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Long ESC wires or long motor wires?

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Old 02-23-2008, 08:42 PM
  #26  
xb36
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Default RE: Long ESC wires or long motor wires?

There were some good points in the last few emails that I would like to reply to.

Let me start by describing the crash of a 17 foot span fuel B-36. There is a reason for me to describe this event here. I had seen this B-36 fly, and it flew fine with plenty of power. It took 4 guys almost 20 minutes to get all 6 motors running about the same..(Now that's a shot!). First, I wasn't there at the crash flight so I pass along what I heard. The plane was sold, so this crash flight was also the new owner's first flight. He was bringing it in for a landing to an asphalt runway and it just wouldn't set down due to ground effect. The 6 engines were idled down so when it was apparent that he was going to overshoot the runway he throttled them up, probably too fast, and the number 1 engine sputtered and stopped. As was mentioned in the previous post, with one engine out the plane had a bit of yaw but was still controllable. It had a wider turn and still had ample thrust. This pilot brought it in for a second attempt at landing and the same thing happened again. This time he lost the number 2 engine. This B-36 was still able to stay in the air but the yaw was so bad that his turning radius was too large to get him back. The airfield configuration (trees) did not allow turning the other way.

I described this event for a few reasons. Originally I was going to use brushed motors with gear drives. I was going to have 3 circuits. Each circuit would have 1 ESC, 2 brushed motors in series, and 1 NiCd battery pack. Motors 1 & 6 would be in one circuit, 2 & 5 in another, and 3 & 4 in the last. I too thought that it would be best to lose matching motors on each wing to avoid the yaw. Just make sure there is ample thrust with 4 motors. This is my first plane design and my first completely scratch built plane. I started the design in the spring of 1999 and I started building it in 2000. It got shelved for a few years for various reasons. Well I am putting in as much time as I can and stay married so I am going to finish it. Electric RC has changed, so I am now going brushless. I asked around about running 2 brushless motors on one ESC and got the same response that Ron mentioned, it usually works but is not recommended. Rich makes a VERY good point: If you lose one motor, so what! From Dick's comments I get: electrics are so reliable that the likelihood that you will lose ESCs on one fllight is inconceivable. So after giving it some thought, I don't think that I am going to worry about losing 2 ESCs or 2 motors in a single flight.

So the only issues left to address is the batteries and the combination of motor current and wire gauge. Century Jet Models was good enough to build a custom set of retracts for this plane. They weigh close to 2 pounds! I thought that this was ok because I originally designed this with NiCd batteries in mind. I reduced weight by changing from NiCd batteries to LiPo batteries. This offset the weight of the retracts. I don't know if I want to add the NiCd weight back in. So for now, let's assume that we are limited to LiPo batteries.

It was suggested that using a large single battery would be an option. If I use LiPo batteries, MotoCalc 8 pointed out that the battery pack would overheat.

I almost always over powered (used more cells for higher voltage) my cheap brushed motors. It made a big difference. Can this be done with brushless motors? A brushed motor will run faster if a higher voltage is applied to it. Can I use 4S1P batteries on a motor rated for 3S? Do I just purchase 6 ESCs with a higher voltage rating? If so, then the suggestion to use a higher voltage and lower current is a very good option!

Rich suggested some build photos. Well I have built up the fuse and I do have photos. I just can't find them right now, but I will. I am building up the wing and I have posted them in a build thread in the RC Scale Aircraft forum. Should I have posted it all in an electrics forum, a multi forum, a scale forum, or a warbird forum. The RC Scale Aircraft thread took off so that is where I update. Look for 2 threads in that forum from xb36.

This is a repeat here, but if you want the plans, I offer them free of charge. I still have a few design mods to make. It was originally going to be a XB-36, now it is a B-36A, B-36B, or B-36C. They are almost identical. If you want the plans, here is a link to download the 87MB zip file: http://www.megaupload.com/?d=AHJ56N2X

I will attach a few images to give you a feel for what I am building.

Don't let the offer for plans take this discussion off track! I am getting some great information here! I knew the brushed motor world. I need to know how to properly run this plane on brushless so opinions are more than welcome.

