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Apr 24, 2014, 03:39 PM
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dbacon's Avatar
MadMurphy,

You will feel at home with brushless motors, as they are 3-phase, and the AC comes from a 3-phase inverter that runs off the DC supply of the battery. It is quite a lot of electronics you get for very little money, I admire what the manufacturers have done.

You should get a Watt meter, which splices in the DC side of the ESC, and most will lock in on the highest number encountered on a test. You want to have at least 150 Watts per pound to keep from having a "doggy" plane.

You can test your plane safely by hanging it from the tail, about 1 foot off the floor, with a digital fish scale to show the thrust developed. If the thrust is greater than the weight, you will have excellent performance. Most of these scales have a "tare" function which will zero out the static weight, and they have a hold function to show you peak thrust.

These items are very cheap, maybe $5 for the scale, $25 for the Watt meter.

You can get rpm gauges pretty cheap too, and you can estimate the "kV" of the motor with that, running a prop that is so small or so shallow that the motor runs with very little load. "kV" is the revolutions per Volt applied, and that is the voltage applied to the ESC, so some of the Watt meters will give you that number too.

Then you can consult some of the prop charts found on the web to see what prop will cause what current draw at what RPM. Then put a prop on and make the above measurements, and check for heat build up, and adjust the prop, or the number of cells accordingly.

It all makes sense eventually.
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Apr 25, 2014, 01:29 AM
Registered User
Eventually...those are famous words.
I got home this afternoon and just pulled the motor out of the SU 31 my son gave me...
it's black.
Other then that there are no markings visible. I'm guessing it had some sort of sticker on it at one time, but for now it's nude and it's a guessing thing for me. I think I'm going to take your advice and visit our LHS tomorrow and see if they have a watt meter. The plane is propped according to what Phoenix Models recommends, but I need to pick up some lipo's with enough cells and mah's and watt's and any other words I need to know, and I think the watt meter is the starting point. I have a pretty good regulated dc power supply that can supply anything up to 100 volts clean dc and a bunch of amps...it came out of a full scale aircraft instrument repair facility. I'm thinking I will use that to power up the esc and see what a watt meter tells me the motors drawing under load. Has to be cheaper then buying a couple of "guesstimated" lipo packs. Maybe as I gain a little more experience with the brushless stuff it won't be such a guesstimate, but for now measurements and numbers are more relate-able for me.
Other then that, is there any way to distinguish between sizes ,or power output of of these little power houses? Numbers of magnets or stators? Because going into the LHS and telling them I have a "black motor" is going to get me laughed at...and I do enough to get laughed at normally that I don't like to compound it if I can help it.
I appreciate your opinions and knowledge, and am looking forward to being around here for a while, either picking brains or showing off my builds. A half built CGB Anniversary Edition Cub is screaming at me "ELECTRIC" from the corner.
It's yellow......
Apr 25, 2014, 12:08 PM
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dbacon's Avatar
MadMurphy,

I think you would be better off using whatever LiPo you have in the plane for the Watt-meter test. Lipos can supply a lot of Amps, whereas a bench supply will probably be limited to 20 Amps or so. Your 100 Volt supply at 20 Amps is 2000 Watts, and that is a lot of power supply, so I think it won't supply 20 Amps, just a guess.

If the Watt meter shows a voltage sag at full throttle, then you can suspect the LiPo is not delivering what it should.

The motors generally fall into categories based on size, and the kA is somewhat based on the wire size. The number of turns around each pole is the key to kA, which is the RPM for each volt applied. If you have 10 to 12 turns, and you may be able to count them, then you would have a high RPM motor, like 2200 kA, meaning you get 22,000 RPM at 10 Volts. If it has a load on it, like a prop, it will still try to get to 22,000 RPM, but it will draw a lot of current to do it, maybe even get hot. That is where you want to reduce the prop size, either diameter, or pitch.

