May 19, 2008

May Day Flight

That's May day -- not "mayday!"

A short flight to do some sightseeing and record performance parameters for the little plane:


Aviatrix said...

Your OAT gauge made me laugh. I had the exact same model in my car. What's the green light right below the HI?

Aluwings said...

//Your OAT gauge made me laugh.//

Yep... simple, cheap and also gives inside temp and Time. Such a deal!

//What's the green light right below the HI?//

Could it be the blue light indicating that my "standby" fuel pump is ON? As the engine has no magnetos or mechanical fuel pump, I was faced with some interesting design choices trying to maintain a similar level of reliability but keep it as simple as possible.

For (electronic) ignition reliability I created a "keep the engine running" bus immediately below the panel. It holds, from left to right, the ignition computer channel selector; CBs for the ignition coils; a switchable CB and annunciator light for the standby fuel pump; and two ignition power switches, each wired directly to one of the main batteries (through diodes to prevent back-feeding to the other battery). In case of electrical smoke or other problems I can switch off the Aircraft Masters without killing the engine. If that doesn't cure the problem, then I can switch off each ignition supply in turn... and if that dosen't cure it, I'm faced with switching off the engine etc...

The fuel is contained in two wing tanks. Because it's auto-gas it is doubly important to reduce chances of vapor lock by pushing the fuel rather than pulling it. So there is a "main" pump located as close as feasible to each tank outlet. Ideally I would have had the "standby" pump wired to run full time, but because it is in series with the main pump, this produces unacceptably high fuel pressure (the carburettor limit is 7 psi at the needle valve). So I have to manually switch the standby pump on for peace of mind when I want to balance wing tanks (i.e. switching a main pump off). A tad more complicated than I wanted. Ideally, I'd just have all pumps running all the time (as they do on cars for 100s of thousands of kilometers). I've thought of adding a pressure restrictor in the feed line to the carb, but that could fail shut (and do regularly from my online research) .... The standby pump is powered of course direct to battery, while the main pumps are powered through the essential bus. So again, I can unpower most of the aircraft with the Aircraft Master switch(s) and the engine will keep running.

I hope this answers your question?