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Old 03-11-2018, 07:14 PM   #1
Vinny7636
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Default Twin Power Question/Help 85" Cessna Bobcat Build

Hello to all,
Some help please with Engine & ESC selections.
I have a wooden kit, twin engine Cessna Bobcat, 85" WS, it should end up approx 17 to 25 lbs.
Being a twin engine model I am reluctant to power this plane with twin gassers. I have 2 zenoah g23's but feel this AC would live longer on E power,

Attached you will find a simple schematic with my calculations after doing some reading on the internet. You will find that I have 1 ESC controlling both engines, I was going to use 2 separate ESC's, but I have an Hobbywing 150A OPTO and would like to use it , not sure if this will work?

?
1 Estimated Flying time with 2, three cell 5000 mah batteries?

2 Will power distribution to motors be balanced with this set up? Anybody tried this before?

I greatly appreciate any input and your time...

Thanks, Vinny


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Old 03-12-2018, 05:29 AM   #2
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Sorry but that would not work.
I have a Deuces Wild, it has 2, 25 size motors as it weighs 10 lbs.

Only very small motors might work that way. I have been told that the Castle Creations ESC might work with small motors.

Even though we think 2 motors are the same, same brand and size ETC. There are tiny differences between them. The ESC's receive a small signal from the motors to tell them when to give the next pulse of power. If 1 motor for some reason turned only slightly slower than the other, the pulse would come in at the wrong time.

In a very small motored setup, maybe the ESC wins and forces the motors to turn as one, but in a large motor setup like yours and mine, things would not workout very well.

My motors each have a 60 amp ESC, I have 2 batteries, both 4 cell, I have a balance lead from side to side connecting the positive input of both speed controls together and the same on the negative. This way the batteries are in parallel when plugged into the plane. If you need the 6 cell voltage, the series wiring should be fine. I was told to use one voltage across both matched speed controls so that one doesn't shut off before the other. Everything is matched from side to side.

My plane runs very well with my setup.

Dave R, KI7MTA Proud PGR rider.
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Old 03-12-2018, 11:37 AM   #3
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Wildflyer, Thanks for your input, Vinny
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Old 03-12-2018, 04:41 PM   #4
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Just a bit of explanation as to why it won't work.
An ESC for a brushless motor sends very short current pulses down 2 of the 3 wires depending on the position of the rotor. To 'time' exactly when to do this the ESC 'reads' the tiny voltage in the third wire generated by the other poles. As the rotor moves round the ESC sends a pulse down the other pair and reads when to this by the third wire.
As you can imagine it does all this incredibly quickly but the timing of the pulses is entirely driven by the speed at which the motor is rotating.
If you have two brushless motors connected to the same ESC it gets confused trying to read the timing pulses unless both are rotating at exactly the same speed which is very unlikely. The result is the pulse timing is unlikely to suit both motors so one will stop - if it even starts.

Brushed motors are different and several can be driven quite happily at the same time but only from a brushed speed controller

I hope this helps.
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Old 03-12-2018, 05:36 PM   #5
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Wow! Allot happening and fast in these little esc/motors! Sometimes I wonder who ever thought up this must be a smart cookie...

Anyway, a perfect and simple explanation and I thank you...

I was trying to get away with using 1 esc that I already have and they don't make them anymore for it's twin that I need, the 150a is overkill.

I will be using two, 80 amp esc with separate wiring except the battery's will remain in series powering both esc's.

If you don't mind..what did you think of the motor selection per performance? Not over nor under powered? I would like a happy medium with good battery time..

Thanks again,
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Old 03-12-2018, 05:55 PM   #6
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Performance will be fine depending on the prop. Flight times of 5-7 minutes.

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Old 03-12-2018, 11:55 PM   #7
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Originally Posted by quorneng View Post
... An ESC for a brushless motor sends very short current pulses down 2 of the 3 wires depending on the position of the rotor. ...
At full throttle, each cycle is one passing magnet


Voltage chopping (pulse width modulation), at part throttle to simulate lower voltages.

Vriendelijke groeten Ron
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Old 03-12-2018, 11:58 PM   #8
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Originally Posted by Vinny7636 View Post
... I will be using two, 80 amp esc with separate wiring except the battery's will remain in series powering both esc's. ...
Keep controller close to battery.
Extend motor wires, and keep the battery wires short.

