Scott Turner wrought;
Jim Hergert comments:


This issue has turned out to be quite intriguing.

If I understand correctly, Tim Ong's idea might have a drawback, in that
when the outside air is cold, would you want to have cold air running into
the cabin to draw air through the venturi?

      At full power on take off you have lots of warm air coming into the
cabin. In fact you have a lot of air coming into the cabin all the time and
that is what the Dukes valve does it lets it out. Therefore, it needs to be
off its seat before power up. JIM

With further thought about Tim's approach, teeing off of some cabin air aft
of the bulkhead and running it through a venturi, could overcome the cold
air problem. However the effectiveness could change by the amount of air
that is selected to go into the cabin.

       The cabin inlet dump valve normally has all the air coming into the
cabin always unless you have smoke or some other emergency to divert the
incoming air. This air is warm as long as you have turbo boost being
generated. Therefore the only time you have cool air coming in is when it is
cold outside and you are flying low with low power settings. I never do low
power settings. JIM

With a better understanding of how the dukes valve works, I have some
thoughts of another approach to get vacuum to the valve. The concept is
to tap off the engine manifold. When the engine is at idle, as every pilot
knows, the manifold pressure is low. The approach would be to T off the
manifold pressure line going to the manifold pressure gauge, perhaps where
it comes into the cabin. Then put a check-valve connected to another T.
One side of the T would go to a reservoir, say around 20 ounce capacity,
mounted to the cabin side of the firewall. The other side of the T would go
to the dukes valve.
This system probably is still used in motor vehicle power brake systems.
The small amount of air being sucked out of the reservoir would have
virtually no effect on the manifold pressure gauge.

Thinking further, it might just be that all the dukes valve needs is vacuum
at engine start-up. Then no reservoir would be needed.

      This is an intriguing thought. If we just had a vacuum reservoir that
would be charged at idle I think that would be enough to unseat the valve and
then it would work like it should for the rest of the flight. Good thinking.
JIM

Some other comments: The IV-P I flew in, the door seal had to be deflated
periodically to get air exchange in the cabin, while others have said it is
a one-time event to get the dukes valve to operate. Could it be that the
pneumatic lines exiting the dukes valve are run differently for those who
have to deflate the door seal periodically versus for those who only have to
experience a one-time pressure surge?

      If you don't unseat the Dukes outlet valve you won't have an outlet
unless you deflate the door seal. Many L4P's take off with the door seal
UNinflated to allow outflow and cooling when it is hot. I have AC which
allows me to inflate the seal and take off with the AC on. If I can only get
vacuum to the Dukes valve I will have a perfect situation. i.e., just fly the
airplane not manage systems. JIM

I'm looking forward to us finding a practical solution to this lingering
problem for our high-end birds. By pooling our knowledge in this open
forum, I feel we will find a good solution and make it available for all in
the future.

Thanks for your help! Scott Turner


I still like the venturi in the cabin idea.
Jim Hergert
N6XE, "An Sex Y" L4P