Arming switch RevA.docx

Tim’s Tips

Tim Wilkerson

Arming Switch Strategies

Ed note:  this is a revision to an earlier article with some new info at the end.

In the electric arena, new safety considerations come into play as compared to glow powered planes.  One such area is the use of an arming switch.   The goal of an arming switch is safe motor operation so that in no case can the prop engage unless at a flight station. 

An arming switch is a means to disable the motor from outside the aircraft and independent of the ESC or transmitter.  Motor disarming is important since, when the motor battery is connected to the control system, the motor can engage quickly just by a bump of the throttle stick.  This is a safety hazard in the pits and why most clubs have a rule to arm only at the flight station.  An arming switch is a definitive and convenient method to accomplish this end.  In this case, the operative term is definitive because it relates to just how safe the system is.  Other methods can accomplish a similar end but depend doing things in a sequence.   

As the size of electric aircraft increase, the power also increases and the use of an arming switch becomes even more important.  Power levels in the 2000-3000 watt range are not at all uncommon these days.  This is a lot of power to have at our finger tips.  While this power level makes for a nice flying bird it also presents a significant injury risk so precautions are simply necessary.

To be clear, the ESC also has a motor arming function built in although this is less definitive than a separate arming switch because it is still under throttle stick control.  On Castle ESCs for example, if you plug in the motor battery with some throttle on the control stick, say half or full throttle, the motor will not turn.  However, once the throttle is brought to the idle position, the ESC beeps in a certain way which indicates the motor is armed and the motor will be engaged by further throttle movement.  This is a good precaution, but still not a fully safe solution because the control stick can still make the motor turn.   Other current day ESCs have a similar arming feature although the beep sequence varies from vendor to vendor.  Using this ESC safety feature can accomplish the motor disable end, but it is not a definitive solution in and of itself so it is not as safe as a separate arming switch. 

Procedural methods can be used to accomplish prop safety.  If the motor battery is conveniently accessible, then the pilot can choose to only plug it in when at a flight station which achieves the safety requirement.  In this case an arming switch is not strictly speaking required, but it remains a good additional precaution.   This safety method requires that the pilot execute the same procedure each time and not get distracted.  In a single battery case this means that both controls and motor become active only at a flight station. 

Use of a dual battery system gives a little more latitude as flight controls can be verified in the pits with the use of an RX battery alone.  The separate RX battery is inadequate in most cases to power the motor so safety is not sacrificed. 

Overall, our goal is safe motor operation and there are a few methods to accomplish this.   The first consideration is if the battery powering the motor is easily accessible without aircraft disassembly.  In this case I would consider disassembly to mean taking off the wing, not just opening a hatch. 

An arming switch is required if aircraft disassembly is required in order to get to the battery powering the motor. 

1. Single battery setup, arming switch required.  With arming switch disengaged, the battery can be plugged in when in the pits.  Assemble aircraft.  When at a flight station, plug in the arming switch and fly. 

2. Dual battery setup, arming switch required.  With motor arming switch disengaged, plug in the RX/servo battery and the motor battery in the pits.  Assemble the aircraft.  When at a flight station, install the arming switch and fly. 

If aircraft disassembly is not required to get to the battery powering the motor, there are a few more choices. 

1. Single battery setup, no arming switch required.  Use the inherent battery connector and only plug it in when at the flight station.  Inherently safe. 

2. Dual battery setup, no arming switch required.  Optionally plug in RX/servo battery in the pits, check control surfaces.  Proceed to flight station.  Only plug the motor battery in when at a flight station.  Inherently safe.

3. While an arming switch is not strictly required in these two setups, an arming switch can be used as an additional layer of protection.   Only plug in the arming switch when at a flight station.

Ok, so we’ve shown that an arming switch is not strictly required as long as good procedures are followed.  However, an arming switch is never a bad idea and its use provides an additional safety layer.  Following are some arming switch implementations. 

Commercially Available Arming Switch

Below is a picture of a commercially available arming switch supplied by Maxx Products/RS Deans.  Both 14GA and 12GA units are available and rated at 30A and 45A respectively.

http://www.maxxprod.com/mpi/mpi-21.html#Harness

Maxx Products Arming Switches

Ok, arming switches are good, but 45A hardly covers all the models out there.  I have seen speed controls up to 160A. 

