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PostPosted: Fri May 29, 2020 12:10 pm 
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Woodstock74 wrote:
But what's the 250.964" indicating (20.8')?


It's the pitch of the blades, which is typically given at 0.7R (70% the way from the central axis to the blade tips).

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PostPosted: Fri May 29, 2020 1:35 pm 
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Woodstock74 wrote:
Regarding the LA's screw diameter, digging into this data base, here:
https://www.parttarget.com/?userguid=72 ... 92E6E12160

Looking at the 688's propeller entry, here:
https://www.parttarget.com/2010-00-106- ... 28E17F6A13

Says:

AGAV End Item Identification Propeller assy 216.000IN od 250.964IN pch rh

216 in OD...= 18'

But what's the 250.964" indicating (20.8')?

Pretty cool it gives diameter of the vortex attenuator (dissipator) at 5.5' and verifies the blade count (11).


'pch' stands for 'pitch': the (theoretical) distance the screw will travel forward in one revolution, as if the medium doesn't flow away, just like a wood screw into wood. 'rh' stands for 'right hand', the direction of turn (clockwise) when viewed from behind, the ship moving forward.

Pitch therefore determines the angle and twist in the screw blades.

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PostPosted: Thu Jul 16, 2020 8:44 pm 
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Just to clarify with regards to the initial, as launched 688 Flight 1's, is it likely they had the clip on STASS bracket on one of their aft planes? I know the other classes were using them but I can't nail down any information on these early boats sonar beyond the fantastic information here. I assume they would have had a similar bracket as those seen on the contemporary SSN's and SSBN's of the era but I wondered if anyone had more information.

I am trying to depict the USS Memphis during the service life of a late acquaintance who once commanded E Division aboard her between 1981-1985. I can tell from this video of USS Memphis coming into port in 1981 that she didn't have the towed array blister:

https://www.youtube.com/watch?v=207JMtbJS4Y

By May 1984, she had been updated:
http://navsource.org/archives/08/687/0868712.jpg

Any help is appreciated!

John


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PostPosted: Sun Jul 19, 2020 2:07 pm 
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That's a good question: Did any of the 688s use STASS? As far as I know, no, and I haven't seen any photos of that tow point on a 688. But I don't have any information to preclude the use of STASS on a 688 and you would think that the Navy would want some towed array capability on their latest submarine class. If I were you, I would probably leave the stern stabilizers bare if depicting a pre-TB-16 boat.

Jacob

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PostPosted: Mon Jul 20, 2020 10:38 am 
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Adding to the above comment, one of the big changes from the Sturgeon to LA classes was the switch from analog to digital conversion in the Sturgeons to all digital in the Los Angeles class. The earlier LAs had to be backfitted as the first LA with the new Mk 117 digital system was Dallas. The earlier analog BQQ-2 sonar was replaced with the digital BQQ-5 sonar. The commitment to all digital eliminated a lot of A to D conversion electronics (which weighed ~3000 lbs.) Later boats received the more capable BSY-1. It is possible that the earlier analog towed arrays (STASS) were not compatible with the LA digital systems. So earlier boats may (and this is sheer speculation) not have had the towed array feature and were later retrofitted with the TB-16 in the forward ballast tank and the hull "hump" to stream it to the stern plane. I also have not heard or seen any evidence of a clip on towed arrays on the earlier LA boats.

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PostPosted: Tue Jul 21, 2020 11:15 am 
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Thanks for the replies. For the time being, I will plan on modeling the earliest 688's without, but it seems hard to believe they didn't have any towed array since it placed a submarine at such a disadvantage to trailing. From what I can see through Navsource images, the last LA class to be launched without the TB-16 hull hump was SSN-699 (USS Jacksonville). SSN-700 (USS Dallas) is the first I could find with the hump at launch.

This overhead image of USS Jacksonville struck me as interesting as it is one of the few that I can find with a look at the stern of a 688 at launch during this time period (http://navsource.org/archives/08/700/0869914.jpg). The thing that caught my eye was the ladder like structure that is clearly affixed in some manner to the stern, aft of the lines connecting her to the dry dock. Any thoughts on what this is ? I recognize the flags on each horizontal plane to give a clear indication of where they were below the waterline while she was in dry dock . They did a pretty good job of hiding the top of the screw.

