Can a jet drive stator stator fully counteract the motor torque reaction?

Discussion in 'Jet Drives' started by Mermaid, Jul 14, 2008.

  1. Mermaid
    Joined: Jul 2008
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    Location: Pererborough, Ontaro, Canada

    Mermaid Junior Member

    :confused: I am a model submariner but have subscribed to this forum because the the basic principles governing the design of model vs full scale vessels are almost identical.
    Model subs need model armament so I am currently designing a 1/20 scale (1.05" x 14.5") RC heavy torpedo. Being RC, it is most important that the unit not rotate due to torque reaction while under power. Most torpedoes have a pair of counter rotating props to negate the torque reaction.
    At this small scale, a counter rotating system will be difficult if not impossible to implement. Consequently, I am exploring the possibility of using a pump jet propulsor (jet drive) in lieu of the props as is the case with the US MK 48 torpedo.
    I am considering a high volume low pressure mixed flow design with a single 3 bladed impeller ahead of a 4 bladed stator. The question is: Is it possible to design a stator that will both a) straighten the water low behind the impeller and b) generate sufficient counter torque to balance the motor's torque reaction?
     
  2. Guest625101138

    Guest625101138 Previous Member

    It is possible. A bit of a fiddle to get it just right.

    Why not just ballast the torpedo so it has enough righting moment to counter the torque on a conventional shrouded prop. All the heavy bits down low and the light bits or space up high. Spin the prop at high rpm to keep the torque down while delivering the required power.

    Rick W.
     
  3. Mermaid
    Joined: Jul 2008
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    Location: Pererborough, Ontaro, Canada

    Mermaid Junior Member

    Dear Rick: The problem is that at 1/20 scale, the torpedo will be only 1.05-1.1"D x 14.5-15"L and displace APR 170g H2O. Within this small package, I will have to accommodate the drive motor, speed control, 4ch micro receiver, 350mAh battery pack, 2 nano servos and micro antenna.:!: All this (including all of the torpedo's structure) must weigh less than APR 170g or the thing will become negatively buoyant and sink (permanently)!! I think that you may see that I have very little latitude in terms of ballasting.:eek:
     
  4. Guest625101138

    Guest625101138 Previous Member

    Where is the problem. Like I said heavy bits on the bottom air at the top. If they can make this thing fly:
    http://www.wirelesshut.com/307772/12616610.html
    at 10g and with remote control I cannot see that you will have a problem with 170g. Monstrous by comparison.

    (Not that I would like to work with such tiny parts. I have one of these. I wonder if the optical control is easier to do than radio.)

    Rick W
     
  5. Mermaid
    Joined: Jul 2008
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    Location: Pererborough, Ontaro, Canada

    Mermaid Junior Member

    Dear Rick W.: ::( The problem is that the heaviest components (motor and battery pack) will have to be mounted on the C.L. of the torpedo. Remember that I am having to fit rectangular components (motor excepted) inside a 1" I.D. tube. I'll have to cram everything else into the remaining space. The weight of the wires will also be significant. I will certainly consider what you have suggested. The proof will be in the pudding so to speak so I'll have to build it to work out the final bugs.:D
     
  6. Guest625101138

    Guest625101138 Previous Member

    The motor in the model plane referenced is about 1/2" in diameter and drives the prop through a 6:1 gear ratio. No need to mount the motor on the centreline. Even AA batteries could be mounted well below the centreline in an inch tube. Tube could be balsa on top and lead on the bottom. Lots of ways to get weight distribution in the best place.

    Rick W.
     
  7. Mermaid
    Joined: Jul 2008
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    Location: Pererborough, Ontaro, Canada

    Mermaid Junior Member

    Dear Rick: What suffices for micro RC aircraft will not work for a high performance model torpedo. Water is 800 times as dense as air and therefor drag/unit speed is about 800 times as great. For a given drag coefficient, for equivalent top speed, you would have a power requirement which is orders of magnitude greater. Of course, the greater power requirement is concomitant with greater battery capacity. (more later)
    I HAVE considered using the Feigao 1208436X motor. This is a 12mm, 17g motor rated for 4A. It performs at 4133rpm/V. I would have to mount it off axis and use a 2.8:1 gear reduction to get reasonable torque and rpm at the impeller. Max power output would be about 32W @ 11.1V at 72% overall efficiency. However, one of the main points of this exercise is to keep things simple as possible. I want to avoid having to building a contra rotating prop drive and am exploring my options. Last thing I want to do is to add a gear train if at all possible.
    My motor of choice is the Feigao 13084141S. This is a 20mm, 43.4g motor rated for 6A. it operates at 2283 rpm/V and could be connected directly to a suitable impeller. Max power @ 11.1V is about 57W @ 85% overall efficiency.
    Lithium polymer cells are the only type of battery with sufficient power density for this project. I have chosen the 11.1V 350mAh Flightpower Evo Lite power pack. This is capable of 18C discharge (6.3A). NiMH's are too heavy, have lower power density and have problems maintaining voltage at high discharge rates. Same (only more so) applies to NiCADs.
     
  8. Guest625101138

    Guest625101138 Previous Member

    You said it. I am suggesting the simplest possible way to avoid complexity of vanes and tuning them is to just put the weight in the right place. Seems the 12mm motor is about what I was thinking. That little plane I have flies for about 15 minutes. Not certain if it uses a capacitor or battery but it has all the bits to do remote control flight. It climbs at about 30 degrees.

