Alternate air is required in case the induction air is blocked for some reason. The most likely is ICE! I guess a really messy bird strike, a plague of locus or volcanic ash are other possibilities where the air filter becomes blocked and the engine gets no air and stops. Ice however is a real possibility and an engine out would add to a bad day.

Alternate air in my design bypasses the air filter and takes warm air (without ice) from around the engine and gets it down the induction pipe to the throttle body.

Some systems are automatic allowing the engine suction, if the incoming air is blocked,  to open a reed valve or membrane of some sort or via a spring loaded door. This has to be just right with a LOT of testing or unfiltered air might sneak into the engine during ground ops.

I have settled on a pilot operated sliding door via a linear actuator (currently on order)

When I made the U pipe I had it a bit longer so I could save some ‘pipe’ as spare material. Here I’ve cut up a piece for the door.

To set the door up I’m starting with a longer piece of pipe to be the mold for side and end rails. First up I’ve covered the piece in duct tape.

Here I’ve overlaid two plies of carbon. Once cured all I have to do is get the mold piece out.

It took a while but I have my ‘window’ and sliding door cleaned up.

Here you see it partly opened. I’m still going to use the smaller piece, the larger was needed to make to side bits and in case I had to destroy it getting the thing out.

Here’s a quick video of the slide action. Obviously I’m going to be cutting a huge hole underneath the door. I’m going to wait for the actuator and do some tests before I make the cut, just to be sure its all going to work.