I would do that but I don't have the raw material on hand so I figured I'd see if someone would try it on one of their hulls for me.
I think the smaller, cheaper motors should be able to turn a prop as well. Remember, these little pager motors swing an eccentric to vibrate, and it appears to be made of lead or potmetal. I wouldn't think a small prop, cut and bent from sheet brass, would be too heavy to swing. Concerning power: It was previously mentioned that a capacitor could be used to store a charge to power the torp, how does that work? Batteries seem to be one of the tougher issues, they are too big, last too long, kinda pricey, and pose environmental issues. Would it be possible to use a capacitor to power one of these little motors for one minute on a charge?
a cap was the first think I thought of when we were discussing this. The problem is I doubt you would be able to discharge a cap slow enough for this kind of 'burn'. BUT caps must ABSOLUTELY be tested... as they provide high output for short periods of time, which is exactly what we want.
I will dig through my fluids books tonight and see if we can guesstamate the power needed to maintain a torpedo at the fastest scale speed I can fathom them going... I.E. fast gun, 100 feet in 22 seconds... since the japanese torps did have a max speed of 48kts. and we can work back from there.
archer, I have a rocket simulator that I can use calculate the needed torpedo power. I'll do that this weekend. Why are you all so concerned about the environmental hazard of batteries? We spew pounds of CO2 into the air from our inefficient guns and many people are using SLA batteries which are terrible for the environment just from a production standpoint, let alone if one of them leaks into a pond. By the way, lithium polymer would react with water and become completely inert. iamscottym
"Why are you all so concerned about the environmental hazard of batteries? " You ever seen pictures of the Love Canal in the 60's? nuff said. CO2 isn't an environmental hazard. Currently the issue on CO2 is trying to be proved one way or the other. Our levels of CO2 are very very slight in any case. SLA's by their nature are sealed and won't leak on the pond. Where they do stand a chance of leaking is during the charging process when charging them at a higher rate than they can take, like if someone uses a car or motorcycle battery charger on their ship batteries. They are also supposed to be recycled and not tossed into a landfill. Yes, they are a problem from a construction standpoint, but they aren't going to poison someone's drinking water, or the catfish someone catches from their fishing pond. The pollution is *hopefully* and mostly trapped during production and recyling. You are correct, with LiPoly, the lithium is bound to the polymer, so it won't escape into the water. Lithium Ion though is not environmentally friendly if tossed into the water as the lithium is free to mix with the water.
A minute will seem like an eternity. Think in terms of seconds, like 10-20, etc. Example: In 1:144 scale, a 40 kt torpedo would cover 100' in about 28 secs. If you want your max. range to be around 20', then your runtime should be about 6 secs. at that speed (not counting acceleration from zero). JM
Prior Disclaimer: Numbers run on a 3/8" Diameter by 5" long smooth rod, just to get an idea. Assuming covering 100 feet in 22 seconds (based on the japanese torps being able to outspeed just about anything at 48 kts and the fast gun max speed, remember I am trying for order of magnitude, thats all) Drag on that cylinder will be on the order of 0.023 N, greatly varying based on shape, front and rear ends. Should we want this thing to run for 3 meters, then we would need to have delivered .069 Joules of energy to the water.. Now we would need to factor in the inefficiencies of the prop/drivetrain, motor, and amount of energy is recoverable from a charged battery or capacitor. I have no hard numbers on this, but just for the sake of a thought experiment, I will deem the prop/drivetrain 15% efficient, the motor 90% efficient, and the amount of recoverable energy from the capacitor/battery as 75% . Assuming I did the math correctly, that translates into a stored energy used to run the torpedo (not accelerate it) of about 0.68 Joules. or a capacitor at 12V with a capacitance of ~10 milifarads , if I did the math right. Could one of the more sparkey inclined fellows check that calc.
velocity: 48knots = 24.69m/s distance: 3m 3/8" OD by 5" long yields: cross sectional area 0.000285 m^2, length=0.127m Quadratic Drag equation: Fd=1/2pv^2*Cd*A where p is mass density of fluid: 1000kg/m^3 (water), Cd is drag coeff and A is cross sectional area: reduces to Fd~=.1425*Cd*v^2 W=F*d, for 3m above W=.4275*Cd*v^2 (J). For v above, W= 260.69 * Cd (J). archer, what did you take to be Cd? This can also be expressed in terms of power output, so as to more easily determine appropriate batteries, etc. P=Fd*v=1/2p*v^2*Cd*A * v = .5p*v^3*Cd*A = 917.28*Cd (W). Based on the above, I either a) did something wrong or b) should not have used quadratic drag but rather linear drag F=bv. Anyhow I'll edit this later. btw, brushed motors are more like 50-60% efficient at best. -iamscottym
Did you use 48 TRUE knots for your calculations, or 48 SCALE knots, as defined by the Big Gun speed chart? Correct me if I'm wrong but 25 m/s looks a lot more like 48 true knots than 48 scale knots.