I think if you took the motor in to the LHS, they would look at the diameter, length, wire size, and in some rare cases, number of rotors stacked in there, and probably come real close to predicting its performance. One thing they may not be able to see, it is "Y" or "Delta" connected. Yes, just like the 3-phase stuff you already know about... A "Delta" connected motor will a different RPM than a "Y" connected motor, with all else equal.
Apr 25, 2014, 02:39 PM
Registered User
.Good advice, and I planed on taking this black thing with me to the LHS..it's not exactly local to me, as I live in one of those places where they electricity ends about 200 yards past my driveway. We still have those places here on the peninsula. So, when parts are needed for anything the rule is take the old one with you. And a watt meter is on my pick one up list when I go in.
I took a look at my power supply, and your guess was about spot on...it will put out 100 volts, and a max of 20 amps. It's an SC1, by ATL...I picked it up from the historical aircraft museum in Port Townsend, and was told they used it to charge full scale batteries, as well as bench work. It's kind of overkill for hobby uses, but I originally got it to power up a hotwire machine for some marine work. It does that! It's big, but it works well.
I think that today I'm going to take the Snowball out and have some fun with it, and the SU13 motor issue can wait a day or two there next to the yellow cub. I know my reflexes and flying skills have dimmed over the last 20 years, and I think having some fun with the pink plane is more on par with my present skill level. It's coming back quickly though, and I'm not banking left when I want to turn right ner as often after just a couple of flights with it. At least I can keep the speed down with the Snowball to the point that if (or when ) I do cross control I have time to say Duh! and correct before it's a fatal landing. Though I have found out after my first maiden in the wind that it's a pretty tough creature..it's rolled or cartwheeled at least 3 complete revolutions before landing right side up, with only a small nick to a leading edge (?). I had a buddy ask me if it's round how do you tell which is the leading edge...I told him it's the one facing forward when it stops rolling.
So far I'm having a bunch of fun, if only driving it around in my pebbled driveway. Today the sun is shining and the winds are about 0 mph, so it's time to gain a bit more altitude, or at least to laugh at myself some more. Another of those things I've learned, beside taking the part with you.
I did measure the thrust with a scale with the HB 2827 and the 10/4.7 on it, and it showed about 645 grms of thrust...and nothing seemed to be getting warm or catching fire. So, considering the auw is just over 500 grms, it should provide for some decent performance...like I said before, anything over a blip of full throttle to get it moving and it's in the air.
Apr 25, 2014, 03:47 PM
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dbacon's Avatar
Since your LHS is so distant, you might go to Hobby King to see what they have. The describe their motors pretty well, and you could figure about 10 Watts per ounce, or per 27 grams, and with the right prop to match the kV of the motor, probably come up with some good replacement motors.

Here is one, stocked in the USA warehouse, that I use at 300 Watts (peak) for a 28 ounce Capricorn, and it is probably less than the gas you would use to get to your LHS.
https://www.hobbyking.com/hobbyking/s...arch=D3128-155 It's $12.30
Apr 26, 2014, 01:39 AM
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I think I might have been uncler. I'm running the HB2827 on my new Snowball, and am more then hppy with it. With a 3 cell 2200 mah I'm measuring about 640 grams of thrust, and my Snowball weighs in at 505 grams, after a tape repair this afternoon. I'm doing about 10-12 minute flight times and landing with battery left. My last flight before the winds came up this evening with a fresh charge..allowing for ample cool down time for dinner, lasted almost 6 full minutes. I just checked with a balance checker nd it reads 11.4 total, 3.82, 3.84, 3.82. With the fresh charge it was 12.6 I think...I didn't write it down in a hurry to get in one more flight. So, from what I've learned on line here that seems to be reasonable performance. I'm more then happy with this setup. When I land I check the motor and esc by touch, and neither seems to be hot, just warm in our cool air here.
My concern with the "black motor" was in my wondering if it would be powerful enough to power the half built cub in the coner. It's listed in the specs as auw at 6.5-7.5lbs. Supposedly the motor from the SU 3`1 my son gave me is a replacement for a .46 glo, which is what is recommended for the cub. So that's the one I'm puzzled with..and still being new I'm not able to even guess at it. I will take it with me to the LHS this coming week and see what he can tell me. It came with a 6o amp esc, and that would seem to be marginal for the size motor for the cub..one of the reasons I question it's identity. I was certain if running it up with a 3 cell 2200 would be ok, so my idea on using my dc power supply. My Snowball is great, and I spend more time laughing while flying it then with any plane I've flown. It's the "black motor" that I need to figure out. If I can find my calipers tonight I'll try and get some basic measurements from it. And again, I do appreciate your help here. Murphy

I'm sorry if I wasn't clear. I confuse myself most of the time.
Apr 26, 2014, 04:19 AM
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dbacon's Avatar
Well that was very clear, thanks.
Converting the Cub to electric would require 800 - 1000 Watts, and that would be 62 Amps on a 4 cell battery. That is pushing the ESC over its limit, many types will not make it. If you ran a 5 cell battery, or a more common 6 cell you would be under the 60 Amp limit, but many ESCs cannot take the higher voltage so check that first if you can find info on it.

