Last up dated: 11 March 2004
Posted on: 17 February 2004
PERKIN-ELMER MARK I DRIFTSIGHT
INSTALLED IN ORIGINAL U-2 AIRCRAFT
by Glenn Chapman
When Kelly Johnson designed the U-2, it was imperative that anything not absolutely
essential to flight not be part of the design. This included an ejection seat,
high-frequency radio system, automatic direction finder systems, radar, in-flight
refueling, and many other systems. Although most other American military aircraft
had many of these systems, it was essential to stay within the “one
pound of weight equals one foot of altitude” rule. Still, because of
the pilot’s inability to see anything directly below him during flight,
and because the U-2 had only rudimentary navigation systems, and to assist
in some systems platform’s operation, some sort of viewfinding apparatus
was needed. However, it couldn’t be too big, must be very, very light,
and be simple in operation.
A company in Norwalk, Connecticut was awarded the contract to devise such
a system. The company was called Perkin-Elmer and was experienced in designing
and manufacturing optical equipment. They designed a thing called a “Driftsight”
which would be operated with a fairly simple electronic hand control device
capable of looking around the underside of the aircraft from about 85 degrees
horizontally to the vertical “nadir” position. (Nadir is a term
defined as “perpendicular to the horizontal axis.”) The hand control
could also provide a “tracking” function that initially was thought
could help in controlling the trip rates of the camera (or Nephographic Systems
as it was originally called) while in flight. The pilot could crank the altitude
and calculated ground speed into the hand control and a V/H or “V over
H,”, signal could be computed. This is the quotient of V, or ground
speed, divided by H, or altitude, giving a ratio to the camera platform. This
was the standard then, and remains ;the standard today, that provides the
camera system with the information needed to allow the shutter to trip at
a rate that provides a 56% overlap of film coverage. He would be able to check
the accuracy of the tracking by viewing the ground movement through the driftsight.
The driftsight system designed by Perkin-Elmer was designated as the “Mark
I Driftsight” and the driftsight hand control as the “Mark II
Hand Control.” Connection of the hand control to the driftsight optics
mechanism was fully electrical through small servo systems that provided an
“A”, or azimuth, and “A+E”, or azimuth plus elevation,
movement of the driftsight optics. Both had to work together in order to move
the optics around properly. The driftsight also had a X1 and X.4 (Times 1
and times .4) magnification of the area being viewed below the aircraft that
was controlled by a switch on the hand control. At the top of the driftsight
was a “viewing knuckle” that was bent about 85° degrees from
the driftsight itself, which was positioned vertically behind the instrument
panel. Inside this knuckle was a fixed upside-down inverted “V”
mirror that reflected image to the viewing glass at the front of the knuckle
This was was an optically-ground glass about six inches in diameter that allowed
the pilot to view non-distorted image through the driftsight. In front of
this glass was another viewing plate called the “drift reticule plate”
with vertical lines on it. At the top of this plate was a small rectangular
plastic tab that was marked in degree increments of up to 7°. Inside the
driftsight itself were a series of optically-correct lenses and a “crosshairs”
reticule that was aligned with the forward axis of the aircraft. By positioning
the tab on the plastic drift reticule plate and aligning it up with the absolute
direction in line of flight, the pilot was able to determine the exact amount
of drift he was experiencing with the aircraft. There was no “drift
string” atop the nose of the aircraft as many in those days had, because
the pilot would not be able to see it because of obstruction due to the placement
of the viewing knuckle. In fact, this knuckle created a blind spot that prevented
almost any forward sight from the cockpit whatsoever. The movable optics inside
the lower part of the driftsight itself was housed in what Perkin-Elmer called
a “hyperhemispherical” glass bubble about five inches in diameter
and optically ground to prevent viewing distortion. This, in a nutshell, was
the original driftsight configuration in the original U-2 aircraft. A long
rubber hood called a “boot” was attached to the viewing knuckle
to prevent any shadows or extraneous light from obscuring the image through
the driftsight. This was in effect doing the same thing that early photographers
did by throwing a hood over their shoulders while viewing an image through
a camera lens.
In addition to the driftsight, an optical sextant was installed at the top
of the aircraft nose just forward of the windscreen. This was the main navigational
system used in the early U-2 aircraft whereby optical celestial sightings
could be made. The sextant, although not an integral part of the driftsight,
was attached to it via a “roof mirror” attached to a sextant pull
knob at the upper right-center of the instrument panel just below and to the
right of the viewing knuckle. This roof mirror was comprised of two rectangular
front-surface mirrors attached in a “V” configuration. A rectangular
access in the top rear section of the driftsight housed this roof mirror assembly.
