Just to start a fire :-) Back Up AI

[cliffy]

With the fact that we have one vacumn pump to drive the gyros and they fail with great regularity and everyone knows of the need 

for a second gyro- considering the high cost of the options for a standby gyro-

Why have we never considered a second AI powered by a bottom mounted venturi?

Safe simple, cheap, always workes as long as we have airspeed

I've got many hours of IMC behind venturi driven gyros. 

One gage, one hose, one venturi. No moving parts, No electricy required. 

 

 

[MikeOH]

And give up 1 kt, or less???  What self respecting Mooney owner would do THAT??

[201er]

Cause most of them have a precise flight standby vac fed off the intake manifold that is lightweight and doesn’t have to stick out. The Venturi sticking out of Jose’s Mooney drives the vac for the piss hose!

[MikeOH]

1 minute ago, 201er said:

Cause most of them have a precise flight standby vac fed off the intake manifold that is lightweight and doesn’t have to stick out. The Venturi sticking out of Jose’s Mooney drives the vac for the piss hose!

Yuup.  I have a Precise Flight Standby system.

[toto]

https://www.aircraftspruce.com/catalog/inpages/venturi3.php

Looks like this is targeting the experimental crowd. 
 

[Marauder]

17 minutes ago, 201er said:

Cause most of them have a precise flight standby vac fed off the intake manifold that is lightweight and doesn’t have to stick out. The Venturi sticking out of Jose’s Mooney drives the vac for the piss hose!

 

15 minutes ago, MikeOH said:

Yuup.  I have a Precise Flight Standby system.

You guys ever use yours for real? I found that the amount of power reduction to get down from altitude and to fly an approach was pretty dramatic. I couldn’t wait to replace mine with an electronic version. 
I also wondered what would happen if the vacuum pump detonated and partially plugged things up.  

[MikeOH]

2 minutes ago, Marauder said:

 

You guys ever use yours for real? I found that the amount of power reduction to get down from altitude and to fly an approach was pretty dramatic. I couldn’t wait to replace mine with an electronic version. 
I also wondered what would happen if the vacuum pump detonated and partially plugged things up.  

Nope.  And, you raise a good point about the power reduction.  I've assumed  that I could maintain level flight...I think I'd best do some testing!

[toto]

4 minutes ago, Marauder said:

 

You guys ever use yours for real? I found that the amount of power reduction to get down from altitude and to fly an approach was pretty dramatic. I couldn’t wait to replace mine with an electronic version. 
I also wondered what would happen if the vacuum pump detonated and partially plugged things up.  

It always struck me as a really difficult workload item to add to a high workload situation.  It made sense in the 80's, but today a portable AHRS is so much better than the precise flight system that it doesn't make much sense even to maintain one (better to put that money in a piggy bank and buy a D3).

https://dynonavionics.com/pocket-panel.php

[MikeOH]

9 minutes ago, toto said:

It always struck me as a really difficult workload item to add to a high workload situation.  It made sense in the 80's, but today a portable AHRS is so much better than the precise flight system that it doesn't make much sense even to maintain one (better to put that money in a piggy bank and buy a D3).

https://dynonavionics.com/pocket-panel.php

Okay, that D3 is pretty cool,....and, less than 1 AMU!

[EricJ]

13 minutes ago, Marauder said:

You guys ever use yours for real? I found that the amount of power reduction to get down from altitude and to fly an approach was pretty dramatic. I couldn’t wait to replace mine with an electronic version. 
I also wondered what would happen if the vacuum pump detonated and partially plugged things up.  

My airplane had one when I bought it.   I had to pull the power back so far to make it work that it essentially turned a vacuum failure into a simultaneous vacuum + engine failure.   I guess you'd have been able to stay right-side-up long enough to, hopefully, break out of IMC or land at a nearby field or at least crash right-side-up.    I got rid of it when I did my initial avionics upgrades.

If you need 6" or so of suction to run your instruments, then your MP will need to be 6" or so below ambient to make it work.    That's not a lot of power.

[cliffy]

32 minutes ago, toto said:

https://www.aircraftspruce.com/catalog/inpages/venturi3.php

Looks like this is targeting the experimental crowd. 
 

Well there are several different venturis and many many gyros were powered by them 50 years ago on certified aircraft. 

Had them on my C 140 that I did IMC in quite  bit. They are an "approved" part. 

The gyros don't erect until just after takeoff.  But once running they never stop until the airplane does. 

The one to get for a stby AI would be the 3 1/2 inch style (3 1/2 inches of vacuum). The 9 inch one requires a regulator. 

[toto]

2 minutes ago, cliffy said:

Well there are several different venturis and many many gyros were powered by them 50 years ago on certified aircraft. 

Had them on my C 140 that I did IMC in quite  bit. They are an "approved" part. 

The gyros don't erect until just after takeoff.  But once running they never stop until the airplane does. 

The one to get for a stby AI would be the 3 1/2 inch style (3 1/2 inches of vacuum). The 9 inch one requires a regulator. 

Sounds like you have way more experience with these than I do...  That aircraft spruce link exhausted my full knowledge of venturi-driven vaccum instruments

 

[toto]

14 minutes ago, MikeOH said:

Okay, that D3 is pretty cool,....and, less than 1 AMU!

Yeah, almost a rounding error in aviation - and they're regularly offered at about $100 off for airshow sales.

