POWER DISTRIBUTION FOR SERVOS ON THE B25

As I mentioned earlier there are seven servos in my B25. Not a lot by some large model standards, but still, I think, a little too much to simply plug straight into the reciever.

In flight under load we could be pushing 6 - 10 amps with all servos working hard.

A company called SM services, near Cranfield, offer a range of 'black boxes' to cater for these requirements.

A device called the 'Powersave' would seem like a sensible solution.

This is a small box with very thick circuit tracks and 12 pairs of universal sockets allowing for a nice way to get power to all the servos. A couple of hunky A123 cells will provide all the amps required OR a 10 amp BEC from UH in HK might also save weight .

This Powersave costs  £29.95

see: http://www.smservices.net/acatalog/Large_Model_Units.html  for datasheets and other stuff.

Got the wing tgether today & strapped it to my trusty workmate. I connected it to 2 X 5000ma 3S

Lipos in series( = 22V)  Props =13 X 8 APC

I ran entirely at full power till the Graupner discharge module tripped the throttles down. Run time was 5 minutes & 38 seconds. So with normal flying conditions I expect that will increase quite a bit.

i have, however, room two more batteries if required.

More testing with different props to follow

Sounds like the DPM is working fine with both ESCs harnessed together then Peter .

A simple power calculator Excel sheet I did a few years ago used a formula of battery capacity / WOT Amps + 30 % .....for approx flight duration, allowing for typical throttle settings during flight. The actual full formula I used was

(((Capacity / WOT in air) [ wot static current - 11% ] X 60 ) +30%) / 1000 = minutes of flight.

Hi.

I have been following this thread with interest as I am building a long term electric project in the shape of a mosquito ic conversion. I have a question about the graupner DPM. The DPM obviously uses the balance connectors from Graupner batteries, where can I get an extension lead and adaptor to a JST-HX balance lead? Thanks.

I am OK with making up some leads but as yet I have not been able to identify the Graupner type connection. Does it have a specific name as I have searched for leads but nothing mentioning Graupner has come up so far.

I will do a thread for the Mossie but it will be some time before I get going. BT plan has arrived and so has the wood kit. Waiting for the cowls etc and to get my current project off the bench.

I think Graupner use a JST EHR plug - see the article from David here

Ironically, I have just been sebnt an adaptor lead (for a 4s pack ) to convert this type of plug to the more common JST HX  and it is NOT what I ordered...any use to you ?

Ironically, the lead in the article looks to be exactlly what I need. a female JST XH (the one on the right in the top picture) with a male graupner JST EHR (the one on the left in the bottom picture). Why didn't my searches find the article?? Never mind.

Timbo, if the lead you have is like the one in the article and as I describe then I will take you up on your very kind offer.

 let me know how much etc.

Thanks.

David. The lead I have is shown below - it has a MALE JST-XH plug which will fit the charger ( I assume your charger / balancer is JST XH? ) and the other end is a FEMALE JST EH which should take the graupner EH style plug on your battery. Please check and let me know if this is correct for you.

PM me please (via the forum), so we dont get this thread off topic. Thanks.

AT LAST I THINK I HAVE GOT IT SORTED!

 Having tested out the rough workings of twin ESC's with two battery packs and a Graupner Discharge Protection Module - and had a working system, things then went SERIOUSLY PEARSHAPED when I attempted to wire things up in the B25 wing.

The wiring nearly burnt the wing out!

WHY?  Well several things I guess. But one thing in particular. I had, as you know, the desire to fit an isolator switch in the battery circuit.

You would think it not important as to whether the switch was in the POS or NEG line.

Actually its SERIOUSLY IMPORTANT! 

Since, for the ESC I am using , The servo negative wire and the battery NEG wire are commoned together, it is VITAL to switch the battery main circuit thru the POS power lead.

Other makes of ESC's with maybe opto coupling might not have this problem

IF you break the NEG lead, all the current routes back up the neg servo lead and through the Graupner Discharge Protection Module

AT 'no load condition', thats  about 2 amps X 2 ESC's = 4 Amps times 18Volts = 72 watts = fried servo wire to the ESC. Also fried the balance plugs / wires into the lipo's.

I have had to perform 'open heart' surgery on the lipo's to replace the balance plugs!

The other matter which finally dawned on me is concerning the connections to the Graupner Discharge Protection Module.

To recap. The DPM has two JST-EHR sockets (marked Battery 1 & 2)  to monitor every individual lipo cell in use. BUT and ita a BIG BUT. If you have two battery packs wired in series to achieve the desired voltage (as I have) Then you MUST connect the balance tap of the lower battery pack to Battery 1. Otherwise the DPM circuit is confused and wont allow the ESC's to control the motors.

Battery 1 NEG wire is at 0 volts. The connection between the two packs is 1/2 total volts. Battery 2 POS wire  is max volts. Battery 1 balance plug is connected to Battery 1 socket on the DPM and the remaining Battery 2 balance plug goes to Battery 2 socket . Just to add to the confusion, the DPM gets its power from the battery 1 blance plug (its only a few milliamps) Battery 2 socket has to be connected first, and then battery 1 socket. This ensures the DPM counts the cells correctly. It does actually explain this part in the Manual.

