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