It seems we are on our own above 45A. 

An alternative, Sung Switch

I was talking to one of the pattern guys, Kevin Sung, and he showed me an arming switch he made using an automotive fuse.  In addition to acting as a conventional arming switch, the use of an automotive fuse adds an additional level of protection should an overcurrent occur from an ESC or motor failure.  In this case, the fuse size can be matched to the current level needed for a particular power system.  That is, if you have a 50A system then you can use a 50A fuse when you build the arming mechanism.  From here on, I’ll call this an arming fuse because it also contains a fuse and to distinguish it from one with a wire capable of full motor current indefinitely. 

I like this idea because it does two things at once, first as a motor arming mechanism and secondly as a safety fuse in case something bad happens to the motor or ESC.  This is a very effective dual purpose safety mechanism.  I like it so much I built one for my 71” Slick and my 51” Slick.

Sung Switch Construction Details

Following are some pictures of Kevin’s arming scheme using an astro flight zero loss connector pair and an 80A automotive fuse on his 10S Neptune. 

Below is a pic on the outside of the plane, installed and ready to fly.  He put some Velcro around it to help in storage. 

Kevin’s Installed Arming Fuse

Below is a pic on the inside plane install, very clean job.

Kevin’s install in his Neptune

Here is the arming fuse itself. 

Kevin’s Arming Fuse, 80A Fuse

Kevin also uses a transmitter tray and incorporated a holding station for the plug in the transmitter try.  A nice safety reminder that the holding spot should be empty when getting ready to fly, and also that the holding station should be full when the plane is in the pits.  This scheme also keeps from losing the assembly and having to dig in the toolbox to find it. 

Ok, so that’s how the Sung Switch works and is installed, now let’s look at the parts to build one.

Sung Switch Construction

Here is a picture of the Astroflight connector pair as it comes out of the package.

Astro Flight Zero Loss Connector

Below is the fuse I bought at Kraegen for $4.  This is sized for my 51” slick as it pulls around 50Amp peaks.  This stock fuse is for a panel mount application, note the large copper flanges with hole for a screw down.

PAL Panel Mount 50A Fuse in Housing

Below is the copper fusing element after busting off the housing of the panel mount unit. 

Actual fusing element

Other types of fuses could be used such as those with spade lugs (0.25” quick connects).  Actually how the fuse is packaged is of little concern as we will remove and discard the packaging and extra copper.   

First, here is a picture showing the 100A fuse element for my 71” Slick, 90A static with 18x8 Xoar. 

100A fuse element

Following is a picture of the parts using a 100A fuse and the additional wire intended for my 71” Slick.  I used some 12GA solid copper wire to act as extensions between the female connector and the fused element as it fits nicely into the connector barrel.  I looked up the outside diameter of 12 GA wire and it fit nicely into the barrel, so I went to Home depot and bought 1 ft.  They thought I was crazy. 

Assembly components with 12GA wire

Mid Assembly with trimmed fuse

100A final assembly

I don’t know why but my camera had trouble focusing on the black finish, but here is the completed arming fuse for 100A. 

Below is the completed arming fuse installed in my 71” Slick.  Works well. 

Arming fuse in Tim’s 71” Slick

I use a flight pad for my TX which supports the TX at two points instead of only one like using a lanyard.  I find it to be more stable that way.  You can see my arming fuses attached with velcro to the flight pad below.   The 51 at 50A is on the left and the 71 at 100A is on the right.  Guess I should label them. 

Tim’s Flight Pad with Arming Fuses

Still another alternative,

Middleton Switch

Jim Middleton has a scheme using a high current FET (Field Effect Transistor) to act as the switching element.  There are a few advantages to using a solid state switching scheme like this as opposed to  something mechanical. 

Advantages

1. Light duty switch.  Since the FET carries the heavy current, only the much smaller gate current needs to be switched mechanically.  So, the on/off switch is relatively light duty even though it is controlling the high motor battery current. 

