1/48 G7 Torpedos

[Bob Gato]

Nick,
As I had mentioned my shop and toys are all in storage.. I can do no further testing or building for a couple of months... But I believe that even if you stopped the spinning , the torpedo would still continue to track in a curved Arc. And your solution of counter-rotating propellers might very well be just the thing to cure that... While I do not have the capability or the eye sight to build the 1:48 let alone a 1:64 gearbox when I get back in operation I will pursue allowing the tube to continue to spin and drive a hard mounted counter-rotationally pitched. prop -

for now will stay tuned. -Bob

[Monahan Steam Models]

Definitely observed the submerged prop walk curve trajectory as well as the paddle wheel effect Yaw in the tests I did. The breaching paddle wheel effect was reduced by adding the 1:1 gear box with low set motor center of gravity but the torp body still spun opposite of the prop being more affected by the prop walk in It’s trajectory.

[Bob Gato]

Propeller 'walking' is pressure phenomena of a still submerged propeller producing a lateral force at the stern (yaw). But if the stabilizers broach the surface, its the paddle-wheel effect of the rotating stabilizers producing a lateral force (yaw).

David

The stab fins are definitely a possibility-in my case and tests the track was consistent with prop walk (using bulk drone props-switching rotation was easy) prop walk will track the torpedo the same way as the lower prop blade pushes(ccw turns torp to starboard-tail of torp goes port nose goes starboard) but if it's the stab fins, the same torp will travein a port track( body tube is spinning opposite rotation to the prop)

[Monahan Steam Models]

I removed my earlier post from last night about my grand father because it was written fairly sloppy as I was headed off to bed. He was a life long Navy man and worked on the the US torpedo development team prior, during and after WW2. They moved around a lot but spent the majority of his time stationed at Mare Island Navy base. He was stationed at Pearl Harbor along with my grand mother and my dad’s brother and sister before my dad was born when the attack occurred. My aunt and uncle although young at the time recall the event and would tell the story of my grandpa running into the house grabbing his rifle and shooting at the planes flying over head. It was a very chaotic story but neat to hear growing up.

Back to the miniature torpedo topic, I always appreciate any wisdom shared!!! Please by all means keep it coming! This is not meant to be a here’s how you do it topic. I’m just sharing the successes and failures I’ve been seeing while attempting to develop these things. My work background is in senior mechanical engineering, machining, fabrication and manufacturing. So I like to tinker and I get a lot of enjoyment from designing and building stuff. The more challenging the problem to be solved is sometimes the most enjoyable.

Nick

[Bob Gato]

Good idea they used to do something like that on car engines for oil control...

By the way great backstory on the family...

So Bob Martin has noted the torpedo tends to curve as it tracks-I believe it's from prop torque or walk. It will curve one way with one rotation and the other way with the opposite rotation of the prop.. these props tend to become surface piercing props because the torp rides so high when it's running so the bottom blade of the prop is scooping the tail of the torpedo one way causing the torpedo to turn the other way like a steering thruster... If you can't go counter-rotating then certainly three blades or more on the prop would help. I really hope this information helps and it's not redundant or annoying... Looking forward to the next post -Bob

[Monahan Steam Models]

One thing I found that fights water ingress into prop shafts is to provide a spiraling profile around the shaft that counter rotates to pump the water out

nick

[Monahan Steam Models]

Some compromises I’ve had add to the current design given the weight of the added by the transmission have been switching from running all brass tubes and stuffing boxes to running a carbon fiber main prop center shaft and aluminum secondary Counter rotating secondary shaft. Both shafts are designed to be supported currently by a bronze bushing which I might switch out for Delran. The battle of weight shaving continues.

[Monahan Steam Models]

Bob, will keep you posted on the build. Didn’t want to go with twisted fins for the exact reason you explained. Yes the torp body spin takes away from the forward trust which has been a thorn in my side. After studying many counter rotating design options, I feel the one I went with on purpose is the most replicated possible for most handy folks once I can get it proven. My thoughts were the same as yours with adding a counter rotating set of fins aft of the propeller. In the tests I did it only added resistance and made the problem worse from what I observed. I could be wrong. Most of my tests and revisions have been to isolate and observe behaviors with each change

[Bob Gato]

Nick-I am so looking forward to seeing how the gearbox works out...if you an pull this off it'll be a game changer! My prototypes were built in the winter and the hard water prevented open water testing, alas I just sold the house With the pool and am presently homeless (waiting on new construction delayed by this damned virus!) All my stuff is in storage.

