Escuadrilla Azul
Tech Sergeant
- 1,870
- Feb 27, 2020
I have a colectible in spanish with that, "El Mundo de la Aviación".
What is your favorite Bf-109?
- Thread starter Just Schmidt
- Start date
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GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
A "hypothetical" Bf109M could not have MGs in the cowl or wing-root where they were mounted historically.
The dual props would not be able to match synchronization plus the gear housing to accommodate the contrarotating props would most likely eliminate a motor-cannon.
So it would be down to rustatz gondolas in the traditional mounting locations on the outboard wing positions.
So in the end, there would be a trade of speed for firepower, not very lucrative, to be honest.
The dual props would not be able to match synchronization plus the gear housing to accommodate the contrarotating props would most likely eliminate a motor-cannon.
So it would be down to rustatz gondolas in the traditional mounting locations on the outboard wing positions.
So in the end, there would be a trade of speed for firepower, not very lucrative, to be honest.
don4331
Senior Airman
A "hypothetical" Bf109M most certainly could have both cowl/wing-root and/or motor-cannons.
All the WWII contra-rotating propeller examples that I know of spin the front propeller at exactly same speed as front. If you mount the propellers on the shafts, so that every revolution of the propellers, they line up with the same desired gap for your synchronized mgs as with a single prop, they can fire away with impunity. As point of reference, Fisher make synchronized MGs work with the 4 - 0.5"s in the nose of the P-75A
For the motor cannon, 1/2 the engine power of the proposed "M" is very close to what early war engines were making at full power. As example: V-1710-C & R-1830 has a SAE #40 propeller shaft for up to ~1,200hp, the early R-2800 has a SAE #50 propeller shaft with single prop for 1,850hp, late R-2800 had SAE #60 for 2,500+ hp but to go to contra-rotating, P&W uses a SAE #40 for the inner and #60 for the outer - front propeller good for 1,200hp and the rear for the rest. If the hollow of the original DB601 allows for motor cannon (Me.109F), then the DB6o5L with inner shaft identical to early engine and larger shaft for outer should be able to.
Now, we will note that Spitfires with contra-rotating propellers were slower than those with single propeller, but they climbed MUCH faster. The P-47D & -51D found that reducing fuselage area for bubble canopy needed to be compensated with more fin area, so I think the hypothetical plane needs a fin
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Um, yeah...let's use American technology to serve a Luftwaffe problem.
For Contra-rotating propeller applications, the Germans had the BMW 803, DB632 and the As413.
All designs (paper or otherwise) had bulky gearboxes, even the XP-75's gearbox is a monstrosity. You are not going to get a motor cannon in there.
In regards to the XP-75's four MGs, the combined fire of four MGs *might* be sufficient to get some lead downrange, but two MG131s trying to squeeze through six blades would be something to see.
But since this is a "what if", I suppose anything is possible if one wants it bad enough...
For Contra-rotating propeller applications, the Germans had the BMW 803, DB632 and the As413.
All designs (paper or otherwise) had bulky gearboxes, even the XP-75's gearbox is a monstrosity. You are not going to get a motor cannon in there.
In regards to the XP-75's four MGs, the combined fire of four MGs *might* be sufficient to get some lead downrange, but two MG131s trying to squeeze through six blades would be something to see.
But since this is a "what if", I suppose anything is possible if one wants it bad enough...
don4331
Senior Airman
Which is the Merlin 130 from the Hornet and which is the Merlin 140 from the Sturgeon with the bulky contra rotating propeller (pictures cropped so you can't see the shafts; no cheating and looking in RRHT #19).
Note: The motor cannon is at the other end of the engine from the reduction gear
Item 24 is the motor-cannon; behind Item 1 is the reduction gear.
don4331
Senior Airman
Way back in the day, my original supposition was that synchronizer mechanism were set up to blocked their firing just went the blades were in front of the MG as that would allow for highest rate of fire. However, I learned that was far to risky for both engineers and pilots (neither wants to see the prop shot off the airplane).
Instead the MGs are only allowed to fire at one position where it can be guaranteed no blades are in the way.
