The Sperry Pilot for Automatic Flying

Excerpt from Your Wings, 1936

The horizon and the gyro point out the exact attitude of the plane relative to the ground as well as its direction of travel. If any changes must be made, their reading is transferred to your mind, and in turn you manipulate the controls so as to bring the plane to the desired position. For automatic flying the reading of the two instruments just mentioned is directly transmitted to power units (servo) and they move the respective controls in identically the same manner as if they had been moved manually by you.

The small spinning gyro, as we already know, is very sensitive to any external forces. Therefore, we cannot use it directly to operate the controls of the plane. Not only that, but the rigidity force of the gyro is by far too small in comparison to the force necessary to actuate the surface controls of the plane. There is no power of any sort derived directly from the gyro for the operation of the controls. As you know, the gyro of the artificial horizon and the one in the directional gyro retain rigidity not in space but relative to the ground. When the plan changes its attitude, air valves open and close, resulting in action of servo units which in turn actuate the elevator, rudder or ailerons. For explanation, we will analyze the function of the aileron controls only, as the other two controls operate in identically the same manner.

Fig. 248. The position of the plane is horizontal. An air valve surrounds the gyro, having two open ports, A and A-1. The action of the ports will be explained later.

Fig. 249. Should the plane assume a position such as shown, it will carry along the air valve, causing port A to close. If the change of position had been less pronounced, the valve would have been partially closed. As you notice, the surface of the gyro is not in contact with the valve. Therefore, any movement of the plane and the valve around the gyro does not induce friction, which might result in undesirable external forces over the gyro.

During the early attempt to harness the gyro principle in the Sperry Gyro instruments the air suction necessary for operating the gyro was derived from a venturi tube, one end of which was exposed to the air stream outside of the plane. While such an arrangement was perfectly sound in fair-weather flying, it caused trouble in bad weather when the gyro instrument was most needed. Occasionally the ventury tube collected ice or was packed with sleet, which meant that it ceased to function.

At present the suction is derived from an air-suction pump operated by the airplane motor, and this pump functions as long as the motor is in operation. Furthermore, in the larger transport planes, where more than one engine supplies the motive power, more than one pump is utilized. That is, if one pump is sufficient to operate the gyro instruments, the other remains as a spare. However, this does not prevent the use of the venturi tube as an auxiliary means for suction in case everything else fails.

The gyro has immeasurably increased the safety of flight and has made it practical for the planes of today to fly through weather in which even the birds stay on the ground. It is the little gyro again which has helped to make flying possible as a commercial enterprise.

Ignorance in flying is the ever-present danger.

Fig. 250. This schematic illustration shows the step-by-step connection between the gyro and the airplane-controlling surface. Gyro A spins in its housing, as already explained, with the air valve B around the gyro. The air valve B changes its position relative to the gyro, a change induced by the plane’s attitude also in relation to the gyro. The diaphragm C bulges to one side or the other, depending on the difference of the atmospheric pressure. The diaphragm in turn actuates, through a rod, oil valve D, the latter admitting oil under pressure into the servo unit as shown at E. The oil pressure is maintained by a hydraulic pump driven by the motor. As you see, the actual power necessary for the automatic manipulation of the controls is supplied by the motor of the plane, as a certain amount of energy is absorbed by the hydraulic pump in order to maintain sixty-pounds-per-square-inch pressure in the hydraulic lines.

Fig. 251. In this illustration the connection between the diaphragm and the oil distributing valve is shown when the diaphragm is in a neutral position, which places the oil valve also in neutral. The ports leading to the servo unit are closed, and therefore the oil does not actuate the servo unit. The diaphragm can bulge on either side of the neutral about one thirty-second of an inch. This small deflection is sufficient to carry the bar R to its two extreme positions, which in turn also causes a maximum travel of the piston in the oil valve.

Fig. 252. A step further is divulged in this illustration. The diaphragm has bulged to the left opening port, P, admitting the oil into one side of the servo, as shown at A. The servo piston under pressure has already moved from the neutral. The by-pass valve at A is closed. If the by-pass valve were open, as shown at B, the piston of the servo would remain in its neutral position as the oil would pass through the open valve and flow through the pipe lines, as shown by the arrows. It is this by-pass valve which you close if the controls of the plane are to be operated by the automatic pilot.

Fig. 253 A and B. Let us put everything together and see how it really works. The plane has changed its attitude relative to the gyro and has assumed position I. As it inclines, the air valve inclines with it, causing one of the ports in the air line to open, resulting in a suction at one side of the diaphragm. Then the piston of the oil valve has moved and opened the port to the servo. The oil moves the servo piston, as explained before, and the servo piston has moved the ailerons. The ailerons in turn will begin to bring the plane toward a normal horizontal attitude. The action of the aileron must be stopped just before the plane has come back to its horizontal attitude, in order to prevent overcontrolling. This is accomplished by having the air valve connected through a cable with the servo unit rod. The purpose of this connection is to cause the air valve always to come back to its neutral position, as shown at B, and cross over to reverse the action of the servo unit while the plane has not as yet reached its horizontal attitude. This causes the control application to be removed by the time the plane reaches its normal horizontal attitude. This cycle repeats itself whenever the plane deviates from a desired relative position to the earth.

One of the best Pals of Progress is Speed.

Fig. 254. When the plane is flown automatically, you can always check up the automatic gyropilot, as you have the visual artificial horizon and the directional gyro in front of you on the dashboard.

Once you throw the manipulation of the controls to the automatic pilot, with the various knobs shown on the illustration you can change the course of the plane, make it climb or descend without touching the controls. What a relief when the flight takes place in fog and your attention is concentrated more on navigational problems!

Fig. 255

Fig. 255. This gives the general arrangement of the Sperry autopilot.

Excerpt taken from The Beacon - Aviation's Golden Age on the Web by Kevin Breen