VS-9 and VS-111 propellers
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Not sure, but I always had the impression that the existing D-13 rather had a VS-111 airscrew like the D-9 as it seems to look so thin.
On certain fotos it even looks like it is clipped.
Anybody know more?
The depiction attributes the clipped propeller to the D-11.
I just read that the clipping was to reduce drag.
On certain fotos it even looks like it is clipped.
Anybody know more?
The depiction attributes the clipped propeller to the D-11.
I just read that the clipping was to reduce drag.
Two items in propeller design; it is quite helpful to reduce the weight of the propeller as far as stress on the engine and aircraft handling are concerned, for instance the four blade propeller on the P51D added a lot of gyroscopic effect; clipping may help ameliorate transsonic drag rise at the tips without going back to a completely new propeller design or changing the engine reduction gearing.
I can't really say about these props, I think they were wooden? From the illustration it doesn't look like the VS 9 and VS 11 are of the same stock, as they are also different part numbers. Ali blades, such as those found on Hamilton Standard props were cut and profiled by hand to match usage. Different blade lengths had different usages, and on the 23E50 props the blade diameter was often cropped by hand.
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- #6
Actually, that "clip" at the propeller tip looks like the sort of action that would be taken to deal with a minor bit of damage to the blade. I'd be willing to bet there's a similar clip on the other blades to preserve balance.
That small a change isn't going to have any great aerodynamic effects on the behavior of the propeller.
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- #8
Yes. But according to the illustration the blades were specified for the Fws. The guy who makes these accurate prop for model kits did research afaik.
GregP
Major
"The guy" and "research" don't always result in correct data. In this case, we have MANY WWII photos from which to get general prop shapes.
Just from pictures, the Germans seem to have been enamored of 3-blade props; MUCH more so than the Allies. That makes sense because the Luftwaffe was adamant that a gun in the fuselage was worth two in the wings. They may or may not have been correct, but the highest-scoring fighter pilots the world has ever seen flew planes with fuselage-mounted guns, and 3-bladed props offer more rate of fire for fuselage-mounted guns than 4 or 5 blades do.
I've always been of the opinion, shared by more than a few, that the Germans were correct, and we'd have been better off with wide, 3-blade props than the 4-blade units we actually flew. But, it was as it was and we can't change it.
Just from pictures, the Germans seem to have been enamored of 3-blade props; MUCH more so than the Allies. That makes sense because the Luftwaffe was adamant that a gun in the fuselage was worth two in the wings. They may or may not have been correct, but the highest-scoring fighter pilots the world has ever seen flew planes with fuselage-mounted guns, and 3-bladed props offer more rate of fire for fuselage-mounted guns than 4 or 5 blades do.
I've always been of the opinion, shared by more than a few, that the Germans were correct, and we'd have been better off with wide, 3-blade props than the 4-blade units we actually flew. But, it was as it was and we can't change it.
The propellers are exactly the same; the blade profiles are different. These are both Hamilton Standard Hydromatic props, the Mustang was fitted with 24D40s if I can remember. The blade types might have differed though, but fit exactly the same hub. One has cuffs and t'other doesn't, but the tips would have been cropped and shaped by hand.
Hello GregP,
What about the Japanese who also used 4 blade propellers with synchronized guns such as on Ki 84, N1K1, J2M, et al?
In theory, it should not make a great deal of difference how many blades are on the propeller because the synchro is supposed to trigger off the passing of a blade and as long as the gun can fire before the next blade gets there, everything is good. No matter how few blades there are (at least two), the gun is not going to fire more than once between two consecutive blades.
More blades are less efficient in theory, but in practice, it does not actually seem to make a difference according to a couple test reports I have seen.
- Ivan.
ThomasP
Senior Master Sergeant
Hey guys,
I can not say anything authoritative about the props for the FW 190, but the P-51D used a Hamilton Standard prop and the P-51K used an Aeroproducts prop.
The Hamilton Standard was a paddle profile with round tips (almost a semi-circle) and was fitted with cuffs.
The Aeroproducts was a semi-paddle profile with close to elliptical tips and did not use cuffs.
Also, there is a difference between a cropped propeller blade and one with a specifically angled 'flat' at the tip of the blade such as on the Ta 152 H-1 in the photo above. As PStickney says above, if there was damage to the tips and the type of blade lent itself to repair, the tips would often be reshaped in the field to allow continued use. The specifically angled 'flat', however, came that way from the factory and was intended to reduce vibration/destructive harmonics originating at the tips at transonic tip speeds.
