Paul,
This probably rates up there with the "Rotary Displacement" question,
but I'm going to ask anyway. When you swap out a certified aircraft
engine for a Rotary, how do you run that panel full of vacuum gauges?
I've never heard of anyone keeping their air pump on an aircraft
conversion. I've always had "all electric" panels in the past, so I
never really thought about it until now.
Don
A few years ago smog pumps were common on car engines. Turns out they
make great vacuum pumps for powering instruments. A bit heavy however
but very very cheap in the junkyards.
Paul Lamar ...No rotor no motor.
Don -
I think the cheapest, lightest, and most reliable way to create vacuum
for your steam gauges is with an exhaust bung.
The exhaust bung would enter the exhaust pipe at a small angle, say 20
degrees, pointed towards the exit, and extend into the exhaust gas
stream about 1/3 of pipe diameter. This creates a venturi, and LOTS of
vacuum.
I googled for references, and didn't find any - sorry.
However in reading up on this solution 3-4 years ago, I read that this
was used as a performance tweak in the 60's to evacuate the crankcase of
those big V-8's, to "pull" on the bottom of the pistons (sounds wacky I
know, but that's not the point...) The problem was that this created SO
MUCH vacuum that it sucked in the valve cover gaskets.
IMPORTANT NOTE: you will have to regulate the vacuum (with a regulator),
but it will work very well. (Almost TOO well.)
So:
lightweight - a few ounces of bung material and tubing
reliable - as long as the engine is running (same as vacuum pumps)
durable - no moving parts
cheap - tens of dollars with no annual maintenance
How's THAT for Experimental Aviation?
- Vince Orton
Wow that might be a great idea pulling on the pistons. Super cheap turbo compound.
How do they keep the oil vapor in there? For drag racing I guess they don't
care. On second thought they lose as much energy as they gain as the pistons
on the compression up stroke are required to pull up against the vacuum so to speak.
Same goes for the exhaust up stroke. Looks like a wash. I have heard about it to reduce windage
on the crank shaft however. Vacuum pumps are sold in the racing parts catalogs for
that purpose.
For use as a vacuum pump to drive the instruments it sounds great.
These are also called "air ejectors" if you want to look them up on Google.
They are used in some composite shops for pulling a vacuum on a vac bag layup.
A simple venturi might generate the -5 psi required for vac instruments without
all the back pressure.
Paul Lamar ...No rotor no motor.
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