Not much else to go on though, but I thought I would share what else I had parsed out.

John


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PostPosted: Tue Jul 21, 2020 3:08 pm 
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Friedman's U.S. Submarines since 1945 says that the contract for integrating the TB-16 into the BQQ-5 system was awarded in 1982. So I think Tom is right that the older analog systems couldn't work with the new digital BQQ-5. It's conceivable that there was a standalone towed array processor, along the lines of the STASS or the BQR-15 towed arrays in earlier submarines which didn't have combined sonar suites. But I don't know.

As for the "ladder," I think it may be two pipes that are connected to the MBT 5 vents. You can see two raised circular object where the aft part of the "ladder" meets the hull, which are in the right pace for the two MBT 5 vents. The "rungs" might be rope or wire keeping the pipes/hoses together. What exactly the function of these pipes are I have no idea.

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PostPosted: Mon Oct 05, 2020 9:46 am 
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Hey Shipmates,

Served on three of them. I am doing a set of molds for use on my 96, 350, 192, 100 scale 688s. Does anyone have an idea where I might get a drawing of the Annhedrals on the Third Flights?

I've been winging it from memory but they are looking too big.

Thanks all! Clear on the broadcast. All stations Conn going Deep.

TMC/SS

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PostPosted: Mon Oct 05, 2020 3:04 pm 
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With respect to the "ladder", it is a series of hoses connected to the aft main ballast tanks to blow them full of air without using ship's systems. There would be similar hoses forward most likely. The shipyard will do this to be able to carefully control the trim of the ship as it is floated off of, or set onto blocks in the dock. With a normal low pressure blow, a boat will sit with a slight up angle. You want to be neutral in a dock.

Hope this helps,

Dave


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PostPosted: Mon Oct 05, 2020 5:15 pm 
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torpedochief wrote:
Hey Shipmates,

Served on three of them. I am doing a set of molds for use on my 96, 350, 192, 100 scale 688s. Does anyone have an idea where I might get a drawing of the Annhedrals on the Third Flights?

I've been winging it from memory but they are looking too big.

Thanks all! Clear on the broadcast. All stations Conn going Deep.

TMC/SS


The only unclassified official dimensional data I've seen on the dihedrals (yeah they really should be called anhedrals) is that they are between frames 129 and 137 (310 and 326 feet aft of the forward perpendicular) and are at a 25 degree angle below the main axis. If you do come across an actual drawing, let me know!

Jacob

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PostPosted: Thu Oct 08, 2020 1:46 pm 
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Vepr157 wrote:
Friedman's U.S. Submarines since 1945 says that the contract for integrating the TB-16 into the BQQ-5 system was awarded in 1982. So I think Tom is right that the older analog systems couldn't work with the new digital BQQ-5. It's conceivable that there was a standalone towed array processor, along the lines of the STASS or the BQR-15 towed arrays in earlier submarines which didn't have combined sonar suites. But I don't know.

Jacob


All of the older towed arrays were analog from the phones back to the boat using multiplexed signals riding a carrier wave. Even the STASS (Submarine Towed Array Sonar System), the BQR-15, and all the way through the BQQ-5 (TB-16 arrays only) worked this way, The Analog to digital conversion took place in the beamformer cabinets after the multiplexed channel signals were filtered, pre-amplified, and then sent to the beam former circuits. All of this circuitry lived in a unit called the TAR (Towed Array Receiver). It was identical on the BQQ-5 and BQQ-6 systems. It might have been Raytheon who built it, but I can't remember. IBM built the majority of the BQQ-5 and 6.

The TB-23 and later the TB-29 TLTA (Thin Line Towed Array) that made it's debut in the late 80's on the Tridents was an all-digital system called the BQQ-9. It was built by Rockwell International in Anaheim, CA. It had a Memory Set, a Tape Drive unit to load the software and a single Processor/Display unit in Sonar. A 32-bit UYK-44 handled the math. It was superior to the "Fatline" towed array in all respects except one - the handling system was initially outboard so the sailors had to work in the superstructure to stream and retrieve the array, which no fun in a high sea state no matter how long you had been underwater and wanted to get some fresh air.

