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8 motor power system, any suggestions?

Old 02-04-2013, 09:37 AM
  #1  
HugoW
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Default 8 motor power system, any suggestions?

Hi,

I'm very slowly designing / preparing plans for a Hughes H4 Hercules, and I really want all 8 motors to turn a 4-blade props. Sizing is an issue I'll deal with later, but first how to set-up / wire them. I have some ideas:
- Go brushed! Put some motors parallel and some motor groups in series. I think it is the easiest way.
- Brushless; each it's own ESC close to the motor, and huge capacity blocks parallel to the ESC, close to the ESC. The wires from battery to ESC could be a long as 1,5 meters.
- Brushless, but the ESCs close to the batteries in the fuse, and use really long motor wires. This should in theory be possible and eliminates the long-batttery wires issue, but requires 3x8=24 heavy wires in the wings.
- Silly idea: mount one motor in each wing, and drive the props with axles / gearboxes / flex-shafts, etc. Not my prefference...

OK, any suggestion is welcome, experiences shared, too. The plan is probably not entering the building phase for another one or two years, but designing and planning is part of the hobby for me, too.

Cheers,

Hugo
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Old 02-04-2013, 09:30 PM
  #2  
Dr Kiwi
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Certainly brushed is easier, but do you really want to stay in the dark ages.. in a few years it might be very hard to even find brushed gear (motors/ESCs).

Brushless with multiple ESCs and long motor wires may be the only sensible solution.
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Old 02-05-2013, 01:52 AM
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Originally Posted by HugoW View Post
Hi,

I'm very slowly designing / preparing plans for a Hughes H4 Hercules, and I really want all 8 motors to turn a 4-blade props. Sizing is an issue I'll deal with later, but first how to set-up / wire them. I have some ideas:
- Go brushed! Put some motors parallel and some motor groups in series. I think it is the easiest way.
- Brushless; each it's own ESC close to the motor, and huge capacity blocks parallel to the ESC, close to the ESC. The wires from battery to ESC could be a long as 1,5 meters.
- Brushless, but the ESCs close to the batteries in the fuse, and use really long motor wires. This should in theory be possible and eliminates the long-batttery wires issue, but requires 3x8=24 heavy wires in the wings.
- Silly idea: mount one motor in each wing, and drive the props with axles / gearboxes / flex-shafts, etc. Not my prefference...

OK, any suggestion is welcome, experiences shared, too. The plan is probably not entering the building phase for another one or two years, but designing and planning is part of the hobby for me, too.

Cheers,

Hugo
Yeah
Brushless motors are going to cost more than brush type motors. On the other hand, you will be able to save significant weight by going to the lighter, much more powerful brushless motors, along with their ESC's and battery packs.

Now days, with the cheap China import motors, cost can be reduced considerably. And even the cheap China motors are much more efficient than those brush type motors. (Yeah, I flew with the whole line of Astroflight Brush type motors, from their Astro geared 05, 15, 25, 40 and their geared 90 motors. Even the $$$$ Hacker motors are 1/2 the price of those geared Astro brush type motors.)

What will be the wingspan, wing area and weight of your model when it's done? If it's going to be a big model, you might want to consider putting in top quality motors/ESC's and batteries for reliability. Yes, they will cost more, but if one of the cheap China motors quits, causing landing issues, the $$$$ saved won't be worth it.
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Old 02-05-2013, 10:15 AM
  #4  
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Personally I'd stick with brushed motors for the simplicity. Mabuchi are still making them in their millions so they're not going to go away any time soon.

Plus with 8 powered motors it's not like you're going to have any real problems if one of them quits in flight. With 3/4 normal power still available on that wing at a guess you won't even notice .

Steve
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Old 02-05-2013, 02:05 PM
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Put two dummy motors on each side, and only power the inside 2 motors on each wing. I'm sure you could find 4 motors that would pull it up into the air.
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Old 02-05-2013, 02:57 PM
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Size likely matters in my choice. What size and estimated weight for the Herc?

If not too large I too would vote brushed. We used them for years and it sure simplifies things.

If you do brushless the motor lengthening is certainly better. This may be required if the size is large. I have seen a couple of these and you can pretty much assure yourself of issues - they just seem to follow large complex setups.

So good quality motors and ESC's are a must.

Mike
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Old 02-07-2013, 03:51 PM
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Thanks for all the input. The plane will be 3 meters span and 1,8 meters long, but in the Ivan Pettigrew style. I'm aiming for 5kg AUW, which is very light.

Some choices made:
1) Brushless. Brushed is easier, but heavier and less efficient. An less challenging.
2) All motors will be functional, no dummies. The idea is great and makes a lot of sense, but I don't want it. Stupid me...

