REFLECTOR: Vacuum Pump

[Brian Michalk]

What I'm about to say does not provide any help to the original poster's 
request.

I have an opinion about an electric over pneumatic solution to the gyro 
requirements.  This, in my opinion is the worst of both worlds.  First, 
it offers no redundancy, and second, it actually increases the rate of 
failures.

We have a split gyro system mainly for the reason that if the vacuum 
goes bad, then the electric gyros would be there to compensate and vice 
versa.  With electric/pneumatic, there is no redundancy.  If electric 
fails, the gyro fails.  If pump, regulator or hose fails, the gyro 
fails.  There is a long string of components in that installation that 
are critical, and their odds are additive.

Off the cuff here ... and I could be wrong.  It's been a while since I 
did the analysis.  Assume three components to make a system.  Each 
component fails once per 100 hours.  System failure per 100 hours is 
.01+.01+.01, which is one system failure per 33 hours.  Now lets take 
that same system and make a redundant system just like it: 
.03*.03=.0009: one total system failure every 1111 hours.  Of course 
you'll be replacing broken components at the rate of once per 16 hours.

I already had a bias towards electrical, but I did a cost analysis 
anyway.  The cost of electric gyros vs a conventional configuration was 
less than $500.  I seem to recall it was more like a $300 difference.  
If one goes all electric, you don't need the pump, hoses or regulator.  
Additionally, if you factor in the cost per hour for the life of the 
pump, the electrics will pay for themselves after a couple of years.

To answer the redundancy question, I have done a probability analysis of 
a multi-battery system with a single alternator.  The odds of a failure 
given a very pessimistic view of battery life at 300 flight hours 
results in a failure rate that is well beyond even the lifetimes of our 
engines; ie., you will have an engine failure (statistically) before a 
total electric failure.  This to me is an acceptable risk; otherwise the 
naysayers are throwing about a bunch of red herring.

Laurence Coen wrote:
> Chuck,
>  
> I'd like to share my opinions and experience on the gyro issue.  The 
> gyros that are in my panel sat for 9 years and have been working fine 
> for three years.  The rest of this is opinion, so be forewarned.  I 
> think that a manufacturer that makes the 6 mo disclaimer has in fact 
> found a neat way of getting out honoring their warrantee.  Secondly, 
> I'd like to see how anyone could, if they wanted to, design a bearing 
> that would self destruct during lack of use.  A shaft sitting in a 
> bearing subject to gravity would logically force out the oil and make 
> metal to metal contact.  What isn't logical is that it would take 6 
> mo.  As soon as the shaft began to move it would wipe oil into the dry 
> spot.  If this was a real problem, which I doubt, you could solve it 
> by inverting the gyro every month and put it back on the shelf.
>  
> Larry Coen
> N136LC
>
> *From:* Chuck Jensen <mailto:cjensen at dts9000.com>
> *Sent:* Wednesday, November 26, 2008 6:12 AM
> *To:* Velocity Aircraft Owners and Builders list 
> <mailto:reflector at tvbf.org>
> *Subject:* Re: REFLECTOR: Vacuum Pump
>
> Several of the mechanical gyro instrument manufacturers will only 
> warrant a 6 month shelf life because bearing/lubrication issues if it 
> sets unused for long period of time and it takes an overhaul to put 
> them back to good condition.  Consistent with the observation that the 
> gyros in a derelict plane often don't work, or work for long, when 
> they are spun up after a couple years setting dead.
>
> Chuck Jensen