The Rotol 3-blade Jablo props (the ones with the wide base and sharp corners) used on the Hurricane I&II had a sharp tip with a similar angle to reduce harmonics also. The later Rotol 4-blade Jablo props used on the Hurricane V and Seafire IIC used a similar tip shape.
The Hurricane and Spitfire DH and Hamilton Standard 3-blade aluminum props all had elliptical blades with elliptical tips (I think).
I can not say anything authoritative about the props for the FW 190, but the P-51D used a Hamilton Standard prop and the P-51K used an Aeroproducts prop.
The Hamilton Standard was a paddle profile with round tips (almost a semi-circle) and was fitted with cuffs.
The Aeroproducts was a semi-paddle profile with close to elliptical tips and did not use cuffs.
Also, there is a difference between a cropped propeller blade and one with a specifically angled 'flat' at the tip of the blade such as on the Ta 152 H-1 in the photo above. As PStickney says above, if there was damage to the tips and the type of blade lent itself to repair, the tips would often be reshaped in the field to allow continued use. The specifically angled 'flat', however, came that way from the factory and was intended to reduce vibration/destructive harmonics originating at the tips at transonic tip speeds.
The Rotol 3-blade Jablo props (the ones with the wide base and sharp corners) used on the Hurricane I&II had a sharp tip with a similar angle to reduce harmonics also. The later Rotol 4-blade Jablo props used on the Hurricane V and Seafire IIC used a similar tip shape.
The Hurricane and Spitfire DH and Hamilton Standard 3-blade aluminum props all had elliptical blades with elliptical tips (I think).
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ThomasP
Senior Master Sergeant
Hey again,
The blades in the second P-51 picture (FB_IMG_1552386755987.jpg) are post-war issue.
The blades in the second P-51 picture (FB_IMG_1552386755987.jpg) are post-war issue.
Hello Milosh,
Why do you believe that?
As long as the lock time of the synchronized gun is quick enough to fire a bullet in the gap of a propeller blade AFTER it has been triggered by the passing propeller, the number of blades doesn't make much of a difference at all. No practical automatic (and I don't mean a motorized Gatling gun) has the firing rate to put out more than one shot in the gap between any two consecutive blades anyway. (At least not for Propeller arrangements typical of a WW2 fighter) Where problems are encountered is when the gun does not have a consistent cycle time or lock time in which case it may not be possible to synchronize or the firing rate may become very low.
The reduction in firing rate would certainly happen with an interrupter system, but those went out of style after the Great War because they were somewhat dangerous.
- Ivan.
ThomasP
Senior Master Sergeant
Hey Milosh and Ivan1GFP,
I may not be thinking this through properly, so correct me if this is wrong.
For this example I assume an engine turning at 3000 rpm with a reduction gear ratio of 2:1, driving a 3-blade propeller turning at 1500 rpm, and assume that the synchronization system triggers the MG at the passage of the No.1 blade.
If we assume the MG can fire with precise control at a maximum ROF of 1500 rpm (or greater) then the weapon could safely fire once per 1 propeller revolution.
If we assume the MG can fire with precise control at a maximum ROF of 750 rpm (or greater) then the weapon could safely fire once per 2 propeller revolutions.
If we assume the MG can fire with precise control at a maximum ROF of 500 rpm (or greater) then the weapon could safely fire once per 3 propeller revolutions.
If I am thinking correctly on this, for a MG to be triggered by each of the 3 blades (i.e. once between each 2 blades) the minimum safely controlled ROF would have to be 4500 rpm?
What if we assume that the MG can fire with precise control at a maximum ROF of 650 rpm? Would this not mean that the MG could be safely fired only once per 3 propeller revolutions? With a resulting maximum effective ROF of 500 rpm? A reduction of 150 rpm?
Could the synchronization trigger mechanism be set to use different timing for each of the 3 blades?
I may not be thinking this through properly, so correct me if this is wrong.
For this example I assume an engine turning at 3000 rpm with a reduction gear ratio of 2:1, driving a 3-blade propeller turning at 1500 rpm, and assume that the synchronization system triggers the MG at the passage of the No.1 blade.
If we assume the MG can fire with precise control at a maximum ROF of 1500 rpm (or greater) then the weapon could safely fire once per 1 propeller revolution.
If we assume the MG can fire with precise control at a maximum ROF of 750 rpm (or greater) then the weapon could safely fire once per 2 propeller revolutions.
If we assume the MG can fire with precise control at a maximum ROF of 500 rpm (or greater) then the weapon could safely fire once per 3 propeller revolutions.
If I am thinking correctly on this, for a MG to be triggered by each of the 3 blades (i.e. once between each 2 blades) the minimum safely controlled ROF would have to be 4500 rpm?
