| Thanks, it makes sense. I think i will go the automatic route and put a flapper door somewhere after the filter.
Alex M
On Sun, Jul 7, 2013 at 4:19 AM, <ARGOLDMAN@aol.com> wrote:
Alex
Remembering back, It seems as if the manifold pressure, at WOT seemed to be
always less than ambient. Ram would increase that. Of course, a charger (turbo
or super) would increase the "ram" pressure even more. In Normally aspirated,
there are two types of alternate air. Manual controlled-- ie levers and cables,
etc, and automatic, which is exactly as you described.(and what I will be
using)
The usual and critical part of the induction system for blockage seems to
be between the air inlet and the filter. The "flapper: door generally has a
light spring holding it closed. Remember that the throttle plate is downstream
of the filter. When the throttle plate is open, the flowing
air between the plate and the trochoids (of course that part of the
induction system downstream of the plate) has the least resistance and
thus the MP is the highest-- going near but not to ambient. As the plate is
closed, the little trochoids are sucking against a partially closed orifice and
the MP goes down. (of course, the MP is measured between the plate and the
trochoid intake port.
The intake manifold pressure upstream of the throttle plate does not suffer
the same fate, as a matter of fact, logic (who says that is right) says that the
pressure in that part of the manifold might even be inversely affected.
Now if you have a blockage before the alternate air flapper (assuming
automatic, the trochoids are sucking against that blockage making the pressure
in the plenum between the blockage and the throttle plate decrease below
ambient, the door swings inward (high ambient vs low internal) and the engine
can still breathe.
In the planes that I have had with the automatic alternate air, if memory
serves me correctly(no guarantees) the flapper plate was part of the air cleaner
assembly.
Rich
Ya don't
want to pay the fuel or maintenance on anything burning kerosene..I take your
point on less airflow. Can you tell me which pressure is greater in the intake
manifold, the influence of ram air pressure or intake vacuum pressure. Or does
it vary from one to the other according to the cycle.
For an "automatic" homegrown system on a 2 rotor would it work to have a
spring loaded door opening inward into the intake manifold, fed by interior
cowling air when the main system is blocked. It makes sense to me to have a
door open for a negative pressure in the intake manifold which i imagine would
be the case of a blocked regular air intake and ongoing intake vacuum
pressure. Would a door like this be kept shut under normal conditions of
positive ram air, to preserve ram air which is what we want?
Alex M
On Sun, Jul 7, 2013 at 12:08 AM, <ARGOLDMAN@aol.com> wrote:
Interesting concept, I can see how that would eliminate a lot of FOD,
however that is not how alternate air systems work for piston/rotor type
engines.
That is almost the reverse. My guess is that the airflow in the PT6 is
hughly greater than in a 2 or 3 rotor engine and the kinetic energy of the
FOD is significantly less. Additionally, the alternate air is to allow air
to flow if the induction system gets blocked. this valve works in the
reverse. The greater the pressure upstream the more it will open and in fact
will vent the presure (FOD/rain etc) overboard
If the intake becomes blocked as in a filter or induction icing, the
pressure is reduced tending to seal the manifold.at this valve.
Would that we could all fly Turboprops
Rich
Hello Lynn
Are you referring to a similar system you see on the Pratt and
Whitney PT6 series engine, basically reversing the flow of air 180 degrees
before entering the compressor (throttle body for us i guess). The inertia
of the FOD or ice particles doesn't let them "make the turn" into the
throttle body and they exit the duct via a bypass flap. In this case the
opening of the flap creates the 180 degree turn and spillover air
exit?
Alex
M
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