I stayed late a bit and ran it on the CFD machine at work... Nothin like access to tools, no matter how suspicious I usually am of them... I think your velocity is way too high. I was using 100 feet in 22 seconds. 48 knots was a historical reference. I am not trying to work on a fast enough torp to be able to penetrate anything without a warhead of some sort. There is a value for Cd for certain rods but our reynolds numbers are too low for it. Here is the table: For a reynolds number based on rod diameter greater than 10^5 (which we do not meet) the Cd is 1.1 for a L/D=0.5, .93 for L/D=1, .83 for L/D = 2, .85 for L/D=4. None of those numbers are valid as our reynolds numbers are far too low and our L/D too high. Our reynolds numbers for a diameter of .375 inches and a speed of 100 feet/22 seconds (1.38 m/s) will be ~13000. Just as a sanity check, one could check the drag force on a sphere in a similar flow. Dia:=.375*0.0254; Dia := 0.0095250 > rho:=1000; rho := 1000 > nu:=1*10^(-6); nu := 1/1000000 > U:=100*12/22*.0254; U := 1.385454545 > ReD:=U*Dia/nu; ReD := 13196.45454 > Cd:=0.45; Cd := 0.45 > A:=evalf(Dia^2/4*Pi); A := 0.00007125573926 > Drag:=Cd*0.5*rho*U^2*A; Drag := 0.03077421132 for a propulsive power required of ~.042W or to cover a 3m course a energy of ~.092 Joules before any derate is considered due to inefficiencies. I was using numbers for brushless, my bad on the efficiency. which would give us a drag on a sphere of 0.03N give or take.. while we are in the ballpark, it makes me a skosh more suspicious of the CFD results. although as far as order of magnitude, I suspect we are in the ball park.
I realized I forgot to scale down my velocity, yikes! I just bought solidworks with their cfd analysis so I'll play with that when I get a chance. From some articles I've read on small-scale flying robots at that scale flying becomes more like swimming so I would expect water to become more like a viscous sludge. That makes me a tad suspicious of your .092J number but I'm no fluids expert. I'm going to order some propellers this weekend so I can make a prototype. -iamscottym
Iamscotty, actual length dimension compared to other things has nothing to do with it. You have to look at reynolds number, first off, and then add in other dimensioness numbers afterwards depending on what problem you are talking about. The .092J was math done using the experimental Cd curve for a sphere in cross flow (as if there is any other kind with a sphere). Please check my math with one of your fluids books.
archer, I only included length in my post as you had, I am aware length did not factor into it. Your numbers appear to be correct and I haven't the experience to be able to justify my gut feeling that the number is a little low. has anyone seen small motors with a hollow shaft so that I may use two motors inline with counter-rotating props?
iamscottym, I haven't seen any small pager motors with hollow shafts. Counter-rotating props would increase the cost/complexity well out out of the range of our goal, a reasonably affordable locomotive torp. I will be constructing a prototype soon as well, it will be very crude... but should give some idea if we are heading in the right direction. It will consist of a brass tube with carved balsa ends, fins and prop cut from brass sheet, with a pager motor and small battery. I'm shooting for neutral buoyancy with forward momentum. What is everybody else's thoughts on dimensions? Of course this varies based on your scale of choice.
I may make another suggestion.. do not make it truly symmetric either externally or internally ballast the torp or give it some form of keel that will keep it always aligned with the top up when running. then we can more easily counter the propeller torque a bit by having appropriately angled rudder fins... for starters on size, for the energy required (although I think I am being a bit pessimistic on the energy needed), I have found springs in the .437OD range with a compressed length of 2" and an uncompressed of 3.5. that may be a overly large starting point... but a point it is
I planned to twist the torp's fins in the opposite direction of the props rotation... It might take some trial and error, but I think it is possible to cancel the props torque if the water moving across the fins is trying to twist the torp in the opposite direction. Its depth keeping I'm worried about... its a much more difficult problem to address. Thats why I'm shooting for neutral buoyancy when the torp is moving... forward momentum is enough to keep it fairly steady at depth... when the prop stops, its slight positive buoyancy causes it to slowly rise to the surface. A question for you sparky guys: How would we use a capacitor to power the motor for a short burst? How do we charge it and get it to hold the charge until we are ready to release it?
"How would we use a capacitor to power the motor for a short burst? How do we charge it and get it to hold the charge until we are ready to release it?" -the same way you would power it with a battery. use a switch... archer- are you proposing a clockwork mechanism rather than a motor/battery?
What size spring is used to launch 1/4" ball bearings in the current spring powered torps? I've heard stories of somebody building a 1/144 PT boat that had spring powered TT tubes. That spring couldn't be too large on such a small ship... Ok... I think I'm understanding capacitors a little better. From what I read you can charge them with something simple like a multi-meter. Putting a current to them will charge them until the reach capacity, then grounding the two terminals releases the charge. The problem is they release their charge instantly... we need a burst of power lasting a few seconds or more. I'm not an electronics expert (cough, cough calling all electronics experts) but if the capacitor held more charge than the motor could except would that mean the would last longer or would it toast the motor???