[cliffy]

I've seen 2 and 3 stacked on one airplane

There is also a 2" one specifically for T&Bs  Its pretty small

[Guest]

I think the venture on the belly has more possibilities than people think.  With a few valves and hoses, Jose can take a pee, Cliff can drive a back up AI and Mike can vacuum the interior on those super long flights.

Clarence

[takair]

I think it’s a practical solution.  The downsides would be weight, drag and I believe they are more prone to icing for similar reasons as a carburetor.  Not sure I recall correctly, but the transport category ones may have had heaters?

[201er]

8 hours ago, Marauder said:

You guys ever use yours for real? I found that the amount of power reduction to get down from altitude and to fly an approach was pretty dramatic.

I pulled it on when I was coming in with a vac failure but I managed to stay VFR the entire time so it was just in case cause it was murky.

It’s not for cruise flight. Obviously at 8000ft, you can’t get the throttle back enough to maintain altitude and run the standby vac. You fly partial panel down to about 4000ft, but at that point you should be able to power back enough in the descent to get it going. You only need it for the approach. You’re easily throttled back 6”MP for the last 2000ft of descent to be able to use it to get on the ground.

[cliffy]

Uness you are out west with DAs of 7,000 or more  MEAs in the teens

If you have one installed you are supposed to do a check flight and record altitudes vs vac and have a placard  on your panel delineating such

You do have that placard on your panel don't you?  :-)

Icing? Ice closing off the venturi? You got way more issues if you're down to flying by the stby AI and you are in heavy icing. Way more. 

None ever had heaters that I know of. Small GA aircraft used them Vast majority of GA used them before dry vac pumps. 

Wet vac pumps were seen on large transport catagory engines. 

[Fly Boomer]

12 hours ago, 201er said:

precise flight standby vac fed off the intake manifold

I have a Precise Flight standby vac on my M20K, but I'm not aware of any intake manifold connection.  Pretty sure mine is just a motor connected to a regular vacuum pump back in the avionics bay.  Controlled by a switch on the panel.

How does the intake manifold connection work?

[kortopates]

On 6/22/2022 at 8:06 AM, Fly Boomer said:

I have a Precise Flight standby vac on my M20K, but I'm not aware of any intake manifold connection.  Pretty sure mine is just a motor connected to a regular vacuum pump back in the avionics bay.  Controlled by a switch on the panel.

How does the intake manifold connection work?

Many of the early K's didn't have electric standby vacuum from the factory. Some had the "poor man's"  Precise SVS standby vacuum system that worked off of manifold pressure. On a turbo it works much worse than on a NA aircraft but still provides marginal capability when you start an immediate descent. The Savvy  pilot creates a table of MAP required at different altitudes to know what to use when needed. (It was never meant to provide operation for cruise at altitude, the AFMS calls for commencing a descent when vacuum is lost.) But the easy way to tell which one you have from the cockpit is to check the control. The Precise SVS had a knob you pull out just left the yoke and a red light that comes on with low vacuum as you'll see at at minimum idle rpm. Whereas the electric standby vacuum pump has its own electrical CB Switch on the panel just like all the others electrics and thus works without the engine running. 

Starting with the '86 K, the 252, all K's had electric standby vacuum with most other options like speed brakes and O2.

Both the electric and SVS should be tested on your checklist before takeoff. The Electric is best tested before starting the engine. The SVS is best tested right after startup before the gyro's have fully spun up. Just pull idle back to minimum, see the red light come on, pull the knob and see red light go out. Then push back in and add MAP to bring up to normal idle rpm.

Edited June 27, 2022 by kortopates

[Fly Boomer]

30 minutes ago, kortopates said:

Starting with the '86 K, the 252, all K's had electric standby vacuum with most other options like speed brakes and O2.

All of the above.  Mine is an '89.

[carusoam]

Precise flight has two different systems…

1) The vac system connected to the engine intake is Not good enough to depend on…. It has limitations over tall hills….

2) The back-up vac pumps are still OK… but not being installed any longer….

 

 

3) Cliffy’s Venturi system is often swapped for a fan powered alternator… somebody has one around here….  The fan gets pushed out into the wind stream on demand…

 

4) TCs are also dead end ideas for decent back-up devices… they wear and fail to be useful in bumpy environments..

 

Soooo….

If you want to go all manual…. And vac driven instruments only…. Go Venturi with a decent vac gauge…

 

Otherwise the D3 seems to be a good alternative for a spare AI….

Pp thoughts only, not an instrument guru…

Best regards,

-a-

[laytonl]

Another option is an Av20. Less than 1 amu and only a couple hours to install.  Lee

[M20F]

I have shot an ILS through a reasonable layer to minimums using precise flight (manifold pressure).  As others have noted you really need to crank it back to get it to work.  That said it does work.  
 
I have that and now a G5 (at some point going to 2 G5’s or adding a 275).  I still feel contrary to most that keeping vacuum is a good redundancy.  2 electric + vacuum with precise flight is a lot of redundancy.  I like redundancy.  

[Andy95W]

5 minutes ago, M20F said:

.  2 electric + vacuum with precise flight is a lot of redundancy.  I like redundancy.  

You should get an STEC autopilot then.  Uses the turn coordinator to keep the wings level.  If you lose everything you can turn on the autopilot and get vectors from ATC.