Now a little math using DB601E and MG131:
DB601E turns 2,700rpm at max rpm with 1.55:1 speed reduction = prop turning ~1,750rpm. (much higher than I expected but it is what it is...)
The MG131 would fire 900 rpg un-synchronized. This being below the speed of the propeller, it obviously can't fire every turn of the prop.
If I divide 1,750rpm by 2, I get 875 rpm. That is sufficiently below un-synchronized speed of the gun for safety, synchronized MG131s fire 875rpg. The 1,750 rounds is a little less than the 1,800 possible but not much.
On the hypothetical Me.109M's SAE #40 inner shaft, I machine 17 of the 18 splines (and same on propeller) and 37 of the 38 on the SAE #60 outer shaft (again same on propeller) and put dots on the reduction gears, it can be ensured they are "timed" the same very time. This would allow using the exact same firing position as any other Me.109 with MG131s and have exactly the same rpg (assuming same engine speed).
p.s. The P-75A with V-3420 turning 3k rpms, and 3:1 reduction in gearbox would be turning the props ~1k rpms. As the M2 fires 600* rpg un-synchronized, the same "fire every 2nd revolution of the propeller" applies... So, the 4 - 0.5" HG would be spewing ~2k rounds per minute, not quite the 2,400 rounds of un-synchronized, but still a lot of lead - all firing straight ahead.
*I know the M2 could be set up to fire faster, but there were also gear boxes for V-3420 with lower reduction ratio which would allow faster firing, but the math would get more complicated and I didn't want to strain myself.
Instead the MGs are only allowed to fire at one position where it can be guaranteed no blades are in the way.
Now a little math using DB601E and MG131:
DB601E turns 2,700rpm at max rpm with 1.55:1 speed reduction = prop turning ~1,750rpm. (much higher than I expected but it is what it is...)
The MG131 would fire 900 rpg un-synchronized. This being below the speed of the propeller, it obviously can't fire every turn of the prop.
If I divide 1,750rpm by 2, I get 875 rpm. That is sufficiently below un-synchronized speed of the gun for safety, synchronized MG131s fire 875rpg. The 1,750 rounds is a little less than the 1,800 possible but not much.
On the hypothetical Me.109M's SAE #40 inner shaft, I machine 17 of the 18 splines (and same on propeller) and 37 of the 38 on the SAE #60 outer shaft (again same on propeller) and put dots on the reduction gears, it can be ensured they are "timed" the same very time. This would allow using the exact same firing position as any other Me.109 with MG131s and have exactly the same rpg (assuming same engine speed).
p.s. The P-75A with V-3420 turning 3k rpms, and 3:1 reduction in gearbox would be turning the props ~1k rpms. As the M2 fires 600* rpg un-synchronized, the same "fire every 2nd revolution of the propeller" applies... So, the 4 - 0.5" HG would be spewing ~2k rounds per minute, not quite the 2,400 rounds of un-synchronized, but still a lot of lead - all firing straight ahead.
*I know the M2 could be set up to fire faster, but there were also gear boxes for V-3420 with lower reduction ratio which would allow faster firing, but the math would get more complicated and I didn't want to strain myself.
GregP
Major
Let's take a Ta 152 with the wide 3-blade prop as our example.
From the hub, adding a small safety factor, each blade takes up about 39.65°. Let's assume it to be 40°, and there are three blades, 3 x 30 = 120°, so we cannot fire for 120° of a 360° circle. That is 1/3 of the prop circle. The gun is unsynchronized, so it can fire at any time, and does so about 15 times per second. So, the prop turns about 116 times between shots (at 2,700 engine rpm, or 1,742 prop rpm).
About 1/3 of the time, a blade will be in the way and will be blocked. So, if we have a synchronizer gear that only blocks fire when the blade is in the way (plus a small safety factor to each side of the blade edge), then we will decrease the gun's rate of fire by about 1/3, and we will drop to around 600 rpm rate of fire.
If we allow the gun to fire at only one point in the rotation, the gun rate of fire is so much slower than the prop that the rate of fire should drop to almost nothing. That is obviously unsatisfactory in the extreme and I thus conclude that we allow the gun to fire freely and block the trigger only when a blade is near or actually in the way.