The Q-9 was a Trident-only system. (I'm reasonably sure a fast boat tested it though.) Later, one of the two OK-276 handling systems just forward of the sonar control room was dedicated to the Q-9 after a couple of sailors were nearly killed working under the turtleback deploying an array. The array was deployed nearly identically as the TB-16. There might have been a modified drogue on the end of the array, (it was significantly longer than the TB-16 but smaller in diameter to the TB-16 which was housed in the TA stowage tubes on both SSNs and SSBNs) to pull it off the extra capstan drive assembly to accommodate the longer array. I lead the installation of the Q-9 systems on the 726-728 around 1988 or '89. Each installation took three (24-day) refits and involved close coordination between many different trades. Rockwell used the same display/processor unit found on S-3 Vikings, and the development time was very fast (<3 years) to get the enhanced system to the fleet. The advanced maintenance school we attended was three months long. The sailors loved the system and it was very reliable. There are numerous pictures of the Q-5/6 CDC's IBM (Control Display Consoles) online, but I've never seen the Q-9 SOD (Sonar operator Display) appear anywhere.

Here's what the nearly always-wrong fas website has to say about the Q-9:

BQQ-9 TASPE Towed Array

The BQQ-9 Towed Array Signal Processing Equipment [TASPE] passive sonar on the Ohio-class submarines is a long range search sonar providing medium detection probability at ranges of some 30nm and Convergence Zone Range: 3(30nm).

As usual, they are . . . in error. (They still list the BQR-19 mast-mounted, collision-avoidance sonar as having been installed on the Tridents!) Never happened. Every US boomer prior to the 726 class had it though. The spherical array on the Tridents rendered it obsolete.


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PostPosted: Thu Oct 08, 2020 2:12 pm 
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Can anyone identify which screw configuration is correct for USS Minneapolis-St Paul SSN-708?

I'd like to model her. There are some very nice 3D-printed screws available in 1/350 scale for 688I boats (and others) from vendor "mulsannemike" (photos below). He offers:
- 7-bladed scythe screw with vortex attenuator
- 7-bladed scythe screw, early (no vortex attentuator)
- 7-bladed screw with annular ring

I'd like to choose the most correct screw for M-SP, if available.

TIA.


Disclaimer: I have no relationship with mulsannemike.


Attachments:
Mulsannemike 688I scythe screw with vortex attenuator.jpg
Mulsannemike 688I scythe screw with vortex attenuator.jpg [ 177.92 KiB | Viewed 1879 times ]
Mulsannemike 688I scythe screw early.jpg
Mulsannemike 688I scythe screw early.jpg [ 121.08 KiB | Viewed 1879 times ]
Mulsannemike 688I scythe screw with annular ring.jpg
Mulsannemike 688I scythe screw with annular ring.jpg [ 116.56 KiB | Viewed 1879 times ]

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PostPosted: Thu Oct 08, 2020 2:39 pm 
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ModelMonkey wrote:
Can anyone identify which screw configuration is correct for USS Minneapolis-St Paul SSN-708?

I'd like to model her. There are some very nice 3D-printed screws available in 1/350 scale for 688I boats (and others) from vendor "mulsannemike" (photos below). He offers:
- 7-bladed scythe screw with vortex attenuator
- 7-bladed scythe screw, early (no vortex attentuator)
- 7-bladed screw with annular ring

I'd like to choose the most correct screw for M-SP, if available.

TIA.


Disclaimer: I have no relationship with mulsannemike.


Hi Steve,

I cannot tell which configuration would be correct for the 708 Minneapolis/St. Paul. But the three examples you show are lacking one important feature: the 'scythe' blades lack the twist from root to tip. At first glance I even thought these were flat PE blades. Disappointing, as the dome and the noise attenuators look very nice.
For exactly this reason I created resin LA screws several years ago, avaialable now from Naval Models. These definitely have the blade twist (but not the super nice noise attenuators, I must admit that!) So if you need 'perfect' screws I think you could best combine them.
(Note: 'scythe' blades is a clear laymans description, but the technical term is 'skewback'. EJ Foeth pointed that out to me and I trust him, he earns his professional money in these matters...)

