Nigel R

Can't add much to the above excellent posts, low angles up to 30deg give lift for takeoff, over 60 deg is an airbrake. Both settings are useful in their own way.

Fitting dual servos for ailerons became commonplace in the 1990s, I would say. I remember at the time (20g size micro servos were still pricey) I used two standard size servos, probably S148 or JR 507, in several 40 or 60 class airframes. Before that, historically, I think central single servo with bellcranks and barn door ailerons were de rigeur for good while (1960's?) before torque rods and strip ailerons became common (70's?). These days of blow moulded foamies and near disposable price electronics, its simply too easy to dig out a hole in the foam and hot glue a 9g sub micro servo in place.

If I'm using two servos I'd use two independent RX channels - no point not... unless I'm stuck with an RX without enough channels. That said... Anything I build myself I usually fit a single standard servo, torque rods and strip ailerons. Differential (if needed) is easily possible via offset servo horn - as Paul's post shows. Old school? Sure, but, works, hidden linkage, looks tidy, happy with end result. On a fun fly or 3d type, you may very well need the two channels for elevator/flaperon mix. On something larger/faster/pricier you probably want redundancy. Something ARTF is probably built with holes for two servos. Horses for courses.

1.8m of Piper Cub will fly on a 0.25 two stroke, so I'd be keeping this as simple as possible, I'd fit a 2000mAh Eneloop AA pack, just one of them. Do get a new pack and bin that old one!

Absolutely this. Businesses provide what people want, if they can make profit on that endeavour.

Might be tiny air bubbles in the oil? Seems odd at first for a bike to spec ATF in the forks instead of regular fork oil, but I guess the viscosity must be matched to the fork's assorted valves.

Agreed on dry storage. I have had one engine stored with ATF gum up. Maybe it reacted with something in the fuel? After that I pushed the boat out, spent £10 on some air tool oil, use that for my infrequent assembly/storage jobs instead. ATF is not hygroscopic.

Cells can fail shorted. Yes, wiring can also short. Failover device like the 'ideal diode' linked by Phil provides protection against both. Even if a first failover devices fails with a dead short, the second failover will prevent the first problem from affecting the second pack. If a first failover devices goes open circuit, the second is there for backup. "chargers can fail" I think I said, or words to effect. Your models, your choice, etc.

What is "enough" power is down to the individual, I'd say. My own stuff, I have airframes from an electric trainer which just about loops from level up to 3d. Make sure it won't fall apart, have at it.

Given the Joker's era, Matt probably expected to use a 12x12 prop?

Far more problematic than the above scenario, is that cells can fail to a dead short. Two cells in a vanilla parallel arrangement have no protection against that.

Quite normal - AC is always rated much higher - the voltage crosses zero so any arcing naturally disappears quickly. The direction of erosion, from the arcing, also varies. DC arcs take longer to die out. The erosion is always in one direction. Hence the lower rating. As noted by Andy and Brian: We only use 5V or thereabouts - not 125V. When you switch off an RX, normal situation, all servos at idle, any current through the switch will be very low, 100mA or thereabouts for "your average sport model" (well within the current rating even at 125V DC). If the current isn't very low, you almost certainly have bigger fish to fry (stalled servo? burning servo?) and any small arc that may result from a 5V (ish) contact break is a bit "down the priority list" as it were. Sure. Many assumptions; it is an abnormal scenario. Nimh cell resistance is usually quoted as between 30 to 100mO - going to assume 30 for now. Cell #1 is charged perfectly to peak. Voltage will be quite high. e.g. it will read (maybe) 1.8V. When it is disconnected it will almost immediately drop to around 1.5V IF Cell #2 has been overcharged for whatever reason. If the peak during charge was (the same as cell #1) 1.8V and then charge continued until the cell was 1.6V. (that would be quite the overcharge, but, chargers do go wrong) When Cell #2 is taken off charge it will drops a comparable amount to cell #1, say to 1.3V. Now parallel them up. Initial current, (assuming the usual simple internal model of a battery) Vdrop = 0.2V, Rtot = 0.06Ohm ( 2 cells in series with 0.03Ohm each). Gives I = 3A The discharge curve at this current (1.5C ish) is sharp and of course, cell #1 terminal voltage will decay quickly. How quickly? Minutes, around 5 minutes or so. Within cell #2, the terminal voltage will also fall, but slower. The situation will self limit. There isn't enough energy in cell #1 to sustain the scenario. Meantime, further overcharge + temp rise within cell #2. Significant? Maybe. Depends. Likely outcomes 1) some further damage to cell #2 2) your power source is now busy self discharging and doesn't have much spare capacity to run the RX + servos.

No, not what I said: Anyway. Do what you want, this is just a discussion, no need to get upset.

