Windmilling Prop & Vacuum Pump

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?

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?

No and no.

Dave Butler wrote:
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?


No and no.

Personal experience, credible sources, or just a guess?


All the best,


David

David Megginson wrote:
Dave Butler wrote:

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?



No and no.


Personal experience, credible sources, or just a guess?

1A. Will it keep the vacuum pump going? No, just a guess. You might have a
better shot at it with a controllable prop. That assumes, of course, that
whatever caused the engine to stop making noise didn't prevent it from spinning
'round.

1B. Will the instruments keep spinning enough? No, just a guess, reinforced by
my experience testing the Precise Flight backup vacuum system when it was
installed in the Archer I used to own. To calibrate the altitude limits on the
placard, you have to fly with varying levels of vacuum and note the instrument
performance. The vacuum doesn't have to be much below specified minimum (is it 4
inHg?) before the gauges get really squirrely. Noting how long it takes the
gyros to spin down while the plane is stationary on the ramp after you shut down
is not a useful measure.

2. It's easy to test. Try it for yourself if you really want to know.

3. I wouldn't rely on a usenet answer if it were 'yes'.

4. Even if I tested it and it worked, I wouldn't rely on it working when I need it.

Short answer: it was a guess.

Dave Butler wrote:

3. I wouldn't rely on a usenet answer if it were 'yes'.

Very wise.

4. Even if I tested it and it worked, I wouldn't rely on it working when
I need it.

Short answer: it was a guess.

Fair enough. My gyros behave fine on an instrument approach down to 1500
rpm, so I'd be pretty comfortable trusting them as long as I could keep my
Warrior's speed up high enough to spin the prop that fast; unfortunately,
with an engine out, that might mean descending a lot faster than I really
need to. As long as my TC were usable, I would find it hard to justify
sacrificing a lot of glide range just to keep the vacuum pump working.


All the best,


David

Dave Butler wrote in message ...
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?

No and no.

No and maybe.

Especially with a constant speed prop, you will wish to pull the
prop pitch to low rpm to get the longest glide. Your pump will be doing
about nothing -- tho the wet ones will make some vacuum even as you crank.

The gyros will typically run for ~ 5 min, but they lose the erection
function caused by the vacuum loss.

But heck, unless you are REALLY high, they will run at least sort
of until you contact the earth. It's not that many minutes.

Bill Hale

Bill Hale wrote:

The gyros will typically run for ~ 5 min, but they lose the erection
function caused by the vacuum loss.

You must have way better bearings than I do... or maybe you're measuring the
time to when they stop spinning. They become useless for attitude control way
before they stop spinning.

Not sure what the 'erection function' is. I'm afraid to ask.

Dave Butler wrote

Not sure what the 'erection function' is. I'm afraid to ask.

Research pendulous vanes.

Bob Moore

Bob Moore wrote:

Research pendulous vanes.

Thanks, Bob, I know what pendulous vanes are, but I was having trouble relating
pendulous vanes to the context of lost vacuum due to engine stoppage. The poster
wrote:

The gyros will typically run for ~ 5 min, but they lose the erection
function caused by the vacuum loss.

Bob Moore wrote:
Dave Butler wrote


Not sure what the 'erection function' is. I'm afraid to ask.


Research pendulous vanes.

Sort of gyroscopic viagra?