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PostPosted: Thu Oct 08, 2020 3:29 pm 
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Here are the 688 propellers and which submarines they can be used on:

I3B - SSN 688-720
I3M - SSN 688-725, 750-765, 767-770
LAH - SSN 751-765, 767-770
IPMP/MOD25 - SSN 766, 771-773

The I3B and I3M are your classic skewed propellers (skewback is an ok term, but I think still slightly colloquial). The I3B is the original 688 propeller, and the I3M is quite similar. I suspect the difference is either that the I3M has improved blade tip geometry or it can mount the hub vortex diffuser.

The LAH and IPMP propellers are both "hybrids" (i.e. ringed or annular propellers; designed for acoustic reasons, not for ice protection) and the difference is probably in the different shaft size of the IPMP boats (SSN 766, 771-773) and possibly different max shaft RPM because those boats had different propulsion machinery than all other 688s.

If you are modeling the 708 nearer to when she was commissioned, I would use a propeller with a plain boss. If you are modeling her nearer to when she was decommissioned, I would use a propeller with the hub vortex diffuser. Hope this helps!

Jacob

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PostPosted: Thu Oct 08, 2020 6:52 pm 
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Thanks very much, fellas. That helps a lot!

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PostPosted: Tue Oct 13, 2020 9:02 am 
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ModelMonkey wrote:
Can anyone identify which screw configuration is correct for USS Minneapolis-St Paul SSN-708?

I'd like to model her. There are some very nice 3D-printed screws available in 1/350 scale for 688I boats (and others) from vendor "mulsannemike" (photos below). He offers:
- 7-bladed scythe screw with vortex attenuator
- 7-bladed scythe screw, early (no vortex attentuator)
- 7-bladed screw with annular ring

I'd like to choose the most correct screw for M-SP, if available.

TIA.


Disclaimer: I have no relationship with mulsannemike.



All my screws, from LA to Sturgeon to Skipjack to Permit to Ohio (and Kilo and Resolution and Type 212, etc...) have blade twist. Yes, as Marten likes to point out, my initial offerings didn't. Fault me for not updating the product photos. Though don't be too surprised when I do and you can't tell; at 1/350 twist is still pretty subtle from conventional angles. My 688 screws have the twist and much more accurate blade thickness at .3 mm (not to mention details like the vortex attenuator, those blades are printed at .2 mm).

I use the 'scythe' term to differentiate from the more conventional looking skewbacks as they are indeed 'scythe-like', certainly compared to a the skewbacks on say a Sturgeon, and denote, at least in my mind, the emergence of the new class of screws with much mode advanced forms that came about starting in the 70s (as CAD and CAM technology started to emerge and be applied to them). And as noted, lacking proper terminology from the source, I've had to create my own, cribbing off of accepted colloquialism.

My screws certainly aren't perfect, but I think they are a step beyond anything that is out there for the most part. I will say they are certainly better than what typically comes with most kits. I mean let's be honest, DML/Dragon's Ohio comes with a tiny conventional screw that would look more appropriate on a tanker; it isn't accurate in diameter or blade shape. I've at least tried to correct both given extremely limited reference material in the case of Ohios. The PE screws were a valiant effort that I think fell flat if only for the their difficulty in forming. And the injection molded screws, while they actually look pretty good in the more recent Hobby Boss Russian kit offerings, still suffer from the blades being overly thick as a requirement for molding. But, I will never crow about being an expert (I am decidedly NOT--I am not a submariner nor do I work in the marine industry nor am I even a CAD guy), and if you have a suggestion for an improvement I'm always open. Furthermore, the beauty is that I'm not tied to conventional tooling; a few clicks in the CAD and large changes can be quickly implemented, with printed results a few hours later. So as better research emerges, I can instantly respond.


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PostPosted: Tue Oct 13, 2020 4:36 pm 
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>I use the 'scythe' term to differentiate from the more conventional looking skewbacks as they are indeed 'scythe-like', certainly compared to a the skewbacks on say a Sturgeon, and denote, at least in my mind, the emergence of the new class of screws with much mode advanced forms that came about starting in the 70s (as CAD and CAM technology started to emerge and be applied to them).

