This idea might have some merit- have a small SS tube, about
1/4" dia
welded
into the last 4 inches or so of the exhaust system, post
muffler. The
tube
would go in and take a sharp downstream turn. This should draw
some
vacuum.
How much, I don't know, but there are NO moving parts to
break, and
the
weight would be very small.
Brian Trubee
Interesting, How about a slight taper in diameter of the tail
pipe,
creating a venturi, installing a second light housing around
the tail
pipe at that location, and plumb a small tube to a vacuum
reservoir.
Drive your instruments from the reservoir. I think this should
be
experimented with. I'll draw something up. Tom Webber
Funny thing this should come up now.... I just spent a couple of
sleepless
nights doing some calculations on venturi eductors because my boss
was
afraid we had inadvertently created one. Here's the limitations
(subject to
my memory limitations... I don't have the reference books here at
home!):
The maximum pressure ratio you can get between the reduced pressure
at the
venturi throat and the pressure source (in this case, figure the
average
combustion chamber pressure while the port is open) is ~0.65. Take
the
average combustion chamber pressure during the exhaust portion of
the cycle
and multiply it by 0.65. If this is equal to (or higher than)
atmospheric
pressure, forget it... it will never "suck" anything.
So it's clear that this needs to be built after any muffler, and
preferably
right at the end of the tailpipe.
Secondly, the pressure ratio used above is a function of the Mach
number at
the venturi throat. The higher the velocity, the lower the pressure
(same
thing that makes high speed wings work with much less camber and
depth). Th
at 0.65 number is good for exhaust travelling at Mach 1. Not
practical for
us I suspect, and it would probably create a higher than desirable
backpressure on the engine exhaust to get it.
I suspect that if this could be made to work reliably, somebody
would
already have done it. The closest thing is the old style
hang-out-in-the-airstream venturis that don't require any direct
power.
They work fine... except when the plane is sitting still (like
before the
take-off run, meaning you don't have a spun-up gyro until after you
left the
ground!), they are draggy, and they foul up with ice (but only when
you
really need a gyro to get out of the ice fast while keeping the
clean side
up).
Sheeesh. My first posting in ages, and it has to be a wet blanket.
Sorry
folks.
Tom Kendall
I remember reading somewhere that venturis used to be mounted on the
belly, &
that they were much less likely to ice up when mounted there. Given
Tracy's
experience with his muffler, it seems that drag from a venturi in the
cowl air
exit path should be negligible & it should be fairly well shielded from
icing.
Any thoughts on this?
Charlie England
I think The venturi is the most practical alternative vacuum source and if
it was mounted anywhere near that muffler, there is no way it would ice up.
I had to put a SS heat shield between muffler & bottom skin to avoid damage
to aluminum skin. If the ventrui could be made to pop out only if needed,
that would eliminate the drag too.
Tracy
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