Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: VIII. - Synchronizing

Section VIII - SYNCHRONIZING

45. General.-The E4 and E8 synchronizing systems provide a means of producing and controlling timed impulses for actuating the firing mechanism. of the synchronized guns on airplanes. The impulses are synchronized with the propeller and transmitted from the impulse generator on the engine to the trigger motor on the gun. The synchronizing system has been divided into four units, each one standard and considered complete in itself.

a. The impulse generator unit originates or generates the timed impulses which actuate the trigger motor. The principal parts of the generator are the housing, cam, cam shaft, and a reciprocating cam follower. The cam rotates at propeller speed and has one or two lobes depending upon whether it is to be used with a three or two bladed propeller, respectively. Due to the lack of uniformity of engine construction, the shape, design, and location of this unit may vary somewhat with different types of engines. However, the means of attaching the impulse tube and wire assembly is the same in all cases.

b. The impulse tube and wire assembly transmits the timed impulses produced by the impulse generator on the engine to the trigger motor on the gun. The major parts of this unit are a steel tube, single strand of steel wire, impulse tube barrel, impulse tube end, wire end screw, impulse wire spring, and two wire ends.

c. The trigger motor unit is mounted on the machine gun and is actuated by the timed impulses transmitted by the wire from the impulse generator. This actuation operates the firing mechanism of the gun. The principal parts of the trigger motor unit consist of a steel housing containing the trigger motor slide, trigger motor plunger, and the trigger motor plunger and slide springs. The trigger motor slide is operated by the angular face of the trigger motor plunger operating against the angular face of the slide, thus camming the end of the trigger slide out through the slot in the housing and into the receiver of the machine gun, tripping the firing mechanism of the gun. The adjusting screw is used to vary the distance the trigger motor slide projects into the receiver of the gun.

d. The electrical control units, when properly connected to the impulse generator, control the generation of the impulses and thereby control the fire of the gun. The major parts of the control units are: The E2A solenoid and the A1 plunger control. The solenoid converts the electrical energy into mechanical energy which is transmitted to the A1 plunger control to cause its actuation in such a manner as to remove the plunger end from the cam follower and allow reciprocating motion impulses to be generated.

46. Operation.-The operation of the synchronizing system will be considered from the generator through the wire and tube assembly to the trigger motor. Under normal conditions, when the machine gun is not in operation on an airplane in flight, the operating plunger of the A1 plunger control will be engaged in the locking notch of the cam follower of the generator. This will hold the cam follower away from the cam, and even though the cam is being rotated by the engine, no reciprocating motion is being developed. When the pilot desires to start fire of the gun, he must set the selector switch to the desired position and close the electrical control switch on the control stick, thus energizing the E2A solenoid. This causes the operating plunger of the A1 plunger control unit to be withdrawn from the locking notch of the cam follower.

a. When the plunger is withdrawn, the cam follower is brought in contact with the cam by the action of the trigger motor plunger spring. This tension keeps the cam follower in contact with the cam at all times, except when the operating plunger is engaged in the locking notch. The contours of the cam cause the follower to be moved in a reciprocating manner and this movement is transmitted through the impulse wire to the trigger motor plunger.

b. As the trigger motor plunger moves inward, the trigger motor slide is cammed into the receiver of the gun and bears against the sear of the release mechanism, thus causing the release of the firing pin at the desired time.

c. So long as the bolt of the machine gun is in battery position when the trigger motor slide starts its inward movement, the firing pin will be released when the slide reaches its predetermined point. If the trigger motor slide has been cammed into the receiver of the machine gun when the bolt is entering battery position, the end of the trigger motor slide will be struck by the projecting end of the sear and pivoted forward. Then, before an impulse can be effectively transmitted to the firing mechanism of the gun, the cam follower must approach low point position of the cam at which time the trigger motor plunger, through the action of the trigger motor plunger spring, has been forced to the rear a sufficient distance to allow the trigger motor slide to be forced back into the trigger motor housing through the tension of the trigger motor slide spring. At this time the trigger motor slide will have been moved away from the sear a sufficient distance for the tip end of the trigger motor slide to have cleared the projecting end of the sear, and the trigger motor slide will then be retracted, or reclined, so that as the cam follower roller passes over low point position on the cam and goes toward high point, the trigger motor slide will be in proper alinement to secure the release of the firing pin at the proper time. In other words, should an impulse be delivered by the trigger motor before the machine-gun bolt is in battery position (that is, closed and locked), the gun will not fire. Further, as the bolt goes into battery position, the gun still will not fire until an impulse is given by the trigger motor after the bolt reaches battery position.

d. When the pilot desires to cease firing, he releases the control switch, thus breaking the electrical connection. This allows the spring tension of the A1 plunger control unit to cause the operating plunger to reengage in the locking notch of the cam follower the first time the cam follower roller reaches high point position on the cam. The timed impulses are thus stopped and the gun ceases to fire.

