The Bombsight

Here is the first, exclusive description of how the famed Norden bombsight works.

Since crashed bombers have provided our enemies with the famed Norden bombsight, the armed services have made available to Flying, for the first time, detailed photographs and descriptions of this precision instrument.

For years, the Norden was so secret it could hardly be mentioned. It was kept in air-conditioned dustproof vaults, surrounded by high barbed wire and constantly patrolled by guards. When a bombsight was taken to a plane it was always carried by two men wearing service pistols and its canvas cover was never removed until the aircraft was in flight. AAF bombardiers had to swear a solemn oath to guard it with their lives.

Early in the war when a Catalina sank near a coral atoll just south of Hawaii, US Naval vessels were dispatched to the scene and divers searched for the plane. Grappling hooks brought it up, the bombsight was removed and destroyed, and the plane was allowed to return to the bottom.

The complete bombsight actually is two mechanisms — a lower unit or stabilizer, and an upper unit consisting of a number of assemblies, most important of which are the telescope, the computer and the gyro.

The stabilizer keeps the line-of-sight direction fixed, or controlled. It also initiates a signal that tells the pilot or auto pilot how the plane is to be maneuvered to solve the bombing problem. The stabilizer also controls the flight of the plane, through the ship's pilot or auto-pilot. So well do the stabilizer and auto-pilot perform this function that, once the bombsight has been set on the bombing run, the plane would continue its flight on a perfectly even keel and drop its bombs at exactly the right instant in time and the right spot in space, even if every member of the crew were killed at that moment. The plane itself would continue on the predetermined course until it ran out of gas or was shot down.

The telescope is used to sight on the target. The gyroscopes hold the bombsight stable. The computer solves the various mathematical problems which confront the bombardier just prior to the bombing run.

What are some of these problems?

First is the speed of the plane through the air, which imparts exactly the same speed to the bomb at the instant of release. Second is the force of gravity. Third is the force of air resistance. Fourth is wind motion and fifth is target motion.

The force of gravity and air resistance are compensated for by a precalculated table which is given to the bombardier. From this table he gets his first setting on the bombsight — the actual time of fall from his altitude.

Since the bomb is traveling at the speed of the plane at the instant of release, but is slowed by air resistance and follows a steadily-steepening curve of descent, this data, too, must be precalculated and given to the bombardier in a table. He, in turn, must set the bomb-sight to make allowances for this additional correction, which is called "trail."

Then comes wind correction. Whenever wind is blowing, the bomber — and the bomb — is in a moving block of air which affects their ground speed and drift. The wind may be blowing from head-on — in which case the ground speed of the plane will be retarded. It may be blowing from behind, in which case the ground speed of the plane will be increased. Or it may be a crosswind, in which case a drift correction must be made.

Correcting for drift is especially complex since, if a plane is crabbing, the bomb does not fall on the ground track of the plane but is carried along in the moving block of air — the wind. In the Norden, this correction does not require a special setting but is made automatically by a mechanical linkage between the trail mechanism and direction control.

Target motion has the same effect as a wind of the same velocity in the opposite direction. In practice it doesn't make any difference whether the wind or the target is in motion, nor does a combination of wind and target motion require any new setting. After the settings for trail (or lag) and time of fall are made, the bomb-sight is concerned only with the resultant "closing" speed and direction of the airplane with respect to the target. It need not be concerned with the causes of the relative motion.

These corrections have solved the basic problem of bombsight accuracy. But there is still a new problem — the fact that the airplane itself is not stable but rotates about three axes and can also move forward, side to side and up and down. In the Norden bombsight, electrically-driven gyros — one with a vertical and the other with a horizontal axis — control the stabilized line of sight.

Now how does the bombsight operate? The problem is comparatively simple in essence, if not in detail. As the bomb is released, its speed (inertia) moves it forward, gravity pulls it down, and air resistance retards both actions. The problem is to determine the correct point to release the bomb.

The first calculation is the length of time required for the bomb to fall to the target. The bombsight then determines the horizontal speed with respect to the target and multiplies that speed by the time of the fall. Result is the distance which the bomb would travel horizontally (with no retardation) during that interval. After mechanically subtracting from that total distance the trail (lag) which was also set in the bombsight from a chart, the bombsight produces a range angle. This is the angle formed between the line of sight to the target and a vertical line from the airplane at the proper instant of release.

The bombsight is basically an automatic speed and distance calculator which interprets its findings in the form of an angle. If you remember your plane geometry, you will recall that in two triangles having the same angles, the ratio of the length of the side of one triangle to the length of the corresponding side of the other triangle is equal to the ratio of another side of the first triangle to the corresponding side of the second triangle.

