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Redundancy in larger models?

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Jon - Laser Engines09/06/2017 12:37:31
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Posted by Bill_B on 09/06/2017 12:08:40:

I would suggest using a Jeti dual redundant magnetic switch and LiPo/LiFe batteries rather than archaic heavy duty mechanical switches and NiMHi cells.

Edited By Bill_B on 09/06/2017 12:09:04

With more than more than double the failure points of my setup i will stick to my battery thanks. Im also not sure the weight of an rx pack is going to make much difference to a 20lb warbird that already has a full pound of lead under the cowling.

Bob Cotsford09/06/2017 12:58:31
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Since the advent of crimped connectors on rc gear to replace the dreadful soldered joints of the 70's I can only think of two models lost to 'radio failure' - one a definite NiMh cell loss and the other a suspected switch. Now I prefer two batteries and two switches even on 120 size models.

While this may have more failure points, the 'point' is that the redundant paths mean that a failure in either path is not critical, and the failure can be found and fixed without the need for a spade as would be the case with a single power feed.

Andy Meade09/06/2017 13:23:58
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Exactly Bob - the complexity goes up, but the SAFETY of the system also does. "...double the failure points.." - quite right!  ALL of those have to fail i norder to bring the aircraft down, not just one.

Simplicity would be great, but doesn't protect the airframe and other people from the number one cause of crashes (after dumb thumbs) - Rx power loss.

And the "my transmitter has a single battery so what's the point" is a poor argument.  The Tx isn't subjected to engine vibration / bumpy landings / oil and moisture ingress / sat in a cold shed all winter.

Edited By Andy Meade on 09/06/2017 13:45:03

Rich too09/06/2017 13:33:37
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Posted by Bob Cotsford on 09/06/2017 12:58:31:

Since the advent of crimped connectors on rc gear to replace the dreadful soldered joints of the 70's I can only think of two models lost to 'radio failure' - one a definite NiMh cell loss and the other a suspected switch. Now I prefer two batteries and two switches even on 120 size models.

While this may have more failure points, the 'point' is that the redundant paths mean that a failure in either path is not critical, and the failure can be found and fixed without the need for a spade as would be the case with a single power feed.

I agreed Bob. This appears to have opened a can of worms!

I am not sure about electric as the vibration killing the electrics is greatly reduced, but I frequent the FG website and redundancy of any type is pretty much universal.

A lot of guys use three batteries - one for ign - and I cannot understand that, as it means an extra battery and no redundancy on the ign!

I guess, as with many things like this, until you experience failure (which fingers crossed, I never have), there is a reluctance to change.

ps I am also going to stop using the old type switches and will move to the pin flag switches in future.

Edited By Rich2 on 09/06/2017 13:35:27

The Wright Stuff09/06/2017 13:42:45
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I think there is an argument that says that redundancy is not just down to hardware, it's also a state of mind. Duplication through increased complexity acts to improve safety ONLY if:

a) the user fully understands the technical implications of adopting a more complicated approach, and

b) the user does not become complacent as a result of the added hardware.

Otherwise, yes, there are situations where the redundancy is well-intended, but overall risk goes up! Can one honestly say that they will check each battery just as thoroughly as if there was only one? Appreciate this may sound cynical, but I have overheard at the field, a conversation along the lines of:

"Are you going to check the 'X' again?".

"Nah, it's alright, there's a backup anyway"

Jon - Laser Engines09/06/2017 13:59:07
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Posted by Andy Meade on 09/06/2017 13:23:58:

Exactly Bob - the complexity goes up, but the SAFETY of the system also does. "...double the failure points.." - quite right! ALL of those have to fail i norder to bring the aircraft down, not just one.