2. Eliminates sparking.   Any sparking from the high current source is eliminated because this is a solid state switch. 

Disadvantages

a. Local power dissipation.  On the flip side, there is a bit of local power dissipation when the FET switch is on.  In a 50A application with a 3 milliohm FET dissipation is 7.5w (I*I*R= 50*50*0.003), not bad.  For a 100A application the dissipation is 30W because the current is squared in the power calculation, and a bit high for my taste.

Jim said he will do an article on his scheme in the near future, but here are some advanced details. 

Below is a picture of the Middleton Switch, note the rather small switch in the upper right hand corner and the skinny wires connected to it. 

Middleton FET Switch

The FET is mounted on 0.093” aluminum with kapton film as an insulator with the beefy, high current carrying wires.

FET side close up

Back Side Wiring

With Heat Shrink Tubing

Nicely packaged. 

Below is the schematic.

Middleton Switch Schematic

Rev A Material

Connector Migration

Recently I’ve migrated away from using Dean’s on the bigger stuff since these connectors top out on current at about 60A.  Another motivating issue relating to arming switch construction is that Astro Flight has significantly increased the cost of their connectors from about $6 to close to $20. 

In looking for a suitable replacement I asked some of my heli friends and then started using Eflight’s EC5 connectors.  These connectors are good for a current level of something like 200A.  The motivation is simply so that the connectors are NOT even close to being the weakest link in the chain.  Here is a pic.  Aero Micro has them at $8/pair locally.  www.epbuddy.com has them even cheaper at $2.50/pair plus shipping from Ohio.   

EC5 connectors

I like the construction of these connectors.  The wire goes into a solder cup which is then filled with solder.  This provides more mechanical stability than the wire to tab connection on the Deans. 

I was expecting around 85A on the Velox using an 18x10, so I initially chose a 100A fuse for arming. Picture is below.  After reading a bit more, some folks were getting too short flights using an 18x10 and were migrating to an 18x8.  So, I did the maiden with this smaller prop and measured only 65A static.  So I’ll make a smaller, maybe 75A arming fuse when I get a chance and use the 100A in the interim.  It’s plenty fast and good hover granularity for now with this bird.

100A Arming Fuse Installed in Velox

A Bit More on the Receptacle Construction

First I constructed a harness which contains the arming receptacle to be mounted in addition to free floating connectors on both ends. The polarity of the free floating ends are such that this harness can be easily added or bypassed, one male and one female. 

To secure the arming fuse receptacle, I use a three layer plywood scheme since the forces for insertion and extraction are not exactly light.  This also helps on side force since these EC5 connectors are sort of long and you can exert some leverage.  Of course, the three plywood layers need to have a hole for the receptacle that all coincide. 

The first layer will be against the covering and is cut to fit an already existing open bay in the side of the fuse. Use material that is of the same thickness as is on the plane.  Install this plate using CA without the receptacle;  I use some thin then back it up with some thick. Ensure there are no CA bumps at this level.   If your plane has a solid side, only use layers 2 and 3 as described and cut the hole in the solid side for the receptacle. 

The next layer (2) is larger so it overlaps the glue joint of the first and extends to the fuse formers to the front, rear and to the top as well.  The third layer is smaller than layer 2, but provides good area surrounding the receptacle when glued to layer 2. 

Putting it all Together, Final Assembly

Regarding assembly process on layers 2 and 3, I load them both on the receptacle to start.  Then push the receptacle through the outside plate and glue flush to the outside of the fuse.  Follow up by sliding layers 2 and 3 toward the outer layer with plenty of CA in between.  Certainly, all the glue joints are free flowing at the same time, so just get the receptacle square and flush to the outside as well as all layers together and then leave it alone.   I let it cure naturally as it seems to me to result in a stronger bond than using kicker.  Allow to dry thoroughly.  Finally, I put a fillet where each layer meets anything else to help to spread the load.  Once dry, this scheme holds the receptacle in a very solid way. 

I put some Velcro around the completed arming fuse as a holder to attach to my flight pad. 

While use of an arming switch/plug is not the only method to achieve motor safety in electric aircraft, it is a definitive method.  Also, its use can increase the safety of any electric aircraft regardless of any other precautions taken.   We can never be too safe especially as the power levels keep going up. 

Happy flying