I've just been snooping around the HK site, and bought a small motor, ESC and battery, and some 2-blade props to butcher and turn into a 4-blade prop. This will be my test set-up. Reason behind the test; I want thrust and rpm numbers (and amps, of course), to see where I am at with this very light set-up. My guess is I will be slightly below the Speed 480 power range, but 4 480s power Ivan's Martin Mars, and I will have 8 on a similar sized plane. Should be sufficient, I hope. I'll know when I have the numbers.

I've just been fiddling with my drawings, I think I can fit a motor, ESC and LiPo in a nacelle. Yes, that means I will run 8 motors, 8 ESCs and 8 small batteries. This makes for very easy set-ups, cheap and light. And only signal wires to run through the wings. I might even power the wing servos from the ESCs. Downside is I cannot get the LiPos very far forward, so CG will shift from the original plans. That is not good, so I need to rebalance. I had 3 servos in the tail to control all surfaces, I might look into long pull-pull systems. Boy, I haven't details any parts yet nor touched a piece of balsa, and already I'm turning the design up side down...

All ideas are still welcome, of course,

Cheers,

Hugo
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Old 02-07-2013, 05:44 PM
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Interesting approach but have you considered that 8 separate batteries means that, unless you fly only short timed flights, you will almost inevitably have to contend with varying motor run times. That might not be much of a problem if only one of eight motors stops early but it's worth thinking about.

Of course just having to charge 8 batteries every time would put me off but I'd never build anything that ambitious anyway .

Steve
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Old 02-07-2013, 06:08 PM
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Yes, that it a valid point. The way I hope to catch that is two fold:
Always fly within safe timing, so never over 50% capacity.
I have designed a piece of electronics to intercept the throttle signal, and relay it to 8 ESCs. It also has 8 reed sensors, one at each motor. The controller has two purposes. It will alter motor rpm per motor to help steer on the water, and should one motor quit, it will shut down the opposite motor on the other wing. And flash a super bright led to let me know something is amiss.

When I get my set-up worked out, I might change to 1 LiPo per 2 motors, if the wire lengths permit. Who knows, it take baby steps in progress to make sure I don't overlook anything.

Hugo
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Old 02-07-2013, 06:53 PM
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Just run your batteries in parallel and all motors will see the same voltage where ever you locate them.

I would just plan on centrally located batteries and ESCs and run long motor wires. Seems like the simplest, most reliable system if not the lightest. At that size, wiring will be a relatively small percentage of the total weight. Plus, you only have to waterproof the area where the batteries and ESC's are.
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Old 02-07-2013, 10:10 PM
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I used 4 brushed motors the first time but no iam switching them out this winter to brushless. I have another friend building a big seaplane now and his 4 brushless motors are wired like this for motor steering on the water hooked up to his rudder on a switch. joe
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Old 02-07-2013, 10:32 PM
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Here is my brushed setup in parrellel and series. joe
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Old 02-07-2013, 10:41 PM
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Here is brushed drawing that a friend did on a spruce goose it was set up for water steering also but using two esc's one for four motors on each side. I hope one of these helps . joe
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Old 02-08-2013, 03:48 AM
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I personally really dislike adding lead to ballance things out. I think you would be better off with one or two large batteries inside the fuse. It would be easy enough to wire two batteries in parellel, then just split it for 4 seperate esc's per wing without out extending the wires much.
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Old 02-08-2013, 02:34 PM
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I have two fly mine with two 3-cell 3200 lipos and a seperate 2600 6volt sanyo pack for my recievers and servos all the way up in the nose to get my cg. It always depends on the shape of the plane ,my fuse has a long rear section tail and i knew i needed to have the batterys all the way up front. But there are other planes with a shorter tail or even a canard type planes where the battery might end up in the middle of the plane just in front of the wing. With my foam scratch built planes i get the cg marked with my esc in the plane then i slide my lipo pack down the fuse till the cg is right and then i cut my battery hatch there.
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Old 02-08-2013, 02:54 PM
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Wow, nice! Is that an Ivan Short Solent? Great looking plane, just add some detail decoration. BTW, what size / type motor and prop are you running?

The H4 has a relatively long fuse, I have no clue where the packs should go yet. I just figured out the really long pull-pull systems will weigh more than small servos in the tail. But the weight will be further forward using p-p.

Well, I will just start building the woodworks. After finishing two more EF1 racers I have on the building table. Once the wood is there, fit some motors and see where the CG is / should be. Right. Another plan.

Please keep the ideas coming, and do not hesitate to post pics like road king 97's!