    Typically flying is more energy intensive than sliding through a medium supported by buoyancy - the plane is quite light but also your torpedo will have a low drag coefficient. I can do the drag and power calculations if you want.

    Rick W.
     
  9. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi mermaid,

    My first choice, as Rick has suggested, would be to rely on ballasting as much as possible- anything that possibly can go below centreline, should.

    But I agree with you that in a 1" tube, getting the weight low enough to make a difference will be tricky.

    2283 rpm/V at 11.1 V is a hair over 25300 rpm. I can find a 13084xxS in Feigao's catalogue, http://www.feigao.com/sdp/85838/4/pd-102732/1336473-51426.html , but you quote 3 digits in the gap where Feigao lists 2. How many turns on the one you're considering? What torque are you expecting it to produce?

    I have a hard time believing you could get the overall CG off centre by more than 25% of the radius- or about 7-8 mm. For a 170 g cylindrical craft that's a righting moment- at 90 degrees heel- of about 0.013 newton-metres. A model aircraft can be made to have some degree of inherent stability; a torpedo is a symmetrical cylinder and has no form stability to speak of.

    Try to get the weight off-centre, for sure. You need the static stability as well as the torque counteraction underway. But I'm not sure that'll be enough.

    The whole idea of an internal jet pump at that scale strikes me as just as complex as a counter-rotation gear would be. Are you sure a conventional shrouded prop, with S-shaped stator vanes downstream of it to counter the torque, is impossible?
     
  10. Guest625101138

    Guest625101138 Previous Member

    Matt
    Did you do the torque calculation at say 15000rpm? I figure a 1" shrouded prop would do wonders with 20W behind it at this rpm. I determine around 50% efficiency.

    Still think just weight distribution can achieve the required result in the simplest fashion.

    Rick W.
     
  11. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Agreed that a shrouded external prop could do very well in this application.

    Not convinced weight distribution alone can handle the torque reaction without some help in straightening the flow, but this could come in the form of vanes aft of the prop.

    How close to upright do you want to keep it?
     
  12. Capt-Ron
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    Capt-Ron Junior Member

    What diameter is the motor shaft? If you could replace the shaft with a hollow one of the same OD then mount two motors end to end and extend the front shaft through the rear hollow shaft. Wire the motors to counter rotate.

    CaptRon
     
  13. Guest625101138

    Guest625101138 Previous Member

    What is your target speed. I am getting low drag numbers for the torpedo. Looks like it will need a fraction of the power you have proposed unless of course you want to do more than scale speed.

    Rick W.
     
  14. Mermaid
    Joined: Jul 2008
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    Location: Pererborough, Ontaro, Canada

    Mermaid Junior Member

    Thanks again for all the feedback folks!

    :D After careful consideration of your suggestions, I have decided to go with a 12mm diameter Feigao brush-less motor (1.5mm shaft, 32W @ 4A and 11.1V = 72.1% efficiency with gear reduction). I am aiming for a top speed of 10mph. To date, model electric torpedo performance has been a bit anemic. I am hoping to change that with this design. Scale speed would be a rather unimpressive 1.75 kt. I agree that incorporating as much static stability as possible would be very beneficial. Offsetting the motor will certainly help. I also agree that I shouldn't rely on static stability alone to counteract the torque reaction. The stator is going to have to generate enough counter torque to handle the balance. I am planning to use an axial mixed flow pump jet (jet drive) with a 0.75" 3 bladed impeller with a pitch of 1.05" with a 4 bladed stator placed just aft of the impeller. Outlet area is to be APR 66% of the inlet area. I estimate that 16,761rpm will be required at 40% slip (if prop design concepts even apply to a pump jet) for a top speed of 10mph. Torque will be 2.582 oz-in. The length/beam ratio is 13.2 to 1. The total surface area is approximately 50 sq in. Rick: I have Apr 32W available with the above mentioned motor. I don't know if this will suffice. It would be a great help if you could estimate the power requirement. I have some of the equations used in designing full sized surface craft but their usefulness in designing model submarine craft is debatable.
     

  15. Guest625101138

    Guest625101138 Previous Member

    I have attached the prop design data for the conditions I calculate at 10mph. I determine the drag to be only 1.1N for a proper streamline shape. My area is 36sq.in and weight is only 131g. This is not quite the shape of a torpedo but not too far off. The power requirement for this is 6W at the shaft.

    If I go up to to 1.15" diameter at the same fineness then the displacement is 175g and area is 44sq.in. Drag goes up to 1.3N and power goes to 7.2W.

    Going to 1.23" diameter gets me to 50sq.in. This gives drag of 1.5N. Corresponding power is 8.4W.

    The prop efficiency is determined to be quite high. This particular analysis uses Re# of 50,000 (lowest standard value) and it could be a bit lower than this depending on the blade chord length. I get only 18% increase in flow through the jet in the lowest drag case and 23% in the highest drag case so proposed nozzle necking to 66% is a bit much. I suggest around 80%.

    To put the power into perspective - If I had 32W on my 24ft 100kg boat I would expect to do 5mph. So it is reasonable that something weighing only 1/600th should easily achieve twice the speed with 25% of that power; 32W would be way more than necessary.

    Rick W.
     

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