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

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