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Why would the
addition of the "trim rings" change the air flow?
The trim rings extend further back into the engine
compartment than the original cowl inlet did. There was a minimal
decrease in the diameter of the cowl inlets. Believe me, I was surprised
by the result as well.
----- Original Message -----
Sent: Wednesday, April 22, 2009 9:26
AM
Subject: [LML] Re: Ram Air Design
I assume because the inlet area is reduced if only slightly
and more likely that the smooth airflow is "tripped" by the sharp edges.
In my opinion, the "ram air" (quotes used deliberately) tube that is on
most Continental-engined Lancairs is the worst I have ever seen. The
tubing is large enough - probably too large, but the sharp leading edge
probably destroys any potential for pressure recovery. And with it
extending significantly into the cooling air inlet, those same sharp edges
have to reduce cooling air flow. What to do? One of my favorites
is to just put another air filter on the second inlet pipe and get rid of the
mechanisms in the inlet system - they worry me anyway, as the small bits can
come loose and go through the engine while the big pieces can come loose and
block the air flow completely. Use the higher pressure in the upper
cooling plenum to give most of the pressure recovery that is possible -
probably 75%. What do you do about an alternate air source? Maybe
nothing, as half of the filter area is pointing backwards and won't be plugged
with snow anyway. Or just through away that pretty "filter" and put
hardware cloth over the inlet. My feelings about the effectiveness of
K&N filters might be showing, I suppose. At the very least just cut
off the ram air tube and put a bell-shaped entrance on it. Your idea of
extending the inlet forward is good, but as Grayhawk said, to be most
effective it has to be away from the hub. This is because the round
propeller sections near the hub are just obstructions to air flow. I put
my chin inlet (bottom-inlet Lycoming engine) close to the hub and it was a
mistake, but I don't think it hurts a lot. Take a look at some of the
turbo 210's - they have a separate ram air inlet on the outboard edge of the
cooling inlet and they have a rounded leading edge. But for your
installation how do you move it away? I'm not sure, but I know what I
would do - use two rectangular paper filters with a smooth transition from
there to the throttle body. Simple, works good, lasts a long time.
It would lose maybe 10% of what ram air could be obtained by heroic
efforts, but a 10% loss in ram pressure probably represents 1% in power, or
0.3% in max speed. And the extra air filter area would probably give 1%
more power at takeoff and reduce the amount of dust by 90% compared to the
K&N solution. FWIW
Gary Casey
ES, Lycoming engine, paper filter from a turbo 402 Cessna.
Posted for "Douglas Brunner"
<douglasbrunner@earthlink.net>:
Recently, I installed two
aluminum cowl inlets (see photo) in my Legacy. I
did this
largely for aesthetic reasons, but also expected that it would
improve my
ram air pressure. I was wrong - for reasons I don't understand my
ram air MP is now less that my filtered air MP by a full
inch. (My ram air is
reversed because my plane had de-ice and
the de-ice alternator is on the left
side).
I have
sketched a new ram air system that I hope will give me back at least
1.5
or 2 inches of ram air. (see sketch). Features:
1.. The
ram air inlet will be flush with the cowl inlet and well out into
the air
stream.
2.. A "flapper valve" to block the filter when not in
use
Your thoughts?
Any suggestions of who could
fabricate this?
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