I have blabbed on too long so I will stop for now.

Mike

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Old 02-24-2008, 08:01 AM
  #27  
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Default RE: Long ESC wires or long motor wires?

Mike,

Dealing with your questions as they come to mind -

Yes you can increase the cell count - this will give you a higher RPM. A higher RPM with the same prop will draw more amps - so you get more power because power = volts x amps and you have more of both. The thing to be aware of is that if you up the cell count you don't draw too much current through the motor (many motors also have a total power rating).

Brushless motors aren't too hard to get your head around. The key thing to watch on them is amp draw - they will have a recommended voltage range. The thing that tends to kill them is putting on too big a prop and drawing too many amps.

So- get yourself a wattups meter so you can measure your amp draws and experiment. There are plenty of notes on how electrics work etc, but you might find these notes handy:

http://www.oz********.com/2007/01/el...ic-flight.html
http://www.oz********.com/2007/09/gu...low-heads.html
http://www.wattflyer.com/forums/showthread.php?t=24238
http://www.oz********.com/2008/02/el...c-numbers.html

Good luck with your multi engine build. If you are faced with the propsect of big expensive lipo packs don't forget you can combine packs in series and parrallel to meet your needs. For example, let's say you need a 6s 4400mAh battery - well you could pay $200 for it, or you could buy 4 3s 2200mAh for $100 and join 2 pairs of 2 in parrallel (to make 2 x 3s 4400mAh) and then join those two packs in series to create a 6s 4400mAh for half the price of a specialist battery - just a smart way to use the mass market power.
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Old 02-24-2008, 10:14 AM
  #28  
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Default RE: Long ESC wires or long motor wires?


This may be quicker to compute I am using... H D servo cable, # 22 gauge wire..... on each B L motor in a GWS A-10. Each pulls 6 amp. @ 12 volts. No real volt drop.
Think & search several times before buying.

Chart for you.

Rich

Edit: Use the CHASSIS amperes rating. More in line with our lengths.
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Old 02-24-2008, 10:52 AM
  #29  
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Default RE: Long ESC wires or long motor wires?

xb36.

You will need some thrust and speed for a scale take off in a "breeze".
As I had thought the 36 is a floater.

So the real issue is what diameter props WOULD LOOK GREAT on the scale engine spacing ?

If geared motors are used, are they really needed with such low KV outrunners now available ?

The big problem becomes a prop with enough pitch and blades in the diameter allowed.
Those planes HAD SLOW turning props.

I have made 2 high pitch props into 1 high pitch prop by machining away 1/2 of each hub area and putting 4 small machine screws Locktighted in place into the hubs. Ran beautifully in the China Clipper on speed 400 motors. The scale spacing at the time required that. Now I just run E props ok.

So a gearboxed brushless may still be needed. Modern ESC's have a SLOW acceleration option. So the sudden start of a motor is NOT possible. Saves gears and avoids the use of expensive motors. A gearbox really shines in letting ANY old motor wind up on VERY high voltages @ very low currents.

If you could find out the weight of the 36 that had the ground effect problem & you are building to a similar weight & airfoils of all surfaces, you can design the motor & battery requirements with a huge safety factor.

Rich

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Old 02-24-2008, 12:17 PM
  #30  
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Default RE: Long ESC wires or long motor wires?

Rich,

Thanks for the comments.

The B-36 is a six motor pusher. If I recall correctly, around 9.5 inch 3 bladed pusher props would be scale. Master Airscrew makes a 9x7 3 bladed pusher prop so I have already purchased six of them. I know that some efficiency is lost with 3 blades, but MotoCalc 8 shows that it is minimal. I thought that it would be nice to keep it scale where I can.

My goal today is to get the second retract installed and possibly get started on the wing mount.



I received an email that someone had problems with the megaupload link to the plans for this plane.

I have also posted the 87MB zip file at http://rapidshare.com/files/94565987...n_2nd_Pass.zip

There is a button at the bottom of the page to download it for free.

Mike
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Old 02-24-2008, 12:58 PM
  #31  
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Default RE: Long ESC wires or long motor wires?

