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Old Feb 23, 2003, 08:07 AM
It's a spiderweb of knit lines
Souderton Pa. USA
Joined Mar 2002
2,651 Posts
del
Can you describe the motor that works for you?
Include things like if it had wires or a contact (shape and size) on the end bell, color, method of retaining end bell (3 crimps or 4 and cut tabs or just deformed metal). Also any electrical info you know like ohms, current at static load and does it draw the same forwards and backwards (if it's easy enough, turn the prop around to do current in reverse). I'm hoping we can find out what makes one work and others that are the same fail.

Billy
The reason for the hole in some motors is to allow
you to push on the shaft to install the pinion. The pinion becomes the thrust bearing when the shaft is being pushed in. On motors without a hole the shaft "bottoms" on the end bell when installing a pinion. The hole is a better method because it allows you to set the endplay so there is no rubbing on the end bell like is possible on motors without the hole.

Anybody have any ideas on how to make a dynomometer for motors this small?

Also, I agree that the easy way is to stick to one vendor (Didel) then the only variable will be unit to unit quality.

Like all science, success only counts if it can be duplicated by your peers!

Mike Cross
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Old Feb 23, 2003, 08:37 AM
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Dave Wulff's Avatar
USA, FL, Fort Lauderdale
Joined Feb 2002
1,902 Posts
Del,

While motor resistance and current draw are tied together, low ohm motors (2-3-4) alone won't cause a problem with the RFFS if geared properly. The key is current draw, the RFFS ESC needs to be kept around an amp, though rumor has it, it can take more. From everything I have seen, pager motors can't deal with much more than 350 mah without burning up. ANY motor, gear, prop combo should be tested on the bench for current draw before hooking it to an ESC, RFFS or otherwise. I have been looking into pager motors for a couple of months, and have been using only "brand" motors that can be replaced (bitcharg-zipzap, etc.) Nothing solid yet, and Gordon has been looking into this alot longer with similar results.

Dave Wulff
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Old Feb 23, 2003, 09:06 AM
Sticky Shepherd
Graham Stabler's Avatar
Oxford/England
Joined May 2001
4,017 Posts
Your absolutely right, a motor is not a resister it is a motor and with the correct gearing ratios they should not burn out. Measure the current and start with a small prop experimanting until you have reasonable thrust. For a 6mm pager you can get a little scale model flying on 200mA I would say that 300mA is about the max. The bitchrgers are somewhat different to the pagers and you should probably edge towards lower gear ratios and smaller props. They should never be run from a dc-dc.

I think the problems we all have with pagers springs from a slight lack of knowledge in terms of power, current gear ratio etc. I include myself in this, I still haven't finished my thrust rig, shame on me. In the past we could always say that the motor got a bit hot so I reduced the prop size now we say the motor got a bit dead.

I'd like to build an index of working pager powered models, motor, gear ratio, model type, weight and wing area and prop. A watt works for the new generation. I forgot my camera at last night aeronutz meet but and there were several model there including a nice bitcharger based model using a 6mm pager, 7:1 gearing, a 40mA cell and it didn't even have the mosfet mod. It was light at about 13g and had around 6g of thrust. Once trimmed it flew very nicely.

Thats that's the other thing, a lot of us from an RC background don't know much about trimming other than on the TX, it is a skill I need to build up and if you want sucess with small pager models it is one we all need to improve (apart from some skillful types).

Here is another question, do we demand that the models have enough power that they wil fly even when trimmed badly? In the days of 2 50mA cell flight trimming was much more important than with the average LiPoly and M20 model. Now with pagers, even with the new cell it is much more tricky but it sure is nice when something does fly.

Cheers,

Graham
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Old Feb 23, 2003, 09:15 AM
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jberg's Avatar
MSC Rödinghausen, Germany
Joined Feb 2001
954 Posts
Hi all,

I've had success today with my Guided Mite and a pager motor. I have built an infrared receiver with a built-in DC booster, weighing 2.7g. The model flies with good power, about 70% throttle are enough for level flight.

