> > > > > > > > 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

 >why not put a venture inside the cowel and duct high pressure to it and
 dump used air from the venturi back out of the cowel. if a negative pressure
 at the exit could be generated so much the better. also you can use intake
 manifold vacuum to drive the gyros when the venturi is not working. all you
 need is to y the various vacuum sources and put one way valves in each

 > source line. I used a large venturi and must use a pressure regulator to

 reduce the vacuum. works fine.
 smale@ncinternet.net

would a venturi in the cooling air duct, screw up the air flow or is
it worth considering?  Jerry

The velocity would be around 40% of what it is outside the cowl.

Paul Lamar
 
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