Sightings through the driftsight or sextant could be made, but not both at
the same time. For viewing through the driftsight, the sextant knob was pushed
inward all the way, moving the roof mirror out of the driftsight’s optical
path. For viewing through the sextant, the sextant knob was pulled fully outward,
pulling the roof mirror into the driftsight optical path, allowing the image
from the sextant to be reflected onto the roof mirror, thence upward through
the driftsight viewing knuckle. Instrument technicians had responsibility
for maintaining the sextant and roof mirror while the driftsight and hand
control were the responsibility of the Nephography Shop technicians.
In order to prevent the driftsight and sextant optics and hyperhemispherical
glass bubbles from fogging at altitude, a purging of all air from the two
bubbles needed to be accomplished. This was performed by connecting a bottle
of -96° ultra-dry nitrogen dew point to a connection that allowed the
nitrogen to enter the bubbles under pressure and push the air from the bubbles.
This took about forty-five minutes immediately prior to engine fire-up and
lasted until approximately ten seconds after the engine had come to idle speed.
Originally, the sextant and driftsight were purged separately, but a modification
was soon made that allowed both to be purged at the same time, allowing a
much more perfect purging cycle to be accomplished.
As the U-2 design progressed, and the systems platforms began to take shape,
it was determined that the camera platforms that the U-2 would be using were
so simplistic themselves that no V/H signal would be required at all. The
camera systems designed originally for the U-2 were the Hycon A-1, A-2, and
73B configurations, and a Perkin-Elmer Mark II 70-mm Tracker Camera. The A-1
and A-2 configurations used a motor-and cam switch system to provide a 12-second
or 15-second camera trip rate, depending upon whether a 4- or 5-digit cam
was used and the 73B had a “programmer” unit of relays that used
a base of 70,000 feet to set trip rates internally. The Tracker Camera used
a “relay logic” binary intervalometer for scan rate, normally
set for 32-seconds. None of these systems required the usual V/H signal, although
future systems might be developed, and ultimately were, that could use it.
The Mark I Driftsight was a very reliable unit, but required a lengthy amount
of time to install and align it with the line of flight. This was done by
first aligning and stabilizing the aircraft laterally and dropping a plumb
bob from the pitot tube under the forward part of the nose, which was about
as close as possible to exact centerline of flight of the fuselage. Using
masking tape, an “X” would be marked on the ground directly under
the point of the plumb bob. Then the plumb bob was aligned with the forward
part of the ARC-34 UHF Radio Antenna directly behind the fuselage equipment
bay and again a masking tape “X” was marked. The antenna, like
the pitot tube, had been determined also as an exact centerline mark. Then
a piece of chalk line or length of masking tape was used to align the two
marks made with the tape to provide a “line of flight” marker
under the aircraft. The driftsight optics were then manually adjusted under
the cockpit floorboards so that they were pointing at the nadir position.
With one technician in the cockpit looking through the viewing knuckle and
a second technician on the ground at the driftsight bubble, the driftsight
was positioned so that the fore-aft lines inside the driftsight reticule were
perfectly aligned with the line of flight mark on the ground. After securing
the driftsight, the optics were checked again to ensure that line of flight
had been properly accomplished. The roof mirror inside the viewing knuckle
also had to be aligned to the cross-hair reticule inside the driftsight. This
was done by viewing as closely as possible through the center of the knuckle
and moving the knuckle right or left until the “swinging image”
of the roof mirror line matched up with the reticule line-of-flight line.
The Mark II Hand Control, as well as the driftsight itself, could be a monster
to work with and very labor-intensive. The driftsight was pretty reliable,
but not so the Mark II Hand Control. The biggest problem seemed to be keeping
the servos inside the hand control properly aligned. It always seemed like
little gremlins would crawl inside it and loosen any security the servos had.
This was a 115 VAC, 400 Hertz system along with the normal 28 VDC to accomplish
some functions. The 400 Hertz power was solely for the servos, while the DC
power was for indicator lights, camera operation, driftsight optical magnification,
and other systems operations. If the aircraft electrical inverter system varied
by only a few Hertz, the servos would not position right. There were many
problems with this hand control, but these were the most important and caused
the biggest problems.
Connection of the Mark II Hand Control to the Mark I Driftsight was in itself
a major job. At the lower starboard position of the driftsight underneath
the aircraft floorboards were the two mechanical drivers for the driftsight
optics. Access was through a panel hole in the right floorboard about five-inches
in diameter.
