daveT
Senior Airman
I welcome any comments on this essay
The Bell P-39 Airacobra fighter aircraft was a powerful weapons system, but it had limitations in its armament. This essay examines the P-39 Airacobra armament, details the machine gun synchronizing system, explains the effects of synchronization with comparisons, and presents the difficulties of operating the nose gun synchronization system.
P-39 Airacobra Armament
P-39 Airacobra aircraft was designed around the 37mm T9 cannon that fired through the propeller hub. The P-39 was also equipped with two synchronized Browning M2 .50 caliber (12.7 mm) machine guns mounted in the nose that fired through the propeller blades. The wing guns were either four .30 caliber machine guns in the wings or later models had two .50 caliber machine guns mounted in pods under the wings. The P-39 Airacobra machine guns were manually charged and electrically fired by solenoid units actuated by two firing switches on the control stick, one for the cannon and the other for the machine guns. An impulse tube type synchronizer was used on the nose cowl guns. The tubes were fabricated from chrome moly steel and clamped to the guns. Exhaust louvers were installed in the nose gun compartment cowling to evacuate fumes. Each of the cowling installed .50 caliber guns had a separate ammunition case containing 200 rounds. Ejection chutes were provided to carry off the links and cases (shells). Both chutes deposited their contents internally into a triangular space between the longitudinal beams and outer skin of the aircraft.
The published rates of fire (RoF) for the P-39 wing .50 caliber guns were 750 rounds per minute (rpm) and for the nose cowl mounted synchronized guns they were rated at 300 rpm. Ammunition loads are listed as 300 rounds per wing .50 caliber gun and 200 rounds per nose gun. The 37mm M4 cannon firing through the propeller hub was rated at 140 rpm with 30 rounds of ammunition. The quoted figures for rates of fire are only an average with the actual RoF for different examples of the same type of gun varying quite significantly depending on age, maintenance and ammunition.
The P-39 Airacobra in general has a favorable layout for the nose-mounted machine guns. The machine gun muzzles are very close to the propellers and the offset of its line of fire with respect to the propeller axis is small. Moreover, the width of the propeller blade is small at its intersection with the line of fire. This minimizes the time the blade takes to cross the line of fire, thus helping to arrange a feasible solution for firing through the propeller blades. From a contrasting point of view, the engine to propeller speed reduction gear was high in the P-39Q model. Other fighters of the time featured lower reductions.
Machine Gun Synchronizing System
A machine gun synchronizer is a device to control a fixed machine gun so that its projectiles may be fired through the arc of rotation of a propeller. It is arranged so that the projectile is fired after a propeller blade passes the muzzle of the gun. According to the Index of Army Aeronautical Equipment with Navy and British Equivalents, Machine Gun Synchronizer, dated 1944. The impulse Tube and Wire Unit worked as follows. The synchronizer included an impulse generator, an impulse tube and wire, a trigger motor, and electrical control. The impulse generator created the timed impulses that actuate the trigger motor. It is mounted on the engine's synchronizer mounting pad and contains a camshaft that is driven by a splined drive from the airplane engine. The cam is rotated at propeller speed, and has a number of lobes, depending on the number of propeller blades. A cam follower generates an impulse when actuated by each lobe.
Impulses are transmitted to the trigger by the impulse tube and wire unit. This unit consists of a wire inside a steel tube with suitable couplings at each end to allow the wire to move freely. One end of the wire is attached to the plunger on the impulse generator and the other end to the trigger motor.
The trigger motor, mounted on the side of the fixed machine gun, is actuated by the timed impulses which are transmitted along the impulse wire. Each impulse pulls the trigger motor plunger forward against the trigger motor slide, forcing the end of the slide through a slot in the side of the gun and tripping the firing mechanism.
The entire synchronizer system is operated by electrical control, which consists of a solenoid attached to the head of the impulse generator, a control handle gun switch, and a safety switch. The type of solenoid used depends on the circuit voltage. When the control handle gun switch is depressed, the solenoid is energized and frees the cam follower in the generator. This allows the spring in the trigger motor to pull the cam follower against the cam. Rotation of the cam produces the timed impulses to actuate the trigger motor. When the control switch is released, the solenoid plunger is returned to its original position by a spring and locks the cam follower in a position where no impulses may be generated.
