Luftwaffe propellers

[Librarian]

Bit of a dumb question but is the reason behind most Luft fighters having three blades because the guns were mounted to fire through the propeller arc (eg 109s, 190s 152s etc)? Could a Ta 152 have four thinner blades?

[Gondor]

Short answer NO

Gondor

[Librarian]

Just remembered the P-63 which has the big hub cannon and two nose guns with four large blades. Should be OK on the 152 if I get rid of the wing root guns.

[rickshaw]

While generally it holds true that the more blades the worse for synchronisation, it shouldn't really make much difference for guns firing through the propeller hub (although it might get a bit crowded in there with pitch control mechanisms).   The Japanese IIRC several four-bladed propeller fighters (Mitsubishi A7M, Nakajima Ki-84) that had synchronised guns firing through the propeller arc, so it isn't impossible but I suspect it would be for five-bladed or six-bladed ones.

[Dizzyfugu]

Some four-bladed propellers were tried, but do not know if the respective aircraft had guns firing through them. The Fw 190 C prototype definitvely had one, as well as the later Ta 152 prototypes. I also know that one Me 410 was tested with four-bladed props - for the latter it brought no benefit, so the idea was dropped. Not sure why the Ta 152 would keep the three-bladed prop, but I'd rather blame it on material shortage.

[Librarian]

Many thanks for the input; will go for a four bladed large hub (look a little like a Sea Fury) with underwing radiators and carb scoops. Need a high altitude excort for the Skyeye.

[Captain Canada]

That's a good question and something I had never thought of ! 3 blades or a turbine.....your choice eh ?

[jcf]

The German guns used electrically ignited ammunition, so synchronization
was very straight forward, and the number of blades was relatively irrelevant.

You can add blades to a propeller to absorb increased horsepower or change
the profile of the blades and keep the blade number the same. The
latter was the preferred approach in WWII Germany.

[Librarian]

Very interesting, many thanks. Would the type of calibre/rate of fire make a difference? I've read about the 30mm gun being nicknamed the pneumatic hammer because of its low rate. It was usually hub mounted or in the earlier 190s pushed out beyond the propeller arc.

[jcf]

The Mk 108 received the nickname because of its appearance, it looks like a jackhammer.
Rate of fire was actually decent at 650 rounds per minute, it's biggest problem was low
muzzle velocity due to the short barrel. It also starts its cycle on an open bolt, which
evidently makes synchronization problematic.

[matrixone]

Dizzyfugu is correct, material shortages was part of the reason German fighters did not use four bladed props, it was found the lightweight wide three bladed wooden props offered nearly as good performance as metal four bladed props so they chose the wooden props to help conserve metal.
If the Bf 109 series would not have stopped at the K-4 version there would have been four bladed metal props used on later versions...why Messerschmitt stuck with metal props on its designs instead of switching to wooden props is unknown.

As far as getting the weapons synchronized to fire through a four bladed propeller that would not be much of a problem, if they can manage to synchronize firing through wide blade wooden props narrow four bladed prop blades would not be much of a challenge.

Matrixone

[kitnut617]

Sometime ago on this forum I demonstrated with some sketchs that you could quite easily fire cannon shells through a contra-prop which had two four-blade propellers.  Can't find where that is right now though ---

But it's all down to the rpm of the propeller which for the most part didn't exceed 3000 rpm.  IIRC, I had worked out the speed the blade was travelling at if the guns are positioned at about 36" diameter (that's the line-of-sight through the propeller arc from the center of the spinner) and the speed of that diameter of the blade was about 320 mph (we are not talking about the tip of the blade diameter and speed). The velocity of a 20mm shell though is 3396 feet per second (2315 mph) so there is plenty of time for a single shell to pass between two of the blades as they rotate, probably quite a few could pass through if the gun could fire fast enough.

But back to the contra-prop with two four-blade propellers, all they had to be is geared so the propellers crossed each other each time in a specific point, say straight up/down and out horizontally either side. But there is another point they cross one in front of the other too, at 45 degrees to the other crossing point.  There is still plenty of time for shells to pass through the propeller arc.

[Librarian]

So could, say, a Ta 152 with two cowling mounted and two wingroot guns have a contra prop with a total of eight blades, or would the maximum be two? I've fallen in love with contras of late and wish to use one on this project.

[kitnut617]

So could, say, a Ta 152 with two cowling mounted and two wingroot guns have a contra prop with a total of eight blades, or would the maximum be two? I've fallen in love with contras of late and wish to use one on this project.

In my opinion, yes!  However, you have to position the guns so they will be in one of the open spaces.

I can't find the four-blade sketch I did on my computer, probably one of the many I lost when my computer crashed last year.  But here's a three-blade scenero I also did at the time



This is a pic of my RR Crecy Fighter, it has two four-blade contra-prop and the cannon position is indicated by the felt-marker line.

[Librarian]

I like the layout of the Crecy, on the Ta it would translate as two wingroots and a hub mount. One 30mm and two  20mms will knock chunks out of almost anything. One 30mm and four 20mms would be greedy (unless you drain two of ammo then switch to the next two).