Here are a couple more pics of the running gear I had posted in another thread. My concept here was to serve several functions in the smallest and lightest package my overworked brain could come up with at the time. This one unit provides: 1) a motor mount 2) a gearbox to drive both props with 1 motor (to do over again, using what I've learned, I would design in counter rotating prop capabilities - it was beyond my true capabilities when I started on this) 3) stuffing tube mounting/ alignment (they will be removeable) 4) Prop shaft to gear box alignment - no universals or dogbones needed. To do over again I would probably also make the two sides the same outside shape and make them a hair bigger in a few areas so that none of the gears were outside the footprint of the gearbox. With that done I would have made one side long enough (or wide enough - depending on how you look at it) to reach the other side, and shelled it out so the gears were totally enclosed. This would have provided seperation from combat debris, enclose the gears so I would better keep grease on them, helped keep down noise (although it is pretty quiet as is), and would have eliminated the small alumunum spacers which would make assembly much easier and eliminated a few parts. There would have been a significant weight penalty though, and I would have had to drill and tap access holes in the outside of the case to get to the set screws on the gears. That part of assembly would have been a pain.
With my considerably lower-tech gearbox, I used two larger driven gears meshing together, with the motor pinion driving one of those. The props were offset fore and aft to fit, given the relatively small diameter of the driven gears (compared to the 1" props). Mine works, but it will not win any beauty contests. Yours looks professional. Very nicely done.
Here is a shot of the new prop shafts . In my infinite wisdom, I made the shafts from super thin walled aluminum tube, and made the bushings fom a brass based alloy. The problem here is our old friend galvonic corrosion. Brass and aluminum are arch enimies when they are joined by the instigator............. water......... which obviously presents a problem! I wouldn't worry about it too much except that the tubing was so thin ( I think It was .015" wall thickness, maybe even smaller). I decided to remake the tubes out of an alternate material. After a bit of hunting the only thing I could find to use without a weight penalty was carbon fiber, so I went for it. Although it was much thicker than the aluminum and ridiculously stiffer, the weight between the two was un detectable on my scale (1 gram increments). Carbon fiber also causes aluminum to corrode, but mostly in salt water. Since I see zero possibility of Uritski being capable of being an ocean going vessel I decided to take my chances. To anyone who thinks its a good idea to machine carbon fiber - don't, but if you must, use super sharp, brand new tools and feed very carefully. Carbon fiber hates impact, and on a microscopic level that is exactly what machining is. The binding resin wants to brake away from the super stiff fibers, leaving some of the fibers intact, causing a sort of "fraying" effect. It was no real big deal and was mostly minimal. I trimmed the excess fibers off with a new shop knife.
Thanks Tugboat, I appreciate the kind words! I don't think the counter rotating props even dawned on me until I had most of the gear box built, an unfortunate side effect of having almost no appreicable experience on the real world side of this hobby. In hindsight, maybe I was better off that way though. I haven't worked with gearbox design, gear terminology, gear clearances or anything of the sort when I dove into this boat. I would have probably came up with some ridiculous beast, snarling with dozens of gears, sounding like a giant hornets nest from one of those old sci-fi movies with hornets as big as dump trucks, sucking up 400 horsepower in frictional losses, generating enough heat that the epa would hunt me down due to my personal contributions in melting the polar ice caps, generating enough of a gyroscopic effect that you couldn't turn the boat with a freight train, but would be so heavy you would need a train to move it, etc etc. :cry:. I learned a lot doing this gearbox, and stand a better chance of designing something functional than I did at the onset, but probably still wouldn't have come up with a solution as simple, light weight, and elegant as yours! Proving yet again that common sense and experience reign supreme . A ton of the things I have implimented on this boat are direct results of seeing your work on Othar. I hope to see you finish him someday!
