Timbo , I have some 3 cell lipos, so two in series is as you say 21v-or so. The exact voltage will depend on amps. which depends on motor, prop etc etc. The Lipos are 5000ma monsters,
With regard to motor choice I have the Turnigy 50-55 already loaded in my 'wish list' . So we are both thinking along the same lines.
In fact I have to confess I ordered a 60Amp Castle phoenix ESC in error. (another project) BUT if I get another one then I have two (obviously!) The thing is I have the USB link to set the things up via PC. The Turnigy 50-55 'A ' version is recommended with a 6C lipo pack at around 60A max to lift a 7lb model. 2 X 7 = 14 which is about the design weight of the B25. And maybe 3 bladers will be good to go, too
Now, bearing in mind this is a twin engine model, several safety considerations are in my mind. If we agree that as a machine, the electric motor can be less prone to 'flame outs' than a 'gas' engine, there is still the possibility of a LVC triggering one ESC before the other. If this happens during a low pass at 50 feet it may be 'hole in the runway' situation. ( Rapid roll into the deck )
Now there is a gadget made by Graupner which will solve this problem . It is a discharge protection module. Primarily developed for use where a large number of Lipo cells makes it unlikely that the ESC Low voltage Cut-off will detect an individual cell 'wiping out' below 3 V.
With the Graupner unit, all the cells are monitored by their balance connectors. The throttle servo output from the RX is connected , thru the protection module, to BOTH ESC inputs. Thus any cell going low (as well as the overall pack voltage) will throttle back both motors . It is important, of course, that both motors are connected to one supply source. The Graupner DPM will cater for up to two 5 cell packs (10 Cells in all) with or without BEC.
I need to check if the Phoenix ESC can have the LVC disabled. and rely on cell monitoring by the Graupner DPM.
The other device for twin engine models is, of course, the 'Twin Sync' This operates using magnets in the spinner with sensors mounted to detect a running motor and 'synchronise' the faster engine to the slower.
The loss of a motor throttles back the other one. However this unit was designed for gas engines. There may not be as great a difference with RPM on electric to worry about. Also the unit is quite time consuming to set up. I read the forum on RCU and thy never mentioned electric even once.
I think I will build a test rig to check all the figures before it gets installed in the B25
I am hoping to order engines on Monday. As you say £22.00 is a 'no brainer'
Graupner Discharge Protection Module connected to two Lipo's
I just realised, before anyone says, You CAN manually set the Phoenix ESC to lower cell count (say 2S). That way the Graupner DPM will always activate first and cut both motors
Personally I always feel people worry a little too much about the dreaded flame out of one motor with twins....I never fly to LVC, and enter finals immediately upon hearing the power drop off. I find that even after this stage, there is usually plenty of time to make a landing, dpending of course upon the LVC setting of the ESC. If using anESC that allows me to set the figure ( and many do not these days ) I used to set it for 2.75V per cell, and still have 2 x 2000mahr KoKams from about 4 years ago which were set like this....and they are still performing quite well today !
However, the DPM sounds like a cool little gizmo, and anything that helps to give us confidence in the rig is a good thing, after all, there is usually enough to worry about on a maiden flight with a large and expensive model, without the guesswork of when the batteries are going to start to sag !
So I take it then you will series link 2 of the packs, and parallel them with the same configuration of the other 2, effectively giving you a single 10amphr 20V battery?
Incidedntally I have the 50-55B motor for my camel, and am aiming for around 70 watts per pound for a scale like flight. Swinging a 15 x 6 wooden prop, on 4s Lipo, it pulled 45A and showed 7500 RPM albeit giving the watts I wanted. However I wish now that I had gone for the slightly lower KV of the 50-55A, as I would really prefer a larger prop swinging slower, I am even considering dropping down to a 3s Pack. The Camel has a huge cowl to mask off much of the prop !
Hmmm, after a nights sleep, I am a little more awake now, realising that my thoughts of you using all packs as one large battery doesnt quite fit the scenario of the DPM. I am not familiar with the device, and the link you gave just sends me to a balancer gizmo? So let me see.....
