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In a message dated 98-09-11 00:53:44 EDT, you Angier writes:

<< Have any of you been shopping for ELT's lately? Just wondering if there is
an
 obvious no-brainer choice among competing products.>>

I picked the ACT ELT because it used "D" cell batteries instead of a fancy and
expensive custom battery. This insures the maintenance costs don't exceed the
acquisition costs.

David's noisy heater.

On the sonic venturi front, the correct size for the choke diameter of a
single hot side venturi is 0.54 inches. If you have two venturies then they
should total the same area (2 at 0.381 inches diameter). Cold side venturies
are about 8% smaller in diameter to compensate for the higher density of the
colder air. These sizes provide the requisite 40+ SCFM (Standard Cubic Feet
per Minute) airflow (10 SCFM per person) for the cabin. The function of the
sonic venturi is to provide a constant volume of airflow into the cabin,
regardless of manifold pressure (as long as it is above cabin pressure). The
sonic part is because the pressure differential that is required to accelerate
the air through the venturi cannot propagate faster than the speed of sound.

If you fly with the door seal off then airflow through the cabin is a lot
higher (300+ SCFM) and any hot air coming in would be diluted. Also remember
that the heat comes from the turbos thrashing about and compressing the air so
to get any heat the deck pressure must be above ambient.

The noise is a plumbing problem. The sonic venturi "features" a standing
shockwave (sonic boom) so the airflow must be carefully expanded (diffused) to
prevent excessive cabin noise. I built a miniature aluminum muffler for my
plane that worked reasonably well.

Regards
Brent