The drivers were nothing more than two circular metal discs about the size
of a nickel that each had a slot machined across them. These attached directly
to the optics drive mechanism. These slots had to be aligned vertically in
a near-perfect position in order for the Mark II Hand Control electronics
adapter to be positioned. Inside this adapter were two discs identical to
the optics driver discs except that in place of a slot, a tab across the disc
was affixed. These tabs also had to be positioned in a near-perfect vertical
position in order to align the adapter to the driftsight. An electrical Cannon
plug was attached at the exterior of the adapter that allowed the connection
of the hand control cable. Inside the adapter were two servos that were attached
to the positioning discs. The entire adapter had to be installed perfectly
onto the driftsight with three Allen head 10-32 machine screws. And this all
had to be done by feel, underneath the floorboards, bending over on hands
and knees, with other cables and stuff in the way. It was possible to secure
the adapter onto the driftsight with the discs not aligned properly. There
was no way of knowing until power was applied to the hand control and carefully
moving the handle around to see if the optics tracked properly. If the discs
were mis-aligned, the optics would not move or would move erratically. More
importantly, the entire driftsight optics driver assembly as well as the electronics
adapter could be severely damaged or, at the very least, need re-alignment
in the shop, requiring even more time. Usually the adapter was initially secured
only “finger-loose” so that if alignment was not right, the chance
of damage might be less. Sometimes it took hours just to get this adapter
properly positioned before the rest of the job could be completed. Alignment
of the hand control was not required because the servos had already been adjusted
in the shop. Once the driftsight and knuckle were installed, properly aligned,
the electrical adapter connected properly, and the hand control connected
and powered up, the system could be checked operationally. A checklist was
accomplished in which the optics were moved around to check for proper positioning
and operation, the driftsight magnification worked right, and tracking was
done as it should be.
After only a couple of years, it was determined that a simpler hand control
system could be used that would cause less maintenance efforts to be expended.
A company called Baird-Atomic developed a purely mechanical unit that used
no servo systems whatsoever. No tracking was done. The unit was about 25%
as big as the original Mark II Hand Control and the only things on the panel
were a Mode Switch (for operation of the camera platforms), a Master Switch
(provided power to the tracker camera or other systems), a “X1-X.4”
Switch (for driftsight magnification), four indicator lights (”A”,
“B”, “C”, and “D”) that indicated camera
modes, and a triangular plastic handle about four-inches long that was used
to position the driftsight optics mechanically. This hand control was designated
as the Mark III Hand Control and was extremely reliable, simple to use, and
much less difficult to maintain and align.
The Mark III Hand Control used two 12-tooth splined cables connected between
it and the driftsight optical drivers to provide optical positioning. In place
of the Mark II Electronic Adapter under the floorboards, the Mark III Hand
Control used two small mechanical adapters that connected to the optics drive
assembly much the same way as the Mark II Adapter had. A splined cable connection
was at the exterior of each adapter. Installing these adapters and aligning
them with the driftsight optics drivers was much easier than the Mark II Adapter.
These adapters were secured to the driftsight with three 10-32 Allen-head
screws much like the Mark II had been. It was learned early-on that the easiest
way to install these Mark III adapters was to use an old piece of the splined
cable about two-inches long as an alignment tool. Again, this was all performed
underneath the floorboards on hands and knees.. The splined tool would be
put into the adapter, one screw with the Allen wrench already attached to
it was put into one of the three holes in the adapter, the whole thing secured
with the right hand, and placed through the hole in the floorboard onto the
.lower optics driver disc. While holding onto the adapter, Allen wrench, and
screw, the splined tool would be moved around slowly with the thumb and fingers
until the alignment tab fell into the alignment slot on the driver disc. This
was easy to feel, unlike the Mark II adapter, which one could not feel at
all. Once the tab and alignment slot were aligned, the Allen wrench would
be turned until the screw was “finger loose.” Then the other two
screws were installed and all three were tightened down onto the driftsight
properly. The top adapter would be installed the same way. The two splined
cables were now connected and it was time to get back to the Mark III Hand
Control.
Aligning the Mark III Hand Control was much easier than the Mark II Hand Control.
Again, a line-of-flight line was installed underneath the aircraft. While
looking through the viewing knuckle, one of the splined cables was turned
clockwise while the image in the driftsight was being observed. If the image
only went around in circles, that cable was the “A”, or azimuth
cable. If it went up and then turned right and kept going, it was the “A+E”,
or azimuth plus elevation cable. While turning the “A+E” cable,
the image moved upwards and to the starboard side of the aircraft until it
was approximately 90° in respect to line-of-flight. While holding the
triangular handle on the Mark III Hand Control fully vertical and pointing
about 90° to the right, the “A+E” cable was moved counter-clockwise
until the image intersected the line-of-flight marks on the ground. The “A+E”
cable was connected to the rearmost splined connection under the hand control
and the “A” cable to the other, foremost, part of the hand control.