Is the temperature change due to these reactions significant Mike?

Usually the manufacturer, as a first port of call. I mean, consider Laser and castor oil - always used to be a fun thread - you would often solidly back the manufacturer's recommendation IIRC - quite sensible I think.

No, the scenario I mentioned occurs when connecting in parallel, after charging. Source for me is simply the well known charge curve of nicad/nimh. I appreciate my previous post was not very clear on that point, apologies for that. But - as Phil previously stated, cell manufacturers do not recommend parallel connection. You have even stated this yourself: I don't think anyone is doubting you did reading and analysis. Devil's advocate - what would it take here to change your mind? It mitigates the possibility of a failure where the cell shorts out - without the diode this would cause you a total loss of power / discharge between the packs / other issue. The diode can fail in several ways. Fail open, you have a redundant power source which can take over. Fail short, that power source shows higher voltage and is thus dominant. i.e. both failures are already mitigated by the redundant nature of this setup.

How much heat though? 0.1C (or 200mA), with a 1.5V terminal voltage (ish), that's 0.3W in each cell. Whilst I don't know the maths to work out what temperature rise that would cause in a nimh, a solid block of aluminium the size of an AA cell would be raised 40degC over 12 hours (says the first online calculator I found) which is significant. But Nicads used to trickle at about 50mA IIRC which is why I suspect is the main reason they were just fine with any trickle charging use and abuse - the physical size is the same as nimh of course, but they were only asked to deal 0.075W of heat when full.

Nice one Phil the pre rolled board with the controller and fets looks perfect.

When you overcharge a nicad or nimh the voltage actually starts going down (think "peak detect") Overcharge further - even more voltage drop, Bigger voltage drop - more current will be pushed in. Rinse, repeat. Will that last forever? Probably not, ultimately pressure and temperature would cause a conclusion. I don't imagine a 4 cell AA pack contains enough energy to be able to make pyrotechnics occur on a second pack, but it certainly would not do the second pack any good. Lithiums as you state, will just equalise.

Or use an ideal voltage source, no need for two packs that way 😄

They are indeed - they also tend to be used in a way such that failure rates are analysed and appropriate mitigation for failures are put in place. There is a positive feedback scenario - past peak voltages after charge - one pack can discharge into a second and then push the second further into the condition. That said - join them together with a couple of diodes, perfectly viable and simple method of failsafe (caveat, diode is correctly rated).

I'd respectfully disagree, too. Trickle charge does not overcharge cells - when they are full the response moves over to a temperature rise in the cell. The cell is also dissipating heat all the time. At a very low current, say 100mA, each cell is only having to deal with about 0.15 of a watt of heat.

Well, cell manufacturers do not recommend parallel connection of nimhs. There is a particular failure mode involving freshly charged nimhs. It's been discussed before on the forum, if I recall some were then of your current opinion, i.e. that the failure mode didn't matter. I would say again that there are better ways of providing redundancy, i.e. using an actual failover device of some sort. If you want to parallel up a lithium battery, fill your boots, there are no failure modes (as far as I know). Are toggle switches are used to distribute power to mission critical units or subsystems? I'd be surprised to learn that they are. They may well be used for signal path applications, if so are certainly going to be debounced when used that way. There are also human factors (ergonomics for one) involved with the design of cockpit units and toggle switches do come out quite well here. Consider that our TX units do use toggles, and don't use slide switches... except for the power switch... which is a slide switch... I also imagine that full size systems are rather more strict with routine maintenance than the average RC flier. Apples and oranges.

Snap. A trickle charge of nimh packs will also perform the task of ensuring they are balanced. Trickle charge is very low tech, but it works rather well for nimh.

Well, maybe I can't convince you Gary... but, checking the voltage is the same doesn't avoid the problem of nimhs simply not being electrically suitable for parallel connection. There does appear to be an amount of misinformation out there, unfortunately. Ultimately - you want more current, fit a different cell type, if you want redundancy, fit a failover device. Another route to putting some redundancy in there (and this is cheap and easy to arrange BTW 🙂 ) would be splitting the batteries across the servos, e.g. one battery runs a single elevator plus the throttle, and the other powers rudder and elevator, for instance. To improve this further you could run the RX from both batteries through diodes, which gives the RX true redundancy. The RX draws very little current and this is a really, really simply method of failover (but not ideal for high current, e.g. servos) As to current capability, I think a single AA pack would be ample for the three 645s on the controls on this model. The worst case - but still recoverable - scenario I can think of here is where you have one stalled elevator; the 645 stall current is 2.5A, and one single AA eneloop pack can put out well beyond 2.5A for an extended period of time, i.e. you're well within capability to get the model back down (assuming you can still do so with one bust elevator).