Statistically speaking, a blade is in the way only about 1/3 of the time, so we have an actually useful rate of fire.
At 2,700 engine rpm / 1,742 prop rpm, the time for one 360ׄ° prop revolution is 0.000574 s. Time per rev we can actually fire without hitting the prop is 0.000383 s. Time we cannot fire is 0.000191 s. These two add up to 0.000574 s. The interrupter MUST be electric to be useful at 900 rpm rate of fire, not mechanical. We only shoot every 0.06667 s, so the gun is very slow relative to the propeller, and we MUST use math to get the possible rate of fire. Actual rate of fire will likely be a bit slower than calculated.
From the hub, adding a small safety factor, each blade takes up about 39.65°. Let's assume it to be 40°, and there are three blades, 3 x 30 = 120°, so we cannot fire for 120° of a 360° circle. That is 1/3 of the prop circle. The gun is unsynchronized, so it can fire at any time, and does so about 15 times per second. So, the prop turns about 116 times between shots (at 2,700 engine rpm, or 1,742 prop rpm).
About 1/3 of the time, a blade will be in the way and will be blocked. So, if we have a synchronizer gear that only blocks fire when the blade is in the way (plus a small safety factor to each side of the blade edge), then we will decrease the gun's rate of fire by about 1/3, and we will drop to around 600 rpm rate of fire.
If we allow the gun to fire at only one point in the rotation, the gun rate of fire is so much slower than the prop that the rate of fire should drop to almost nothing. That is obviously unsatisfactory in the extreme and I thus conclude that we allow the gun to fire freely and block the trigger only when a blade is near or actually in the way.
Statistically speaking, a blade is in the way only about 1/3 of the time, so we have an actually useful rate of fire.
At 2,700 engine rpm / 1,742 prop rpm, the time for one 360ׄ° prop revolution is 0.000574 s. Time per rev we can actually fire without hitting the prop is 0.000383 s. Time we cannot fire is 0.000191 s. These two add up to 0.000574 s. The interrupter MUST be electric to be useful at 900 rpm rate of fire, not mechanical. We only shoot every 0.06667 s, so the gun is very slow relative to the propeller, and we MUST use math to get the possible rate of fire. Actual rate of fire will likely be a bit slower than calculated.
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
Wow
don4331
Senior Airman
Greg:
Your calculations are exactly what I thought they did and I was 100% wrong.
Better example might be the Bf.109E with MG17; 2,700 engine rpm / 1,742, MG 17 fires 1,200rpm unsynchronized.
Blade 1 is straight down once per revolution - 1,742/minute. When blade 1 is straight down, there is enough space between blades 2 and 3 for the MGs to fire. But MG17 does fire that fast. So, divide by 2 = 871, or 3 = 580 as blade one is also straight down at those times.
As MG131 fires 1,200rpm it is ready to fire at the 871 that the blades are in correct, so that is the synchronized rate. And you could make it happen with cams and rods, but electrical is so much easier.
While the MG131 doesn't suffer much reduction in rpg, the MG17's rate is significantly reduced (1,742 for the 2 mgs in cowl vs 2,400 in the wings). But still very useful RoF.
Your calculations are exactly what I thought they did and I was 100% wrong.
Better example might be the Bf.109E with MG17; 2,700 engine rpm / 1,742, MG 17 fires 1,200rpm unsynchronized.
Blade 1 is straight down once per revolution - 1,742/minute. When blade 1 is straight down, there is enough space between blades 2 and 3 for the MGs to fire. But MG17 does fire that fast. So, divide by 2 = 871, or 3 = 580 as blade one is also straight down at those times.
As MG131 fires 1,200rpm it is ready to fire at the 871 that the blades are in correct, so that is the synchronized rate. And you could make it happen with cams and rods, but electrical is so much easier.
While the MG131 doesn't suffer much reduction in rpg, the MG17's rate is significantly reduced (1,742 for the 2 mgs in cowl vs 2,400 in the wings). But still very useful RoF.
glennasher
Senior Airman
That math stuff makes my head hurt, I know it has to be done, but it still hurts my pitiful brain. Thanks for doing that, though, that was good smart work.
