sfaust

My Feedback: (11)

Dual revceivers? Dual batteries? Should I run 6v or 4.8v. Do I need a battery backup device? What about an Isolator? Should I increase the wire size on the leads? Will all digital servos exceed the power handling capability of my receivers bus? Will a Y connector be ok to split my two rudder servos on one channel?

Questions? This thread might help.

As the result of a couple other threads, I have been on a mission to validate some long held convictions on how to setup a 40% aerobatic plane, and the issues involved with adequate power, redundancy, reliability, 4.8v or 6.0v systems, the receivers ability to handle the power of all digital servos, the use of battery backup devices and isolators, and so on.

I had my own options that were formed over the years based on information I have heard, airplane configurations that I have seen fly successfully season after season, my own limited background, and the benefit of a close friends background who graduated from MIT with an electrical/engineering degree. But, for the most part, much of the details escaped me.

Recently, I posed various questions to a lot of different individuals with various backgrounds, some from the manufacturers, some from known and respected giant scale pilots and builders. The following is a summary of how to effectively setup a safe and reliable 40% sized aerobatic airplane. While there may be differing opinions and various pieces of this, this is at lease backed up with solid electrical engineering practices. Further, I have seen airplanes that have not be set up this well, and violate many of these practices, yet fly season after season without issues. It would then be safe to assume this is a conservative, well engineered, and very acceptable solution to setting up the larger aircraft.

The following is a summary of the recommendations. Most of this came from Simon Van Leeuwen from the Giant Scale and IMAC mailing list. It was derived from a question I posed to him regarding the ability of a single receiver to handle the loads of the larger aircraft running all power hungry digital servos. Much if it was collected from various posts over time, and general concensus of inputs from many giant scale pilots. While Simon admits to putting all on a couple pages in one place, he acknowledges many of the other participants as providing a lot of the input over time.

It is broken into a couple parts, because he only had a few minutes , and his 1600 words would not fit in one reply as limited by RCU. I thank Simon for taking the time to respond to me on this issue, and for the detailed response.

sfaust

My Feedback: (11)

Here is part one of his post (with my lead in). Some Dos and Don'ts.

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Ok, some feedback on the power bus issue, isolators, etc. It confirms what a few of us have been saying here, but goes into great detail on why, and backs that up with electrical engineering experience. We don't have to say, "It's works because I have seen airplanes fly that way" anymore.

I received lots of replies and information to the request on the IMAC and GiantScale lists. One was from Simon Van Leeuwen, whos is very knowledgable, and very respected on those lists as 'knowing his stuff' in this area. I included his reply here (with his permission), however, for easier reading I moved his summary on what we should be doing on our larger planes electrically here, and put it under the summary heading. The rest is added below under refereence. In his post to me, and the lists in general, this was reversed. However, I feel most of us will want the 'meat' up front, and for those that want to know the 'whys', the can read the reference section.

It does pretty much say that burning up a receiver bus is a non-issue. Power splitting is not required for a single receiver 40% airplane with digital servos. Coupled with the response from JR's head research and development person (via a reply from Mike Hurley), that their receivers can easily handle the load even if you loaded all channels up with digital servos, a 40% airplane with a single receiver, dual batteries, and heavy duty switches and wiring is safe and reliable. Further, using dual 6v packs reduces the occurance of depressed voltage, adds reliability, and these packs do not need to be regulated. Going with dual receivers increases the overall reliability, reduces the depressed voltage issue, adds additional redundancy, reduces the number of connectionsand Y adapters, increases the available voltage to the effected servos, and cross wiring the packs at a heavy duty DPDT switch adds battery redundancy for cell drop outs and pack problems. It also confirms my crude testing that one discharged pack will not draw enough current from a fully charged pack to cause any issues during multiple flights. This should be picked up between flights with an ESV anyway.

There are a few changes that I will be doing in my planes based on this. While I feel I had much of this covered, I have left some holes here and there. I know I will be doing much more soldering, and making less connectors. And I'll dump the dual receiver bus jumper, and go with the DPDT cross wired battery packs.

Anyway, here is the gist of his post

Summary
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- Use two 5-cell packs through 2 quality DPDT self-cleaning slider-type switches (which have been internally paralleled) in parallel to power the receiver. Use position "1" and the last (usually BATT) on the RX to place power at either end of the buss, or more conveniently any open (unused) O/P as it's not that important.

- DO NOT use toggle, pushbutton, pin/jack-type, magnetic, etc type switches on RX or ignition. Again, the saying "well it has worked for me" just means not if...but when it will fail. Toggle switches in particular can vibrate to the point where they are making/breaking contact, and cause major EMI/RFI and the associated headaches for aircraft and owner alike. why risk it?

- On systems utilizing 2 RX's, one can purchase HD DPDT switches which would allow each battery to be cross-wired to each RX. This would allow both packs to run both RX's. You will have to add separate charge leads though. This redundant process has 2 battery packs operating through 4 separate switch contacts (within 2 switches), through 2 RX's, which then allows pairs of servos to be operated off of the same channel.