The Effects of Synchronization and Comparisons
Aircraft propeller blades sweep past the gun muzzle at a much faster rate than any gun can fire. For example, a three-bladed propeller rotating at a typical 1,200 rpm, a blade will pass the muzzle 3,600 times per minute. Gun synchronization was therefore about choosing the correct instant for firing each shot, not about occasionally interrupting the automatic fire. Calculations show that a gun firing at 1,200 rpm would have its firing rate slowed by an average of 12.5% depending on the synchronization system and aircraft.
A practical example of the effect of synchronization was graphically provided by comparative tests held by the U.S. Navy in 1926/7 of the .30 inch (7.62 mm) M1921 and .50 inch (12.7 mm) M1921, both on a test stand and in synchronized mountings. These also shed some light on the differences between claimed and actual rates of fire, and between different installations of the same gun. The .30 had a claimed RoF of 1,200 rpm, but proved capable of between 800 and 900 rpm on the test stand. When synchronized, the RoF went down to an average of 730 rpm (a fall of about 15%), with a range of between 667 and 818 rpm for different installations and propeller speeds. The .50 had a claimed RoF of 600 rpm, and did rather well to achieve between 500 and 700 rpm, depending on the recoil buffer adjustment (although a contemporary British report put this at 400-650 rpm, the difference possibly caused by belt drag when installed), but this fell to an average of 438 rpm when synchronized, varying between 383 and 487 rpm. As the synchronized guns were adjusted for maximum RoF, this represented a reduction of around 37%.
There was a big difference when comparing the P-39 nose-mounted guns to the wing-mounted unsynchronized guns and to other aircraft. The P-39 nose-mounted machine guns averaged around 300 rounds per minute which equated to 5 rounds per second (rps) for each gun in the P-39 nose cowl. The P-39 wing guns averaged 750 rounds per minute (rpm). Synchronizing the .50 Cal. Browning P-39 nose guns caused about a 37% reduction in the rate of fire compared to unsynchronized guns. This speed was slow when compared to other fighters. As a comparison, the P-40B nose cowl synchronized guns averaged around 425 rpm. The discrepancy between the P-39's .50 Cal. Browning RoF nose gun and other US fighters synchronized RoF was due to the P-39's reduction gear-driven design which was unlike the other propeller drive sync designs.
The P-39Q equipped with the V-1710-85(E19) engine, the published propeller reduction, and synchronizer gearing was .449:1, both relative to crankshaft speed. The P-39 Gun Synchronizer Impulse Generator had the following entry in their manuals.
When compared with the P-40B/C aircraft equipped with V-1710-33(C15) engine, the propeller reduction, and synchronizer gearing was .500:1, both relative to crankshaft speed. The P-40 Gun Synchronizer Impulse Generator had the following entry in their manuals.
Synchronization Difficulties
There were some difficulties with the synchronizers, as you'd expect of any mechanical system functioning at such a high rate of oscillation. Generally, mechanical systems were inferior to hydraulic or electric ones, but none were ever entirely foolproof, and synchronization gears at best always remained liable to occasional failure. Several factors affected the synchronization of the machine guns to the propellers.
The linkage between the propeller and machine gun had to be precisely synchronized. Many early gears used an intricate and inherently fragile bell crank and push rod linkage that could easily jam or shake itself to pieces, especially when required to work at higher speeds than it had been designed for. Poor maintenance of the system was reported as the number one cause of synchronization problems, but there were other causes.
The aircraft machine guns had limitations that if exceeded could cause malfunctions. The maximum burst of 75 rounds may be fired from the standard 36-inch machine gun barrel. After one minute, firing may be resumed at the rate of one 20-round burst per minute. The gun should be cooled for at least 15 minutes before another long burst is attempted. Firing longer bursts could overheat the gun barrels and cause malfunctions.
The same type of machine gun could vary quite significantly depending on age, maintenance, ammunition, and environment. Depending on the fit of the parts, smoothness of the bearing surfaces, and the strength of the springs (and recoil buffer) two machine guns just one serial number apart could, in theory, fire at opposite ends of the rate of fire range. A good armorer could swap parts or polish surfaces to help even things out, but the usual goal was smooth running guns, not peak rate of fire.
Machine guns might vary their rate of fire depending on the ammunition used. Inaccuracies in bullet manufacturer and quality meant that some bullets would fire at the wrong time. Tiny variations in ammunition quality could cause bullet synchronization problems. Special "Red Label, Synchronized Guns" ammunition was sometimes used in aircraft synchronized cowl machine guns. This type of ammunition had better quality control and tighter tolerances than standard ammunition.