Too long battery wires can kill ESC: precautions, solutions & workarounds - RCG
Contents
  • Solution I & II, rules of thumb
  • Problem
  • Capacitor type & polarity (orientation!)
  • How & where (not) to add extra capacitors
  • DIY capacitor pack pictures
  • Capacitor & pack suppliers
  • Expert/manufacturer opinions, they all say the same ... & their rules of thumb.
  • Explanation, water hammer/knock analogy, theory, references, measurements

Vriendelijke groeten Ron
Without a watt-meter you are in the dark, until something starts to glow
e-flight calculators watt-meters diy motor tips Cumulus MFC
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Old 03-13-2018, 01:26 AM   #9
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Thanks Ron, the ESC and batteries will be accessible and in the removable nose of the AC and there will be no wiring extensions to esc +- connections. Approx 24" wiring or maybe shorter will be from the esc's to motors. All motor wiring (6 Total) will be cut in exact lengths to both motors.

Thanks again,
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Old 03-13-2018, 02:25 AM   #10
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Ron's thread on "too long battery wires" is a MUST read! SO much good stuff in there.

I love twins but there are lots of variables that can make them a bit more challenging than a single. I've had two of the same exact off brand motors that were purchased at the same time that caused a yaw situation due to KV variance that I could never resolve. I replaced them with two Scorpion motors and the bird flew perfect. Lesson learned. NEVER use El Cheapo motors on a twin! I've also had two of the same Castle ESC's that caused a yaw problem. Once I wised up and checked them both with Castle Link I found they had different firmware. Updated them and all was good.

There is more to a twin than just bolting on an extra motor and ESC. You MUST do some testing before you ever go out for maiden. Ensure both motors are producing even RPM and thrust. Make certain you have a LOT of rudder authority just in case something goes wrong. If one motor is weird you'll be glad the rudder can help compensate so you can at least land it in one piece. Otherwise you'll watch it circle around all the way to the crash site.

The few twins I have are true gems and I love them. Worth every effort to make them fly just right.

Joe

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Old 03-13-2018, 03:24 PM   #11
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Originally Posted by Turbojoe View Post
Ron's thread on "too long battery wires" is a MUST read! SO much good stuff in there.

I love twins but there are lots of variables that can make them a bit more challenging than a single. I've had two of the same exact off brand motors that were purchased at the same time that caused a yaw situation due to KV variance that I could never resolve. I replaced them with two Scorpion motors and the bird flew perfect. Lesson learned. NEVER use El Cheapo motors on a twin! I've also had two of the same Castle ESC's that caused a yaw problem. Once I wised up and checked them both with Castle Link I found they had different firmware. Updated them and all was good.

There is more to a twin than just bolting on an extra motor and ESC. You MUST do some testing before you ever go out for maiden. Ensure both motors are producing even RPM and thrust. Make certain you have a LOT of rudder authority just in case something goes wrong. If one motor is weird you'll be glad the rudder can help compensate so you can at least land it in one piece. Otherwise you'll watch it circle around all the way to the crash site.

The few twins I have are true gems and I love them. Worth every effort to make them fly just right.

Joe
Thanks Joe and to all, and for advise on those motors. It's a shame there isn't much trusted consistency with that manufactures quality. I have a few of those engines in single engine set ups that would not be noticeable.

Yes agreed, twin engine thrust can be extremely critical. To be honest I wasn't putting quality into the picture here, and we all know that can be a big mistake.... And being a big gasser guy I should know better considering I have flying friends I help all the time who have a 200 lbs. 200" b 25 and several Ziroli P 38's. Engine out or sagging....no circles most of the time, just a yawing into a lawn dart is the normal outcome. It depends on the type of plane, flight mode, and when it happens. Although I've seen big A10's fly like nothing happened...depends on the set up.

A little history here for this build. A long time older friend of mine who is a true artist gave me this Bobcat 75% completed. It is a museum piece and he gave it to me because he knows I would finish it, the detail of what he finished is wonderful. And...I want him to see it fly and he has allot of time in it. To really do this plane justice with 1st class devices I guessed approx $600 to $800... that ain't happening, it ain't in the budget. My money is in giant scale..don't get me wrong the last time I counted I had approx 25+ electrics from Giullows kits to 60" in sizes. My GWS ME 262 is awesome that I've had for years. From my 30+ years in this hobby, electrics have revolutionized this hobby and kept it alive, because on my end of giant scale this hobby is what I think is experiencing a slow death, in my area anyway. Not much building happening like the old days, and few kit manufactures anymore, it's leaning out and killing me to see it!

I posted these questions on this wonderful website because you guys been there..and done that! And I appreciate your time! I guess what I need to do is drop back and think this thru, and maybe quit being so cheappo..for this plane.

