Smoke Pump Controller

Obviously I have too much time on my hands.

While reading some posts on Van’s Air Force about wiring up smoke pump control switches, I thought there must be a better way than most of the ideas presented. I know if I were putting smoke in my airplane (and I fully intend to do so), I’d want an ARM switch and a pushbutton on the stick or throttle. What I would NOT want is to have to keep my finger (or thumb) on that switch all the time.

So, I designed a little circuit board with an ultra simple microcontroller to act as a multi-mode switch. It does everything I want it to do…

  • An indicator that uses a bi-color LED to show the ARMED / ON status
  • On/off toggle action with a simple “click” of the pushbutton
  • Constant ON mode while the pushbutton is held
  • Sufficient to control a 10 A pump with 100% duty cycle
  • Easy and intuitive to use
  • Simple and inexpensive to produce
  • Small and light weight, easy to mount wherever there’s space

The switching MOSFET was really the only part I was concerned about. I contacted Marvin at Smoke System Helper, who helped with testing to validate that it will drive a 7A pump constantly with no detectable rise in temperature. All the parts are rated for 10A or more, and the MOSFET has a ridiculously low Rds(on), less than 4 mOhm at the 5V gate drive voltage used. It’s rated for 160A at 30V, so it’s snoozing. The max current limit is actually imposed by the terminal block.

Overall I’m very pleased with the result. The case is not perfect — no mounting tabs — but really, the thing weighs so little that some double-sided foam tape will hold it in place, maybe with a zip tie for high vibration environments. My plan for the Celebrity is to tuck it behind the panel.

Spar webs

The plans and BOM call for plywood spar webs, cross-cut, 60″ long. The webs are butt-spliced mid-wing, with what seem to me to be fairly narrow blocks of spruce on either side.

Problem is, you can’t buy 60″ wide sheets of aircraft plywood in the US. Aircraft Spruce has ONE size, and not a size I need. Everything else I’ve found is in 48″ wide sheets. And since the web needs to be crosscut, it’s not as simple as just buying a 4′ x 8′ sheet. I’ve contacted Fisher Flying Products as well as another supplier in Canada. Fisher’s tab will be roughly $340, the other place was pretty reasonable for the plywood but wanted over $500 for crating and shipping. To put that in perspective, ACS will sell me the plywood I need for about $100, delivered.

The question is, can I make 48″ long spar webs work in a way that is at least as good, or preferably stonger and better, than the original design? I am no engineer. Especially a mechanical engineer. I’m certainly not qualified to make such a call. Fortunately, I have someone local who is an engineer, and is qualified to help make that decision and recommend another way to do it — if there is one. I’ll be taking the wing plans to the February EAA chapter meeting to go over them with him and see what we can figure out.

If that doesn’t work out, Spruce does sell 5/32 ply in 61″ x 61″ sheets. The spar webs are specified as 1/8″ for the main spar, and 1/16″ for the rear spar. I could just make both webs 5/32 with a slight weight penalty. The plywood would weigh 3 to 5 pounds more, but I’d probably also need to trim 1/32 to 3/32 from the spruce stiffeners, so that might make up for some of it. Again, I’ll need to consult with a real engineer to see if that wold work or not. Ending up with a plane that’s a couple pounds heavier but significantly stronger is OK. Ending up with a plane that is in any way less strong or less safe is absolutely not. And if the ribs need to be modified a little to clear a beefier spar — I’m OK with that.