With precise timing requirements, changes in the environment could result in small changes to the synchronization. Temperature changes in the metal parts caused thermal expansion/contraction and resulted in small changes to the synchronization. The effect of the low temperatures experienced at high altitude in congealing the gun lubricants and on the variable G-forces consequent on maneuvering also could cause small changes to the synchronization.
Propeller speeds also varied during the flight. While you might think that it wouldn't matter since the cam rotated at the same speed as the propeller, the firing of the gun took a length of time that was independent of the propeller speed. With some systems, the pilot had to keep an eye on the tachometer indicating the engine speed to know when it was safe to fire.
There were complaints by U.S. airmen about the slow synchronized rate of fire for the P-39 nose-mounted browning .50 Cal (12.7 mm) when compared to other fighter aircraft. According to the ordnance chief of the 68th Fighter Squadron on Guadalcanal and other Pacific islands, P-39s, and P-40s both had synchronized guns using hydraulic synchronizers. The synchronizers only worked properly within a certain engine RPM range. He stated that several pilots came back with holed and dinged propellers because they fired outside their recommended RPM range.
Just because the synchronization failed and a round went through the propeller doesn't mean immediate failure. There is at least one occasion where synchronization failed on a fighter and a hole was shot through the propeller and the plane continued to fly. This was mentioned in the book "Some Still Live" by Frank Tinker, the aircraft was the Soviet I-15. The famous Luftwaffe ace Adolf Galland in his memoir of the war period "The First and the Last" describes a serious faulty synchronization incident in the Messerschmitt Bf 109 that occurred in 1941. Just because occasional problems with the synchronization systems occurred, most did not cause catastrophic failure.
The increased speeds of the newer aircraft meant that the time available to deliver a sufficient weight of fire from synchronized machine guns to bring down an enemy aircraft was greatly reduced. Later aircraft models used more unsynchronized wing-mounted guns with higher rates of fire. The retention of fuselage-mounted guns with their synchronization gear which slowed their rate of fire and still occasionally failed became increasingly unattractive.
The Bell P-39 Airacobra fighter aircraft was a powerful weapons system, but it had limitations in its armament. This essay examines the P-39 Airacobra armament, details the machine gun synchronizing system, explains the effects of synchronization with comparisons, and presents the difficulties of operating the nose gun synchronization system.
P-39 Airacobra Armament
P-39 Airacobra aircraft was designed around the 37mm T9 cannon that fired through the propeller hub. The P-39 was also equipped with two synchronized Browning M2 .50 caliber (12.7 mm) machine guns mounted in the nose that fired through the propeller blades. The wing guns were either four .30 caliber machine guns in the wings or later models had two .50 caliber machine guns mounted in pods under the wings. The P-39 Airacobra machine guns were manually charged and electrically fired by solenoid units actuated by two firing switches on the control stick, one for the cannon and the other for the machine guns. An impulse tube type synchronizer was used on the nose cowl guns. The tubes were fabricated from chrome moly steel and clamped to the guns. Exhaust louvers were installed in the nose gun compartment cowling to evacuate fumes. Each of the cowling installed .50 caliber guns had a separate ammunition case containing 200 rounds. Ejection chutes were provided to carry off the links and cases (shells). Both chutes deposited their contents internally into a triangular space between the longitudinal beams and outer skin of the aircraft.
The published rates of fire (RoF) for the P-39 wing .50 caliber guns were 750 rounds per minute (rpm) and for the nose cowl mounted synchronized guns they were rated at 300 rpm. Ammunition loads are listed as 300 rounds per wing .50 caliber gun and 200 rounds per nose gun. The 37mm M4 cannon firing through the propeller hub was rated at 140 rpm with 30 rounds of ammunition. The quoted figures for rates of fire are only an average with the actual RoF for different examples of the same type of gun varying quite significantly depending on age, maintenance and ammunition.
The P-39 Airacobra in general has a favorable layout for the nose-mounted machine guns. The machine gun muzzles are very close to the propellers and the offset of its line of fire with respect to the propeller axis is small. Moreover, the width of the propeller blade is small at its intersection with the line of fire. This minimizes the time the blade takes to cross the line of fire, thus helping to arrange a feasible solution for firing through the propeller blades. From a contrasting point of view, the engine to propeller speed reduction gear was high in the P-39Q model. Other fighters of the time featured lower reductions.
