Building a Sling TSi in Under 8 Seconds

[Skepilot]

I made short video of the building of my airplane, "One Alpha Mike," a Sling TSi. About a year's worth of work consolidated into less than 8 minutes. Many thanks to Evan Brunye for expert Build Assist!

[Eric Page]

Wow, that's a very nice plane -- congratulations on completing the build. Your paint and interior are beautiful, and I loved the signatures by your wife and daughter for their rivet pulls!

That said, I have a concern about your aircraft. You're probably not going to like what follows, but I pray that you'll take it in the constructive, safety-minded way that it's offered.

At 3:48 in the video, I count 36 relays on three modules, with wires that seem to be terminated by inserting bare wire into screw-type wire-to-board terminal blocks. If the bulk of your electrical system is wired through these relay modules, then I would advise you in the strongest possible terms to re-think your wiring scheme.

The screw-type terminal blocks used on those boards are notorious for loosening due to temperature cycling and vibration, and they provide zero strain relief to the wires where they exit the terminal. They're intended for use in static industrial control cabinets, not in vehicles, and certainly not in aircraft. Aircraft wiring is normally done with crimped connections that include an insulation grip; either the FASTON type or ring terminals secured with a lock washer and nut.

The Songle SRD Series relays on the red relay boards shown in the video were rated at 7A for resistive loads and only 3A for inductive loads (i.e. motors). An old version of the SRD datasheet (here) included those DC ratings, but on the current version (here) the DC ratings have been removed. Why is that? Did Songle remove the DC ratings in response to reports or tests showing poor performance switching DC loads?

Most alarming is that a couple of relays on the same board appear to be labeled "PUMP 1" and "PUMP 2." If those are your fuel pumps, then I would absolutely not fly this plane until you get the power to those pumps re-routed. If the deleted DC ratings on the relay datasheet and their low inductive load rating aren't reason enough, then consider this: Each of the shared connections on the relay boards (i.e. DC+ and DC-) are single points of failure that can take down multiple systems. If either of those wires comes loose, every relay on that board will switch open and you will lose both fuel pumps simultaneously. This alone should terrify you! The same goes for the engine's ECU and Fuse Box, if they're similarly wired.

Using all of those relays has introduced multiple unnecessary points of failure. There are fourteen connections needed to power a single device: three at the bus (device power, control circuit power, relay board DC+), two at the switch (not counting illumination), five at the relay board (DC+, DC-, control signal, power in, power out), two at the device being switched, and two at the ground point (device ground and relay board DC-).

Removing the relays would reduce the connection count to six: one at the bus, two at the switch, two at the device and one at the ground point. It would also eliminate the risk of terminal block screws loosening due to temperature and vibration, remove the failure risks associated with inadequately rated relays, delete the unnecessary (and possibly fragile) opto-isolated control circuitry on the relay boards, and remove multiple single points of failure.

Generally speaking, there's no good reason to include that many relays in the electrical system. Relays are great for controlling a large current with a small one (battery master, starter motor), but almost none of the other switching tasks in a light aircraft are difficult for an appropriately rated toggle or rocker switch to handle.

It appears that you may have been driven to use relays by your choice of low current pushbutton switches with LED ring illumination. If you want to stick with that kind of switch, there are some options that might be suitable. A quick search turned up two manufacturers with relatively high current DC-rated metal pushbutton switches with blue LED rings. The second one would probably handle any load in your plane.

Bulgin MPI005 Series (5A @ 12VDC)

Switch Components PD Series (10A @ 14VDC)

Best wishes, my friend. Fly safely!

[Skepilot]

Thanks for the reply. I probably shouldn't have included that particular pic in the video. That was an early prototype for testing. System has since been changed, addressing all concerns. Relays are for dimmers on the switch lights, and there's a redundant relay bank behind the one you see. All wires are connected with strain relief. As for the switches themselves, they don't carry much load as in most cases they're just sending a signal to the VP-X .

[Eric Page]

Excellent! Sounds like you've got it all sorted out. Congrats again; hope you enjoy the plane!