Quick Question on German Gun Synchronization

[SpicyJuan11]

Hi, I was wondering how well would the gun synchronization work on aircraft models with a four bladed and up, or contra-rotating props? I know the Bf 109 K-14 had this, and the same armament that previous types had. Would it have cut the rate of fire down enough to consider replacing them with those of the slower firing 30mm, or would it not have been worth? Many thanks.

 

[Shortround6]

Not sure what you are asking, the 30mm went through the prop hub and didn't need to be synchronized. The 30mm guns couldn't be synchronized in any case.

 

[Juha]

Hi, I was wondering how well would the gun synchronization work on aircraft models with a four bladed and up, or contra-rotating props? I know the Bf 109 K-14 had this, and the same armament that previous types had. Would it have cut the rate of fire down enough to consider replacing them with those of the slower firing 30mm, or would it not have been worth? Many thanks.

109K-4 had 3 bladed prop as the all common WWII 109s had.

 

[mikewint]

From the mid-30s on German fighters used electrical solenoids to fire their fuselage mounted guns so 2, 3, or 4 made little difference. Contacts were mounted on the prop shaft which opened and closed. Closed sent a signal to the solenoid which pulled the trigger. The guns were actually being fired in a semi-auto manner, i.e. one trigger pull one shot. The guns had to fire from the closed breech position and be mounted close to the prop. The working of the bolt took time as did the bullets flight time to the prop so the gun was often told to fire when it could not. Thus a second gun was used to increase the rate of fire. A third would increase this even more but space was not available. As aircraft became metal and even armoured and speed increased it became very difficult for two fuselage mounted guns firing rifle calibre ammunition to bring down the new monoplanes or the even bigger and heavier bombers. The monoplane's single much more rigid wing was an ideal spot to mount true machine guns which could be harmonized to produce a fairly narrow cone of fire. The larger wings offered more space for more guns thus the fighter delivered more weight of fire.
Only the UK and Soviet Union made use of the Contra-rotating props.

 

[SpicyJuan11]

109K-4 had 3 bladed prop as the all common WWII 109s had.

I know Juha, but the K-14 had a four-bladed prop.

From the mid-30s on German fighters used electrical solenoids to fire their fuselage mounted guns so 2, 3, or 4 made little difference. Contacts were mounted on the prop shaft which opened and closed. Closed sent a signal to the solenoid which pulled the trigger. The guns were actually being fired in a semi-auto manner, i.e. one trigger pull one shot. The guns had to fire from the closed breech position and be mounted close to the prop. The working of the bolt took time as did the bullets flight time to the prop so the gun was often told to fire when it could not. Thus a second gun was used to increase the rate of fire. A third would increase this even more but space was not available. As aircraft became metal and even armoured and speed increased it became very difficult for two fuselage mounted guns firing rifle calibre ammunition to bring down the new monoplanes or the even bigger and heavier bombers. The monoplane's single much more rigid wing was an ideal spot to mount true machine guns which could be harmonized to produce a fairly narrow cone of fire. The larger wings offered more space for more guns thus the fighter delivered more weight of fire.
Only the UK and Soviet Union made use of the Contra-rotating props.

But wouldn't the ROF go down at all?

 

[stona]

I don't believe that any K-14s aircraft existed, not least because neither the engine nor the four bladed airscrew entered series production (though a version of the DB 605 with a two stage compressor may have been available in early 1945).
There are many misspellings and transcription errors in the sometimes very difficult to read records and K-4 to K-14 is a simple error, probably explaining why some sources suggest two aircraft, designated K-14, were in fact delivered.
Cheers
Steve

 

[mikewint]

Yes, ROF was down. The fuselage guns were actually firing semi-auto. Also while machine guns fire at an essentially constant rate, props spin at different RPMs depending on the throttle. Fuselage guns tend to be more accurate as they are firing straight ahead while wing mounted guns are toed in so that fire converges to a point or rectangle. The rectangle's size and distance vary with policy or at times with pilot preference. German Major Erich Hartmann, set the wing guns (later cannon) of his Bf 109 to converge at 50 m (160 ft) because of his preference for waiting to attack until very near his opponent.

 

[SpicyJuan11]

Yes, ROF was down. The fuselage guns were actually firing semi-auto. Also while machine guns fire at an essentially constant rate, props spin at different RPMs depending on the throttle. Fuselage guns tend to be more accurate as they are firing straight ahead while wing mounted guns are toed in so that fire converges to a point or rectangle. The rectangle's size and distance vary with policy or at times with pilot preference. German Major Erich Hartmann, set the wing guns (later cannon) of his Bf 109 to converge at 50 m (160 ft) because of his preference for waiting to attack until very near his opponent.

Ok, thanks.

 

[SpicyJuan11]

Could someone tell me how the M2 Brownings worked in the nose of the P-75? I'm especially curious because the a/c had contra-rotating props.

 

[Koopernic]

The main German guns the Mg 151/20 and MG131 had electrically ignited primers for the propellant charge rather than a percussion system. This made electrical synchronisation relatively simple since only a rotating electrical switch was needed.

A 4 bladed propeller turning at 1800RPM provides a gap suitable for firing every 33.33 milliseconds. Assuming the Blade Area is only 20% near the shank that means there is maybe a 6.66ms period every 33.33ms that is blanked off. An MG151/20 has a cadence of 750RPM so wishes to fire every 80ms.

However only every 2nd or 3rd blade blade will block the gun for maybe 6.66ms so the loss is less than 10% (ie the loss on the Fw 190 was less then 10%)

The hydrostatic percussion system used on the 50 calibre browning on the early P40 lost about 40% rate of fire since the system was less precise than the electrical fuses.

As far as your question regarding coaxial contra rotating props is concerned. It would depend on whether the propellers can rotate independently or not.

If they are locked to a common drive shaft then the meshing of the props could be organised to suit the gun position and little cadence lost.

if they rotate independently I think the reduction would be substantial, at least twice as much; say 40% since the blade area to interblade gap has doubled.