<<  Most gyros need vacuum, not pressure.

 > I thought they just needed a 4 inch pressure differential, be it either a
 > vacuum on the exhaust side, or a pressure on the intake side.
 >
 > Bob Darrah >>

The instrument can't tell the difference. (is there a difference), they use a
vacuum because it was free. Just a venturi outside to run everything. There
were no electrical systems to worry about. You can't use intake vacuum
because at full tilt there isn't enough. You can't use exhaust because the
acid and water will destroy the instruments. So it's vacuum pump or venturi
or both.
Lynn E. Hanover

I think I've posted this PBI before, but the subject of vacuum has been running for
awhile now so I thought I'd bring it up again.

NASA has investigated the idea of placing a venturi inside the cowl so that cooling
air is exhausted though it. Presto, vacuum without the drag of the venturi mounted
outside the aircraft.  Of course you have no vacuum below some critical airspeed
just as you don't if you mount it outside and suffer the drag penalty.

AND questions have been asked about using augmented exhaust to draw air through a
radiator. Paul recently posted a picture of Charlie Airesman's use of it in his
Subaru powered Vari EZ.

The next step is to combine the two ideas.  Use an augmented exhaust to pull air
through the radiator and put a vacuum tap on the augmentation venturi to power the
instruments that require vacuum. You now have vacuum at low / no airspeed and
cooling air flow through the radiator under the same conditions by extracting
double duty from the exhaust energy you're literally throwing away.

Rick Girard

 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org