47. Inspection and maintenance.-a. General.-The E4 and E8 synchronizing systems installed on aircraft, will be given complete inspections by personnel familiar with the installation and functionings of the systems and their various component parts. If nonstandard, worn, or otherwise. defective parts are found during any inspection, they will be replaced by serviceable standard parts. In each instance, after completing inspections and replacement of parts, assemblies, etc., and the adjustments of such parts and assemblies, the entire system will be checked, and all additional adjustments required will be made to correctly time the zero shots and obtain proper firing. All periodic inspections will be made in accordance with Air Corps Circular I5-43 pertinent parts of which are as follows :

Technical inspections of aircraft armament pertaining to a specific aircraft will be recorded on Air Corps Form Number 43. These inspections consist of two types : Operating and maintenance. Operating inspections will be known as Job No. 1; maintenance inspections will be known as Job No. 2.

(1) Operating inspections are designed to determine whether the equipment is in usable condition and will be performed each day the aircraft is flown immediately before the first flight. In no case will more than one week be allowed to lapse between operating inspections unless the aircraft is in "Storage Status."

(2) Maintenance inspections wiIl be made of equipment pertaining to each aircraft every forty (40) flying hours regardless of whether or not the equipment has been used during this period. Insofar as possible, the armament maintenance inspection will be performed at the same time the 40-hour maintenance inspection is performed on the aircraft. In no case will more than one month be allowed to lapse between 40-hour armament inspections regardless of whether or not the aircraft is flown unless the aircraft is in "Storage Status."

(3) The inspection periods prescribed herein wilI govern unless shorter periods are prescribed by technical publications relating to the specific item of armament equipment.

b. Positioning of zero shot gap.-The guns are so mounted that the line of bore of the gun will intersect the plane of propeller rotation. This necessitates the accurate timing of the release of the firing pin in such a manner as to insure the safety of the propeller blades. Before any airplane of a particular type is allowed to be used in a tactical organization, exhaustive tests are made of the gun installations peculiar to it and the results tabulated and forwarded to various organizations having this equipment for their use in correctly placing and timing the guns. The position of the propeller blade in relation to the line of fire at the instant of firing pin release is known as the zero shot gap. The measurement of this gap is in inches around the circumference of the circle of fire. In some cases the placing of the blade will be directly in front of the muzzle of the gun at the instant of firing pin release. In a case of this kind it will be necessary that the propeller be turning at some speed in order to prevent possible damage to that blade by the projectile. For this reason in these cases the minimum speed at which firing may be conducted is    r. p. m. Also, there is a probability that the propeller blades may be turning fast enough that the following blade will be struck by the projectile. In order to prevent this the publication from the experimental laboratories states that the blades cannot be turning faster than      r. p. m. In all cases of this kind the notice as to the minimum and maximum r. p. m. will be conspicuously posted in the cockpit of the airplane governed by these regulations.

c. Overtravel.-In order to insure positive release of the firing pin, the trigger motor plunger is forced into the housing a greater distance than is absolutely necessary to secure the release of the firing pin. This added movement of the plunger is known as overtrarvel. There are definite limits given for this movement, and under no circumstance will any firing be conducted when these limits are exceeded, since an excess in either direction may produce an unsynchronized shot and possibly damage the propeller.

48. Duties.-It is the duty of the armament officer to see that the equipment used by the armorer, in properly synchronizing the guns on the airplanes in his organization, is in proper condition, and to assure himself that the work performed by the maintenance crews is properly accomplished. It will also be his duty to initial forms certifying the condition of the equipment at stated intervals. He will also be charged with the proper coordination of the various men under him to see that the work necessary is done in an efficient manner, and all necessary inspections properly performed.