Integration of the Automatic Pilot System and the Norden Bombsight

J B Nealey
Associate Editor

[Editor's note: A page with the captioned photos is provided.]

The fabulous Norden bombsight, as normally operated, is but one of a group of instruments that has put bombing among the exact sciences. Integrated mechanically and electrically, this set of devices constitutes the automatic pilot.

Highlighting the accuracy of the automatic pilot is a recent flight from Norfolk, VA, all the way to Iceland, in which the plane was flown mechanically the entire distance and without attention from the human pilot except for minor corrections for variation and wind drift. Regardless of this, the most spectacular contribution of the automatic pilot to the war remains in bombing, to which operation it has added sufficient accuracy to justify the term "pinpoint bombing." The automatic pilot referred to is the Stabilized Bombing Approach Equipment (SBAE) developed by Carl L Norden in 1935 so that full advantage could be taken of the accuracy of the Norden bombsight which he invented in 1928. The Carl L Norden Co. has been manufacturing both ever since.

Shortly after its adoption by the Navy in 1935, the SBAE was used as an automatic pilot and is so used today in the Flying Fortresses, the Superfortresses — in fact, in all multi-engined bombers. In this auto-pilot system, the Norden bombsight is attached directly to the housing of the azimuth stabilizing unit, which is also called the directional gyro stabilizer.

While there are many different types of automatic pilots, they all embody three basic groups of equipment, consisting of gyros, servo motors and suitable follow-up systems to make the control surface displacement proportional to the deviation of the airplane.

Mock-up of the SBAE

A mock-up of the SBAE is shown in fig 1, which is a simplified version of the actual setup of the Norden equipment in a bomber.
Fig 1

The system, represented in fig 1, includes the bombsight (1) which sets on the directional stabilizer (2), the flight gyro (3), also shown in fig 3, sector box (4), also fig 2, bank motor (5), also fig 4, and three servo motors (6, 7, 1)), also figs 5 and 6. The system is electrically operated from a 26VDC power source. Aileron, elevator and rudder control surfaces are represented by 9, 10 and 11.

The directional stabilizer (fig 1-2) can stabilize both the airplane and the bombsight in azimuth, simultaneously or separately. When the SBAE and bombsight are used together the bombsight course knobs (fig 1-12) are turned so that the SBAE will point the bombsight toward the target and at the same time turn the airplane in the proper direction for establishing a collision course at any ratio the operator desires. Also visible in the illustration is the pilot's control panel (fig 1-13) and the pilot's attitude control knob (fig 1-14) as well as the bombardier's control panel (fig 1-15).

The flight gyro (fig 1-3 and fig 3) controls the airplane laterally and longitudinally by means of an electrical gyro which employs the bail and roller system of erection. To it are fixed the brush-sector units controlling the elevators and ailerons and the attachments completing the follow-up system.

Fig 2

The sector box (fig, 1-4 and fig 2) is mounted on the directional stabilizer housing and serves as the connecting link between the stabilizer and the rudder servo motor (fig 5) and banking servo motor (fig 4).

Fig 3

The banking motor (fig 1-5 and fig 4) introduces bank to turns by moving the aileron sector in the flight gyro an amount proportional to the displacement of the rudder brush, thereby activating the aileron servo motor (fig 1-9) which applies aileron.

Fig 4

Fig 5

The three servo motors (fig 1-6, 7, 8 and figs 5 and 6) furnish the power to displace the airplane control surfaces. Each of these motors continuously rotates two clutch gears in opposite directions. The clutch gears, through a differential (44 in figs 5 and 6), drive the control surface drum and cable in either direction, depending upon which clutch gear is engaged by the clutch solenoids (49 in figs 5 and 6). Basically, the three units are the same except for follow-up design.

Fig 6

Follow-up System Explained

The follow-up governs the amount of control surface applied for any airplane deviation and returns the control surfaces to neutral after the correction has been made. This is accomplished by moving the sectors of the brush-sector units. This followup system can be traced in the various illustrations by the rods between the flight gyro and the servo motors and cables between the drums in the stabilizer sector box and the rudder servo and banking motors. In further explanation of the brush and sector unit it may be said that sensing airplane deviation from present flight or attitude is accomplished by the electrical contacts made between the brushes and sector terminals. Since the gyro remains in a fixed position a brush attached to it will move in relation to its sector when the plane and gyro case rotate off course, or vice versa.

Establishing an electrical contact immediately energizes the corresponding servo motor which responds by moving its respective control surfaces. The direction of movement is determined by which terminal on the sector is contacted by the brush as the terminals on opposite ends are in circuit with the opposed clutch-engaging solenoids of the servo motor, The directional stabilizer normally is placed in the bombardier's compartment in the nose of most bombers but in the belly of the torpedo bombers. The gyro is mounted in a vertical cardan and the stabilizer housing which is bolted to the airplane turns around the cardan when the plane deviates left or right from its course.

