The Evil Doctor's Next Project

[Dr. Jet]

Back to the next project going to the paint shop, i.e. the MHZ Miss Madison. This one will be simple: White with the "Miss Madison" livery.

The motor in it.

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[Dr. Jet]

It says right on the endbell. It's an 1107/1.5Y/H/3.2. The -/H means it's a helicopter version of an 1107/1.5Y with a finned case and a built-in cooling fan. My program says it's slightly less than 300 Watts with a Graupner S-31 prop at just under 27,000 rpm and 47 amps (2S). Calculated speed is a bit over 40 mph. Real world testing shows all those numbers to be in the ballpark.

The program shows it would be 1 or 2 mph slower with an S-29 prop and the current drops to about 36 amps. Efficiency goes WAY up with this smaller prop at only a small loss of top speed.

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[Dr. Jet]

I can only tell you what the voltage of the pack is when I put it in, and the voltages/temperatures when I take it out. I have no way to measure current and voltages while underway; I don't have any data recording devices; and since I have so many boats that use the same battery, I don't keep track of the amps that go back into any one particular battery. It is because of those factors that I rely on prediction programs to at least have an idea what the current draws will be/are.

[Dr. Jet]

As I recall, Steve had an over-run of the 1107/1.5Y/H motors a LONG, LONG, time ago and I bought several of them in a "Blow Out" sale. (That's the inside track I was talking about: I occasionally know when a good deal is coming down the pipe.) The helicopter motors with built-in air cooling work really well in boats. In fact, the "Secret Project" mentioned above that is currently in the paint shop is powered by the 1107/1.5Y/H as well. Stay tuned for photos and discussion.

From an old page on Steve's website:

•The 1107H/1.5Y motor has stronger magnets than does our 1107/1.5Y giving it higher torque and lower RPM making it ideal for the Trex450 on 3S Lithium. The 1107H/1.5Y is a new version that is intended for the very hard core 3D flier. It has very low resistance coupled with very low idle current which results in a motor that can be run at very high peak power without generating excessive heat. Use a 10 or 11 tooth gear for 3S. This motor is too high of RPM for 4S use.

Using my motor analyzer, I measured the rpm/V at 7.4V as 4175 rpm/V.

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[Dr. Jet]

Hubert,

Remember, this is in a 17" hydro. I don't have a lot of room in there for additional hardware. I had to hide the motor way up in the cockpit area to get it close to balancing. Adding the vertical fins and wing are going to move the CG back and it won't be easy getting it to balance again. I run two 1000mah batteries in parallel or 2000mah total. At 35C, I can only safely get 70 amps from this battery arrangement, and would prefer to keep it a bit below that number.

[eXoNerated]

Hubert,

Remember, this is in a 17" hydro. I don't have a lot of room in there for additional hardware. I had to hide the motor way up in the cockpit area to get it close to balancing. Adding the vertical fins and wing are going to move the CG back and it won't be easy getting it to balance again. I run two 1000mah batteries in parallel or 2000mah total. At 35C, I can only safely get 70 amps from this battery arrangement, and would prefer to keep it a bit below that number.

Hi Dr Jet,
I understand but based on your setup I suppose you will have to decide if you trust the calulator neu uses and its sailor or not. Based on a target power and rpm you propose a certain amount of torque is required for that work and what the calulator is saying as based on the torque constant of the motor and it's other parameters this is what it will take to do that. It aint my fault Doc. You will have to run the boat and see what reality has to say.

Regards
Hubert.

[Dr. Jet]

I have run this boat many times and it seems close to the calculated results. It is quicker on the S-31 prop, but the motor comes out much cooler on the S-29 prop (it's been a while and I don't recall specific temperatures).

Remember, this is not an all-out race boat. It is intended to be a "Fun Runner" with good speed, handling, and duration. Something quick enough to make the crowds go "Whoa....!!" , but squeezing every last bit of speed out of it isn't the goal. Not causing an on-board fire or releasing ESC smoke is my primary goal.

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[Dr. Jet]

I do have an infrared thermometer and motor temps were in the 90s with the 29mm prop and over 100 (and less than 135) with the 31mm prop. Exact numbers I don't recall. This boat used dedicated batteries so I could tell the amps out by the amps in when recharging. By factoring in the run times the current draw can be reverse-engineered. Granted, it is hours later when I get that data, and it doesn't account for partial throttle running, current spikes at start-up and such; so again, it is an estimate.

With the bigger boats, when you want to try a motor/battery/prop combo, the chances are somebody else has already tried it and the information is out there. For example, let's say you have a PT Sport 40. A hundred guys will chime in and tell you what they have run and their results. You want a 1/8 scale hydro? Rules limit the motor choices and another hundred guys can tell you their experiences with props, batteries, hardware, and such. If you have a 17" MHZ Miss Madison, you're pretty much on your own.

That's why I use the calculators: Not for specific numbers, or to maximize anything, but rather to know in advance if I'm running the risk of breaking anything. Then I dial it in the old-fashioned way: Make minor changes and see what happens in "The Real World". I don't care about exact numbers or how close I can get to them; in "the big picture" that's a meaningless pursuit. The real test is: Was it fun? Did you break anything? Do you want to do it again? Did the spectators think it was cool?

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[Dr. Jet]

You have real problems determining rpms or peak amperage by those methods........

Of course that's true. I'm just saying that is the only way I have of ESTIMATING the peak amperage using the only real-world "data" I have. I normally don't bother with those estimates as it's not something on my "Need to Know" list. In the many decades I have used FE Calc, I have found the predictions good enough to select a starting prop and an ESC for a given motor. I don't care what the actual numerical value of the peak amperage or RPM is because that's all theory; all I want to know is will this prop load up the motor too much and is my ESC big enough to handle it?

.......... A log showing the rpm and mechanical power out would have really been interesting.

Yes, it would, but........ It's not data I really need. Remember, there was a time before data loggers where modelers still got it "right enough".

You are trying to analyze the sound wave structure of a guitar solo in hertz and decibels and break it down note by note; I just want to know if it sounds good.

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