Hopefully this is the final version of the propulsion system. On Tugboats suggestion I decided to look into brushless. After days of Research (I have never used brushless and knew nothing) and tons of digging thru various websites, I found what was basically a drop in replacement for the brushed motor I had. This one met all the criteria - same shaft diameter, same mounting pattern and bolt size, similar outside diameter, and drew low enough amps to use a lighter ESC than the brushed motor did. I could'nt be happier. This one is much smaller in length, has waaaayy more power, runs smoother, should have less likelyhood of causing radio interference, should draw less amps than the brushed (though I haven't verified that yet), and is considerably lighter than the brushed motor. Switching to this motor in itself reduced the propulsion system weight by more than 20%, and the ESC is about 8 grams lighter than the brushed MTronicks was on top of that. Total weight savings is around 34 Grams ! I might get to have a superstructure after all!!
Also, the cheap HobbyKing car ESCs have a very nice low-noise battery eliminator circuit, so you can run the receiver off the main power via the ESC and get no noise. Lots of experience with this Love love LOVE me some brushless tech from HK The nice short motors also make it easier to fit a pump under the deck! Which rules set are you building it for?
Building for big gun rules, no particular club since there isn't one nearby, so I'm doing my best to make it conformable to most clubs with only minor modifications. I'm really glad you had suggested the brushless..... so far I'm totally impressed. Nice small package and lots of power, and the outrunners have gobs of torque in a tiny package. I just ordered a little outrunner for my pump today! Hobby king is the way to go if you can catch it in stock and can wait for the slow, slow, slow boat to get here with the goods!
Finally got an ESC figured out. I had decided to go with the Hobby King Blue Series 12A ESC. Again, after much hunting aound, this one was $7, is good for 12 amps, and with connectors and everything weighed in at 10 grams on my scale. Of course there is a problem.... it doesn't have reverse. I spend the next couple weeks studying ESCs on the internet, and find that there is some firmware out for Atmega based ESCs that give you that option, as well as some control over some other operations. Hey, what could possibly go wrong???? I ordered that bad boy right up, and in a matter of only 3 weeks it was at my doorstep. Arriving along with it was a usbasp programmer (the same that can be used to flash the Turnigy 9x radios) and hobby king even offers an interface cable that plugs into it on one end of the programmer while you hold the other end over the chip..... steady..... easy..... careful.... press a few buttons on the ol' PC.... hocous pocous... and WHALLLA!!!!! Instant smoke show in my den.... as if by majic. Since I always order extras, I have another ESC victim waiting nervously nearby, but decide to hold off..... what the h*&& went wrong?? Ultimately thru some forum guidance and much soul searching, I decide to go with a different ESC (the one Tuggy just mentioned). After a few days of hem hawing around and much more internet studying I decide that before I place an order and wait another 2 or 3 weeks, I only have $7 to lose, so I try again.... this time WITHOUT a battery hooked up to the esc!!!! Say the majic word and ..... WHALLLA!!!!!! Nothing seems to happen. I hook it up anyway and holy cow, it actually works!!!..... Great!!!! I feel like I can go out in public again!! Successfully Flashed ESC on top, under it the first victim, then the programmer, and on the right the Atmega to usbasp interface cable: Interface cable has pogo pins to contact the proper spots on the Atmega chip: Just plug the programmer into the pc, plug the interface cable int the programmer, hold the interface on top of the chip in the proper orientation, press a couple buttons and.... WHALLLA!!!:
That's very interesting -- I had lazily assumed that ESCs for aircraft which were forwards only with standard firmware didn't have circuitry which could do reverse.
au contraire sir Reversing thrust is a great idea which is why so many aircraft have just that capability, some can even use it in flight.
Ah, but precious few of them are propeller-driven, and ZERO of them acheive thrust reversal by reversing the direction of the engine's rotation!
True, however that doesn't make it any less of a good idea. If you can figure out controllable pitch props on out boats I need to talk to you..... thrust reversing aside I'm rather liking that reprogramming dongle. Is there a code library for these esc's? Where were you able to find different versions of the software to flash? You could probably do all sorts of clever things with these ESCs is you had access to the code and could reprogram them.
