[simpits-tech] Zero point on a two needle altimeter

[Craig Rochester]

I really have no experience with these instruments.  I was using this link
as a reference: http://home.swipnet.se/~w-65189/AP_html/pitot_static.htm  I
do have a lot of experience with pressure interments and pitot tubes.  I do
process engineering and HVAC at work.  I was interested in your discussion
on the instruments and from my standpoint, it seems a lot simpler to rig two
pressure(or one pressure and one vacuum) systems to drive 4 instruments
rather than modifying 4 instruments and linking to 4 discrete servos.  If
you're building a two seater, you get 8 instruments with two control rigs.

If you already have D/A voltage output from your control system, you can
hook it to a V/P like this one:
http://www.fairchildproducts.com/ic/xt_download.asp?table=fip%5Feptpdf&id=9&
filename=Model%5FT5700%2Epdf I bought this on eBay for  $30.  I'm using a
servo linked to a standard regulator because I don't want to pay the money
for an analog I/O card.  The Servo controller I use only costs about
$50(runs 8 servos),  and the regulators are real cheap on eBay.  Compressed
air and/or vacuum is an additional expense but if you're only using it for
the instruments, you could get by pretty cheap(very small capacity).

If you could offset the instruments' bellows adjustments to allow only
pressure to be used, it would be no contest compared to modifying each unit.
Also there may be some aesthetic difference on how the needles move from a
servo drive and  pneumatic actuation.

I could set you up with a servo/regulator test rig pretty easy... you could
try it.

Craig R.

-----Original Message-----
From:	simpits-tech-admin@simpits.org [mailto:simpits-tech-admin@simpits.org]
On Behalf Of Matt Bailey
Sent:	Saturday, October 26, 2002 10:06 PM
To:	simpits-tech@simpits.org
Subject:	Re: [simpits-tech] Zero point on a two needle altimeter



Craig Rochester wrote:
>
> The device is not for an altimeter, it's for a G-suit regulator.  The
> pressure range is 0-5 psi as configured.  I found a table that lists the
> barometric pressure vs. altitude:
> http://www.usatoday.com/weather/wstdatmo.htm#us It's about 30-3.4 inHg
> 0-50,000'.  If you convert that to vacuum at sea level, 50,000' would
> require about 27 inHg vacuum.  That's a pretty deep vacuum.  Maybe the
> "zero" setting of the instruments could be offset to read, say 50,000' at
> ambient pressure and the pressure rig could maintain a positive pressure
> above ambient to achieve the same thing.

	Yes that would probably be much easier. I *think* this would require
modifying the altimeter some if you wanted to be able to use the
Colesman window, if you just had to have this feature. I think it would
be "cool" but not really necessary.

> In PSI this is in the 1.6-14.7 PSI range.  That's pretty easy to do with
> stuff off eBay.  I don't know what the "velocity pressure" is on the pitot
> tube, I'd guess it's in this same range.
>
> Craig R.

	Well the airspeed indicator compares static air pressure with air
pressure at the pitot tube, if I'm not mistaken. If you did use an
unmodified ASI you would need to either calibrate your sim output for
the current barometric pressure in your sim room, or seal the static
port on the ASI so that it's always at the same pressure. If you wanted
to get fancy I guess you could have a single air chamber that is
pressurized to the atmospheric pressure your sim is indicating (like the
static port on a real plane), and hook all the static instruments to
this. Then make another chamber that is pressurized to the pressure at
the sim plane's pitot tube (just use the sim's indicated airspeed).
Cool, but seems unecessarily complex. I think the onyl other static air
instrument is the VSI, correct? I have no idea what's inside these
things but I guess if you did have this functional simulated static
system you could just hook the VSI up directly to it.

	-Matt Bailey
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