Now the hand control was positioned into the right cockpit console directly
aft of the Auto Pilot control unit but not yet secured into the console. Again,
the handle was rotated and moved up and down carefully to check operation.
Then the handle was pulled full downward and rotated until the detent in the
hand control was felt. The “A” cable was dis-connected and the
image aligned by rotating the cable until the pitot tube was aligned with
the driftsight reticule line. The “A” cable was re-inserted and
the hand control was checked again for full operation up and down and full
around the underside of the aircraft. If fine-tuning of the cables was needed,
it was usually only one tooth plus or minus. A last check was made whereby
the handle was put into the detent position and straight up with the optics
pointing at nadir position. While holding the handle full up, the handle was
pulled in a full 360° circle. The image at nadir was checked to move in
either a very tight circle or, better yet, no circle at all. At this point,
the hand control was considered to be in alignment.
Eventually, the line-of-flight alignment was discontinued and the “quarter
method” began being used. This was highly illegal at the time, but saved
a lot if time and was much easier. I was the one that initiated the quarter
method way back around 1960 or so, scared to death I would be caught, and
then was caught by none other that Colonel “Big John” DesPortes,
our Wing Commander. All he told me to do was re-write the checklist and be
sure to “---put a FOD (foreign-object damage) phrase in it.” It
finally evolved into a small brass Hand Control Alignment Tool with a “Remove
Before Flight” streamer.
The Mark II Hand Control was very precise, but was extremely
over-designed for use in the U-2. It was a relatively unreliable unit, was
difficult to install and align, and very time consuming for both flight line
and in-shop maintenance and alignment, and was expensive. It was used much
more by the CIA pilots than with the Air Force U-2s.
The Mark III Hand Control was not as precise as the Mark II, but was extremely
reliable, could see almost a full 90° up and down, and could see 360°
around the underside of the aircraft. It was not difficult, except for the
adapter installations, to align, did not require a large maintenance and alignment
time on the flight line or in-shop, and was inexpensive. It was used almost
exclusively with the Air Force U-2 program.
Eventually, six of the Mark III Hand Controls were modified with a special
detented collar around the handle. This collar was easily positioned by the
pilot to any of the four positions. The reason for this modification was to
allow the pilots to use the detents while the hand control was positioned
to one side or the other. Each detent corresponded to a specific camera angle,
especially the three Hycon 73B camera “look angles,” so he could
determine whether he was properly on track to get the best oblique photography.
When the hand control was positioned fully to starboard or port, a detent
notch in the collar could be selected, the handle pulled down into this slot,
giving the pilot an extremely good view of his track, allowing him to move
the aircraft as desired to get optimum photo coverage with the camera platform.
This hand control was designated as the Mark III Modified Hand Control or
as was commonly called, “Mark III Mods.”
As the U-2 evolved, so did the driftsight. Eventually the Perkin-Elmer Mark
I Driftsight and Mark II Hand Control and the Baird-Atomic Mark III and Mark
III Mod Hand Controls were replaced with a much better driftsight system manufactured
by the McDonnell-Douglas company. As the U-2R aircraft were modified into
the U-2S digital “Glass Cockpit” models, the driftsight system
was removed. Technology had evolved to the point where Global Positioning
Systems and other high-tech systems were installed, relegating the driftsight
and hand control into antiquity. For it’s time, however, it was a useful
photographic and navigation aid for U-2 pilots. There are a few pilots, and
Driftsight Dragons also, out there today that will bemoan the loss of this
“old stovepipe” and reminisce endlessly over it. I am one of those
Driftsight Dragons and miss it, sometimes with tears in my eyes, and other
times, as I remember my times with it, like I would miss a toothache. The
old “Driftstick” is now in Optical Heaven for sure. Or would it
be Optical Hell?
Who knows, and who really cares? Only us old Driftsight Dragons and a few
old Dragon Pilots.
Perkin-Elmer Mark I Driftsight in Early U-2 Aircraft
Driftsight Part Number Was PE-151-0001
Baird-Atomic Mark III Mechanical Driftsight Hand Control
Hand Control Part Number Was BA-501-0005
©2004 Glenn Chapman
Many thanks to Glenn for writing down this excellent history and set-up of the driftsite used on the U-2A....He is a truly the "Driftsight Dragon" -John
Back to the main U-2 page