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On my Lancair 360 I use a simple alternate air door.  My system works like
this.  Air enters the inlet at  the cowling.  Behind the cowling inlet is a
fiberglass bee hive shaped affair that expands to a 6 1/2 inch diameter where a 1/4" wire mesh is installed.  On the front of the wire mesh is a
standard 1/2" thick foam type filter cut to fit over the wire mesh.  The
increasing diameter of the inlet portion of the bee hive holds the filter
against the wire mesh although I found I needed a single wire keeper to
hold the filter in the event of a back fire during engine start.  After the
wire mesh, the bee hive shaped air box chokes back down to a diameter that
matches the inlet to the throttle body.

It is just behind the wire mesh that I installed a 2 inch diameter door
that is spring loaded closed but will open into the airbox in the event of
a clogging of the filter or air inlet.   This system keeps my expensive engine filtered 100% of the time with a very small pressure loss across the
filter and of course provides an alternate air source in the event of a
problem.  The bee hive air box is built into the lower engine cowling.

The above is definately lighter, simpler, and more reliable that the many
aluminum air box designs I have seen on other aircraft.  Unfortunately, you have to fabricate the bee hive shaped air box from scratch and this
takes half a day.

The bigger the engine in displacement and RPM the bigger the filter area
needs to be.  To minimize the pressure drop across the filter you want to
make the filter area big and the filter thin.  The thinner the filter the
less filtering effect. The smaller the area of the filter the more often it
will need to be changed and the larger the pressure drop across the filter.
As I recall for my IO-360 and the 6 1/2" diameter Bracket 1/2 thich foam
filter, I have less than a 1/4" MP drop across the filter at max RPM and
power and the filter needs to be changed every 50 hours in theory.  The alternate air door does not neeed to be very big.  Mine is only 1 3/4"
in diameter which will get me home with only a slight reduction in power
even if the normal inlet were 100% clogged which is unlikely. Jim Frantz  

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Please send your photos and drawings to marvkaye@olsusa.com.