I don't even remember how I came across the information that flashing these was possible, probably on HK website, it was probably mentioned in one of the reviews or in a discussion - I tend to read all that stuff when I'm looking for something I don't know anything about. The firmware is called SimonK (was written by a guy named Simon Kirby). He has a fairly good sized number of different firmwares for different ESCs, and has a listing of ESCs each version is known to work with. Do a search for SimonK firmware and you will find several tutorials and how to blogs. If anyone is interested in trying to do this, I recommend reading as many of the informational write ups as you can find. Then read them again. The code is essentially open source, so anyone who knows how can modify it to their liking I would imagine. I have to believe you could get a compatible ESC to behave pretty much any way you want with the proper coding. The Atmega chip has capabilities far above and beyond what the ESC uses it for. You should be able to set throttle ramp ups and downs, brake on/ off/ force/ behavior, behavior if overheat or overload is detected..... the sky should be the limit. The original reason he came up with the firmware was for multi rotor flyers. It had to do with the way ESCs typically use an average to output throttle amount and multi rotors tend to fly better with and instantaneous throttle response with no averaging or something to that effect. I can say that with the configuration I currently have mine flashed with (you can toggle on and off several options and set limits on some things as the firmware is supplied) the response is super fast - almost too fast.... it is so instantaneous it seems to put the driven components under extra stress.
@Snipe - I actually considered doing variable-pitch props and did some drawings. It became apparent that at my current capabilities, more than 2 blades was not feasible. But definitely possible. That would raise interesting rules questions, because I could be running all motors at one speed, same voltage, but effectively turn tighter, turn while going in reverse, AND have super reverse, on demand, while staying within the letter of the rules. Definitely not in the spirit of the rules, but it'd be fun to do once to show off. Maybe do one, take it to Nats, and show off on Sunday during testing, but have a second model programmed into the transmitter that doesn't do those things (i.e. still variable pitch but not doing fancy mixing). But it's well within the stock programming capabilities on the 9X transmitter. Maybe on the 1/96 Baltimore, where we allow differential thrust...
Seeing Marks pump in his destroyer build thread promted me to get some input on one of the numerous designs I have been working on trying to finalize lately. I need a small pump, but have zero knowledge of fluid dynamics. I put tgether a few possibilities of impeller design for everyone to see and critique. Let me know which would be best, whats right about it, what's wrong with it, and what I can do to improve it. I'm assuming the designs that keep one side seperated from the other would be best, but since I know so little about the subject I thought I would get as much input as possible. I have seen lots of pumps manufactured with curved vanes, so I'm guessing there is a significant advantage to that, again I don't really know. Ease of manufacturing is essentially irrelevant, I wouldn't have designed any of these this way if it would be too much bother to build them, so consider "this one would be easier" an un-necessary factor. I'm building for big gun, so I only need 1/2 gallon per minute capacity. The diameter of the impeller as drawn is 5/8", but I can change it to whatever, I just don't want it way bigger than necessary. Also, as drawn the vanes are .150" tall. I haven't done any calculations on impeller displacement/ rpm/ outlet size ect. to get an estimate of capacity. I figured it would be nearly as easy to build the housing sort of "modular" and if there wasn't enough capacity I would re make the impeller with taller vanes, and modify the housing to accept it . These are all drawn as they would be machined, with the vanes to the top, but when assembled they would be down, and rotate clockwise (a mirror of what you see here), only matters on the curved ones of course. Seems to me the one in bottom center would be best on an impeller this tiny, but would be best without the two satellite vanes? Here is a 3d rendering just to give a better idea of what one would actually look like:
I think more than 2 blades is a lot of trouble for little gain. Might be wrong. But I got 1.4gal/min through a 1/2 unit restrictor in my Tug-pump MkI (in Scharnie at Nats). I think it was a 1.25"dia by 3/8" high impeller. Standard BC pump discharge hose and stock BC 1/2 unit restrictor. I need to measure the new pump (Mk III) to see if I've improved any. But it's 2 blades, too. At really high speeds, I think more blades become an impediment to good flow, as there is less time for the flow to pass through the entry orifice before another blade is coming around, disrupting flow. I read somewhere that the ideal (efficiency-wise) is a one-blade pump (but not practical for obvious reasons).
