Lipoly batteries in Transmitters

[John Wheater]

There are Lipoly sets being advertised to replace the 8 x AA battery packs in transmitters. I don't think this is a very good idea. How many times have you left  your Tx switched on? Particularly after installing the radio in a new model on the bench? Go on, we have all done it.  Result, entirely flat NiCds or NiMhs. No problem, recharge them. Do the same thing with a Lipoly pack installed and you will have to buy a new one.

[Tim Mackey]

Well actually John - it doesnt work like that. These packs have special DPM ( discharge protection modules ) these cut off the battery whn it reaches a preset level ( usually around 10.0V or so ) Problem is, that most tr alarms do not sound off until around 9.6V and therefore, without warning...your tr just shuts down! Now I have had a lipo fitted to my DX7 for around a year now, and found this out the hard way. Luckily Itwas whilst flying a tough EPP slope soarer, and It landed without incident - just a long walk

I immediately set to work fitting a bright red LED to my set to help aviod this happening again - and I have subsequently made and fitted an external piezo alarm to my tr, and set this to sound at 11.3V which is as low as you reaaly want to let your pack get in a transmitter. It works well.

If you are interested further, then take a look at this thread of mine which has info on these and other mods done

Transmitter modifications

[John Wheater]

Thanks for that Timbo, I had no idea. Mind you, I will stick to the 2200 mAh NiMh that I know and love using the overnight standard charger rather than quick charging! On the subject of DPMs though, I have read somewhere that they are essential for twins but it was all a bit vague. I am building a TopFlite Dakota using a 3300 3S on each motor and I have read somewhere that DPMs are advocated. Could you let me have a bit of gen on that aspect of their use? Thanks.

[Tim Mackey]

Check this thread here...we had a lot of discussion about it, but not sure where it ended up

DPMs

[John Wheater]

Thanks Timbo, I have printed it off for study.

[John Wheater]

OK Timbo, I have studied;

Quote: It is important, of course, that both motors are connected to one supply source.  Surely if both controllers are linked to the same power source and this power source goes down (in whatever way) they will both be affected in the same way obviating the need for a DPM? Can't the DPM monitor two batteries? THAT'S what is needed, certainly in my proposed application, 2 x batts, 2 x controllers and 2 x motors. I will, of course be using a separate Rx battery. Thanks in anticipation.

[Tim Mackey]

Hi John...its not so much the "power source going down" more a case of the 2 seperate ESCs having slightly different tolerances. It is IMPOSSIBLE to get two ESCs even of the exact same brand and model to have the exact same tolerances due to component differences etc. The upshot is, therefore, that one ESC is almost certain to invoke its LVC before the other. It is this instant albeit possibly brief cutting of the one motor that causes the potential torque effect of course. Now personally ( and I mean PERSONALLY ) I just never fly to LVC anyway, so it would not be a big worry for me. However I can appreciate that some peeps may have a huge investment in a very complex and valuable twin motor model, and will be reluctant to risk not hitting the LVC. There does seem to be some doubt as to whether the DPM in this thread was capable of monitoring the two batteries. Of course, again IMO the risk of this happening is still smaller than the risk involved in twin IC engined models having a problem and that hasn't stopped people from flying some VERY big and expensive models.

I also happen to think that flying a twin calls for maybe a little more respect, and the schedule should be designed to reflect that - IE dont do inverted low passes when you are 3/4 way into your flight time etc

[John Wheater]

Thanks Timbo. It's late do you have your PC next to the bed tuned in to Model Flying? I have been flying electric seriously now for six years or so, (although I played with MFA stuff in the early eighties) and I have never, never ever got down to the LVC on a single flight. We are at one on that. However, I am miffed that you suggest I cannot do inverted low passes 3/4 of the way into the flight time with my Dart Dakota! LOL.

[Tim Mackey]

 Ah you see ...someone has to be around to moderate you lot after baby David has gone to bed !

I have 4 PCs around various locations in the house, office, garage, lounge, and oh yes, a laptop in case I am not in any of those places

I didnt say you cant....just make sure you have a black bag with you

[John Wheater]

I will be flying this mother in bare balsa first. I don't want to spend six months covering and finishing it only to have an engine out on a low inverted pass. Confucious he say, "Man who fly low inverted pass, need stalls of beel" Or was that Doctor Spooner?

[Tim Mackey]

I am off to beddy byes now m'self, so I will bid y'all good night ..... Now, behave while we are gone

[Nicholas Booth]

Hi John

 Flicking through this thread I noted that you are building a Top Flite Dakota and converting it to Electric. I have just finished my exact same conversion and although I do not class myself as an expert in any way, I am pretty sure that if you have any setup concerns plans etc, I reckon most of my bad planning meant I have already experienced them. Interesting, this is the second time I have posted this link today but  http://www.modelflying.co.uk/gallery/image.asp?sp=&v=5&uabn=244&uin=1944

I followed some very interesting threads on another forum that helped with a lot of ideas for hidden controls for the flaps, and covering methods etc.

 I spoke to Timbo re DPM'sand tried source them locally but could not find them. In the meantime I finished the build and am not sure of how I would integrate them into the power setup I have. I (despite the warnings from Timbo ) will possibly try without the DPM. I have now completed 15 ground runs with all indicating that one motor is very likely to cut out prior to the other (6.5 min runs from 3/4 to full throttle) all leaving around 100 Mah difference in draw. However, I have used varible pitch three bladed props which are manually setup and still need a little more tweaking as the rpm is a couple of hundred out at the moment. Once they are more balanced I think that I will be able to safely fly for 6 minutes and have 1 minute for approaches etc. I actually expect to have a lot more in reserve but won't comment until she has been in the air.

One thing I wills ay is that there is a lot of chat and videos of these just rolling over straight after take off and crashing with no way to stop it. From the ground tests I have made on my setup, the 1.5 degrees of up incidence built into the tail gets the tail up and airborne very quickly, that is the mistake I think of the many as it may bee seen as a plane ready to be airborne and causes almost instant stalling. My  guess is without flaps I will need to be looking at 3/4 throttle at the very least and a slightly extended take off run to ensure adequate speed for take off.