On the upper cardan gudgeon of the stabilizer two clutches are mounted: (1) a directional clutch (fig 1-16) which is engaged during a bombing run; and (2) a secondary clutch (fig, 1-17) which is engaged when the bombsight is not in operation. Sensing airplane deviation in azimuth is accomplished in the sector box.( fig 1-4 and fig 2), in which are located the rudder sector and brush (fig 2-18 and 19) which operate the rudder servo motor (fig 5) and rudder, and the aileron banking sector and brush (fig 2-20 and 21) which control the banking motor (fig 4) to motivate the ailerons through the flight gyro.

While both brushes are mounted on a single slide (fig 2-22) in order that they may move in unison, the aileron circuit can be interrupted so that rudder alone may be applied if desired. The sectors are moved by the follow-up drums (fig 2-23 and 24).

To follow through the operation of the stabilizer, assume that the airplane is being controlled in flight by the SBAE. When the plane deviates from its course it causes the gyro housing to rotate with it around the gyro and cardan, and a connecting arm between the secondary clutch (fig 1-17) and the brush slide (fig 2-22) moves the brushes to make electrical connections on one end or the other of their respective sectors. Assuming for the moment that the banking circuit is "off," the contact is between the rudder brush and sector alone and this energizes the proper clutch solenoid of the rudder servo motor (fig 5) which applies the rudder (fig 1-11) through the drum and cable (fig 5) . This starts to bring the plane back on its course.

Attached to the rudder servo motor drum is another with a cable to the follow-up drum in the sector box (fig 2-23). Rotation of these drums moves the rudder sector and this action, rudder displacement and follow-up, continues until the neutral segment is under the brush, thereby breaking the electrical circuit to the rudder servo motor. This determines the amount of rudder displacement. When the airplane responds, the gyro housing rotates the other way to move the rudder brush in the opposite direction, to make contact on the opposite terminal of the sector. This energizes the other clutch solenoid, thereby reversing the drum of the rudder servo motor and causing the rudder to return to a neutral position. The result of all this is that the angular motion of the plane is actually stopped when the plane reaches its original course.

The flight gyro (fig 1-3 and fig 3) senses deviation in attitude, that is, in roll and pitch, and corrects same by applying the aileron and elevator control surfaces. The flight gyro (fig 3) is bolted to the fuselage. The gyro proper (fig 3-26) is mounted on two gudgeons which are attached to the cardan (fig 3-27) and the cardan is carried by two gudgeons mounted in the housing (fig 3-28). The upper erecting bail (fig 3-29) is parallel to the longitudinal axis of the plane and the athwartships erecting bail (fig 3- 30) at right angles. The aileron sector (fig 3-32) is so fixed to the differential lever that it will rotate at right angles to the longitudinal axis of the airplane.

When the plane and flight gyro housing are tilted laterally or when a wing drops, the aileron sector is moved under the brush (fig 3-32) which is attached to the gyro (fig 3-26) . This contact energizes the proper relay and clutch solenoid on the aileron servo motor (fig 6), which then drives its control cable drum in the right direction for applying aileron (fig 1-11) for lifting the wing. To control the amount of aileron applied and bring it back to neutral, the sector, which is integral with the followup differential lever (fig 3-33), which in turn is mounted on the gyro housing by spindle, must be moved correspondingly. This is accomplished through a rod (34 in figs 3 and 6) attached to a lever on the aileron servo motor. This lever is actuated through a gear train to the cable drum of the aileron motor and moves when the cable drum moves. Thus, as aileron is applied the aileron sector on the gyro is moved, and when lateral displacement of the plane ceases the sector continues to move until the neutral segment is under the brush. This breaks the circuit and stops the aileron servo motor.

Now, when the plane responds to the applied aileron and the gyro housing returns to the vertical, contact is made on the opposite end of the sector, the opposite relay and solenoid of the aileron motor are energized and the cable drum rotation is reversed, which returns the aileron surface to neutral and the aileron sector of the gyro is brought back to its original position by movement of the followup connecting rod.

The elevator sector (fig 3-35) in the flight gyro is parallel to the longitudinal axis of the airplane. When the airplane is displaced longitudinally, the sector is moved until an electrical contact is made with the brush (fig 3-36) mounted on the gyro. This contact energizes the proper relay and solenoid of the elevator servo motor (fig 1-7), which causes the elevator servo motor drum to rotate in the right direction for moving the elevator (fig 1-10) of the airplane to correct for this displacement. The elevator servo motor follow-up lever is connected by a rod (fig 3-37) to the elevator differential lever (fig 3-38) on the flight gyro, which moves the elevator sector in the proper direction until the neutral gap is moved under the elevator brush. When the airplane responds to the control applied., contact is made on the opposite end of the sector, energizing the other relay and solenoid which causes the elevator cable drum to rotate in the reverse direction, return the control surfaces to neutral and also bring the elevator control sector back to a neutral position.

The aileron servo follow-up arm and the flight gyro aileron differential lever are so designed that a mechanical adjustment may be made of the followup ratio.

There is a difference of opinion as to the desirability of skid as against bank turns, so for those desiring a bank turn there is mounted in the bombardier's compartment a double pole, double throw switch (fig 1-15) which controls the bank motor (fig 1-5 and fig 4-5).