Its primary role is presumably as a cell monitoring device such as those found elsewhere, including those on some smart ESCs now available.
The Graupner DPM however, you say is capable of monitoring two seperate packs of up to maximum 5 cells in series., and this would seem to be the key for this application. However you said also
" It is important, of course, that both motors are connected to one supply source".
This is where the tiredness kicked in I thought the idea was to monitor both batteries individually for either cell droput, or general low voltage, whereupon the gizmo shuts the throttle to BOTH escs, thereby avoiding the yaw problem.
I have attached a crude sketch of how I think you are planning to use it. Is this right ?? Cheers....
Just because I am not paranoid dosen't mean it wont happen ! . Have a look at all the B25 model crashes on YouTube!
In essence, your sketch is the same. . The module doesn't care what you do with the batteries, be they in series or parallel. If it sees any cell in any pack dropping to danger point it will pull the throttle pulse to a 'close' signal.
I think it would also work if you 'connected two DPM's in series on their servo leads. IE daisy chain. You could then have four batteries of up to 5 cells each connected in any way you wish. (probably a series parallel combination ) . I know four 3 cell 5000ma packs will fit in the bomb bay, but I think thats probably un-neccessary
Yes thats absolutely correct. One point to remember, though, unless you fit a switch, there's a minute current draw from the DPM itself.
You connect the balance plugs one at a time. You then switch on the RX. The DPM then signals a count of the cells in each battery, by separate green & red LED. It then passes the throttle signal to the ESC's. ((You can use from 3 to 5 cell packs on each input)
All its looking at is the individual cell states. How you connect the main battery cables has no effect on its cell monitoring abilities.
I think is a good sixteen quid investment especially for large battery packs / models. If you read some of the articles by Greg Covey on a certain well known Forum across the pond, he always uses the CellPro DPM.
Unfortunately for us they seem to have attracted a three-fold hike in price from a certain UK distributor. So I am afraid I'm not going for that. The Graupner one is the only European unit I have found .
Thanks Peter, I found the link now works. However, I thought the device was designed specifically for twin ESCs ...but I now have my doubts, thinking instead, that you are actually just going to use a "Y" lead type connection to both esc throttle cables. Is this right, and if so....are you sure the device wil be happy on this arrangement ?
Sorry to keep on about it, but I do like to ensure I fully understand what is going on here !
PS It looks like having now read the pdf on the link, that it can indeed be used up to 10s battery, one would just need to do a little modding of the battery balance plug to convert to twin plugs as it were.
There you go then. I suppose its easy for me. I've been in electronics for over 40 years, and I definitely dont lnow it all - yet.
I got a couple of adapters to connect my batteries to the Graupner DPM. The plastic of the connector is sufficiently flexible enough to allow ,say, a 3 or 4 cell plug to connect to the 5 cell socket securely. You just need to ensure that the negative wire aligns to the ground pin, which is clearly marked. The DPM counts the number of cells each time, so it always knows where its 'baseline' is.
Lets suppose you were using two 3 cell batteries, and you decided to go up to 4 cells on one or both batteries. Assuming your motor & ESC would handle this, the DPM would note the extra cell and monitor it quite automatically.
It makes this little gadget very flexible. I intend to build and test the engine setup, on a test stand by running it till the motors quit (hopefully) both together.
There is no way the setup will be installed in the B25 until I am completely satisfied. I don't particularly like the idea of flying to a stopwatch. You never know when you might need that extra minute to, for example, 'go around' due to a lousy approach. This solution means I am never in danger of a single engine out, or a lipo balloon
I don't know what others do, but this is my way.
Engines will be ordered tomorrow . 2 X TURNIGY 50-65A 400kv Outrunner
Fascinating stuff .I learn something everytime I browse through the "black art" forae ( Forums ?) Its nice & comforting to know that you experts out there don't know it all & its still a continual learning process. Makes my first 'lectric venture look positively amateurish but I shall succeed "Witha little bit of help from my friends "
Grumpy (not getting much sleep cos of all this) Myron Good tho' innit?
