Windmilling Prop & Vacuum Pump

On Wed, 13 Oct 2004 04:39:47 GMT, Matt Young
wrote:

Ok, a curious question just popped in my mind. Say that one was flying
IFR in a piston single, maybe a 172 or 182. While enroute, either in
actual or above a cloud layer, the engine fails. Will the windmilling
prop keep the vacuum pump going enough to make the AI and DG usuable
during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

Your 're going to get answers all over the spectrum, but my take would
be it might happen, but don't count on it to save your bacon. IOW,
I'd not count on them working, or staying accurate which is
_far_worse_ than not working when you are in the clouds.

There are many variables such as the actual engine, prop, and vacuum
pump combination as well as best glide speed.

Its been my experience (which may not be typical) that a wet pump will
do better with the low RPM than the dry pump. This is assuming you
still have RPM which with a catastrophic failure you probably won't.

In my particular airplane which has a wet pump and a constant speed
prop, once the instruments are spun up even vacuum at idle (which is
well below the minimum) will keep the AI and DG working. The AI stays
accurate. The DG will start precessing but slowly although it does
hold well with a prolonged idle on the ground. Actually it's close to
being in tolerance. With full vacuum I don't have to reset it during
a 3 to 4 hour flight.

So, I'd expect to see the vacuum instruments "on mine" hold for some
time, but even knowing them as well as I do, I'd still depend on
partial panel and consider any use out of the vacuum instruments a
bonus.

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