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Multi motor confusion

Old 03-05-2008, 09:47 AM
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Lysander
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Default Multi motor confusion

I have been flying brushless powered, single motor models for several years and I am entirely at home with installing such power trains. However, I am now putting together the Modell Studio (Czech) Parternavia Victor. http://www.puffinmodels.com/partenavia2020.htm

Wingspan 2020 mm
Length 1610mm
Airfoil Clark Y2
Wing area 50,5 dm
Flyweight c. 4000 - 4500 gr.

I am struggling with figuring out the power consequences (watts per pound) and Amps drawn by a twin system. I think that two Mega 22/30/3E motors would be suitable (110kv, Max Amp 50). I will be driving each motor through its own 40A ESC off its own Kokam 3200mAh 3S lipo although it has been suggested that both motors drawing off a single 3S2P pack might be wiser should one pack begin to fail for example.

This is where I get confused! For the sake of this discussion, let's say that a single 22/30/3E on a fullish throttle delivers 33A at a nominal 10V giving 330W. We have two motors giving that output. Our aircraft weighs about 10lbs. Questions:

1. I want to work out how many Watts per pound I have at my disposal. Do I add the output of the two motors together totalling 660W or 66W/lb? If I do and that is the answer, I wonder if these motors have enough grunt as even at say 40A, totalling 800W of output I still get only 80W/lb which is hardly enough for our grass field.

2. At the same throttle giving 33A draw from each motor I calculate that with a single, ganged pack of 3200mAh (total 6400mAh) 3S2P or each motor wired to its own 3S 3200mAh the endurance will be the same. I am right am I not?

3. How do I roughly calculate the endurance of this aircraft on this twin installation?

I think that'll do for now.
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Old 03-06-2008, 06:30 AM
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Larry3215
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Originally Posted by Lysander View Post
I have been flying brushless powered, single motor models for several years and I am entirely at home with installing such power trains. However, I am now putting together the Modell Studio (Czech) Parternavia Victor. http://www.puffinmodels.com/partenavia2020.htm

Wingspan 2020 mm
Length 1610mm
Airfoil Clark Y2
Wing area 50,5 dm
Flyweight c. 4000 - 4500 gr.
((sniip))
1. I want to work out how many Watts per pound I have at my disposal. Do I add the output of the two motors together totalling 660W or 66W/lb? If I do and that is the answer, I wonder if these motors have enough grunt as even at say 40A, totalling 800W of output I still get only 80W/lb which is hardly enough for our grass field.

Yup - add the power from the motors together to get the total, then devide by the weight.

I agree - your going to be low on power - especially for ROGrass. Also, I would use more like 10.5 volts as a base line. Your packs should hold at least that well or you need bigger or better packs

2
. At the same throttle giving 33A draw from each motor I calculate that with a single, ganged pack of 3200mAh (total 6400mAh) 3S2P or each motor wired to its own 3S 3200mAh the endurance will be the same. I am right am I not?


Right again

3. How do I roughly calculate the endurance of this aircraft on this twin installation?


I think that'll do for now.


You need to know your average power usage. Hard to tell that untill you fly it, but Id guess you'll need about 40-50 watts per pound to keep it in the air and fly scale like. More for aerobatics. It will entirely depend on your usage of throttle tho.

I would recomend you power UP some though. Id be tempted to run those motors on 6S packs and gear down to where your drawing 30-40 amps if your esc's will handle it. Most of the cheeper 40 amps esc's cant handle 6S though. You will also need a seperate BEC to run at higher voltages.

What gear box were you planning to use?

Larry
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Old 03-06-2008, 06:34 AM
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Oops - I just looked at the link you gave and it looks like the motors are intended be used as direct drive spinning small props. Forget my 6S suggestion.

Your stuck with what you have it looks like unless you go to a higher Kv motor - maybe something in the 1300 range spinning the same props but with larger packs - maybe 3P to handle the watts.
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Old 03-06-2008, 11:00 AM
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Lysander
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Default Head for square one perhaps!

Thanks for that - I feel a little more confident now. If you don't mind doing so, perhaps you would look at the manufacturers stuff on this url: http://www.modellstudio.cz/planes.pdf

That is their catalogue and you'll need to select the Parternavia Victor 2020 to see their advice. That is what really puzzles me as they recommend a pair of 600 motors on 8 cells. I reckon that a 10lb model would barely be able to taxi on such a setup let alone RO grass.

I have yet to buy the motors. So, if you will perhaps you would begin by imagining that you had this same model and with a clean sheet were having to select a drive train. What motors would you buy? My packs are either Kokam or Worley Parsons (Cellpro/FMA compatible balancing plugs) so I know they have quality. I simply chose to take the pessimistic, worse case when I adopted the nomical 10V under load. I do not presently have 4S packs but could remedy that and if in doubt I confess that I usually go for more Volts.

I have several single motored aircraft - all brushless. The nearest I have from which to draw comparison, a Multiplex Alpha 27 glider weighs 4.25lb, runs off a 3S 2500mAh pack driving a Mega 22/30/3 on a direct 10 x6 prop (Yes I know that isn't the most efficient arrangement but it goes up like a rocket). So in my simple way I think that a model weighing a little more than twice that and powered by a pair of similar motors really should fly reasonably well - until I start making calculations which is where we came in.

This Partenavia Victor will only be asked to take off and land with scale like circuits in between. No flashy aerobatics for this model.
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Old 03-07-2008, 02:45 AM
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Well, many mfg's under rate the power systems. I suppose it might take off with the recomended brushed setup but it would be dicey! They cover there asses with this statement.

This modell plane has been designed as an electric flight model, using two E-motors. Speed 550 Neodym like cheap propulsion may be used.
Brushless motors are recommended for full performance.
To start with the power system/motor choice you need to begin with the items you cant change - what is the max prop size you can spin?

I didnty see any prop recomendations anywhere.

Larry
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