- pager motor 6x15mm, 8 Ohm (from epilot; I don't know the source at the moment)
- DC booster providing 5,3V constantly
- prop GWS 4,5x4
- gearbox with Didel gears, 6,6:1
- weight 20g all-up
- 1x 145mAh Kokam LiPoly (I didn't measure the flight time, I flew until it became boring )

Mike,

you don't need a dynamometer if you follow the methods and hints in my motor constants article that will be included in the next issue of the "Inside Story". I hope it will show up really soon now.

Regards, Jochen
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Old Feb 23, 2003, 09:59 AM
I don't want to "Switch Now"
pmackenzie's Avatar
Toronto (Don Mills), Canada
Joined Dec 2002
17,100 Posts
Gordon, I have posted details about my plane in both the BIG thread, and the still far too short ()
"Working bitchargers" thread. The plane is a Minnow, AUW 14.9 grams, 16" span, 44 square
inches.

The 4" prop is pretty pitchy. At the tip it is approx. 30 deg. which means about 7.25" pitch. Since my
model was a shameless copy of mcross, I ordered the 6" Ikara prop from HL. Before it arrived I
tried various rubber model props I had before settling on the 4" one. Even the 5" ones were too big
for the motor, so I never tried the Ikara.

At this power level (about 1 watt) and at 15 grams the model flies great. I tried ballasting the model
to see how heavy a scale model could be and still fly well. At 19.5 grams (one nickel for ballast) the
model would barely ROG and climbed very poorly. At 17.5 grams (1 penny) it was better, but the
cruise setting would not maintain level flight.
The conclusion I drew was that at the low power levels you can get from these motors weight is
critical. The fixed weight of all the equipment is 9 or 10 grams . Motor with gears 3, battery 4, rcvr
2, actuator 1. This leaves about 4 or 5 grams for the airframe.

With the clones you don't get the details of which motor they give you. The DCr is about 4 ohms. My newer cars (different brand) are about 2.5 ohms. If I do any motor testing I will try to
measure Kv and Io. I will also check to see which direction performs better. They may require taller gearing, or a smaller prop like the U-80. I would still try to stay at about the same current since it seems to be the brushes that are the weak link.

For what it is worth, here is a shot of my power train.

Pat MacKenzie
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Old Feb 23, 2003, 10:55 AM
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Dave Wulff's Avatar
USA, FL, Fort Lauderdale
Joined Feb 2002
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Graham is right, watts are a much better value. Voltage and current draws constantly change. All of my testing on the bench has been direct from a freshly charged LiPoly. When an ESC is put in the circut everything changes and amp readings need to be "tempered" by voltage. I will try to start using, and more importantly thinking in watts.

Dave Wulff
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Old Feb 23, 2003, 11:38 AM
I don't want to "Switch Now"
pmackenzie's Avatar
Toronto (Don Mills), Canada
Joined Dec 2002
17,100 Posts
Brain fade- 30 degrees at 4" dia is 7.25" of geometric pitch, not 6" as I posted earlier. I am going to edit it.

pitch = d*pi*tan(theta)

Pat MacKenzie
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Old Feb 23, 2003, 02:48 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
Pat,
Thanks for the details. I should have looked at the Bit thread. You've confirmed what I believe, that a very light floater is a good use for these motors, or a very light scale model. My main point is that we routinely fly models around 30g with an M20. These motors require substantially lighter planes and are not substitutes for an M20. There is no free lunch here.

If you have 4 ohm motors you probably have something roughly comparable to a Bit 1.0. Your newer ones might be more equivalaent to a Bit 2.2.

I've posted my preliminary 6mm motor static tests here again.

Gordon
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Old Feb 23, 2003, 03:08 PM
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Dave Wulff's Avatar
USA, FL, Fort Lauderdale
Joined Feb 2002
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Gordon,

Since you probably have been working on the room flyer longer than most, how about an updated opinion on necessary thrust to AUW. I know drag has alot to do with it, but most of what I am seeing from David Liu to Mike Cross seem pretty similar in that respect.