- use 5-cell packs to increase voltage. Increase pack A/Hr rating by 25% to address the extra current consumed as a result. Using 4-cell packs on large-scale aerobats invites RX drop-out from depressed voltage, and pretty well ensures a crash if (in the highly unlikely event) the onboard system experiences a single-cell failure (you are using 2 RX packs in parallel aren't you???). BTW, a single cell failure in 1 pack of a 2-pack system will have little consequense other than (hopefully) you discovering reduced control surface "zip". I'm getting tired of those uninformed people out there who insist that a cell drop-out is gonna somehow discharge the 2nd "good" pack while airborne! Do yourself a favor and DITCH any devices which claim to have smarts to decide whether pack "A" or pack "B" should be supplying power to the RX based on O/P voltage. Configure both packs in parallel, straight into the RX. If you are one of those who believes that somehow operating consistancy due to a regulated voltage is an imperative, then employ regulators. Apparently there are still some RX's out there that do not like unregulated 5-cell packs, especially fresh off off of charge, these would be candidates for regulators as well. Oh ya, use linear-type (not switching - electrically noisy) regulators that can supply adequate current to meet your needs without coming close to thermal cut-off. If you do not prescribe to either of these theories, than don't use regulators!

- DO NOT add electrical gadgets, for the sake of adding electrical gadgets! The most efficient and trouble-free electrical systems world-wide...are the simplest.

- Get rid of as many connectors as possible! This is "really" important. The electrical losses through connectors is significant. Using OEM extensions for convenience's sake is compromising the performance of your onboard electrical system. It's that simple. Long extensions should be soldered at the servo end at the very least, and remain unbroken all the way to the RX.

- Ganging (paralleling) 2 servos and expecting some skimpy OEM extensions or "Y" to supply adequate power is inadequate. Signal and power will be split, with the resulting decrease in operating performance. The lead material upstream from where the servos are "Y'ed" needs to be able to adequately handle the potential current loads of both servos. Don't skimp!

- If required, the backside of the power buss (within the RX) can have 22-24awg buss wire soldered directly to the bottom of the O/P pins, more than doubling it's ability to pass current. Never use solid conductor for anything other than this though.

- Keep all extensions a short as possible

- The use of twisted cabling increases RF headroom at the RX. More headroom is possible if properly grounded sheilded cabling is employed instead.

- Plug-and-play type connectors that are cycle-rated that allow you to plug/unplug stabs and wings without touching or manipulating any cabling or connectors adds integrity, longevity, and safety.

sfaust

My Feedback: (11)

Here is part two, with a summary of his recommendation on setting up the electrical system.

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Here is what I would consider a robust system for a 40% aerobat on digitals that sees a lot of aggressive flight maneuvering:

I would start with two 5-cell NiCD packs with at least 4/3 A cell size (you choose the capacity from there based on flight duration required). This offers adequate internal reactive surface area within each cell to at least offer power that the system may potentially consume. The elevated operating voltage will negate RX drop-out (~3.5-4V) and allow a respectable voltage to be present at the most important point...the servo.

Each of these packs would then have 3 pigtails, 20awg. One for charging, and two that would be soldered and strain-reliefed to the I/P posts on one half of each of 2 DPDT quality slider switches. The O/P side of both switches would have 2 pigtails 20awg, one would go to each of the 2 RX's that have been employed. At this pont I might consider soldering some buss wire to the backside of PWR/GND within the RX itself. This removes all doubt as to whether the RX itself will handle the extra current that will now flow as a result of upgrading the entire system. Next I would make extensions from twisted 22awg, making sure to twist all of the leads going to the same physical location (say the tail), so they form one bundle. I might consider using specialized shielded (or
double-shielded) cabling that enhances EMI/RFI suppression.

These extensions would utilize connectors with spring contact material identical to the header material used on the RX PCB. These aftermarket spring contacts would be configured to ensure that as much contact area between the RX post and the spring contact itself was achieved, thereby further reducing series resistance. At the end of each of the extensions, I would directly solder the servo pigtail (cutting off the existing OEM connector close to the servo case). If the servo happens to be mounted in a removable portion of the airframe (eg wing, stab), I would employ a high quality, low resistance, high cycle-rate drawer-type connector that would allow automatic PWR/SIG hook-up. This elliminates brain-fade, increases electrical system reliablility (no physical manipulation), and therefore airframe longevity and safety!

Up to 40% aerobats power or opto isolation is not a valid requirement in my opinion. Two RX's have demonstrated time and time again viability to minimize paralleling (Y'ing) servos, enhancing power distribution, and redundancy. Pretty simple actually, as the majority who already do this...already know. I think I addressed all your points Stephen, if not let me know...

sfaust

My Feedback: (11)

And here is some background information to support Simons recommendation

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A couple of other readers pointed out some valuable information, first that the perceived problem is overblown, and secondly that the measured voltage at the RX buss lands/traces themselves under load before even reaching the servo(s) is depressed. To what degree depends on what gauge and length of lead, switches, and/or connectors are present between the RX and the battery. The result is that actual current present during the most extreme loading is not only lower than anticipated, but also finite in duration.

