On Mon, 31 Oct 2005 07:17:02 -0500, "Guillermo"
wrote:


"Marc J. Zeitlin" wrote in message
...
Jon Kraus wrote:

So what you are saying is that a windmilling engine is going to
produce enough vacuum to run the gyros? I think not...

Since the vacuum pump on most of our engines (an O-360 in my plane -
something similar in your Mooney) are run by a gear on the engine, as
long as the engine is spinning, the pump will be spinning. When I do my
runup, I get 5" of vacuum at anything over 1500 RPM or so - certainly at
1700 RPM or above.

So, as long as my engine windmills at more than 1500 RPM, I'll have more
than enough vacuum to run my gyros. And it does - I've tried it.

You've tried turning off your engine and have your propeller spinning at
more than 1500 RPM?
That seems unlikely, when you do a simulated engine failure with the engine
idling, you do get more RPM than in the ground, but not even close to 1500
RPM... I can't imagine that the engine will spin more than 1000 RPM (or a
lot less) in a real engine failure

That's one of the reasons I fly approaches faster than VFR patterns.
At idle (with windmilling prop) the vacuum drops below the red line.
Now that doesn't mean I lose instruments immediately, but it'd get
pretty iffy doing a complete ILS at idle. With an engine failure I
wouldn't want to have to let down through a thick layer without a
second vacuum pump or backup electrical AI.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com