In regard to the difference between the Los Angeles and and Sturgeon (and before) propellers, I suspect it is not due to CAD/CAM, but rather due to the difference in the submarines' machinery. The Skipjack and Thresher originally used a 15-foot diameter, five-bladed, unskewed propeller operating at a maximum shaft speed of 200 RPM. Around 1960 was discovered that skewing the propeller blades (and increasing the number of blades from five to seven), decreased the unsteady forces on the propeller, greatly reducing low-frequency noise. Seven-bladed skewed propellers were developed, but they kept the same 15-foot diameter for the Skipjack and Permit classes because a greater diameter would lead to higher blade tip velocity and thus earlier inception of blade tip cavitation. The combination of pitch-relieving at the blade tips and the greater number of blades led to a substantial drop in efficiency that could only be compensated for by enlarging the diameter of the propeller. BuShips considered using a larger propeller on the Sturgeon, but the maximum shaft RPM was still 200, so they were forced to keep the same 15-foot propeller as they could not easily change the speed of the turbines or the reduction gear ratios at that point in development.

The Los Angeles had completely new propulsion machinery and thus likely had a lower max RPM, perhaps 150. This allowed the propeller to be enlarged to 18 feet to regain the efficiency lost in the initial move to skewed propellers. Because the wake of a submarine gets more uneven farther from the main axis, the propeller likely had to have a greater skew to reduce low-frequency noise to a sufficient level. Keep in mind that the Los Angeles is a late-'60s design and CAD/CAM was likely not used in the design of the initial propellers. The same is true for the Ohio propellers, which look quite similar but are even larger (hence an even lower max RPM, perhaps just 100).

The first propellers to take advantage of CAD and CFD probably were the Seawolf propulsor and the "hybrid" propellers on the 688I. Photos of the 688I propeller (the LAHII or IPMP/MOD25) show that the blades have a more complex geometry reminiscent of modern turbofan blades.

Also, not that it matters really, but the most proper terminology is "skewed" whereas "skewback," though widely used, appears to be more colloquial. Of course, if you use "scythe" or "skewback" everyone knows what you're talking about anyway :smallsmile:

Jacob

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PostPosted: Tue Oct 13, 2020 8:01 pm 
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Vepr157 wrote:
>I use the 'scythe' term to differentiate from the more conventional looking skewbacks as they are indeed 'scythe-like', certainly compared to a the skewbacks on say a Sturgeon, and denote, at least in my mind, the emergence of the new class of screws with much mode advanced forms that came about starting in the 70s (as CAD and CAM technology started to emerge and be applied to them).

In regard to the difference between the Los Angeles and and Sturgeon (and before) propellers, I suspect it is not due to CAD/CAM, but rather due to the difference in the submarines' machinery. The Skipjack and Thresher originally used a 15-foot diameter, five-bladed, unskewed propeller operating at a maximum shaft speed of 200 RPM. Around 1960 was discovered that skewing the propeller blades (and increasing the number of blades from five to seven), decreased the unsteady forces on the propeller, greatly reducing low-frequency noise. Seven-bladed skewed propellers were developed, but they kept the same 15-foot diameter for the Skipjack and Permit classes because a greater diameter would lead to higher blade tip velocity and thus earlier inception of blade tip cavitation. The combination of pitch-relieving at the blade tips and the greater number of blades led to a substantial drop in efficiency that could only be compensated for by enlarging the diameter of the propeller. BuShips considered using a larger propeller on the Sturgeon, but the maximum shaft RPM was still 200, so they were forced to keep the same 15-foot propeller as they could not easily change the speed of the turbines or the reduction gear ratios at that point in development.

The Los Angeles had completely new propulsion machinery and thus likely had a lower max RPM, perhaps 150. This allowed the propeller to be enlarged to 18 feet to regain the efficiency lost in the initial move to skewed propellers. Because the wake of a submarine gets more uneven farther from the main axis, the propeller likely had to have a greater skew to reduce low-frequency noise to a sufficient level. Keep in mind that the Los Angeles is a late-'60s design and CAD/CAM was likely not used in the design of the initial propellers. The same is true for the Ohio propellers, which look quite similar but are even larger (hence an even lower max RPM, perhaps just 100).

