too much gyro vacumn?

[Ray Toews]

I am removing the vac T&B and putting in an electric and thinking of
removing the regulator and then apply full venturi vacumn to the
horizon. Is it possible to over rev a gyro with an outside venturi?
I am doing this because even after a 15 min runup I am several miles
off the runway before the horizon stabilizes, not that great on a nite
takeoff.

Ray

[Orval Fairbairn]

In article >, Ray Toews wrote:

> I am removing the vac T&B and putting in an electric and thinking of
> removing the regulator and then apply full venturi vacumn to the
> horizon. Is it possible to over rev a gyro with an outside venturi?
> I am doing this because even after a 15 min runup I am several miles
> off the runway before the horizon stabilizes, not that great on a nite
> takeoff.
>
> Ray

Vacuum gyros are meant to operate at a specified vacuum level, in order
to maintain their proper RPM. Applying extra vacuum would cause them to
spin too fast and cause premature wear on the bearings.

This is one of many reasons for vacuum pumps, instead of venturis.

[Bill Daniels]

"Orval Fairbairn" > wrote in message
...
> In article >, Ray Toews wrote:
>
> > I am removing the vac T&B and putting in an electric and thinking of
> > removing the regulator and then apply full venturi vacumn to the
> > horizon. Is it possible to over rev a gyro with an outside venturi?
> > I am doing this because even after a 15 min runup I am several miles
> > off the runway before the horizon stabilizes, not that great on a nite
> > takeoff.
> >
> > Ray
>
> Vacuum gyros are meant to operate at a specified vacuum level, in order
> to maintain their proper RPM. Applying extra vacuum would cause them to
> spin too fast and cause premature wear on the bearings.
>
> This is one of many reasons for vacuum pumps, instead of venturis.

Uh..don't think so. There is a vacuum regulator in the system.

I had an emergency vacuum system on my Archer made by Precise Flight that
used full engine manifold vacuum in place of the vacuum pump should it fail
and the regulator still delivered a steady vacuum at 4" H2O. I don't think
a venturi would ever produce as much vacuum as an engine at idle.

Now that I think of it, the pitot/static pressure differential at 80 knots
is 4" of water column - the same as the vacuum instruments need. Over 80
knots, you don't need a venturi, just ram air and a static port outlet - and
filters, of course.

Bill Daniels

[COUGARNFW]

Ray...

Not enough information. You did not mention which venturi...four inch, 8 inch,
9 inch?

Older gyros required 4 or a bit above...and the horizon unit required all of
the volume of flow the four inch could give, so no other device should be
paralleled with it. (a four inch could supply a DG and, through a flow limiter
which you did not mention but might still be there, a T&B)

If an 8 inch or 9 inch venturi, then you either keep the regulator (and measure
that it is set for the gyro you use and what it needs...newer ones like 5.x
rather than the four inches) or ensure early failure of the gyro.

Neal

[Kyle]

(COUGARNFW) wrote:

>newer ones like 5.x
>rather than the four inches) or ensure early failure of the gyro.

Did I read this correctly? Low vacuum will cause premature failure of
a gyro instrument? If true, I'd be interested in the mechnism of
failure. Thanks.