Can you all recommend maybe a medium quality engine & esc manufacture without busting the budget? It's been a while since I've set up a E plane.

Thanks again, Vinny


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Old 03-13-2018, 03:37 PM   #12
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Just a little comment ...

Not a good idea to poser same question in two different sections ..... I was totally confused reading above considering I had already answered - then noticed its a different listing of exactly same first post :

https://www.wattflyer.com/forums/sho...50#post1012050

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Old 03-13-2018, 03:47 PM   #13
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As I posted in other same thread ... DIFFERENTIAL ..... that puts to bed inbalance of motors / ESC etc.

I use it on most of my multi's ... it gives me yaw authority far more than a rudder alone.

Second when motors are out of synch - you can hear it easily. With Differential ... its easy to adjust one motor to other.

Wiring : Yes having too long battery to ESC wires are not recc'd - BUT there are many factory built models out there with long battery wires. I agree its not good practice but so far I have not seen any failures due to it. My Lancaster has 4 motors with ESC's out in the nacelles and long lead from centre fuselage separating out to all. Its years down the road and still flying.
My Catalina has ESC's out in the nacelles ... long leads back to fuselage mounted battery. Still flying.

My TU144 and Concorde both have normal battery to ESC lead length and then solid copper core wires out to motors.... Tu144 has twin Floaterjet motors and the Concorde has twin 50mm EDF's .... the solid core leads are domestic cable inners and I like them because I can bend and form to shape they sit in. Current / Volts capacity far exceeds needs giving virtually no voltage drop.

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Old 03-13-2018, 04:26 PM   #14
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Originally Posted by solentlife View Post
Just a little comment ...

Not a good idea to poser same question in two different sections ..... I was totally confused reading above considering I had already answered - then noticed its a different listing of exactly same first post :

https://www.wattflyer.com/forums/sho...50#post1012050

Nigel
Sorry about that can i remove the other thread?
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Old 03-13-2018, 06:13 PM   #15
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Originally Posted by Vinny7636 View Post
Sorry about that can i remove the other thread?
I'm no moderator ... I was just commenting so we all know they exist and have posts in relevant.

Better we all stay in one now.

Me .. I reckon just leave other one - maybe ask mods to close it ?

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Old 03-13-2018, 09:14 PM   #16
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So being a novelist with twin E's, I am assuming that you mix throttle and rudder? How do you program that or do you hook esc in different channels on the Rx? Would your rudder trim be used as a balance?
I brought your question over to here ...

It relies on using a modern programmable radio with more than 4ch's

Yes - I hook one ESC to throttle channel and other ESC to an Aux channel.

I then mix the aux ch to the throttle ch. and throttle ch to aux ch.

But each has a second line in that links rudder stick movement in ... so I can set throttle level for both by moving throttle stick. Then by moving rudder stick - the throttles are modified for one to be higher than other ... I have usually set the max difference to about 15% and only +ve .....
Reason I do not allow -ve is to avoid having one motor go below minimum flight speed as in landing etc.

Yes rudder trim will affect the two and you have to be careful to make sure rudder is aligned properly at centre trim.

Nigel

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Old 03-13-2018, 10:31 PM   #17
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Originally Posted by solentlife View Post
I brought your question over to here ...

It relies on using a modern programmable radio with more than 4ch's

Yes - I hook one ESC to throttle channel and other ESC to an Aux channel.

I then mix the aux ch to the throttle ch. and throttle ch to aux ch.

But each has a second line in that links rudder stick movement in ... so I can set throttle level for both by moving throttle stick. Then by moving rudder stick - the throttles are modified for one to be higher than other ... I have usually set the max difference to about 15% and only +ve .....
Reason I do not allow -ve is to avoid having one motor go below minimum flight speed as in landing etc.

Yes rudder trim will affect the two and you have to be careful to make sure rudder is aligned properly at centre trim.

Nigel
Sorry about that 2 thread deal, it won't happen again. That is so cool! and I can imagine that this would help in a better coordinated turn also like your suppose to..Ailerons,Elevator and then Rudder that nobody uses. I have flown large Cubs and/or high wings that had high aspect ratio's that required rudder, it was a must or you banks are forever coming around... When I was teaching guys I would always mix in rud with ailerons an shown them the difference in banking an airplane.

Now that I think about, for some reason if an engine was to sag in power that would be the probably the fastest way to recover. And if I had a KV problem rudder trim would compensate both thrust and on the axis.

Not to disqualify what others have recommended, I value all comments.

Whats your thought on the motors I selected...not so good? And do you see any problems with using HK's less expensive esc's?


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