What if we assume that the MG can fire with precise control at a maximum ROF of 650 rpm? Would this not mean that the MG could be safely fired only once per 3 propeller revolutions? With a resulting maximum effective ROF of 500 rpm? A reduction of 150 rpm?
Could the synchronization trigger mechanism be set to use different timing for each of the 3 blades?
Hello ThomasP,
I believe your general idea is pretty close but there are a couple other factors to consider.
First of all no machine gun has an absolutely consistent cycle time and the cycle time of the first rounds which were loaded by spring pressure tends to be a bit longer than the cycle time for the rounds loaded by a combination of spring pressure and the rebound of the last round / recoil amplifiers, etc.
Second, there is a second trigger controlled by the pilot who does not know where the propeller blades are when he decides to fire.
The point is that the propeller blades and gun cycle time are not going to align precisely.
Therefore:
Some of the times when the pilot pulls the trigger, there is a very slight delay because the propeller is not aligned properly to fire the gun. Also, when the gun cycles and is ready for the next shot, the propeller may or may not be aligned to trigger the shot resulting in another delay.
These delays are the cause of the firing rate reduction. As you have already observed, the majority of the time that the propeller's synchronizer can trigger the gun to fire, it will not be ready to fire.
- Ivan.
I believe your general idea is pretty close but there are a couple other factors to consider.
First of all no machine gun has an absolutely consistent cycle time and the cycle time of the first rounds which were loaded by spring pressure tends to be a bit longer than the cycle time for the rounds loaded by a combination of spring pressure and the rebound of the last round / recoil amplifiers, etc.
Second, there is a second trigger controlled by the pilot who does not know where the propeller blades are when he decides to fire.
The point is that the propeller blades and gun cycle time are not going to align precisely.
Therefore:
Some of the times when the pilot pulls the trigger, there is a very slight delay because the propeller is not aligned properly to fire the gun. Also, when the gun cycles and is ready for the next shot, the propeller may or may not be aligned to trigger the shot resulting in another delay.
These delays are the cause of the firing rate reduction. As you have already observed, the majority of the time that the propeller's synchronizer can trigger the gun to fire, it will not be ready to fire.
- Ivan.
ThomasP
Senior Master Sergeant
Hey Ivan1GFP,
Does that mean the synchronized guns would 'stutter' (I currently can't think of a better word) and fire irregularly?
Does that mean the synchronized guns would 'stutter' (I currently can't think of a better word) and fire irregularly?
Hello ThomasP,
Now keep in mind I have never actually heard a synchronized MG firing but I don't believe the change in firing rate (although it really would be a slight "stutter") would be very noticeable.
Here is why I believe this:
Note that if we are discussing a typical .30 cal type, the free firing rates are usually around 1000 to 1200 RPM.
At 1200 rounds per minute, the shots come 0.05 seconds apart (1/20 second).
Now with a 2500 RPM engine driving 3 blade propeller at a 0.500 reduction, we would have 1250 propeller RPM or 3750 blades per minute or a spacing of 0.016 seconds between blades.
I figure if the gun is ready to fire but a blade is in the way, the average delay before the next blade triggers the round would be about half of 0.016 seconds and that doesn't happen with every shot, so I don't think it would be so noticeable.
If the gun's cyclic rate were even lower, it would be even less noticeable.
Perhaps someone here has actual experience with these guns. I have tried programming a simulation to test the changes in firing rate, but my conclusions there depend a bit on my preconceptions being correct. My discussions thus far are based on what we all know must happen rather than how I think the numbers are affected.
- Ivan.
Now keep in mind I have never actually heard a synchronized MG firing but I don't believe the change in firing rate (although it really would be a slight "stutter") would be very noticeable.
Here is why I believe this:
Note that if we are discussing a typical .30 cal type, the free firing rates are usually around 1000 to 1200 RPM.
At 1200 rounds per minute, the shots come 0.05 seconds apart (1/20 second).
Now with a 2500 RPM engine driving 3 blade propeller at a 0.500 reduction, we would have 1250 propeller RPM or 3750 blades per minute or a spacing of 0.016 seconds between blades.
I figure if the gun is ready to fire but a blade is in the way, the average delay before the next blade triggers the round would be about half of 0.016 seconds and that doesn't happen with every shot, so I don't think it would be so noticeable.
If the gun's cyclic rate were even lower, it would be even less noticeable.
Perhaps someone here has actual experience with these guns. I have tried programming a simulation to test the changes in firing rate, but my conclusions there depend a bit on my preconceptions being correct. My discussions thus far are based on what we all know must happen rather than how I think the numbers are affected.
- Ivan.
