Underwater Quadcopter

Hi

This may sound crazy.  How about a quadcopter designed for underwater use.  Brushless motors operate perfectly well submerged and the rest can be waterproofed quite easily.  Idea being that one would use small props, basically the quad would operate in a different medium but essentially it should still stabilise as it does in air.  I thought about escs (which don't like water) getting hot but they could be exposed, covered with tectyl or some other waterproofing spray.

As a fail-safe, waterquad would be slightly buoyant so in case of power loss, it would merely float to the surface.  One would then have keep throttle on all the time to stay submerged and power up to sink as opposed to normal operation.  I know that 2.4ghz does not work underwater but the older 35mhz systems do.  For fpv, 1.3ghz could be used ??  Also I wonder which flight controller would work best, kk2 maybe for cost-effectiveness. 

Has this been attempted before and if so, any success?   Couldn't seem to find anything on this concept.  I'm itching to start building but somebody please stop me if I'm wasting my time. 

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            • The way a motor is wound determines it's characteristics.  You can't have a motor thats good at both and has the weight ratio to be able to fly.

              The only way to do it would be with some kind of gearing or swappable props.  I think this would be too much complexity and extra weight on a small craft. (And also certainly much harder than one specialised vehicle carrying another specialised vehicle)

              The premise at face value is ok but its a bit like saying why can't we have an apple that is also an orange? Even if you could cross them, you end up with something that is neither.  It simply can't be both.

              • This is true, however the power requirements underwater and in the air are very different. With a quadcopter-sized propeller, you'd only need a few watts of power to maneuver underwater.

                • IF you had a motor with the right torque and efficiency peak to run that slowly, but you don't. It will stall and burn out your ESC.

        • if you are in fresh water , don't bother water proofing. If in salt water then rinse well with clean water after each dive and just be prepared to change the motors every now and then. Changing the motors is probably less hassle and cheaper than trying to seal them. The normal way around this by using bilge pumps etc wont work in flight so don't bother with them. 

          Those blue thrusters are good but are too expensive and too heavy for this application. 

          Standard slow outboard brush-less are the way to go, as mentioned above you can put controllable flaps on the fan outputs or actually rotate the motors to get vectored thrust and I thing this will be the way to go..

    • you are right..but i think same propellers will do the work..  still i am finding ideal propellers which could work more efficiently..

  • An underwater quadcopter is certainly technically possible, but beyond the bouyancy and waterproofing issues it is important to understand some other items as well.

    Yes brushless quadcopters can work underwater - for a while.

    In salt water (ocean) once or twice would be a fair guess due to the corrosion playing absolute havoc with the teeny tiny ball bearings / let alone the aluminum and steel bits that touch each other and form great electro galvanic batteries (dedicated to corrosion).

    In fresh water you can probably make quadcopter motors last a lot longer if you clean and re-lubricate their bearings after every submersion.

    Then there is the matter of the difference in density of water and air, the requirements for a motor propeller combination to be effective both in the thin stuff (air) and water are hugely different.

    Much more torque and much less RPM in water and really an entirely different set of requirements for each.

    Various inventors (and even the military) at various times have designed and tested assorted flying submarines and for the reasons mentioned here and by others there have been no significant successes. 

    It can probably be done, but my guess is that to produce a practical implementation is - unlikely.

    Best Regards,

    Gary

    • As long as the water is pretty clear of minerals or salt it is surprising how long those brush less last. I must have used one for at least 20 hours under water doing tests etc and it still runs great, all I do is rinse the thing off and leave it in a breezy area in the garage to dry out. I have even used a hair dryer to dry it but of course don't use any heat or you maybe driving any grease left in the bearing out. Also WD40 isn't the best as it'll wash out the grease. A quick re-grease after drying would probably help to.

      Vectored thrust is the way forward if your insistent on using the same props and motors, as mentioned above the same prop and motor set up will work providing the motors will give enough torque at very slow speeds. Maybe dream up a cunning magnetic coupling thing and use different props. The up and down is handles OK by the quad props but maybe an extra one for forward / reverse and a rotation prop or two thrust and use them to turn as well (skid steer)  it would involve increasing the weight by two motors and props (or coupling system) but may work out better in the end...

      surface float idea works, tether control is a problem. One idea i tried was a multiplier fishing reel with a motor for retraction but this may be too heavy if you are going to carry this on a quad copter .. The other idea I thought of but didn't try was a normal little Dc motor and a magnetic coupling to a spool , that way it would put a constant pressure on the tether retraction and slip if it pulled too hard (ie snagged).  

      • thanks for your advice.. but i still confused whether to waterproof the motors or not ?.. and if i should which kind of sealing i should do? mechanical or oil filled?.. today i met a guy.. who can help me to build these things (since i don't have any practical experience)..  he has a good knowledge about quadcopters and electronics.. but he says that water proofing could be costly.. and he also said he never worked on underwater robots.. i think if i can fix waterproofing problem with his help i can do this project.. 

  • Make sure you consider the difference in physics between a quadcopter supporting its weight in air and a marine vehicle with near-neutral buoyancy.

    Remember that a quadcopter tilts to generate a forward force and accelerate. When you're maintaining altitude, this force is basically your (thrust * sin(tilt angle)). That's a lot for a quadcopter that has to hold it's weight up in the air. For example, a 2 kg quadcopter tilted at 10 degrees will generate 0.35 kgf forward.

    Underwater vehicles, on the other hand, are usually closer to neutrally buoyant so the "thrust" term is much closer to zero. For example, if you are close to neutral and need just 0.1 kgf to maintain depth, then a 10 degree tilt would only provide 0.02 kgf. That won't do much.

    That basically means that you'll have to tilt the vehicle much further to get the same lateral force. If you're neutrally buoyant, you'd have to turn a full 90 degrees to change direction without also ascending or descending.

    Also, if you're positively buoyant, then all the tilting dynamics work out in reverse, i.e. you would pitch nose up to go forwards instead of backwards.

    Just some things to think about.

    -Rusty

    • yes.. i have seen this part.. i am using same method as you said.. it will be neutrally buoyant and.. and propellers will turn 90 degrees.. now i have to work on its buoyancy factor.. i have to build it by myself i think.. because the ballast tanks which are readily available in market which could get into a depth 25 to 30 m are out of my budget.. The depth was given to me by my project guide because he thinks idea is  more important than its limits also we have less budget and for my for my project its more than enough if it goes a max of 30 depth..  

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