<< We have installed a large NACA scoop in the top of the cowling, operated by
a choke cable. This enables us to introduce cold air (or at least luke warm)
into the area the carb intakes are located. We figured this was a much
cheaper alternative to a plenum chamber and also keeps the regulator box and
electrics cool at the same time.
>>
It is possible and inexpensive to build a plenum chamber from fiberglass. I
built one to accept the air from the Europa NACA inlet added on the top of
the Classic cowl. Two stubs of 2" dia aluminum tube were glassed into the
plenum sides to allow connecting to the carbs via SKEET tube.
A third stub was added to the bottom of the plenum to eventually allow carb
heat using air from under the cowl or connecting to the muff if needed.
Fittings were placed inside the plenum to allow installing a plate fitted
with a butterfly valve for control of carb heat -- the carb heat option
hasn't been installed or tried yet, however.
A small pocket inside the plenum connects to the carb float bowl tubes to
ensure the pressure matches the carb intake pressure as specified by Rotax; a
drain tube connects this pocket to the slipstream so should fuel come out the
float bowl tubes it is handled properly. Brass 1/8 tubing stubs were glassed
into the plenum to connect the float bowls to the pocket.
Building the plenum wasn't too difficult but there were a lot of steps needed
to handle all the little details. Probably 8-10 hours scattered over 2 weeks.
In its non-carb heat mode the plenum seems to work fine. The carb heat
option should get added and tested this spring if the snow ever melts and
flying weather returns.
One point I haven't seen mentioned is the mounting arrangement for auxiliary
controls like carb heat and cabin air. To allow easy removal of the
instrument panel I added a "sub-panel" which is simply an "L" shaped bracket
about 10 inches long made from fiberglass with a core of 1/8" ply. This
sub-panel is secured to the bottom of the panel in the pilot's footwell so it
is a bit forward of the panel face using Southco fasteners. This simplifies
instrument panel removal by allowing all the controls actuated by cables to
be disconnected via two 1/4 turn fasteners; the sub-panel just dangles in the
footwell when the instrument panel isn't in place.
John N44EU
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