Puffin - Trial and error engineering

[Subculture]

Loaded and unloaded is referring to the rpm of the motor running uncoupled in the air, loaded is in the water coupled up to the propeller of your choice. Also worth mentioning that a statically held propeller in a test tank will draw more than one that is dynamically moving through the drink.

[Occupant]

Thanks for all the info

RPMs would be interesting , not readily done - my drivetrain is in the wet so optical is out, hall effects units would have to be waterproofed. FRSKY had an ESC that provides all the data RPM, V, A but not sure want to invest just for a quick measurment - kinda pricey.

"To get the best efficiency from your set up aim for a loaded rpm to be around 85-90% of your unloaded rpm."

Sorry if this sounds dumb but explain what you mean by loaded and unloaded At first blush i thought you meant dry vs wet, Dry i'm pulling .8 amps max speed tested (have output weighted to about 70%) that can't be what you meant by unloaded ?

Think i get the concept loaded vs unloaded the closer they get the higher the efficiency less work ont he motor and ESC. for example a square tire vs a round one, very inefficient under load. add facets and the square becomes a hex, and octagon and such until it approaches a circle and the efficiency of a circle.

FYI completed a couple more tests this afternoon, double the pitch - added 30% to current draw to produce nearly the same thrust. they i went half the pitch i started with and got a fraction of the thrust at a lower current. The peak so far seems to be with a pitch .75 of the diameter. Next test is smaller diameter with the 75% pitch see if it can come close to thrust and drop the current draw. will try first in the 2: tube then sleeved to a smaller closer to the 1.5" diameter see what having free space around it does.

[Subculture]

Alongside measuring current and thrust it’s also worthwhile measuring rpm if you have the capacity to do so. To get the best efficiency from your set up aim for a loaded rpm to be around 85-90% of your unloaded rpm. If it gets below 80% then the combination is poorly matched and you should either reduce the load by changing the prop size or look at some form of reduction, fit a larger motor and/or one with a lower kv.

[RCSubGuy]

Im my big Alfa, I was drawing 1.2A at cruise speed and 8A at full throttle. The Seawolf by Thundertiger draws 13A ful-tilt. Current consumption increases exponentially with speed through the water. I run the FAS40 current/voltage sensor for live telemetry in my personal boats.

[Occupant]

Thanks for the info, i wanted to somehow quantify results and came up with a makeshift setup to get me better info. Attach the drive to a yardstick with a pivot, one end is the drive the other end a finger to push on a scale. Add to that telemetry giving current measurements. To start using ~2 inch prop - already have the mount for it. From there modeled and printed two props a 2 blade and a 3 blade - both with the same pitch. the two blade gave almost double the thrust with only 20% increase in current. Now starting with the 2 blade as a reference do another with double the pitch and then finally smaller 1.5 inch diameter to see what is the best balance of current and thrust .



FYI the throttle is being limited to 70% output via transmitter curve. I use the same 650KV motor for my ballast pump , limit it to 25% output and it draws a little over an amp. Anyone else measure current on your drive? what kind of current draw are you geting for normal running?

[Subculture]

Speed controllers whether brushed or brushless control speed by ‘chopping’ the current using a microcontroller to pulse it on and off. Generally the frequency of the pulses is in the thousands of hertz. With a brushless control the commutation is handled electronically, and handled mechanically with a brushed motor.

The pulsing does create some losses in the system, as transistors take time to switch on and off, and in that time span they have a much higher resistance and therefore generate more heat. However modern transistors are so efficient and fast this is barely a concern.

With the 11000RPM I’m basing that on full throttle. I would say for that motor you want a prop diameter of no more than 1.5”. For a a 2” prop running direct drive, I would look more towards a 35mm outrunner, but you’ll probably be okay with a lower kv 28mm unit. I’d look at running a simple two blade prop.

