VS-9 and VS-111 propellers

[grampi]

Hey again,

The blades in the second P-51 picture (FB_IMG_1552386755987.jpg) are post-war issue.

I believe those are non-cuffed, chisel tipped blades...

 

[ThomasP]

Hey grampi,

Thanks for the info. I do not know exactly when this type of blade came into service, but I do not recall seeing any photos of them until post-WWII around the Korea war period (I think). Any more info would be appreciated.

 

[spicmart]

What were cuffs for anyway?

 

[ThomasP]

Hey spicmart,

The cuffs are what could be called broader and thicker airfoil sections, usable because they are located closer to the propeller axis and are going at a lower speed (same rpm but covering a shorter distance per rotation because of their shorter radii). They were designed to do one or both of two things. Help cool the engine/feed more air to the engine accessories, or act like a larger blade area (and more efficient) propeller. The problem with them is that at high enough forward air speeds the cuffs can become a source of parasitic drag. Cuffs are still used today on some modern turbo-prop aircraft, but are becoming rarer as newer materials and more capable computer modeling allow more and more capable one piece blade designs.

 

[grampi]

Hey grampi,

Thanks for the info. I do not know exactly when this type of blade came into service, but I do not recall seeing any photos of them until post-WWII around the Korea war period (I think). Any more info would be appreciated.

I believe you are correct...

 

[Ivan1GFP]

The blades are much wider than they actually are. Out there is a photo of a disc mounted on a prop hub of a 109. It is marked for when the gun is fired and when it can't be fired.

Hello Milosh,
I hadn't missed your post. I was trying figure out the best way to answer you without relying on data from my "synchronizer" simulation which in my eyes will always be suspect until I find out more about actual synchronizer timing.

Please pardon my rather long winded reply.

First of all, you need to answer the question of whether or not a synchronized MG or cannon can be fired between the blades of a 4 blade propeller and how different it would be from firing through a 3 blade propeller.

I believe that a lot would depend on the gun.
From generally available literature, the Oerlikon cannons apparently could not be used in a synchronized arrangement. Browning HMG are described as synchronizing badly (losing a lot of their firing rate) and the .30 cal Vickers and Browning types seem to work very well.
The characteristic of the gun it would depend on is what I have been calling "Lock Time", but there may be a more correct term.
What I mean is the delay between pulling the trigger and having a projectile pass through the propeller arc.
This time may be a bit longer than what one would normally expect because these are open bolt machine guns we are dealing with and there isn't even a round in the chamber when the trigger is pulled. This time may also vary slightly because of random maintenance, wear, lubrication, fouling, etc. and will certainly vary from the gun's automatic reloading cycle.

So, knowing this, what kinds of guns have we seen successfully synchronized through a 4 blade propeller?
7.7 mm Vickers on Japanese fighters
Ho-103 12.7 mm on Japanese fighters
Ho-5 20 mm on Japanese fighters (Ki 84)
MG17 or MG 131 on the FW 190 prototype that had a 4 blade propeller.
,50 BMG on the P-39 / P-63 'Cobra

For a 4 blade propeller, each 1/4 rotation would be divided into an arc during which the synchronizer can trigger the gun followed by an arc during which the gun may fire safely before the next blade arrives. The difference between a 3 and 4 blade propeller would be three wider arcs per revolution during which the gun may fire or four narrower ones. I don't believe this would make a great deal of difference.

- Ivan.

 

[Milosh]

I Know you weren't Ivan.

Say each blade has a 5 degree arc. 3 blades = 15 degrees and a 4 blade is 20 degrees. Add 5 degree on each side for safety and that becomes 15 x 3 = 45 degree for the 3 blade and 4 x 15 = 60 degrees for the 4 blade..

The 4 blade has 15 degree less firing time. The 3 blade has 15 degree more firing time.

Only so many bullets can be fired safely.

 

[Shortround6]

Only a fast firing gun will fire once for each rotation of the propeller, a slow firing gun and fast propeller might see the prop go through 3 full revolutions before the gun fires once (early Buffalo with ungeared engine turning 2100-2200rpm with 600rpm .50 cal in the cowl).

geared prop might turn 1200-1600rpm depending on engine and gear ratio. Guns that fired over 1200rpm were rare.

It is the "lock time" that may have caused the trouble. How long (time) from sear release (firing pin starts moving) to bullet reaching the prop disc?
How long from the "sensor" sending the 'all clear" signal to the sear release mechanism?

Even in WW I you had 1200-1800rpm engines with 2 bladed props ( a few 4 blade) spinning in front of 600-700rpm machine guns.