Glen wrote “In my
opinion, the "ram air" (quotes used deliberately) tube that is on
most Continental-engined Lancairs is the worst I have ever seen.”
I concur. And
having all those flapper valves with attachment hardware to swallow is asking
for trouble in a high vibration environment.
I have attached a photo
of my arrangement on my IO-550 Lancair IV. The alternate air inlet is
deliberately set back so that rain water is not inducted directly into the
intake manifold. The filter is one of Glen’s esteemed K&N
models, not a paragon of filtering efficiency, but OK (not great) for airplanes
operating in clean air in my opinion. The flapper valve is attached to
its shaft via epoxy and fibreglass wrap-around, no hardware to come loose and
get sucked into the engine.
The alternate air inlet
was included to eliminate any air filter loss and get that last little bit of
pressure rise.
It was a waste of time.
There is no benefit compared to the air filter tucked inside the cowl.
I modified my cowl inlets
to have 6 inch inlet diameter. They are set away from the hub as shown in
the second photo to maximize ram recovery, removing the low energy air stream
near the spinner. The ram pressure on top of the engine is the ram
pressure delivered to engine intake manifold.
By reading full throttle manifold
pressure and comparing to indicated air speed I find that I am getting about
75%-80% ram efficiency with these inlets on my IO-550, but that is with the
cowl flaps closed. This equates to about 1.5 inches of Hg at 200-210 IAS
when the total ram pressure is about 2.0 inches of Hg. I get a bit less pressure
rise with the cowl flaps open, perhaps half as much.
I get virtually no
benefit by opening the alternate air door and bypassing the air filter.
Sometimes I can see 0.1 in. Hg on the digital manifold pressure gauge, but that
suggests that most of the time the benefit is less than 0.05 in. Hg. The air
filter, sized per K&N guidelines, has virtually negligible pressure drop.
One of my hangar mates
has a new (to him) Cirrus SR-22 with the IO-550N engine. It uses an oiled
foam conical air inlet filter installed much like my K&N, but on the left
side, and has an alternate air hose that sucks unfiltered warm air from the
lower cowl area. I suspect the warm air inlet is to meet certification
requirements for flying in snow and ice crystals which may block the
filter. No reports yet on ram efficiency. However, the engine baffling
set up is a thing of beauty, well thought out; no leaks, precision formed sheet
metal, and extremely tight fitting rubber trim. It is the best factory
baffling set up I have ever seen. If you see a Cirrus with the cowl off,
have a very close look.
Fred Moreno