GregP
Major
Hi don 4331.
Actually, I think I calculated wrongly above, but it's late. Will check it in the morning, Cheers!
Actually, I think I calculated wrongly above, but it's late. Will check it in the morning, Cheers!
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GregP
Major
My calculation above was incorrect. The only excuse is too many distractions.
I attached the spreadsheet below. I assumed a Ta 152 with the wide propeller. If you use a ruler and measure the arc of a propeller blade in degrees, it comes out to just under 40° per blade (39.7°). I gave it an extra 10° in front of the advancing blade edge and behind the retreating blade edge for safety. The total was 49.7° per blade when we cannot fore or we hit the blade.
The Ta 152 had a 3-bladed propeller, so the total arc was 3 * 49.7°, or 149.1° out of 360° when we cannot fire. That is 41.41667% of the prop arc when we cannot shoot, and also 41.41667% of the time we cannot shoot.
The gun fires asynchronously with regard to the propeller. That is, we cannot control how fast it fires since it fires at the natural synchronous fire rate for that particular gun. We are assuming 900 rounds per minute synchronous fire rate, or 15 rounds per second. So, it fires every 1/15 of a second, or every .0667 seconds.
Assume the engine turns 2,700 rpm with a reduction ratio of 1.55 : 1. So, the propeller turns 1,742 rpm. That is also 29.032 revolutions per second. We shoot 15 rounds per second, so the prop turns 29.032 / 15 = 1.935 turns between every shot. Each propeller revolution takes .0344 seconds, and we cannot shoot for just over 41% of that time. The time we CAN shoot during each revolution is .0202 seconds (59% of the time for 1 revolution) and the time we CANNOT shoot during each revolution is .0143 seconds (41% of the time for 1 revolution).
All of the above assumes we are trying to allow the gun to shoot as often as possible during each propeller revolution. But, as you can see, the gun is very slow by comparison with the propeller. If we try to allow the gun to shoot once per revolution, the gun is simply not fast enough to do so. The prop turns 1,742 rpm and the gun can shoot at 900 rounds per minute. The best we could do in this circumstance is to allow the gun to shoot once per every two revolutions (assuming we shoot once per revolution when we CAN shoot). So, we allow the gun to shoot once every two propeller revolutions, or every 2 * .0344 seconds, or every 0.0689 seconds. That equates to 14.516 rounds per second, which is within the capability of the gun. That turns into a rate of fire of 870 rounds per minute, which is within the synchronous rate of fire of the gun's capability.
There ARE circumstances when it doesn't work out quite so well, such as if the prop is turn slower and the synchronous rate of fire is slower. If our gun only fires at 450 rounds per minute and the prop is turning 1,290 rpm, and we fire every 3 revolutions, we get 430 rounds per minute. So, synchronization does NOT seriously detract from the rate of fire ... most of the time.
Using the spreadsheet, you have to enter how many revolutions between shots, and you have to go high enough so the rate of fire is within the gun's capability. So, you step up that number until your rate of fire is less than the synchronous rate of fire.
Interesting, at least to me. I had never thought it would come down to firing once per revolution, but it seems like that is the way it is done.
I attached the spreadsheet below. I assumed a Ta 152 with the wide propeller. If you use a ruler and measure the arc of a propeller blade in degrees, it comes out to just under 40° per blade (39.7°). I gave it an extra 10° in front of the advancing blade edge and behind the retreating blade edge for safety. The total was 49.7° per blade when we cannot fore or we hit the blade.
The Ta 152 had a 3-bladed propeller, so the total arc was 3 * 49.7°, or 149.1° out of 360° when we cannot fire. That is 41.41667% of the prop arc when we cannot shoot, and also 41.41667% of the time we cannot shoot.
The gun fires asynchronously with regard to the propeller. That is, we cannot control how fast it fires since it fires at the natural synchronous fire rate for that particular gun. We are assuming 900 rounds per minute synchronous fire rate, or 15 rounds per second. So, it fires every 1/15 of a second, or every .0667 seconds.
