[simpits-tech] Using real instruments
Craig Rochester
simpits-tech@simpits.org
Sun, 26 Jan 2003 15:46:45 -0500
Yes, the syringe concept is sweet; on paper. ;) It's near impossible to
have leak free sliding seals. My altimeter case doesn't really seem to have
a 100% air tight seal around the barometric adjustment knob. I think the
mechanics of making a bellows that could pull below 20 in Hg would be real
challenge as the 15 psi atmospheric pressure on the sides would tend to
collapse the bellows. To pull from ambient to 28 in hg would require roughly
a 15:1(29.92:2) volume increase of the Altimeter and piping dead space. The
altimeter volume is about 1 cup(1/16 gallon) so the bellows or syringe would
have to have about a gallon capacity! :O
I read up a bit on vacuum regulators and I think I can do a simple mod to
one of the eBay pressure regulators I have. It involves porting the vacuum
source to the regulator's exhaust port...pretty simple change. I'll try it
at work as I've no vac pump at home.
BTW, I made a mistake here:
"So with one vacuum signal and one pressure signal you could drive the
Altimeter(vac), ASI(vac and press), and AOA Indicator(vac). Where vac =
barometric pressure and pressure = pitot dynamic pressure."
I meant VVI, not AOA Indicator would have the same vac signal. I think the
AOA uses two pitot pressures, I'm not sure exactly how this works.
Craig R.
-----Original Message-----
From: simpits-tech-admin@simpits.org
[mailto:simpits-tech-admin@simpits.org]On Behalf Of Marv De Beque
Sent: Sunday, January 26, 2003 9:04 AM
To: simpits-tech@simpits.org
Subject: Re: [simpits-tech] Using real instruments
I had considered a bellows or syringe style vacuum pump. The only problems
I see is generating the force required to activate the bellows and
controlling the force so you can regulate the amount of vacuum.
That and not having the system leak. ;-)
Marv
On 1/26/03 12:14 AM, "Craig Rochester" <johncraigroch@msn.com> wrote:
> Thanks Marv,
>
> I'll get some pics and details posted soon.
>
> Before you modify your altimeter, you might want to consider using vacuum.
> VVI(VSI) would also use the same vacuum signal. The VVI measures the rate
> of change of the barometric pressure. I'm pretty sure it's a pneumatic
> gauge, most are. The vacuum pump wouldn't have to be very big as the
amount
> of air being pumped is tiny. I would need to pull a pretty deep vacuum;
> about 28 in Hg for 80,000'. Like this HVAC(refrigeration)unit:
>
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3308108396&category=26198
>
> The vacuum regulator would be very similar to the one I'm using for my
> G-suit rig, so the servo mod would be the same. It may even be possible
to
> modify a pressure regulator to regulate vacuum. The principles are the
> same, but everything is reversed(sort of).
>
> Another advantage with vacuum is that the instrument would be operating
> exactly as it was designed with no jury-rigging required. It sure would
be
> great to have the real instrument perform identically as in real flight.
So
> with one vacuum signal and one pressure signal you could drive the
> Altimeter(vac), ASI(vac and press), and AOA Indicator(vac). Where vac =
> barometric pressure and pressure = pitot dynamic pressure.
>
> Craig R.
>
> -----Original Message-----
> From: simpits-tech-admin@simpits.org
> [mailto:simpits-tech-admin@simpits.org]On Behalf Of Marv De Beque
> Sent: Saturday, January 25, 2003 6:50 PM
> To: simpits-tech@simpits.org
> Subject: Re: [simpits-tech] Using real instruments
>
>
> Great work, Craig.
>
> Once again, you have taken a lead role in this effort and have come up
with
> an innovative idea.
>
> I have the same type of altimeter and look forward to your final write up
so
> I can do the same.
>
> Marv
>>
>> Ok, I bought the Altimeter. I took it apart(a delicate task). It could
>> definitely be driven by vacuum, but I'm going to try to modify it to be
>> driven by pressure. The ASTEK meter I have works by the expansion of two
>> sealed metal bellows expanding as the atmospheric pressure decreases at
>> higher altitudes. I'm going to drill a hole in each bellows, port them,
> and
>> try pressurizing the bellows. In theory this will be equivalent to
> applying
>> a vacuum to the outside. If it works I'll post pics of the results.
>> Otherwise...anyone interested in a ruined Altimeter? :)
>>
>> Craig R.
>>
>> -----Original Message-----
>> From: simpits-tech-admin@simpits.org
>> [mailto:simpits-tech-admin@simpits.org]On Behalf Of Craig Rochester
>> Sent: Wednesday, January 08, 2003 8:39 PM
>> To: simpits-tech@simpits.org
>> Subject: RE: [simpits-tech] Using real instruments
>>
>>
>> I'm thinking driving the real instruments from two(2) electronic
> regulators
>> would be simpler than modifying each instrument. Particularly if you
were
>> doing a dual pit. I don't know what the leak rate is on the VSI, but the
>> rest could be run from a tiny compressor(and vacuum pump if required).
>> Barometric pressure varies from 14.7 psia(0 gage pressure) @ sea-level
and
>> decreases to about 1.1 psia @ 60,000 feet. If you could modify or adjust
>> the Air Speed Indicator and Altimeter to be off-set down by about 15 psi,
>> you could use pressure instead of vacuum.
>>
>> I know the pneumatic altimeters uses a sealed metal bellows that expands
> as
>> the barometric pressure falls with altitude. The movement of the bellows
> is
>> converted to dial rotation with gears and levers. Maybe there is enough
>> adjustment in the works to have the instrument read 60,000 feet with no
>> applied pressure and then output 1-15 psi from the regulator. Or maybe a
>> fitting could be fashioned to the bellows and connected to the regulator.
>>
>> I see electronic pressure regulators on eBay pretty often:
>>
>
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26210&item=3105137770
>> They're called an I/P transducer(eye-to-pee) current to pressure or
>> V/P(vee-to-pee) voltage to pressure transducer.
>>
>> Craig R.
>>
>> -----Original Message-----
>> From: simpits-tech-admin@simpits.org
> [mailto:simpits-tech-admin@simpits.org]
>> On Behalf Of Matt Bailey
>> Sent: Wednesday, January 08, 2003 5:04 PM
>> To: simpits-tech@simpits.org
>> Subject: Re: [simpits-tech] Using real instruments
>>
>> I remember talking about using an air compressor to control real gauges
>> several months ago. Sounds like a GREAT idea to me, a little more complex
>> than just using servos but the extra realism in gauge movement would
>> probably
>> be worth it. Not to mention the utter coolness of being able to drop a
> REAL,
>> UNMODIFIED instrument directly into your sim. The instruments themselves
> are
>> pricey though, unless you found an old junk instrument but that would
> defeat
>> the purpose of making your sim accept real instruments since the
> instrument
>> would not work. :)
>>
>> Driving gyro instruments like the turn coordinator I suppose could be
done
>> with servos driving the gyro cages (although the ball is just a ball in a
>> glass tube, it's not driven by any mechanical device in real life so it
>> would
>> probably have to be totally reworked) or just removing the gyros
> completely
>> and hooking a servo directly to the small aircraft representation. Anyone
>> done this?
>>
>> -Matt Bailey
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