As stated early in this article, with the banking motor switch in the "on" position the rudder and aileron brushes in the stabilizer sector box operate both the rudder and aileron control surfaces simultaneously. Aileron displacement is accomplished by transmitting rotation of the bank motor (fig 4) to the aileron brush-sector unit (fig 3-31 and 32) in the flight gyro and from there to the aileron servo motor (fig 3-6 and fig 6). In this chain of movements the aileron follow-up differential on the flight gyro (fig 3-33) is employed, together with the aileron follow-up rod (34 in figs 3 and 4) and banking motor connecting rod (39 in figs 3 and 4,) The operation is as follows: With the switch in the bank position, the banking brush on the slide (fig 2-21 and 22) in the sector box will control the relays and solenoids (fig 4-40) in the banking motor (5 in figs 1, 3 and 4). When energized, this motor will cause the banking cam and arm (fig 4-41 and 42) to displace the aileron sector (31 in figs 3 and 4) in the flight gyro, causing the aileron servo motor (6 in figs 1, 3 and 6) to apply aileron (fig 1-9) in proportion to the displacement of the banking sector.

Cable to Banking Sector

A follow-up drum (fig 4-43) is mounted on the banking motor from which a cable runs to the banking sector follow-up drum (fig 2-24) on the sector box. The sensing is such that it will move the banking sector in the same direction as the brush (fig 2-21). Because the rudder and banking brushes are mounted on the same slide (fig 2-22) and move the same distance at the same instant, it is possible to coordinate, by the use of the dash-pot (fig 2-25) on the sector box, the control of the rudder and aileron (fig 1-11 and 9) to make a bank turn.

Bank turns may be made as follows:

  1. The "turn" switch in the bombardier's or pilot's cockpit, when placed in the right or left position, will cause the slide in the sector box to move fore or aft, making the proper electrical contacts for a right or left turn, as desired.
  2. When the secondary clutch (fig 1-17) is disengaged and is moved manually.
  3. When the directional clutch (fig 1-16) is engaged and the stabilized brush is displaced by turning the course knob (fig 1-12) of the bombsight. The two latter operations are performed in the bombardier's cockpit. The units composing the SBAE are so designed that the proper sensing is accomplished regardless of their positions relative to each other.
Important points to remember when checking the action of the SBAE on the ground are:
  1. Moving the secondary clutch to the left causes the slide in the sector box to move to forward position so that the rudder and banking brushes make contact on the forward terminals of their respective sector to drive the motors in the proper sense for a right turn.
  2. Elevator brush contact on the after terminal of the elevator sector in the flight gyro drives the elevator servo motor in the proper direction to raise the nose.
  3. Aileron brush contact with the left aileron sector in the flight gyro drives the aileron servo motor in the proper direction to raise the right wing.

In the bank motor the banking cam, lever and lever spring are so designed that they can be relocated on the opposite side of the case, if it should be necessary to reverse the sensing of the aileron sector in the flight gyro.

The four telltale lights on the pilot's instrument panel (fig 1-13) indicate the plane's position laterally and longitudinally with respect to the position of the flight gyro. Also mounted on the pilot's instrument panel is a pilot's directional indicator (PDI), which indicates the position of the rudder brush in the sector box.

As soon as the airplane is in the air, the pilot turns on the master SBAE (AFCE) and after a short warm-up period, during which he centers the PDI and extinguishes the telltale lights, he engages the SBAE.

The speed of the autopilot in displacing the airplane and returning it to rigid level flight during evasive action or a bombing run increases the maneuverability of heavy bombers far above that attained by human control. Contributing to this are the power, 225 lb, speed, 5 reversals per sec, and positive action of the servo motors in displacing the control surfaces. This makes possible rigid control of the plane at the slightest deviation from the spin axis of the gyros.

The flight gyro's spin axis is maintained within 3/8 of one degree from the vertical at all times by the erecting bails. The azimuth gyro is of the restrained type and does not precess with the earth's rotation. This feature enables a pilot to maintain a selected course indefinitely.

The bombsight stabilizer and SBAE instruments have been modified slightly to receive certain electronic follow-up features. However, all units of the Norden SBAE or autopilot remain basically the same as manufactured in 1935. The modifications were made to adapt the installations for the following types of planes:

Army Navy
Heavy B-17 PBM
Bombers B-19 PBY
B-24 PB4Y
Light B-25 TBD
Bombers B-26 TBF
A-20 TBM

In conclusion, it may be said that Norden's foresight in developing the automatic pilot resulted in a design which is adaptable to many uses. Its simplicity has permitted various attachments which make possible its use in other fields.

The veil of secrecy that of necessity has surrounded all Norden equipment can be lifted for a moment. When the war is over, the Norden automatic pilot, which has so unerringly carried thousands of bomber crews over enemy targets, will be made available to transcontinental and transoceanic airlines. Additional features have already been perfected and flight-tested, and the manufacturer expects that the pilot will have wide application in the commercial airline field.

This article was originally published in the June 1, 1945, issue of Aero Digest magazine, vol 49, no 5, pp 98-101, 236, 238.
The article contains the six photo/figures referenced above.
Photos are shown on their own page.
Photos are not credited.