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

Depends on your pump and what RPM's you can expect the prop to windmill
at. My pump will produce 4" at idle and if you just shut off the fuel
to the engine, the prop will windmill in the glide at well above idle
(actually, you can't really tell by the RPM's that the engine isn't
running).

Of course, if your engine siezes, you're not going to get any windmilling.

during descent through the clouds, or will the gyros keep spinning fast
enough long enough to make the vacuum pump irrelevant?

It's amazing how FAST the gyros spin down (and how long it takes them
to spin up). I had a pump crump on takeoff roll and before I was at
pattern altitude the AI had started to lean over (fortunately in VMC).