Your first electric venture will not be amateurish at all Myron....in fact you have started off - if not in the deep end, at least midway down the pool.
Dont worry about keeping the questions coming on your thread.... we all learn all we know from someone else originally remember
And PS yes it is good, as you shall experience for yourself when you finally open the tap on that 70mm 40000RPM fan on your lovely Red Arrows Hawk. It may not sound or smell like an IC engine, but the magic of the thing working the way it does will still give a nice feeling of satisfaction.
PS depending on the current draw that you achieve with the fan and motor off Scott, that ESC I have for you may now fit the bill. PM me via forum again .
Aeroplanes have 'Engines'. Electric motors are in gramaphones ! So its engines 4 me.Here is the link for the 'Engine' : http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4913With regard to Chinese New Year. I have just had a commercial delivery from Shenzen in only 6 days. I have also had a delivery from HC. The order was placed on 29th Jan and arrived here last Saturday 9th Feb. That’s OK in my book. I think they must be keeping a skeleton staff these days.
Thats right ! I got the wrong one in the cart! Thats a pint I owe you.
The one I am now ordering is the Turnigy 50-65A / 400KV. The order is ready to go, but as they are off till the 15th, any attempt to order by EMS blows back to Standard Air parcel (ie: 'mule train!!) So I shall hold off till the 15th before pressing the button. As for 3 blade props HC don't seem to have anything suitable, so I will have to look elswhere.
In the meantime, I have enough bits to run up a spare motor thru the DPM with the Phoenix controller programmed to not cut out. It is actually set to 4 volts. I will then be relying on the DPM to cut the motor back / off before the lipo becomes a 'pigs bladder'.
I look forward to the testing of the twin ESC and DPM rig...please keep us posted
I think you will be well pleased with the motors...the Turnigy I have for the Camel is excellent ( given the few pennies It cost of course ).
I also am well pleased with the Turnigy Plush 60A ESC from them - 3A switched mode BEC, and it throttles up as smooth as the proverbial infant's derriere again all for a nippers pocket money price.
So can you program the DPM trigger point ?
I guess there is a case here for someone to do an ESC with NO LVC feature..... for use with the DPM.
The Graupner DPM, and probably all other makes, is pre-programmed to cut the throttle(s) if ANY cell falls to the threshold for lipos which is 3 volts. It doesnt matter what the cell count is (from 3 to 5 cells on each battery). It should allow you to fly confidently to the safe minimum limit of the pack.
With some speed controllers, for example the Castle Phoenix, it is possible to manually set the cell count to a much lower voltage cut-off thanthe pack voltage you have in the plane.This would be lipo suicide, Without a discharge protection module, but I dont want the speed controllers making independant decisions.
On a twin engine aircraft, no two speed controllers will cut off at exactly the same moment, and you need something that definitely will cut BOTH throttles when the battery is becoming exhausted. The DPM can do this. The alternative for gas powered models is the 'Twin Sync.
And thats a pint of coffee . As for sticky bun, you may have to grovel
Yes I understand all that....I just wanted to know if the DPM was adjustable - I am usualy happy to let my cells get down to 2.75V or thereabouts under a heavy load.
Of course using the DPM you will need to ensure the esc LVC is disabled or at least fooled into thinking you have a 2s instead of 3 etc, otherwise it could defeat the whole object.
heres a couple of photos while waiting for Hong Kong to get back to work, so I can order the motors, and servos!
A few guns added. I sprayed the visible areas under the top gunner dome and side gunner windows with matt black before fitting. It gives a better look I think
here you can see the nose wheel steering connected to the servo by a closed loop linkage. I will have to add a rubber band tensioning device to pull the closed loop wires out of the way as the leg is retracted
denotes 
) 
I thought the idea was to monitor both batteries individually for either cell droput, or general low voltage, whereupon the gizmo shuts the throttle to BOTH escs, thereby avoiding the yaw problem.
. Have a look at all the B25 model crashes on YouTube! 
The Graupner one is the only European unit I have found .
Electric motors are in gramaphones ! So its engines 4 me.
Thats a pint I owe you.