Machine Gun Synchronizing System
A machine gun synchronizer is a device to control a fixed machine gun so that its projectiles may be fired through the arc of rotation of a propeller. It is arranged so that the projectile is fired after a propeller blade passes the muzzle of the gun. According to the Index of Army Aeronautical Equipment with Navy and British Equivalents, Machine Gun Synchronizer, dated 1944. The impulse Tube and Wire Unit worked as follows. The synchronizer included an impulse generator, an impulse tube and wire, a trigger motor, and electrical control. The impulse generator created the timed impulses that actuate the trigger motor. It is mounted on the engine's synchronizer mounting pad and contains a camshaft that is driven by a splined drive from the airplane engine. The cam is rotated at propeller speed, and has a number of lobes, depending on the number of propeller blades. A cam follower generates an impulse when actuated by each lobe.
Impulses are transmitted to the trigger by the impulse tube and wire unit. This unit consists of a wire inside a steel tube with suitable couplings at each end to allow the wire to move freely. One end of the wire is attached to the plunger on the impulse generator and the other end to the trigger motor.
The trigger motor, mounted on the side of the fixed machine gun, is actuated by the timed impulses which are transmitted along the impulse wire. Each impulse pulls the trigger motor plunger forward against the trigger motor slide, forcing the end of the slide through a slot in the side of the gun and tripping the firing mechanism.
The entire synchronizer system is operated by electrical control, which consists of a solenoid attached to the head of the impulse generator, a control handle gun switch, and a safety switch. The type of solenoid used depends on the circuit voltage. When the control handle gun switch is depressed, the solenoid is energized and frees the cam follower in the generator. This allows the spring in the trigger motor to pull the cam follower against the cam. Rotation of the cam produces the timed impulses to actuate the trigger motor. When the control switch is released, the solenoid plunger is returned to its original position by a spring and locks the cam follower in a position where no impulses may be generated.
The Effects of Synchronization and Comparisons
Aircraft propeller blades sweep past the gun muzzle at a much faster rate than any gun can fire. For example, a three-bladed propeller rotating at a typical 1,200 rpm, a blade will pass the muzzle 3,600 times per minute. Gun synchronization was therefore about choosing the correct instant for firing each shot, not about occasionally interrupting the automatic fire. Calculations show that a gun firing at 1,200 rpm would have its firing rate slowed by an average of 12.5% depending on the synchronization system and aircraft.
A practical example of the effect of synchronization was graphically provided by comparative tests held by the U.S. Navy in 1926/7 of the .30 inch (7.62 mm) M1921 and .50 inch (12.7 mm) M1921, both on a test stand and in synchronized mountings. These also shed some light on the differences between claimed and actual rates of fire, and between different installations of the same gun. The .30 had a claimed RoF of 1,200 rpm, but proved capable of between 800 and 900 rpm on the test stand. When synchronized, the RoF went down to an average of 730 rpm (a fall of about 15%), with a range of between 667 and 818 rpm for different installations and propeller speeds. The .50 had a claimed RoF of 600 rpm, and did rather well to achieve between 500 and 700 rpm, depending on the recoil buffer adjustment (although a contemporary British report put this at 400-650 rpm, the difference possibly caused by belt drag when installed), but this fell to an average of 438 rpm when synchronized, varying between 383 and 487 rpm. As the synchronized guns were adjusted for maximum RoF, this represented a reduction of around 37%.
There was a big difference when comparing the P-39 nose-mounted guns to the wing-mounted unsynchronized guns and to other aircraft. The P-39 nose-mounted machine guns averaged around 300 rounds per minute which equated to 5 rounds per second (rps) for each gun in the P-39 nose cowl. The P-39 wing guns averaged 750 rounds per minute (rpm). Synchronizing the .50 Cal. Browning P-39 nose guns caused about a 37% reduction in the rate of fire compared to unsynchronized guns. This speed was slow when compared to other fighters. As a comparison, the P-40B nose cowl synchronized guns averaged around 425 rpm. The discrepancy between the P-39's .50 Cal. Browning RoF nose gun and other US fighters synchronized RoF was due to the P-39's reduction gear-driven design which was unlike the other propeller drive sync designs.
