so pairs of these
attachment clips and am probably up to 200 or so flights using them. The clips are
serving quite well. When I attach them to the kite line I put a single turn around
the pair of bolts on the right and another single turn around the single bolt on the
left. They have yet to slip -- even when the line goes quite slack.
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Comments to author: crisp@socrates.berkeley.edu . All content,
graphics and
images contained throughout are Copyright (C) 1995 - 2005 by Charles C. Benton
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All rights reserved. Revised: Saturday, June 26, 2010
URL: http://www.ced.berkeley.edu/~cris/kap/equip/picavet.html