The first propellers to take advantage of CAD and CFD probably were the Seawolf propulsor and the "hybrid" propellers on the 688I. Photos of the 688I propeller (the LAHII or IPMP/MOD25) show that the blades have a more complex geometry reminiscent of modern turbofan blades.

Also, not that it matters really, but the most proper terminology is "skewed" whereas "skewback," though widely used, appears to be more colloquial. Of course, if you use "scythe" or "skewback" everyone knows what you're talking about anyway :smallsmile:

Jacob


CAD, CAM, and CFD all have deep roots. So while no, I can't say for certain, I do know that as these tools became available to us in the automotive industry in the 80s they had been well honed decades before in defense industries. So I'd be surprised if virtual airflow hadn't licked a virtual 688 screw and they had benefited from their development, at least in the follow on screws if not the original fit, as it certainly has the look of something defined using advanced tool as compared to the Sturgeon family of screws which are decidedly conventional looking. We were using CFD in the early 80s in motorsports (1983 is the first motorsports usage of CFD that I'm personally aware of), it was nascent and crude (it was NOTHING like what was available in aerospace at the time), but it was here to stay (it took about another 10 years to become really useful for us however--though even today I like to joke Can't Find Downforce). That all being said, if CFD indeed wasn't used to develop say the 688 screw with the vortex attenuator I'd be very curious the tools they DID use to arrive at the shape of the blades, positioning and number of blades of the VA, etc., as that would be one interesting model!


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PostPosted: Wed Oct 14, 2020 4:27 am 
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Personally, I don't really see a fundamental difference between the 688 and 637 propellers. The 688 propeller has higher skew and the chord is significantly reduced at the tips, but this is pretty typical as you increase skew. I also don't necessarily think the vortex diffuser benefitted from any sort of advanced computational techniques either. Contrarotating propellers (and pumpjets for that matter) were designed by the U.S. Navy in the 1960s, and the vortex diffuser is similar in principle.

Where I do see a fundamental difference is in the shape of the hybrid propellers, which were developed in the '80s. The Virginia rotor blades appear to be similarly shaped. For acoustic reasons, the number of blades between the rotor and stator is dictated by having a large least common multiple, which leads to each typically having a prime number of blades. For the rotor, the number of blades is typically lower because if you have too many blades, the roots will be quite thin and flexible/weak. A large (and odd/prime) number of blades is also beneficial for an open propeller, and seven is typically the best balance between acoustic signature and strength. The stator typically has a larger number of blades because they are supported at their tips by the duct and thus can be thinner.

Jacob

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1/350 Typhoon
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1/350 USS Nautilus
1/350 Tang
1/350 November
1/350 Hotel II
1/350 Alfa
1/350 George Washington
1/72 Type VIIC


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PostPosted: Wed Oct 14, 2020 7:00 am 
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Joined: Sat Nov 07, 2015 11:02 am
Posts: 160
Vepr157 wrote:
Personally, I don't really see a fundamental difference between the 688 and 637 propellers. The 688 propeller has higher skew and the chord is significantly reduced at the tips, but this is pretty typical as you increase skew. I also don't necessarily think the vortex diffuser benefitted from any sort of advanced computational techniques either. Contrarotating propellers (and pumpjets for that matter) were designed by the U.S. Navy in the 1960s, and the vortex diffuser is similar in principle.

Where I do see a fundamental difference is in the shape of the hybrid propellers, which were developed in the '80s. The Virginia rotor blades appear to be similarly shaped. For acoustic reasons, the number of blades between the rotor and stator is dictated by having a large least common multiple, which leads to each typically having a prime number of blades. For the rotor, the number of blades is typically lower because if you have too many blades, the roots will be quite thin and flexible/weak. A large (and odd/prime) number of blades is also beneficial for an open propeller, and seven is typically the best balance between acoustic signature and strength. The stator typically has a larger number of blades because they are supported at their tips by the duct and thus can be thinner.

Jacob



Very insightful. To my eyes it looks a massive leap, 637 to 688. Side by side, one appears rooted in convention, the other much more highly advanced as it is so detail oriented and very intentional; it's the details that stick out as they appear very well honed for the era in which it was introduced. That's what drives my perception of the potential influence of a tool like CFD. But without an eye for the iterative steps to get there it seems a giant leap.


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