What’s the best system for a 1/4 scale cub?
|David Rivers||10/10/2018 17:08:06|
|33 forum posts|
im looking for ideas and suggestions please. I am gurrently fitting out a quarter scale cub and wonder what the best system for the various power supplies is. I need a 4.8v supply for the petrol engine ignition, so thinking a 4 cell NiMh would be good. Can I use a AA size pack or should I go to sub-c. For the receiver/servo battery I am thinking that 2 packs would be good to give some security. What is the best way to connect them and what are the alternatives for battery backer? Power box are good but a bit expensive. Does anyone else make a battery backer? What type of cells do I need. The plane has 7 high torque analogue servos (Hitec HS-645MG) these can run at 4.8 or 6v so I suppose a 2 cell LiFe would be ok. Be grateful for any thoughts.
561 forum posts
Hi I believe simplicity is best (KISS)
the system i have had no problems with(the only one ) is using this ultra ibec and two batteries to suit rx and servos with two good switches, so either sub C 5 cel or life should be good. and with the nimh 5 cell two packs of same type/ age can be connected together with no problems what so ever( some use diodes), charge separately though.
I am certain others will be along with their systems of choice
Edited By flight1 on 10/10/2018 18:06:56
|john stones 1||10/10/2018 18:11:59|
10039 forum posts
Ignition 4.8v only, if not I use 6.6v life, nimhs o.k though, sub C maybe helps with balancing.
SM do battery backers, don't know if they still trade but they come up regular on BMFA
Would have no probs with one battery myself on a 1/4 scale Cub. 645s will be o.k on a life.
7999 forum posts
THIS page might give you a few ideas. For safety’s sake, always have a separate battery for Rx and Ignition. Always use an opto isolation switch...Rcexl do an excellent item.
Edited By cymaz on 10/10/2018 19:10:52
|Peter Christy||10/10/2018 19:22:19|
|1130 forum posts|
My experience with my (older) RCexel ignition is that AA NiMhs cannot provide sufficient oomph! The engine will start OK, and seem to run, but will be well down on power. Sub-Cs work fine. (The internal resistance of AAs is too high to allow the capacitor to charge fully between firing strokes.)
I believe the later RCexel will handle more than 4.8 volts, in which case a 2-cell LiFe pack would be ideal. Sadly, mine won't, so I have to use Sub-C NiMhs (model needs the ballast, anyway! )
Like Flight1, I'm a great believer in simplicity! The more you put in, the more there is to go wrong! I've known more supposed radio issues eventually tracked back to battery backers, regulators or diode protectors than almost anything else!
For the receiver, my advice would be to use a single, adequately large battery pack, but perhaps using two switches in parallel. Switches, being mechanical, are more likely to fail than a good quality battery.
Paralleling up battery packs only really works with LiPos, and that means adding a regulator (yuch!). Paralleling NiMhs up won't help if a cell fails low voltage. The bad pack will pull down the good one. If your servos will take 6 volts (or just over), a 2-cell LiFe pack is ideal.
I fly a large scale gasser helicopter using two Sub-C packs, one for the engine, one for the radio. The only problem I've had with it is the vibration from the strimmer engine continually shaking the airframe to pieces.....!
|David Rivers||11/10/2018 17:31:12|
|33 forum posts|
Thanks for the info. I was looking at the power distribution boxes which look interesting. Hobby king do them as well as 4 max. I soon realised that where there have facility for connecting two batteries, there are just connected directly together in parallel with no circuitry between them. So if one battery fails or shorts etc then it would take the other with it. Has anyone used a diode on the two battery connections so that one does not backfeed the other?
The other benefit of the distribution boards seems to be that you take the servo loads away from the receiver. Is this necessary on a 1/4 scale cub with one servo per surface. I wonder what the rating of the supply rails on an 8 channel Spektrum receiver are?
|Bruce Collinson||11/10/2018 18:19:27|
|184 forum posts|
The Zenoah 20 and RCxl whatever in my big Mentor run on a 1s LiPo as part of a Toni Clarke package which came with the (borrowed) engine. The rx runs on a biggish NiMh. They’re totally separate and have been entirely satisfactory.
The DLE 20 in my Seagull Yak has 2 2500 LiFes which are again completely separate and have been fine but with far fewer flights so far (silencer issues). Same ignition system. Isn’t it the case that most modern rxs and servos will run at over 6v, if you delve?
Both have electronic kill switches which are cheap and comforting and carry monitor lamps which are helpful.
|Frank Skilbeck||11/10/2018 18:38:25|
4181 forum posts
My 32cc petrol model with 7 high power and one regular (on throttle) servos uses a 4 cell sub C Nimh on the ignition with a mechanical and electronic Rexcel switch and a single 2s Life for the receiver and servos with a FET switch (Multiplex safety switch). All my 1/3 and 1/4 scale gliders use single 2s Life or 5 cell Sub C Nimh with FET switches (where a switch is fitted).
I do use large receivers on these models with either a high current (MPX 6 pin) connector or dual regular servo plugs (the 12amp safety switches have MPX 6 pin connector to the battery and 2 outputs to the Rx)
|Paul Marsh||11/10/2018 18:42:59|
3398 forum posts
I use the HK Failover switch, even on 15cc models, now.
|Steve J||11/10/2018 19:07:11|
807 forum posts
My Rcexl ignitions are fed by 4 cell Eneloop 2000 AA packs. No problems. If memory serves, the Rcexl pulls around 0.5A.
