Posted for "Bob Pastusek" <rpastusek@htii.com>:

Cabin press flapper valve

Phil,

This is a common problem that I've observed in every IV-P that I've flown. I
believe it is a result of the dynamics of the pressurization start up
process.  The flapper closes when you start bleeding engine air into the
cockpit with the door seal inflated, but as soon as it does, pressure builds
up in the passageway coming from the tail and forces the flapper open. As
soon as this ram air pressure dumps into the cabin, the increasing cabin
pressure snaps the flapper shut again. Eventually, the cabin pressure builds
up enough to hold the flapper closed against the ram air pressure from the
tail scoop and it stops cycling.

I have an automatic air valve in the tail in my (uncompleted) airplane--part
of the airconditioning system--that should eliminate this problem for me,
but before I went this way, had considered installing a Ray Allen (or
similar) trim servo to positively open and close the flapper valve. This
could be tied to the cabin door seal pump switch, or the pressurization
controller power switch (probably better) so it would essentially operate
"automatically" when you pressurize. Some added weight, expense and
complexity to overcome a small problem, but this should do the trick if you
want to eliminate this noise.

Good luck, and let us know what/if you find a good solution--many could
benefit.

Bob Pastusek