Cheers,

Hugo

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Old 02-08-2013, 03:50 PM
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Originally Posted by HugoW View Post
Wow, nice! Is that an Ivan Short Solent? Great looking plane, just add some detail decoration. BTW, what size / type motor and prop are you running?

The H4 has a relatively long fuse, I have no clue where the packs should go yet. I just figured out the really long pull-pull systems will weigh more than small servos in the tail. But the weight will be further forward using p-p.

Well, I will just start building the woodworks. After finishing two more EF1 racers I have on the building table. Once the wood is there, fit some motors and see where the CG is / should be. Right. Another plan.

Please keep the ideas coming, and do not hesitate to post pics like road king 97's!

Cheers,

Hugo
Ha Ha hows this.Its Ivans 480 size or 99 inch wing span .I figures if i was going to build his seaplane the 80 inch size was not going to do it for me so i went big. joe
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Old 02-25-2013, 12:43 PM
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How about this for a power set-up:

Each motor and ESC their own very small pack, 3S500mAh, in the nacelle. This pack will not be suffcient to fly on, but to suppress the problems with long leads. Parrallel on these packs, with longer wires, will be the realy power packs. 3S8000 or something like that. Would the packs work as really big capacitors, and keep the ESCs happy?

Cheers,

Hugo
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Old 02-25-2013, 04:19 PM
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Never EVER use separate batteries for each motor - that can be a recipe for disaster. You want one battery (or several but wired in parallel) so that power distribution is the same across all the motors and more importantly that they run out of juice at the same time.

Mike
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Old 02-25-2013, 05:15 PM
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Originally Posted by rcers View Post
Never EVER use separate batteries for each motor - that can be a recipe for disaster. You want one battery (or several but wired in parallel) so that power distribution is the same across all the motors and more importantly that they run out of juice at the same time.

Mike
Mike is right ,only asking for trouble that way. BTW i fly with a extra 2600 6 volt nimph battery pack to run my radio receiver and servos so in case my lipos stop working or motors i can still get her down safely. I have had my esc cut out my motors once already and i was glad i had backup . . joe
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Old 02-25-2013, 06:18 PM
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Well, that is exactly what I'm trying to achieve, here. One main battery in the fuse, 8 separate (one in each nacelle), and all of them parallel. So the lot should run empty all at the same time. If a small pack is drained faster, it's voltage drops and it is automatically charged by the main pack, which has a higher voltage still. That's the theory, at least.

Hugo
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Old 02-25-2013, 08:40 PM
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Originally Posted by HugoW View Post
Well, that is exactly what I'm trying to achieve, here. One main battery in the fuse, 8 separate (one in each nacelle), and all of them parallel. So the lot should run empty all at the same time. If a small pack is drained faster, it's voltage drops and it is automatically charged by the main pack, which has a higher voltage still. That's the theory, at least.

Hugo
Just a note for some of the newer people flying these electric models. Parallel connecting these LiPo, LiFe or A123 cells works very well.

But, parallel connecting those old NiCads, or Nickel Hydride batteries is NEVER a good idea.
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Old 02-25-2013, 09:11 PM
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You would still have all the disadvantages of all of the extra weight in wire, batteries, and have youpicked motors yet?

A 500mah 3cell will get hot if it ever draws more current then the battery can handle, and could cause a failure if too much throttle is used and a motor/esc/ battery shuts down.

I would really lean twoard one or two large batteries in parellel in the fuse. What will the spacing be between motors, approx prop size, weight, ect? I think you should build up the wing and fuse so you can guess at weight and cg as well as clearances and wiring before even thinking about power setup.
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Old 02-25-2013, 10:31 PM
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I run my esc's in my fuse by my lipos but others are running large wire to the necceles and esc;s in there . I know I know your supose to keep your esc's as close as you can to your lipos with short leads but others like Neons are doing it the other way so they have less wire to run, and he is getting away with it. . Iam running two 3-cell 3200 packs for four motors plus my nimph battery just for my radio and servos. joe
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Old 02-26-2013, 03:23 AM
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I se nothing wrong with adding 6" or so of wire, like many parrellel and series conectors are wired up. This should be enough to get all of the esc's mounted in the wing.

Supposedly, adding length to the motor wires isn't really an issue.

Dc greatly increases load and loses efficency, requiring a lot of power loss the longer the leads get. They get around this by going with much larger wire when you increase length. The motor wires are esentually ac voltage pulsed to allow the motor to spin. The speed control converts it to a higher voltage pulse. The higher voltage the ac is, the less loss there is overdistance.

In the early days of electrical power, it was way more expensive to run dc electricity. It could also only be transported about 2 miles. This is why the first places with power were industrial areas and big citys. Now that we rely on ac, power is sent from my home town in utah, to california.
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