David, If you want dual receiver batteries, then have a look at the Spektrum Powersafes. If you want two batteries, but regard the Powersafes as excessive, then a use a couple of Schottky diodes (e.g. a FYPF1545DN).
Edited By Steve J on 11/10/2018 19:11:24
240 forum posts
I've been flying a 1/4 scale cub for 6 years, 120 four stroke, single 4.8v nimh 2000ma rx pack. 4 slightly beefy multiplex servos. Single standard switch. Only extra is an onboard glow and battery. My favourite all time model. I know you have the added complication of a petrol ignition but why all the other complications...… as long as you make sure it's charged up...just go fly. It's a Cub, just needs to be helped around the sky....
|Martin Harris||12/10/2018 00:43:11|
8021 forum posts
My Cub is traditionally built (from a Premier kit) powered by an ASP 160 twin with onboard glow and certainly no lightweight at (IIRC) around 18.5 lbs before adding 14 ozs of fuel - and on occasions payloads of 10 - 15 ozs. I know it's "only a Cub" but as this puts it firmly into the >7kg category with considerable kinetic energy even with relatively modest airspeeds, I consider it important to take precautions against easily avoidable loss of control, so have adopted the simple solution (to my mind, potentially the most reliable) of using twin receiver batteries with two separate switches and connections to the receiver.
Preflight checking of the individual batteries by the "mag check" method and telemetry monitoring to alert me of any unusual conditions give me confidence that I have a reasonable chance of surviving a power supply problem.
|Percy Verance||12/10/2018 08:07:23|
7108 forum posts
I might be wrong, but I'm sure the owners of S M Services retired and called it a day some time ago....... and Mainlink systems used to some similar items, although I'm not sure if it's a similar situation there as well. There hasn't been any change with the Mainlink site for years....
Edited By Percy Verance on 12/10/2018 08:12:57
|Piers Bowlan||12/10/2018 08:36:01|
1518 forum posts
You remember correctly Percy regarding SM Services - gone. I haven't heard of Mainlink for years but there website seems active. Aren't they a rail network?
Edited By Piers Bowlan on 12/10/2018 08:38:33
|Nigel R||12/10/2018 09:03:26|
1982 forum posts
There is much wisdom in Pete C's approach of parallel switches on a single big pack. You've doubled up on the least reliable link in the chain by doing that.
Nimhs are not suitable for directly connecting in parallel, you need a widget to isolate / monitor / switch between the two. Connecting directly in parallel reduces the reliability.
"The other benefit of the distribution boards seems to be that you take the servo loads away from the receiver. Is this necessary on a 1/4 scale cub with one servo per surface"
The HS645, if one stalls, the current draw is 2 1/2 Amp (stated on hitec website). Outright worst case is thus 7 x 2.5 = 22.5A. Clearly this is a highly unlikely scenario, but there it is.
I would consider going with a basic power distribution unit.
Have a think about RX lipo capacity and the cabling as well, standard servo cable is 5A (ish) capacity (from memory, might be a bit more or less).
Switch capacity also worth thinking about.
Edited By Nigel R on 12/10/2018 09:04:45
|Frank Skilbeck||12/10/2018 09:27:46|
4181 forum posts
Bear in mind if you use any form of safety switch that these continue to pull a very small current when in the off position, so if you leave the battery plugged in for any appreciable period it will flatten it.
|Percy Verance||12/10/2018 09:56:00|
7108 forum posts
You may possibly be thinking of Northern
Switch capacity can indeed be key Nigel. I normally use the Multiplex Safety switch in both 6 and 12amp versions. Not too pricey for the piece of mind they offer..... Some of these so-called heavy duty aftermarket switches are absolute rubbish. The one I took apart a few years back had the chunkier body and heavier wires, but exactly the same type of contacts as the smaller normal type of switch. Fit for the bin and not much else.....
And 5amps for standard servo cable? You sure? I always thought it was around 2amps.......
Edited By Percy Verance on 12/10/2018 10:09:32
|Martin Harris||12/10/2018 10:14:54|
8021 forum posts
And another reason to heed this advice!
|Percy Verance||12/10/2018 10:34:41|
7108 forum posts
There is in fact a warning in the (Multiplex) instructions regarding that Frank. Although not everyone reads the instructions do they?
Edited By Percy Verance on 12/10/2018 10:44:49
|Nigel R||12/10/2018 11:39:02|
1982 forum posts
Percy - as ever "it depends" and "you answer may be more or less conservative than mine".
26 gauge standard cables are 0.04Ohm per foot (standard copper resistance).
How long is the battery cable? Maybe a foot between battery and RX. This is the worst case section, the heaviest loaded piece of cable. The round trip to that point is therefore two feet, or 0.08Ohm total.
At 5A that gives a volt drop of 0.4V. You call on whether that is significant. I believe most any RX can cope with a 1/2V drop without issue.
The cable also has to dissipate a couple of W of heat. At 5A, things are getting a bit toasty for some PVC insulation types. Silicon types can cope with 7A in this size cable, in free air (as our installations are).
Or to sum up -
Real bog standard PVC standard extension cable and switches I would say, yes, 3A tops to stay cool.
Good quality standard size silicon extension cable will deal with 5A quite happily.
If you jump to the HD cabling, 22gauge, the numbers are 8A (PVC) and 13A (silicon).
Most of the time we are operating our kit so far away from that worst case we are plenty safe. And the 'worst case' loadings are very contrived - all surfaces completely stalled, simultaneously.
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