Edited By Andy Meade on 09/06/2017 13:45:03

If the box of tricks in that jeti setup falls over then its dead meat, and there is still only a single plug connection to the rx. The model can still be brought down by a single point failure and a number of models have been lost due to the failure of a powerbox or similar. At the end of the day there is no totally failsafe system, but adding more and more things to a model is likely to increase the rish of a failure (even if its a non fatal failure), and the only dual system i would use on any of my own stuff would be the setup i mentioned earlier with two completely independent rx's with their own power supply. Then if i loose one in flight i have the other.

in 25 years i have only lost one model to radio failure and it was the rx itself that snuffed it. Most other potential failures are best guarded against with proper maintenance and inspection of all vital connections.

Martin Harris09/06/2017 14:06:16
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Posted by Rich2 on 09/06/2017 13:33:37:

A lot of guys use three batteries - one for ign - and I cannot understand that, as it means an extra battery and no redundancy on the ign!

I think this may be to isolate the ignition power supply from the receiver's - I've seen many reports that ignition systems seem to generate some nasty spikes which can upset a receiver. Losing the engine is an inconvenience - losing the receiver/servo power is a disaster!

MaL09/06/2017 14:51:32
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Two Batteries and two switches here. The simple addition of a diode or a bridge rectifier in each battery line between the battery and the switch removes any possibility of a duff cell pulling the other good battery down. Common the two sides of the slide switches to reduce the possibility of a switch failure. Using Li-Fe batteries allows for the forward voltage drop across the diodes and supplies the Rx with just under 6v. Use diode/bridge rectifiers commensurate with the expected loads. If you don't need to drop so much just substitute single diodes for the bridge rectifiers

Test each battery supply individually at switch on and switch off..its pretty obvious if you have a duff cell in a pack. follow this regime and there is not much that can cause a catastrophic power failure.

Bob Cotsford09/06/2017 14:58:59
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There's only one certainty, and that is that this discussion will reappear at regular intervals!face 1

I've gone the receiver redundancy route on occasion, this guards against signal loss on either receiver but as Jon points out it still leaves a single point of failure in the redundancy control unit itself, whatever form it takes. There is no way of eliminating all risks so I just try to avoid those most likely to occur. In my own experience this has taken the form of battery and switch failures, though with clever radio systems giving 'low rssi' warnings at unexpected moments my paranoia level has climbed several rungs. Too much knowledge is a Bad Thing.

Robin Colbourne09/06/2017 15:23:01
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When you consider switches, its not hard to see why they are the weak point in the system. The receiver and battery are generally wrapped in foam, the servos are mounted on flexible grommets, whilst the switch is more often than not hard mounted on the side of the fuselage. If this is fibreglass or plywood, then its going to resonate with the engine, and vibration is what kills electro-mechanical parts, not to mention all the oil, dirt and moisture that get into switches as well. If the switch is being vibrated, then the contacts could be arcing and sparking too, which is going to cause them to get hot and lose their springiness. In fact its a surprise that they last as long as they do.

An internally mounted switch in a soft mount, with, say a piece of fishing line through it to turn it on and off, would go a long way to improving its reliability.

A twin battery/twin switch set up does make sense on any model which can cope with the additional weight.

Engine Doctor09/06/2017 16:00:51
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Posted by Martin Harris on 09/06/2017 10:56:20:
Posted by Engine Doctor on 09/06/2017 09:26:09:

Having lost one expensive model a few years ago to battery (one cell in the pack failed ) failure I now use a Two battery and switch set up for added security . Not a lot of weight in a big model and doesn't affect flight performance I understand other post who choose to use a simple set up and use the KISS principle but for me any larger model with considerable investment in time and money I use and recommend a redundant RX power supply.

Telemetry saved my Chipmunk in similar circumstances - a cell failed in flight and a low battery voltage warning prompted me to make an immediate landing. There are many who try to tell me that telemetry is just a gimmick...

Hi Martin I don't think telemetry wouldn't have saved my model, the battery showed good voltage prior to flight and at the postmortem. Only when tested with any sort of load did it switch off completely ! Prior to the flight it worked with load.

The plane just suddenly glitched while in knife edge flight then nothing . Fortunately the rudder blew back sufficiently to let it descend into the next field .A second back up battery would have saved the model .So all my bigger models now have dual battery and switches with diodes fitted in the leads to stop a duff battery draining the good one .