Assume the engine turns 2,700 rpm with a reduction ratio of 1.55 : 1. So, the propeller turns 1,742 rpm. That is also 29.032 revolutions per second. We shoot 15 rounds per second, so the prop turns 29.032 / 15 = 1.935 turns between every shot. Each propeller revolution takes .0344 seconds, and we cannot shoot for just over 41% of that time. The time we CAN shoot during each revolution is .0202 seconds (59% of the time for 1 revolution) and the time we CANNOT shoot during each revolution is .0143 seconds (41% of the time for 1 revolution).
All of the above assumes we are trying to allow the gun to shoot as often as possible during each propeller revolution. But, as you can see, the gun is very slow by comparison with the propeller. If we try to allow the gun to shoot once per revolution, the gun is simply not fast enough to do so. The prop turns 1,742 rpm and the gun can shoot at 900 rounds per minute. The best we could do in this circumstance is to allow the gun to shoot once per every two revolutions (assuming we shoot once per revolution when we CAN shoot). So, we allow the gun to shoot once every two propeller revolutions, or every 2 * .0344 seconds, or every 0.0689 seconds. That equates to 14.516 rounds per second, which is within the capability of the gun. That turns into a rate of fire of 870 rounds per minute, which is within the synchronous rate of fire of the gun's capability.
There ARE circumstances when it doesn't work out quite so well, such as if the prop is turn slower and the synchronous rate of fire is slower. If our gun only fires at 450 rounds per minute and the prop is turning 1,290 rpm, and we fire every 3 revolutions, we get 430 rounds per minute. So, synchronization does NOT seriously detract from the rate of fire ... most of the time.
Using the spreadsheet, you have to enter how many revolutions between shots, and you have to go high enough so the rate of fire is within the gun's capability. So, you step up that number until your rate of fire is less than the synchronous rate of fire.
Interesting, at least to me. I had never thought it would come down to firing once per revolution, but it seems like that is the way it is done.
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SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
I'm glad you admitted it. I didn't want to call you out on it.
don4331
Senior Airman
The way it was explained to be was: You had 100s of individuals who had never seen an internal combustion engine (ICE) before, and were now being asked to maintain them. So, the engineers needed to apply KISS, so that mistakes would be minimized.
So, while the initial solution which
GregP
and I came up with allowed the highest firing rate, it is also a level more complex = a level easier to get wrong. As the consequences were extreme if you got it wrong, the simpler solution was implemented.
p.s. The upper engine in post #47 is the Merlin 140 for the Short Sturgeon SHORT STURGEON
So, while the initial solution which
GregP
and I came up with allowed the highest firing rate, it is also a level more complex = a level easier to get wrong. As the consequences were extreme if you got it wrong, the simpler solution was implemented.p.s. The upper engine in post #47 is the Merlin 140 for the Short Sturgeon SHORT STURGEON
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
I have no idea what what you guys wrote but I love the way you say it.
GregP
Major
Hi SaprotRob.
If I calculate something wrong, feel free to chime in. It's nice if you point out the error without calling me an idiot, but my intention is to make correct calculations.
Last night, I just calculated rpm as if it were revolutions per second ... but there are 60 seconds in a minute, so I was off by a factor of 60 on some calculations (116 / 60 = 1.933 revs per shot). I was trying to do this and two other tasks at the same time and, as it happens, that doesn't always work for me. All I had to do was to make sure my units were consistent, and I didn't the first time. Shame on me.
It's all good. Cheers! If that is the worst mistake I make, I'll be doing well.
If I calculate something wrong, feel free to chime in. It's nice if you point out the error without calling me an idiot, but my intention is to make correct calculations.
Last night, I just calculated rpm as if it were revolutions per second ... but there are 60 seconds in a minute, so I was off by a factor of 60 on some calculations (116 / 60 = 1.933 revs per shot). I was trying to do this and two other tasks at the same time and, as it happens, that doesn't always work for me. All I had to do was to make sure my units were consistent, and I didn't the first time. Shame on me.
It's all good. Cheers! If that is the worst mistake I make, I'll be doing well.
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
You do realize I have absolutely no clue. I was bored.