Red said: If you are running both batteries in parallel there is no way one can go down before the other. They will discharge in proportion to their avialable capacity.


Red, I'm still trying to fully digest your comment. Let's see if my understanding is correct. If I have two batteries in parallel, by definition their terminal voltages must be the same. If one of the batteries is at a lower charge state, it will try to operate at a lower terminal voltage. But current will flow from the stronger battery to the weaker battery. So the two parallel units will discharge in the same total time, but the weaker will discharge at a lower amperage. Do I have it right?

The only variable beyond this is that my crossover cable is 22" long, so there will be some voltage drop. It is 16 ga., which I sized for slightly over 0.1 volt drop at 10 amps. This design assumed that I might be supplying the full current of one motor through the crossover cable. But from what you're telling me, the reality is that I'll be putting a small amount of current through the cable continuously and both batteries will reach cutoff at pretty much the same time. Short of doing some measurement, I guess I have no idea of the current levels in the crossover cable. But it's probably much lower than I first thought.

Thanks for the education you're giving us,

Dick
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Old 02-24-2008, 02:23 PM
  #32  
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Default RE: Long ESC wires or long motor wires?


ORIGINAL: otrcman

Red said: If you are running both batteries in parallel there is no way one can go down before the other. They will discharge in proportion to their avialable capacity.


Red, I'm still trying to fully digest your comment. Let's see if my understanding is correct. If I have two batteries in parallel, by definition their terminal voltages must be the same. If one of the batteries is at a lower charge state, it will try to operate at a lower terminal voltage. But current will flow from the stronger battery to the weaker battery. So the two parallel units will discharge in the same total time, but the weaker will discharge at a lower amperage. Do I have it right?

Dick
More or less, you have it right. Except for the fact that it is literally impossible to charge one battery from another with the same number of cells unless one is practically fully discharged, even at that, there will be minimum transfer of energy. when you look at the discharge curves and compare with the charge curves it will become quite clear. It takes more voltage to charge than is available anytime you are discharging one pack. You need 4.2 volts/cell to fully charge a LiPo pack, but no LiPo pack will supply 4.2 volts/cell while being discharged.
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Old 02-24-2008, 02:52 PM
  #33  
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Default RE: Long ESC wires or long motor wires?

A brushless motor in combination with a brushless controller acts the same as a brushed motor, the same constanst apply, notably RPM = Kv* voltage. Motor currents wants to go up with voltage squared. So if you go from e.g. 2 cells to3 celss, voltage increases by factor 1.5 and current wants to increase by factor 1.5^2 = 2.25, that's 125% extra current. To keep the current within the motors limits, you probably have to prop down. Motors, brushless or brushed, don't have a voltage range. They can run at any voltage, provided it does not exceed its rpm or insulation limits (the latter only when using too dangerous a voltage). It's the current that can kill. A voltage rating is just something the marketing departments come up with.

If you double your voltage, and use motors of the same type, make and size but with half the original Kv, nothing changes, rpm stays the same, power stays the same. Except the current, it goes down by a factor two. Voltage losses in the wires will go down by factor two, powerlosses in the wires will go down squared (Ploss=I^2 * R). If you can live with the original losses, you could cut wire cross section area (=weight) by factor 4.

@cyclops2/Rich
Why no LiPo's in large planes? Overkill? NiMH will do just as well, because wing load is save anyway?

@ozrboy
Your lightspeed explanation does not apply, or better, it is not relevant in this case. Furthermore, the speed of electrical signals in wires is the same as the[link=http://en.wikipedia.org/wiki/Lightspeed]lightspeed in vacuum[/link] lightspeed in vacuum, 3*10^8 m/s. You have to brush up a bit on 'selfinductance in lines' I think.

Vriendelijke groeten Ron
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Old 02-24-2008, 08:12 PM
  #34  
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Default RE: Long ESC wires or long motor wires?


Ron.

I just hate seeing a big plane have a minor crash and damage the LIPOS. The delayed fire that starts later is avoidable. Not to mention the car or truck that goes up in flames with the plane.

The amount of LIPO the 36 will have on board, means nothing can grab it, or put it out.

Plane could probably fly with a lead acid or a gel cell or anything else easily. Certainly cheaper.