The diagram in the DPM manual  is a physical Hook - up  photo. They don't include a theoretical wiring diagram at all. I think this is a great pity.

I have nearly re-wired the wing, with everything marked clearly '

While I was sulking over the smoked wiring I knocked up a nice little Scottish Aviation Bulldog in the Swedish livery. Still waiting for the weather    !!!!!

Here is a link to the first proper engine run with all the bits in place (sans cowls)

I have ordered 2 X 4S X 5000ma to replace the current 3S packs

Still getting the settings right on the Futaba radio.

http://video.google.com/videoplay?docid=-2727582645500784189&hl=en

Nearly time to fly!!

Im sure we are all  eagerly waiting for the next installment, i am getting the B25 for my winter building project and am hopeing you guys get things sorted, so I being a numpty can learn from your vast experiance, come on peter, its worse than waiting for my next delivery of RCM&E.

Hi Peter

I see your B25 is still shiny. I have just dirtied up my H9 P-47 Thunderbolt.

Firstly I gave it a thorough cleaning with isopropyl alcohol (rubbing alchol) then I gave it light coat of Prymol so the next coat of paint will stick. I then weathered it with Flair enamel thinned with white spirit (doesn't dry as fast which is better when spray weathering) and finally I gave it a light coat of Flair satin finish fuel proofer. Yes it's odd putting fuel proofer on an electric model.
The satin finish has made a real difference to the look of the model. It now looks like a model and not like a large toy.

George

B25 Update

I have been waiting for weeks for items to come into stock at Hobby City

The all up weight of my B25 is up to @ 17lbs. This is a bit more than advertised.

Funny thing that- when all you do is screw the bits together out the box, thats what they weigh!

To be fair, the published weight is probably without retracts.

So I need 850 watts per motor.  The max prop is 13 X 10 (without hitting the ground!) To achieve the necessary wattage , I must increase the volts to 8C. The Turnigy Plush80 is only good for 6C. SO-ooo I have got the Turnigy 100HV  These wil take up to 12 C. The good news is the amps should be a bit less.

Interestingly, the new ESC has the heatsink securely screwed on (although thescrews need threadlock!)

Did you know that on the previous ESC the heatsink is a small slab of metal held in place with the heatshrink covering. As an engineer, I dont like that. Nuff said!

During the course of the year I have refined various practises relating to electrc flight, using a CAP 232 as a sort of test bed for these.

I now have a Futaba 7C 2.4ghz. The multi model memory is always a useful feature, except that the TX battery is used up more quickly. I recently got the Futaba Lipo 2600ma from Our Little Chinese Friends.

Leaving the TX on for THREE HOURS used up 25% !!!  And it recharged in30 mins. For £12.00 thats the best investment I ever spent.

Next thing, then, is to re-design and fit new engine mounts to accomodate the new ESC,s 

George, your TB looks well 'dirty'. Nicely done, sir !

"So I need 850 watts per motor. The max prop is 13 X 10 (without hitting the ground!) To achieve the necessary wattage , I must increase the volts to 8C. The Turnigy Plush80 is only good for 6C. SO-ooo I have got the Turnigy 100HV These wil take up to 12 C. The good news is the amps should be a bit less."

Sorry Peter, lost me there. general practice is to discuss"C" rates in relation to battert capacity and ability, NOT voltages - this electrickery is confusing enough for beginners, so please explain

PS I do of course understand the philosophy  of using more volts and less amps to achieve the same watts, and sound idea it is too....however " I must increase the volts to 8C" - nope dont understand .

Hi Timbo

As Peter has a very limited prop diameter and the model has turned out a bit heavier then expected. he needs to "up" the watts needed to give the model the performance he requires.
Where prop diameter is no problem we would normally load up the motor with a larger diameter and possibly a greater pitch propeller (larger diameter = more thrust, Larger pitch = more speed).
Also larger diameter props are a lot more efficient.

As Peter has a limit on the physical diameter of propeller he can fit, he has to spin the largest prop he can fit,  faster.
To get the same motor to spin faster, you need a greater voltage, hence going from 6S to 8S.
I think this is where Peter confused you, I'm sure he meant to say "S" instead of "C"
In this case a 3 bladed prop would also help to transfer more power. In this case a nice 3 bladed varioProp from Ramoser would really look the part.
http://www.eisenbahnarchiv.de/cgi-bin/ramoser/e/his-webshop.pl?f=GROESSE&c=12C&t=temgroup_12C

As an example Spitfires started off with a 2 bladed prop, then as RR increased the power of the Merlin and later the Griffin, the only way to transfer this extra power, as again they were limited on prop diameter was to increase the number of blades. That is why the later mark Griffin engined Spits had 5 bladed props.

Hope that puts it in an easy to understand way

Cheers n Beers
George