PS, didn't make it to the WRAM show, still working on the project that has to go to NJ.

Dave
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Old Feb 23, 2003, 07:09 PM
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jberg's Avatar
MSC Rödinghausen, Germany
Joined Feb 2001
954 Posts
I've made some measurements for the 6x15mm pager that I have used today to drive my Guided Mite.

Idle curent 44mA

Measurement 1 (GWS 4,5x4):
- 4,84V
- 257mA
- 7g thrust
- 3620/24133 rpm (prop/motor)

Measurement 2 (GWS 4x2,5):
- 4,87V
- 198mA
- 5g thrust
- 4550/30333 rpm (prop/motor)

From this I calculated:
- Ri=9,7 Ohm
- ns=10288rpm/V

For 5V I predict by calculations:
- Pout,max=0.54W at 23520rpm with 280mA and 38% efficiency
- max.efficiency 50% at 36400rpm and 150mA with 0,377W output

The 4,5x4 prop is about right, a bit on the small side (or we could use a slightly lower gearfactor). Since the Guided Mite flew well I think this prop is a good match. This is also a confirmation for the old rule of thumb that you need one third of the model weight for thrust to fly at least ok.

Hope this helps. I think this special pager is a good one. I will try to find a source.
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Old Feb 23, 2003, 07:27 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
Jochen,
Nice work. I have a couple of those pagers too, and a gearbox with the same ratio, but haven't put it on the test stand yet. Your results are encouraging. Perhaps I should recessitate the Cub you built when you were here and put this propulsion set on it.

Gordon
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Old Feb 23, 2003, 07:31 PM
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jberg's Avatar
MSC Rödinghausen, Germany
Joined Feb 2001
954 Posts
Gordon,

yes, do so! If you manage to keep the weight at 20g or lower then it is almost forced to fly (but you *need* the 5V, it does not work just from the LiPoly cell, I have tried it).

Regards, Jochen
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Old Feb 23, 2003, 07:53 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
Jochen,
Well, luckily I have a number of boosters, including yours. So, that's not a problem. I'm more or less reaching the conclusion that for semi-scale planes below the M20 size you either have to use a pager motor with a booster or straight from a LiPoly a Bit motor or one of the low resistance Didel pager motors. That's the reason I tested some of the motors shown above at the 5 volt level.

In case that Cub isn't light enough, I've started making its little brother for testing small motors.

Gordon
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Old Feb 23, 2003, 08:36 PM
Nickel what?
Phreakish's Avatar
Prescott, Arizona, United States
Joined Jan 2003
2,365 Posts
one thing I've noticed after I've autopsied several motors after killing them in my modified micro cars is that different manufacturers use different motor contstruction too. I had a panther clone, and it is one of the better liked clones in the micro car world... Anyhow, I killed my panther motor after running 12 AA's worth of charges through it, then disassembled it. It was a coreless design, and had a lot more power than any of the other motors I've played with... Where as the microsizer and bit motors are of the standard construction.

Has anyone thought about trying the new micro brushless motors? I've seen them as small as 2-4mm in dia. And unloaded RPM around 36k... and Im sure they can take a lot more punishment than a standard pager motor... unfortunately I havn't found any retailers for them, and the control board Im sure adds weight... but if you get enough power out of the motor, it may not be an issue... just a stray thought.
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Old Feb 24, 2003, 01:54 AM
del
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del's Avatar
north of Chicago
Joined Feb 2003
1,827 Posts
Graham.

You are absolutely right when it comes to really light models.
They have to be trimmed like Free Flight models before they will fly
r/c. Then ya' use throttle to make 'em go up or down, and rudder to keep 'em
from smackin' the walls..

But, hey, if ya build light enough, the model will bounce off the wall..

--del
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