Now imagine the right triangle formed by the line directly beneath the bombsight to a point on the ground, a line from that point directly to the target, with the third side of the triangle formed by the line of sight from the bombsight to the target. Then imagine another very tiny right triangle within the bombsight itself created by the same line of sight to the ground beneath and to the target ahead, but bounded on the bottom by a horizontal wire. The ratio of the sides of these two triangles is identical.

In the so-called timing type of bombsight, the target is "tracked" by moving a crosswire back along this horizontal wire for a number of seconds corresponding to the estimated time it will take the bomb to fall. The crosswire is kept in line between the bombardier's eye and the target and, at the same time, a second crosswire is moving forward from the vertical point directly beneath the bombardier's eye. The two crosswires across the vertical wire look like an "H" laid on its side. The speed of the first wire corresponds to ground speed. The distance which it moves along the horizontal sighting wire corresponds to the distance on the ground over which the airplane travels during the time of tracking.

If the rear wire travels forward at the same speed as the front wire moves to the rear, and if the tracking time is equal to the time of fall, it is apparent that the rear wire will move ahead of the vertical a distance corresponding to the actual distance which the airplane will travel during the time of fall.

But that is not the correct range because trail has not been subtracted. To accomplish this, we make an adjustment. The rear wire is initially set to the rear of the vertical a small amount corresponding to the trail. It starts forward with a handicap which causes it to go beyond the vertical a distance corresponding to the correct range. Now the line from the bombardier's eye through the rear crosswire is the line of sight which should pass through the target at the instant of release. The bombardier directs the airplane along the proper course as the target and the rear crosswire approach each other. Just as they come in line, the bomb is dropped instantaneously.

In the Norden bombsight as now used, the line of sight is a telescope, power-operated by a variable speed drive. This telescope moves automatically to keep the line on the target. The mechanism sets up within itself a release-line-of-sight angle corresponding to that generated by the rear crosswire in the sight previously described. The lateral crosswire is kept on the target, as the telescope is driven. When the telescope reaches the right angle the bombs are released electrically. While the bombs could be released manually, an interval of only one-fifth of a second at 200 mph would cause the bombs to go 60 feet beyond the point which they would hit with an automatic release.

American precision bombing technique calls for a check of the bombsight before arrival at "initial point." The initial point (IP) is an easily-identified check point in the terrain, usually four to five minutes from the target. The C-1 auto pilot and the bombsight are checked and running at this time. The bombardier takes a drift run to obtain wind direction and velocity at the operating altitude. By using a modified E-6B computer, or an automatic bombing computer which is attached to the stabilizer, he can obtain the drift on any heading and also the dropping angle. He presets the data obtained into the sight to save time during the bombing run. Meanwhile he takes a final check on the target charts and maps.

The formation turns and the bomb bay doors are opened. The bombardier has previously placed all the switches and levers of his instrument in their proper position, Now, by means of the automatic pilot, the bombardier guides the aircraft into the attack heading by sighting over the trail bar. He rechecks the drift and dropping angle. It is a simple matter for the plane to be turned over to the bombardier. The pilot can turn on the auto pilot and let the bombardier know he has done so, or when the bombardier operates the bombsight clutches for bombing purposes, he automatically connects the sight into the auto pilot for the run.

As soon as the target is at a 70° sighting angle, the bombardier throws in the telescope drive clutch, starting the lateral or range hair driving on the target. If evasive action is necessary, the bombardier unclutches the auto pilot clutch (a secondary clutch) and maneuvers the plane by turning the bombsight or using a turn control knob. The degree of bank is limited mechanically by stops on the auto pilot, preventing the bombardier from making excessive banks which might break up the formation. His turns should not take the plane more than 15° off course. Evasive action is continued until the rate of index indicates that the bombardier has the prearranged time of bombing run remaining. This time is measured by noting the distance between the rate and telescope indices and converting to minutes.

Now the bombardier guides the plane onto the target by looking into the telescope and when the vertical hair bisects the aiming point he engages the direction clutch, thus stabilizing the bombsight directionally. He uncages the gyroscope and levels the bubble, which is centered when the gyro is operating properly, by using the two precession knobs. He places the range hair on the target by using the outside range knob. If any ground motion is apparent, he synchronizes the movement by using the inside range and course knobs. When the motion ceases the bombsight is synchronized and takes over automatically after the trigger is raised and locked. Bombs are released automatically when the rate and the telescope indices cross.

After the bombs are loosed, the bomb bay doors are closed and the control of the automatic pilot is switched back to the pilot by engaging the auto pilot clutch and disengaging the directional clutch. The bombsight is turned off and all switches are opened. The pilot takes over direction of the plane again.

This article was originally published in the July, 1945, issue ofFlying magazine, vol 37, no 1, pp 28-30, 124, 126.
The PDF of this article includes, along with the photos above, photos of men operating the Norden sight, an example telescope image, and a diagram showing the various corrections the bombsight must compute.
Photos credited to British Official, AAF, Press Association, Inc.