Tim Kearsley09/06/2017 16:18:49
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As the creator of this thread, can I just say a big "thank you" to all those who have contributed. There's much food for thought here! Having worked for a long time in I.T. and having spent a lot of energy in looking at disaster recovery and single points of failure I can appreciate the points being made. Eliminate one single point of failure and you move on to the next. I think in any R/C installation there will always be a single point of failure, be it a mechanical switch, a battery or a piece of electronics. I think the skill is in deciding where the elimination of a particular risk is worth the expense in doing so.

Again, thanks.

Tim.

Ultymate09/06/2017 16:24:22
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Haven't read the full thread, but no one seems to have mentioned "failsafe switches" ie switches that should they fail do so in the "on" manner

Matt Jones09/06/2017 17:03:55
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I did, smart switches. Probably the result of not reading the full thread.............

Andy Green09/06/2017 17:08:24
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Mechanical switches are point of failure, and with so many fakes and poor ones out there I have over the past few years moved away from mechanical and uses these Hall effect ones. Rated at 20A and fail to on, they do give peace of mind. That said I've never experienced a switch fail.

Andy

Bob Cotsford09/06/2017 17:27:43
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Supposed "heavy duty" switch fail

exit jabberwok.jpg

Edited By Bob Cotsford on 09/06/2017 17:29:20

Chris Walby09/06/2017 17:33:31
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So not withstanding Martin's setup where the telemetry monitors the batteries, hence you actually know something has failed in real time.

How do you know with twin battery and twin switch that one is not faulty, okay test it on the ground, but once in flight?

Think of it as links in a chain...the more links the more likely there will be a failure, having two chains in parallel are only a benefit if one can take the entire load & you know the other has failed...

Why have a switch on an electric model? When you want to fly battery plug RX supply, proceed to starting area, plug main battery leads to ESC in & carry out pre flight checks?

Edited By Chris Walby on 09/06/2017 17:36:04

Bob Cotsford09/06/2017 18:55:39
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If one goes faulty in flight it doesn't matter - you should pick it up the next time you switch on. You would of course always try each switch in turn before each flight! As for electrics, my Curare for example has a 2S LiFe pack powering the radio. If the esc fails I can at least glide it in somewhere safe.  The rx and it's battery are out of reach of the flight battery bay.

Edited By Bob Cotsford on 09/06/2017 18:56:51

Martin Harris09/06/2017 20:00:58
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Posted by Chris Walby on 09/06/2017 17:33:31:

So not withstanding Martin's setup where the telemetry monitors the batteries, hence you actually know something has failed in real time.

How do you know with twin battery and twin switch that one is not faulty, okay test it on the ground, but once in flight?

Think of it as links in a chain...the more links the more likely there will be a failure, having two chains in parallel are only a benefit if one can take the entire load & you know the other has failed...

Why have a switch on an electric model? When you want to fly battery plug RX supply, proceed to starting area, plug main battery leads to ESC in & carry out pre flight checks?

The whole point of a 2 battery / 2 switch set-up is that they are running in parallel and give true redundancy in that losing any one component will not result in failure of the system. Telemetry is the icing on the cake but pre-flight checks give a high level of security.

I wouldn't trust an expensive or valued electric model to a BEC - too much to go wrong...just last year I was flying a friend's EP Extra when it went dead stick (yes it happens!) - as I touched down, all control was lost and it rolled to a halt. The ESC had failed (comfortably over-rated BTW) and got so hot that the solder on the battery lead terminations had melted!

Allan Bennett09/06/2017 21:17:43
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Since the OP is talking about an electric model, I'm a little confused why there's so much discussion about switches. None of my electric models use any switches other than the main battery power plug. These days I fly many of them with twin stand-alone BECs, or the BEC in the ESC plus one stand-alone BEC, connected via Schottky diodes so they don't interfere with each other. LEDs confirm that both BECs are providing power and, in the event of failure of one, the other will continue to supply.

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