draw, thrust, and components

[kenl]

I've finally done some proper tests on my thrusters.

Components:
Motors = Turnigy D2836/11 750KV Brushless Outrunner
ESCs = HobbyKing Brushless Car ESC 30A w/ Reverse
Props = 52mm Ebay props and a 75mm CPU cooling fan

Test rig is in the photos

Sorry about the Kgs (1 Kg = 35oz)

52mm prop in forward
2.4kg@23.5amps
2.2kg@19.5amps
1.6kg@12.0amps
1.1kg@7.1amps
0.6kg@3.3amps
0.35kg@2.0amps

52mm prop in reverse
1.5kg@20.1amps
1.0kg@10.0amps
0.7kg@6.2amps
0.5kg@4.7amps
0.4kg@3.1amps
0.3kg@2.3amps

75mm Cooling fan FWD
2.0kg@40amps
1.8kg@33amps
1.0kg@10amps
0.7kg@6.1amps
0.4kg@3.0amps
0.3kg@2.3amps

75mm cooling fan reverse
1.5kg@40amps
1.4kg@33amps
0.8kg@11.8amps
0.7kg@9.2amps
0.6kg@6.5amps
0.22kg@2.2amps

I think the 52mm prop outperformed the cooling fan especially in the top range, and it required no extra effort to fit, whereas the cooling fan had to be stripped from it's motor and the adapted to fit.

Next week I will modify a prop shaft so that I can fit 2 x 52mm props back to back, one facing forwards and one back. The aim is to even up the FWD and REV thrusts?

[rossrov]

Good post Ken. Are we having too much fun yet? I'm wondering how the thruster body is affecting flow and ultimately thrust when in the flow (downstream) from the prop? In one of the photos it looks like the concave face of the prop is facing away from the body. Maybe stick it on the shaft the other way round and re-test forward vs reverse, if you haven't tried this already. My thrusters have the props on this way but I haven't done any measurements to confirm.
A prop of more symmetrical blade or foil section would be better. Traditionally of course one would cast a blank then mill to the finished shape..... Good application for a_shorething's 3-D printer, but would the material be strong enough?

[kenl]

I've thought about that a lot too Ross, the prop is smaller than the housing? I'm not sure if I have the prop on the right way around and will try the other way, work is getting in the way this week!!

There are many options I want to explore, one being the props in series (back to back), then I want to see the difference a nozzle makes and lastly a prop at both ends of the thruster housing, but that is mainly as a bow thruster set up.

cheers

Ken

[a_shorething]

<snip>
A prop of more symmetrical blade or foil section would be better. Traditionally of course one would cast a blank then mill to the finished shape..... Good application for a_shorething's 3-D printer, but would the material be strong enough?

Good question. I've had some success setting up a prop blade with the inner shaft hole matching a bilge pump shaft shape (not quite a semi-circle, but a 3mm shaft with an off-center flat) The initial versions had problems going from 0 to full power because the hub I was using wasn't printed at 100% infill so it was 'stripping' the hole. Since then I've gone to 100% infill and it's as strong as any other solid plastic part of the same size as nearly as I can tell.

Another option is to set it up to accept one or more set-screws like this 3d Printed extruder gear: (the slot in the top is for a 4mm nut and the bolt would go in from the side, through the nut and press against the flat on the prop shaft).


If I have trouble with the shafts stripping I may try this technique.

I'm going to try ABS instead of PLA plastic soon (just ordered some) this has two benefits: 1. It's stronger than PLA although not as environmentally friendly and 2. You can really make a nice smooth finish by placing it in an acetone tank (above the acetone, but in the vapor above it) for a few minutes after the print. It melts the outer layer just enough to smooth it out. This would be perfect for a propeller and I think Olegodo on here has done it successfully on his 3D printer already.

If I can get that to work consistently I may open a side business printing custom props for people on here if anyone is interested. I can print props and nozzles to order once I work out whether or not it can handle the strain and the pressure (which should not be an issue if there are no voids in the print).

[rossrov]

Tried to find out a bit about the different printing materials a while back and your post is more informative than what I found then. The solvent fumes sounds a good trick. Maybe another thread just for 3-D printing?

[a_shorething]

Sorry, I didn't mean to hijack this one.

I started one asking about whether anyone thought 3D printed parts could handle the pressure but apparently it hasn't really been tried much yet.

[rossrov]

No, wasn't implying any hijack, just that until you started posting the mag-coupled stuff there hasn't been much practical printing info on the forum. I thought about starting a lathe thread over in "the tool room" because I suspect some folk think that lathe use is somehow an expert's field. Can be, but in my case for example, I mostly machine plastics and aluminium, which is very easy

[olegodo]

I managed to get some testing done today.

Motor: Turnigy D2836/11 750KV Brushless Outrunner
ESC: HobbyKing 30A Boat ESC 3A UBEC (water cooled, ordered the wrong one)
Prop: Graupner 2308/65L

Tank size unknown, see pictures.

I basically only measured the max force i could achieve with this motor/prop combo and only tested in forwards direction.

It maxed out at 2.3kgs while drawing ~190W and @12v that ads out almost 16A right? This is measured from the mains power socket, since I only have the equipment to measure at the mains socket, so this is including what is lost in the PSU.

And yes, I live in a small apartment that I share with one other person, so all my ROV building and testing is going on in my bedroom

[a_shorething]

Hi Ole,

That's a lot of thrust but 16A! Wow. If you end up putting in 5 motors for vectored thrusts that's going to require some serious power.


I'm glad you got back into it though. I've been looking at some new options and now I'm even thinking of moving back over from the Raspberry Pi idea to using Arduino for control and a webcam sending signal back up directly instead of through the Pi.

I got some parts but I need a few more (naturally) before I can do more design and testing. The holidays really set me back and I also had a change in the ownership of my company so work is kind of a distraction right now.

Keep us posted on progress, looks like you're moving forward.

[marcane]

Hi, have any of you experienced that the motor is stalling when you are running at low rpm? My last thruster setup stalled and made a squeeking noise after a very short period of time running in the test bench.. This was a outrunner 450kv turnigy and 30a esc.