Propeller detachment and brownouts

#1

Hi,

Last weekend I was finally able to get my rov in a test tub and start to test its various performance properties. I really learned a lot, definitely a blast to get it in the water and see it bobbing and spraying around.

I'm really impressed with the power and speed. I'm using a joystick with proportional power control and I can't imagine trying to control it without having that fine control.

Two major things occurred that I found interesting and will be mentally and physically prepared for in the field, brownouts and propeller detachment.

With brownouts I've experienced two different styles, one where the rov recovers and powers back up automatically, and one where the rov stays in the off state until I physically detach power from the batteries. The brownouts only occurred under heavy load and I wasn't sure how fresh the batteries were.

As for the propeller detachment that comes as a surprise. What happened is that the propeller literally unscrewed itself off the shaft when I applied hard reverse thrust at full power. This didn't happen right away, but after a while of me being aggressive with the driving. I think this just highlights how much thrust is being generated, it's waaayyyyy more than I was expecting :)

I'm not quite sure the best way to put the propeller back on once it get's ripped out. For now I've coated the shaft with silicone and slide the prop back one (still a firm fit), then some more silicone between exposed shaft and hub.

-Marcus

#2

To avoid brownouts, I recommend running the ROV at power level 3 and below (hit the numbers on your keyboard to set the power level in the cockpit)

Also since the batteries in the tubes are in parallel, if only one tube of the batteries is connected it will still power on, but only be able to source 1/2 the current. Make sure that both battery tubes are firmly closed; When I have brownout issues it is often caused by me not putting them on tight enough :P

For attaching the props to the shafts, we've found that the Hysol E-90 used for potting the wires is also great for firmly sticking the props to the shafts. I recommend sanding down the motor shafts to roughen them up in order to get better epoxy bonding strength! 5 minute epoxy and other quick set epoxies didn't work that well, they seemed to be a bit too brittle

hope this helps!

-Colin

#3

I ended up using a JB weld like substance to attach the motor hub to the propellers.

It feels very secure and the substance dries rock hard. It may not look pretty, but I feel extremely confident the thruster will no longer be separating from the hub.

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#4

That will definitely works. The unfortunate thing is that we've since found props that are make the ROV about 200% more efficient. Don't worry, though, Marcus - I'll send you another set of motor bells.

#5

Thanks for the motor bells, I wondered what their official name was.

200% more efficient!!?? That's awesome and insane at the same time. Like I said before the thrust that is being currently being produced is more than respectable.

Just out of curiosity what makes it 200% more efficient? I can't imagine a different propeller design that produces the same amount of thrust I'm seeing at 50% the power consumption. Maybe I've got my math wrong, percentage increase always confuses me.

#6

Walt or Colin or Eric was going to post a big write up about the experiment. I actually thought it was already up. I'll get the photos and data - or nag them to post.

You're right on, though. It's not about more power, and more about efficiency. The ROV can now go between 2-3 hours on one charge. That's much better.

#7

her is the link to the propeller testing:

http://openrov.com/profiles/blogs/preliminary-propeller-efficiency-testing

#8

Thanks. Excellent write up and great to see hard numbers.

One aspect that I'm running into now in my testing is the ESC thermal shutdowns. This occurs when I run the motors hard (just under brownout conditions) for more than 30 seconds.

So although I've got the power adjusted to avoid brownouts, there still is the threat of having an ESC shutdown for about 15 - 30 seconds while it cools down. I was happy to read that the efficiency of the Graupner 2308.65 will cut the current draw and thus hopefully stay away from the thermal shutdown conditions.

Where can you buy these new Graupner 2308.65 props?

#9

you can get the props directly from Graupner. There are other model boat sites like cornwall model boats that have them, but they're currently sold out of the Right hand turn props (you want a left and right hand set in order to negate torque roll)

with the 2308/2309 props, the ESCs are much happier since they are drawing half the current to get the same amount of thrust. Thus, thermal shutdowns become a problem of the past (hopefully)

#10

Can we program the default power setting to 1? Where would I look for that in the software? I to am having serious brownout problems. I also keep getting video failure that necessitates refreshing the browser. The refreshed browser defaults to 5 and I then brown out and have to retrieve the unit for reboot.

#11

Did the testing of the different props include the duct?

#12

I also had issue with props coming off. I used the Hysol E-90 to attach the props securely. I taped off the sides to enclose the area between the prop and the bell housing. I fashioned some painting tape on the inside of the bell so that I did not get epoxy on the magnets. I then applied epoxy with the correct mixing tip directly to the cavities in the props through the bell housing and continued to fill until the bell struts were covered in epoxy. Just enough to cover the struts and below the center ridge that fits against the bearing. They are going nowhere now. Sucks I cannot change props but they are in full drive now. It did create a different buoyancy that had to be corrected with a combination of weight in the front and flotation in the back. The ROV actually handled the best it has ever been.