The P-39Q equipped with the V-1710-85(E19) engine, the published propeller reduction, and synchronizer gearing was .449:1, both relative to crankshaft speed. The P-39 Gun Synchronizer Impulse Generator had the following entry in their manuals.
When compared with the P-40B/C aircraft equipped with V-1710-33(C15) engine, the propeller reduction, and synchronizer gearing was .500:1, both relative to crankshaft speed. The P-40 Gun Synchronizer Impulse Generator had the following entry in their manuals.
Synchronization Difficulties
There were some difficulties with the synchronizers, as you'd expect of any mechanical system functioning at such a high rate of oscillation. Generally, mechanical systems were inferior to hydraulic or electric ones, but none were ever entirely foolproof, and synchronization gears at best always remained liable to occasional failure. Several factors affected the synchronization of the machine guns to the propellers.
The linkage between the propeller and machine gun had to be precisely synchronized. Many early gears used an intricate and inherently fragile bell crank and push rod linkage that could easily jam or shake itself to pieces, especially when required to work at higher speeds than it had been designed for. Poor maintenance of the system was reported as the number one cause of synchronization problems, but there were other causes.
The aircraft machine guns had limitations that if exceeded could cause malfunctions. The maximum burst of 75 rounds may be fired from the standard 36-inch machine gun barrel. After one minute, firing may be resumed at the rate of one 20-round burst per minute. The gun should be cooled for at least 15 minutes before another long burst is attempted. Firing longer bursts could overheat the gun barrels and cause malfunctions.
The same type of machine gun could vary quite significantly depending on age, maintenance, ammunition, and environment. Depending on the fit of the parts, smoothness of the bearing surfaces, and the strength of the springs (and recoil buffer) two machine guns just one serial number apart could, in theory, fire at opposite ends of the rate of fire range. A good armorer could swap parts or polish surfaces to help even things out, but the usual goal was smooth running guns, not peak rate of fire.
Machine guns might vary their rate of fire depending on the ammunition used. Inaccuracies in bullet manufacturer and quality meant that some bullets would fire at the wrong time. Tiny variations in ammunition quality could cause bullet synchronization problems. Special "Red Label, Synchronized Guns" ammunition was sometimes used in aircraft synchronized cowl machine guns. This type of ammunition had better quality control and tighter tolerances than standard ammunition.
With precise timing requirements, changes in the environment could result in small changes to the synchronization. Temperature changes in the metal parts caused thermal expansion/contraction and resulted in small changes to the synchronization. The effect of the low temperatures experienced at high altitude in congealing the gun lubricants and on the variable G-forces consequent on maneuvering also could cause small changes to the synchronization.
Propeller speeds also varied during the flight. While you might think that it wouldn't matter since the cam rotated at the same speed as the propeller, the firing of the gun took a length of time that was independent of the propeller speed. With some systems, the pilot had to keep an eye on the tachometer indicating the engine speed to know when it was safe to fire.
There were complaints by U.S. airmen about the slow synchronized rate of fire for the P-39 nose-mounted browning .50 Cal (12.7 mm) when compared to other fighter aircraft. According to the ordnance chief of the 68th Fighter Squadron on Guadalcanal and other Pacific islands, P-39s, and P-40s both had synchronized guns using hydraulic synchronizers. The synchronizers only worked properly within a certain engine RPM range. He stated that several pilots came back with holed and dinged propellers because they fired outside their recommended RPM range.
Just because the synchronization failed and a round went through the propeller doesn't mean immediate failure. There is at least one occasion where synchronization failed on a fighter and a hole was shot through the propeller and the plane continued to fly. This was mentioned in the book "Some Still Live" by Frank Tinker, the aircraft was the Soviet I-15. The famous Luftwaffe ace Adolf Galland in his memoir of the war period "The First and the Last" describes a serious faulty synchronization incident in the Messerschmitt Bf 109 that occurred in 1941. Just because occasional problems with the synchronization systems occurred, most did not cause catastrophic failure.
The increased speeds of the newer aircraft meant that the time available to deliver a sufficient weight of fire from synchronized machine guns to bring down an enemy aircraft was greatly reduced. Later aircraft models used more unsynchronized wing-mounted guns with higher rates of fire. The retention of fuselage-mounted guns with their synchronization gear which slowed their rate of fire and still occasionally failed became increasingly unattractive.