Regarding the spinning- which as you know (and for the other readers) causes the torp to lose fwd thrust because a good portion of the motor's torque is wasted on spinning the body tube and not the prop. My thinking is to not fight it and attach a reverse rotation prop to the back of the tail cone such that when the body tube rotates CW it provides fwd thrust and when the shaft driven prop rotates CCW it also provides fwd thrust conserving the motors power to drive the torpedo, in essence with counter rotating props.(CRPs) I just need some time and water to build it.

I have resigned to allow the coreless motor to run wet and seal the interior of the torp from fwd of the motor, eliminating the shaft waterseals and pretty much eliminates water ingress. The motor being full aft causes the CG to be finicky-but with the capacitor full fwd and only a reed switch in between, the torp just floats slightly nose high.

Crap Batteries are going on this computer(I have to find the power pack) -keep up the progress-Bob

[Monahan Steam Models]

BG,

Yes sir you are absolutely correct. I like your design and choices of materials and components. The smaller you try to build these things the challenges go up exponentially as you already know!!!

From the very beginning even before making the first part, I’ve been fighting to shave hundreds of a gram anywhere I can while keeping the center of gravity negative in the x axis, zero in the Y and zero to slightly positive in Z axis. Right now the Z axis is adjustable. So far it has been a successful design with buoyancy, running mostly in the direction it was fired (I’ll get to that later), mechanically dependable, water tight and robust. The stupid Bass fish in my pond keep attacking them and swimming off with them, only to spit them out later. The nose has been kept removable and empty for when the time comes to switch out the practice rounds.

So far during all the testing and revisions the biggest and last problem to solve has been the torque spin of the torpedo about it’s center axis that is counter to the rotation of the propeller. The original goal was to keep the design simple with running one motor, one propeller and most importantly to me, keeping it’s dimensions and appearance true to the full size originals.

The torque spinning is mostly what has driven all the minor revisions since the original design. Things I’ve done to solve this so far are as follows. Keeping the weight as low as possible. I even moved the motor all the way down to the bottom. One revision I machined a miniature spinning flywheel of the appropriate weight and had an active mechanical gyro stabilizer on board. This did actually help but not enough. I’ve played around with different pitches on the propeller blades and again this did help but was not enough to solve the problem. I’ve added varying degrees of twist to the tail fins which I was hoping not to do and it didn’t help as much as I had hoped for. Up until the last revision, I’ve been doing all the testing with a anti clockwise spinning motor and screw. The last revision I made and tested had a two gear 1:1 ratio transmission. This time I used the anti clockwise screw and a clockwise spinning motor. I thought maybe just maybe this might solve the opposing rotation. Hahahahaha hopes and dreams flattened and the stupid Bass grabbed it during one of the test runs and took it down deep before giving it up. Time to arm the damn things!!!

Okay so this brings me to the current revision I am designing at the moment while waiting for miniature pieces to arrive from near and very far. I decided to stop screwing around and attempt what I was trying to avoid from the beginning. The latest design has the most tech and hours of design time put into it so far. This version will have a scratch built 2 stage transmission. The first stage is a 5:1 ratio planetary gearbox which drives the 2nd stage counter rotating shaft gear box. I will have lots of machining time invested into this one. The whole 2 stage gear box is roughly only 1/4” in diameter and 1/2” long!

I am close to finishing the current design. Most of the work is in correcting the last few hundredths of grams to get everything balanced back to where I had it before adding this transmission. I’ve attached a sneak peek of the almost fishes CAD model assembly. The layout of components will stay basically in the same location with only minor tweaks but the chassis that holds all the bits will likely change a lot by the time the revision is completed.

Nick

[Bob Gato]

M.S.M.
I'm sure by now you have found that buoyancy and weight is critical-so much so that I wound up ditching the aluminum body tube and winding a composite tube from 1/2 oz glass-as a result I saved many grams and gained about a full mm inside for mechanicals. If further weight reduction is needed, consider going with a cap for power which will also eliminate the need for a timer and greatly shorten recharge time to about 30 seconds-but my case was different as I was shooting for 1:64 , good luck with this project-BG

https://www.rcgroups.com/forums/show...91&postcount=6

[george]

Again Thanks for posting/sharing info.

[Monahan Steam Models]

George,

Got the timer from Bob at nautilus

[Monahan Steam Models]

Beautiful work Mr. Merriman! Thank you for sharing!! Very nice!

[george]

That is really nice work!!!