It does not need LIPO.

Rich
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Old 02-24-2008, 08:18 PM
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Default RE: Long ESC wires or long motor wires?



Red is right.
All chargers have a NOT connected voltage of at least 30 % higher than a fully charged battery. If they did not , It would take several days to charge a run down battery.

A FAST charge charger can have 2 to 3 X the voltage of the battery. It can also overheat the cells and ruin the battery in a short time.
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Old 02-24-2008, 09:32 PM
  #36  
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Default RE: Long ESC wires or long motor wires?

Mike,

Back to the airplane design, which is the part I understand. The accident with the 17 foot B-36 underscores a feature of this airplane which you should keep in mind. It is a pretty streamlied machine with a nice, high aspect ratio wing. That makes it a good glider. It's also pretty heavy for a glider. Heavy airplanes fly fast (lots of kinetic energy) and efficient gliders take very little energy for a nice, flat glide. Also, efficient gliders take very little power to maintain level flight. Put all this together and you have an airplane that refuses to slow down and land if you approach too fast (stored kinetic energy) or if you have high idle thrust (gasoline energy).

Do you remember the full-size B-47's that deployed a drag chute while in the landing pattern? They did that because they, too, were efficient gliders and also because the early jet engines needed to run at a high approach rpm to facilitate acceleration in case of a go-around. The parachute added drag to offset the idle thrust so they could bleed off speed during the landing flare. The full scale B-52 had much better engines which accelerated well from low power and also the B-52 had high deflection flaps for drag.

Your electric powered B-36 will have the advantage of being able to stop the props on final if necessary. But you will still have to maintain good airspeed control in order to not cross the fence at too high a speed. This is opposed to something like a Stearman biplane, which can literally be dived at the end of the runway without picking up much speed. Once over the runway, the high drag of the Stearman permits rapid bleed-off of any remaining speed and then a prompt touchdown.

Dick
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Old 02-24-2008, 10:05 PM
  #37  
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Default RE: Long ESC wires or long motor wires?


Come in at full power. Pop the chute. Full flaps. EJECT just before touchdown.


That only works 1 time.


Rich
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Old 02-24-2008, 10:26 PM
  #38  
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Default RE: Long ESC wires or long motor wires?



I too have thought of the B-36 as just a big powered glider. That appealed to me and my flying skills.

You phrased it better than I, but this is just a quick glider. I am building it as light as possible. I don't know if it will float, but it will fly. If I remember the numbers correctly, MotoCalc 8 puts stall speed at 20mph, and full throttle level flight at 61mph.

A while back I read an article about running multi motors. I don't recall the details but I recall that you can really scale the size of the motors down on a multi.

I have purchased 6 of the E-Flight Park 480 brushless motors. They can swing a 12x8 2 blade prop so I am sure that a 9x7 3 blade shouldn't be an issue. MotoCalc 8 is ok with it.

Century Jets just sent me an email with a solution to fitting my retracts in the thin wing so I should be able to move ahead now. Changes, always changes.

Thanks for the glider comments. They kind of confirm what I had speculated at the start.

Full flaps... possible, Full throttle.... I hope not, chute.... does not apply, Eject.... not an option

Mike
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Old 03-01-2008, 12:27 AM
  #39  
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Default RE: Long ESC wires or long motor wires?


ORIGINAL: xb36

I am building a 114" span 6 motor B-36.
Mike, I LOVE the B-36. I spent much of my boyhood near the SAC base at Altus, OK and remember hearing the power of those engines as they flew over.

My dad used to take me there for the open house/air shows. I remember seeing helicopters doing a 'square dance'.

Just some great memories of a 10 year old.

We wanna see pics!!
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Old 03-02-2008, 02:20 PM
  #40  
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Default RE: Long ESC wires or long motor wires?

I have posted some photos in the RC Scale Aircraft forum.

I have almost finished putting together the complete set of photos with comments. They should be ready for download tonight or more likely tomorrow. I have about 55 pages of photos and comments for this plane.

Take a look at the RC Scale Aircraft forum if you have an interest in seeing some of the photos.

Mike
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