[Occupant]

Thanks for the info, going to try a different prop first (only changing one part ) simple easy test doing with the 2" just as a oint ot start change the pitch and duration of the blades. pitch of .75 duration of about 130 degrees. while thats printing will look at using the 650kv motor i have in the peristaltic pump in the design se if it fits and if i can quickly just adapt the end cap vs having to do all the parts again. that way if the new prop improves things the slower motor would be another step in the right direction.

should i read by your comment "11000rpm+" that your running full throttle? how does limiting the motor speed via transmitter effect the battery and ESC, concerned the throttling may generate heat in the ESC nd heat up the internal WTC cavity with the battery in it.

[Subculture]

That's your problem then. Huge amount of pitch at unloaded speed of 11000RPM+

You want a motor around 300kv-500kv for a prop like that on 3s. It should still be rapid. or consider an inline gearbox of 2:1 or higher. Also consider reducing the pitch of the prop to square or less.

Alternatively, with your existing drive train consider running a much smaller diameter and pitched prop, something around 1.5" diameter and 1-1.25 times pitch.

[Occupant]

D2830, 1000kv, 2” diameter, 3”pitch - 3 blades not the two blade pictured

[Subculture]

Okay, so what's the motor size, KV, and diameter and pitch of the propeller?

[Occupant]

Type-Ohhh damn... 2200mah

[Subculture]

Yeah I think it's a typo, he has a 2200mAh in the picture so I'm going to go with that pack. that should give you a continuous current in excess of 10-15A or more, so I reckon your prop/motor combo is probably a bit overcooked if that battery is getting too hot.

What's the motor size, KV, and diameter and pitch of the propeller?

[RCSubGuy]

200mAh? Whoa... Even my smaller boats usually run at least a 2200mAh pack at 3S.

[Subculture]

If the pack is puffing then you are pulling too much current. I suggest taking some current measurements when the system is under maximum load. Unless your pack is exceptional then pulling more than 3 or 4A from a 200mAh pack continuous is likely to cause it some strain.

[Occupant]

More trial, less error after fixing a few things that went wonky got pool time again today to see if latest changes were helpful at putting issues to bed.

Fixes:
1. Reprogrammed drive ESC to be fwd/rev no brake and 100% output both directions - works very nice now no dead band or wait for brake time between fwd/rev
2. Tried a different impeller 3 blade vs two and 45 degree pitch vs 30 - not as fast
3. Used syringe to push water thru ballast pump to prime along with a little water in the ballast tank (syringe) to start vs trying it dry to start
4. Slow down, check everything out before hand vs gotta get it in the water. Have a shrader valve installed int he WTC to check seal, give it a few pumps and then push the pin listen for air to come out. this time got a different indicator, since I had water in the syringe the air pressure started pushing the plunger and got water out the intake fitting - huh, cool way to check now.

All that being said was a bit slower than the last set of runs, I can fix that with the impeller. At this speed the dive planes didn't have enough reaction to do much of anything and the nose was too buoyant. For a quick test I had just zip tied a 2# weight middle bottom and its does a decent job getting it close to where I want it in the end. Turning radius is remarkable for a 2 foot boat it does a U turn in about a 3 foot diameter. I tried using the ballast pump but it wouldn't pull it down, needs more lead to get it within what the syringe displaces. When opened up afterwards the pump had worked and filled the syringe. Towards the end of the runs the TX was showing 7V then 0V and varying between on the RX as I hit the drive voltage go to zero and then come back up as I let off. along with that some of the servos were starting to chatter - could see little movement on the dive planes without input.

I have some questions that may solve some of my issues:

1. What percentage throttle do you use for most of your run time at? I"m currently running full speed to see what it will do - that creates a big load on the ESC - generates heat.
At the end of my runs the battery has been warm/hot (Zeee 3S 200mah Lipo) should that be normal? Just noticed the battery is starting to look bit "puffy" don't think its gotten wet - maybe heat effecting it? time to use another battery.
2. For quick testing tin the pool I haven't been using an antenna or upping the power of the TX ( saw a setting for output) , when you do use the TX with an antenna in a larger area what power output do you set it to? FYI running a FrSky Tandem 18.
3. Is it normal for the servos to start chattering as you lose power or do I have something else going on?