It is clear that indeed 1-2 RX's can address electrical loads in 30-40% aerobats, as few (if any) failures have been (publicly) documented. The act of setting up test parameters is a bit sticky. Without some hard data to show just what max/min/avg loads are present during a typical(?) flight for a given size aerobat, opens up a can of worms. Every aircraft is different, and it is these significant differences that would only allow a ballpark figure for people to gauge whether or not they should have 1 or
2 RX's, or some other form of power/signal distribution system.

As far as testing to destruction to determine current-carrying capabilities of a typical receiver, this will only confirm what can be extrapolated from existing data which is used by myself and other PCB designers to determine trace width and thickness. Add to this the fact that I could say "I can extract 100 amps from any RX", and you'll see what I mean. Yes, I CAN push 100+ amps through any RX...for about 20 milliseconds. I could also push 30A through any RX, for about 600 milliseconds. The information is meaningless.

Most modern RX's I have seen have power/GND buss traces both top and bottom, and are ~0.100" in width and most likely a 1/2oz copper pour. After plating, this can increase to up to 1oz which equates to ~500 sq/MILs of conducting surface area between both upper and lower buss traces...if they are indeed plated.

For our purposes, lets say the traces are not plated and stick with a total MIL area of 120MILS (2 x 60 for upper/lower traces). So with no plating, 120MILS of copper equates to ~5 amps @ 20C. This means it will handle 5A all day without an increase in temperature. Unfortunately, given the substrate material (the actual printed circuit board), is actually acting as an insulator, we need to derate to say 4.5A. Given a typical RX board's dimensions, and learned observations regarding heat dissipation for a board of this size and mass, one could expect to be able to pass 5 amps continuously with only a small increase in buss trace temperature. Say ~30C, or 6A @ ~45C, or 7A @ ~60C, etc.

As you can see, this is pretty good, but is it good enough? Who knows...and who cares? OK, I do...sorta. Although I could modify a LoLo to measure current, then download the data after landing, what would I do with all this dynamic info? It's only going to change on the next flight, and the one after that, and...well you get the point. One flight could consume current that when average trending was applied the value will be towards the top of the system's limit, while another more docile flight that would see trending towards the lower end of current consumption.

All I can say for sure is that as the level of current consumption increases "PAST" the nominal limit (if we use the info above, then % amps), so does the RATE at which heat is developed. What level of heat is acceptable? I would suggest ~60C+. After this the PCB itself and surrounding components begin (notice I said begin) to become affected.

So, at what point in a 40% Fantasia XX345's aggresive flight maneuvering would this occur? It's not that simple:

a) not all servos simultaneously, or any for that matter, are drawing anywhere near their rated power.

b) Another reader (correctly) pointed out that similar to water pipes, the total system will only pass current relative to the weakest link, or more accurately circular MIL area before significant heating takes place. Heat is lost electrical energy BTW.

c) Although the cells within battery pack(s) can pass significant amounts of current (single cell NiCD AA=50A+, NiMH AA=40A+, LiXX 18650=significantly less), the spot-weld tabs between cells are lossy.

d) From a servo's point of view, or worse 2 servos operating from a single RX O/P, see only a shadow of the pack voltage and control signal O/P. Here are some reasons for this inherent series-resistance:

- OEM (original equipment manufacturer) connectors. These are rated for ~2A, and are not meant to handle lots of cycling (the act of connecting/disconnecting).

- Differing pin and spring-contact material between differing brands of OEM connectors causing an electrolytic reaction do to galvanic action when plugged together. This creates deposits which significantly reduces conductivity

- inadequate size (gauge) of electrical lead employed over significant distances

The electrical losses within large-scale aerobats are significant enough to allow the RX to at least continue to operate without experiencing undo heat-loading. The actual voltage present at the servos will dictate current consumption. More voltage, more current available, more consumption. Here are some tips that will increase electrical performance:

- Current consumption is highest upstream. In other words, use adequate gauge lead material (the term "wire" refers to a single strand in a lead, and the term cable refers to >1 lead) for the expected load. Depending on strand-count in 22awg lead, the circular MIL are can be from 640 to 700, 20awg can be as much as 1216, and 18awg can be as much as 1900. The current carrying capacity of some lead material (expand your viewing window if misaligned) of a single conductor in Free Air at 30C ambient temp):

SIZE Polyethylene Polypropylene Polyvinylchloride
Kynar Kapton Neoprene Polyethylene PVC (irridated)
Polyethylene Teflon Polyurethane (high density) Nylon (cross
linked) Silicone PVC (semi rigid)
Thermoplastic Elastomers
at 80C at 90C at 105C at 125C at 200C
26awg 4 5 5 6 7
24awg 6 7 7 8 10
22awg 8 9 10 11 13
20awg 10 12 13 14 17
18awg 15 17 18 20 24


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Simon Van Leeuwen, Calgary, Alberta
RADIUS SYSTEMS
Cogito-Ergo-Zoom
IAC25233*MAAC12835*IMAC1756*LSF5953*IMAA20209
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