To run 6 minutes will require 5000 mAh, so we are talking some serious batteries, and that will add a lot of weight to the Cub, making you go back to calculate the wattage requirement again.

Then you may get into the problem of wing loading, and high wing loading requires high speed, so it may not be fun anymore. Calculate the wing loading on your SnowBall to see how it compares, and you might want to leave the Cub as glow.
May 01, 2014, 12:05 PM
I call it 'landing'...
Plattenwerk's Avatar
Strange behavior of my snowball: on the ground (not matter grass, concrete or sand) he turns to the left like hell and I trimmed nearly 15 rudder to move him straight on. But: once in the air the rudder trim is way too much. He spins - and noses in. Taking off without the mentioned trim is inpossible: insane pirouettes.

Motor is straight aligned and so the rudder is. CG is 6" behind the front, diameter 24", 6" winglets with 25 dihedral.
Any ideas?
May 01, 2014, 12:22 PM
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dbacon's Avatar
This is normal behavior for any tail-dragger airplane, because the propeller shaft is not parallel to the ground. Since it is pointed up, the propeller gets a bigger bite of air on the right side than the left, so pulls harder on the right side.

Once it is in the air, actually when the tail rises on a tail-dragger, the apparent angle of attack is almost in line with the propeller shaft, and the effect goes away. Some people call this a "torque effect", I don't think that is the right name. Some people think it is due to whirling air coming around to hit the rudder on one side, again, I don't think so.

In the case of the SnowBall, the motor would be pointed up to make up for the fact that the thrust line is higher than the center of drag. When flying, if the drag center is below the thrust center, it pulls the nose down.

So a couple of things you can try:
1. Move the motor as low as you can, and reduce the upthrust to just a couple degrees.
2. Get used to giving right rudder during takeoff, and trim it for straight flight in the air.

If you have a big prop, which is very efficient and all that, you can replace it with two smaller props bolted on in "X" fashion, and get the same thrust for the same current. This will allow lowering the motor even more.
I don't find it is less efficient, due to my thrust/current measurements, but everyone else tells me it is...
Last edited by dbacon; May 01, 2014 at 12:29 PM.
May 01, 2014, 01:30 PM
I call it 'landing'...
Plattenwerk's Avatar
Ok, I'd thougth I messed it up because I didn't see this behavior on any posted video.
Motor mount is 3" tall, floats too and I followed the instructions with 5 motor angle.
This I will decrease while I fix damages (diamonds are the girls best... fo me the glue gun is ) . Second is to build it lighter: floats and motor mount are really tank-like: it's nose-heavy and I had to move my battery fairly to the tail to get a proper balance. AUW ist 492g with 3S 1300 , Emax 2812 and 7X6 prop.
May 01, 2014, 02:41 PM
Registered User
dbacon's Avatar
Well, if you have to make new floats, I just tested "pool-noodle" foam, and it is indeed closed cell, meaning it won't suck water! The only other foam I found that truly doesn't take on water i Depron. In all fairness I haven't tried my MPF (Model Plane Foam) yet, it feels like it would be good. Just too been too busy...
My pool noodle was about 3.5" in diameter and a 3" section (I am making wheels) weighs only about 0.4 oz or 13 grams.

The lightest floats I have made are Depron, hollow, and with no back on it. I use them on my two Capricorns. They sit a little low in the water, but jump right up when there is any forward movement.
May 01, 2014, 02:59 PM
I call it 'landing'...
Plattenwerk's Avatar
I read about your pool-noodle floats a few posts before and will keep this in mind.
But first I will cut some holes in my existing floats using a Dremel tool and seal them with laminate. Try not to weaken structural strength.
May 01, 2014, 03:22 PM
Registered User
dbacon's Avatar
If your existing floats are solid foam, and you cut holes and seal them, you need to make sure that you get immediate indication if they leak.

My built up (of) 3 mm Depron floats would leak and make my take-offs veer to the side and crash. I would try and try to seal them,a constant battle. Finally decided to just remove the back panel, problem solved!
May 01, 2014, 03:41 PM
Registered User
Or glue on thin depron over the holes or use covering over the holes instead of tape
May 02, 2014, 03:19 PM
I call it 'landing'...
Plattenwerk's Avatar
I think I'll try something different: the holes are done (saved me 10g each float), but they're not glued to the wing: I have very strong velcro and will do changeable floats. For land I use the ones I already have. A secound pair made of depron for water.

Instead of a cutted bottle I used a laminating pouch for the floats. Works good.


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