[simpits-tech] Brainstorming a static & pitot system

[bjones at pipecomp.com.au]

Matt,

Ive been at work all day so my brain is a little fired at the moment but
here is something to think about.

I'm certified to perform maintenance on gliders(sailplanes) in Australia,
when i certify ASI's its simple.

Using (i think a 200ml) syringe to apply pressure to the ASI i'm easily
able to dial up speeds around the 160knots mark, if you drove the syringe
with a stepper motor you would be able to drive the ASI.

Simple as.

Regards

Ben
West Oz


-----Original Message-----
From: simpits-tech-bounces at simpits.org
[mailto:simpits-tech-bounces at simpits.org] On Behalf Of dabigboy at cox.net
Sent: Wednesday, 25 January 2012 2:24 PM
To: Simulator Cockpit Builder's List
Subject: [simpits-tech] Brainstorming a static & pitot system

Ever have one of those ideas that is trying to bust out of your head so
bad that you have to talk abut it? Alright, so I've got this idea which I
have decided to follow through with....and I can't wait to figure
out/order the parts and start experimenting, so I've decided to post about
it.

I had the idea years ago to make a pressurized system that would be
controlled from a computer, so that you could connect real pneumatic
gauges in your simulator. But the success of servo and stepper
motor-driven gauges, plus the complexity of the system, led me to lean
towards those methods instead. Now that I have more knowledge under my
belt, and have actually started to experiment with more "traditional"
methods, I think I'm going to give the real thing a shot.

Specifically, we're talking about the three main instruments that use the
pitot/static air supply in an airplane: the altimeter (static pressure),
vertical speed indicator (static), and the airspeed indicator (static and
pitot). I looked up the formula and a benchmark table for determining air
pressure at various altitudes, to give an idea of what sort of pumps I
would need (and of course, the formula is what's needed in the control
software). It looks like I can get by with a fairly cheap pump, even an
AutoZone-variety tire inflator would do the job (tapping into the intake
portion of the pump, of course). One experimenter who needed a vacuum pump
for other things did just that, and reports that such a pump will give
around 25hg max, but more practically 20hg without burning out early.
Atmospheric pressure doesn't hit 9.92" hg until close to 30k feet, which
is higher than I typically fly (I could always get a better pump and
easily hit 25" or so regularly if this idea works 
 out).

My plan is to get one fairly large pressure tank and use the
aforementioned pump to drop its pressure as low as possible, say 10" hg or
so (as mentioned above, this corresponds to a little under 30k feet). I
will tap the tank to mount a cheap pressure switch in it to switch the
pump on when pressure goes up to, say, 11", and stop at 9.5" or so
(hip-shooting the numbers at this point). This big tank would be connected
to a much smaller tank, with a servo-controlled valve in between. The
smaller tank would be our "atmosphere" for the altimeter and VSI to tap
into. As the simulator aircraft climbs, the valve would open, venting high
pressure from my "atmosphere" tank into the big vacuum tank. A second
servo-controlled valve would open the smaller tank to the (real) outside
atmosphere. When the plane descends, the intermediate valve would close,
and this outside valve would open, allowing the pressure in the
"atmosphere" tank to rise.

For the pitot system, I will probably just vent the static portion of the
airspeed indicator to the real atmosphere to start with, and make a second
two-tank system like the first. The difference, of course, is that the
larger tank would be held at higher-than-atmospheric pressure, and air
from this tank will be piped to the intermediate tank, which is where the
pitot line from the airspeed indicator will tap into.

I *think* if things go well, I might try running the static portion of the
airspeed indicator to my first "atmosphere" tank, and adjusting the pitot
tank's pressure to account for the drop in atmospheric pressure. This
would have the handy benefit that, if I'm not mistaken, fancy airspeed
indicators with built-in Mach indicators will actually display an accurate
Mach number....which would just be way too cool. :)

Why so much trouble? A couple reasons:

1: I have built a servo-operated airspeed indicator, to start with, and
while it works well, I find the movement is just a little too notchy and
unnatural for the amount of movement (and speed) this gauge typically
makes (my turn coordinator is servo-operated and mostly looks great, but
that's an instrument that doesn't move so quickly or so far as the ASI). A
true pneumatic indicator would be far smoother....assuming a relatively
large tank (allowing for more precise pressure adjustment), the ASI, VSI,
and altimeter, all of which make fairly rapid and/or large movements,
would be incredibly smooth....essentially, exactly the same as real
gauges, since they are, in fact, real gauges being driven exactly the same
way they would be on a real plane.

2: Cost and complexity: if you think about what it takes to build and
drive these three instruments using servos or stepper motors, the pressure
system is actually far simpler, and might even end up costing less (unless
you manage to do everything with very cheap steppers and an Arduino with a
couple of shields, and fabricate your own instruments, of course).

3: You get to use real, unmodified aircraft instruments for three of the
most critical instruments on the panel. How cool is that? :) It does also
mean, of course, that extra "features" of the instruments (like the Mach
indicator, or an analog altitude readout that is common on older
turboprops, jets, and airliners) would be available and not require any
fancy mechanical trickery inside the gauge.

4: Since we are using real gauges without any modifications whatsoever, it
would be possible to quickly swap the airspeed indicator with another one
to reflect a different speed range of aircraft you may want to
fly.....four screws, two air lines, and possibly an electrical hookup for
lighting. Or you can switch from a 2,000ft/min VSI to a 6,000ft/min model,
for instance.

It also means I would be down to two of my primary instruments being
glass......attitude indicator and HSI. I already see how to hack up a real
attitude indicator (or make a new one) going off the example in Mike
Powell's book, but since I like my fancy glass software with a flight
director and other niceities, and since the HSI is a complex instrument to
build, I would probably keep these as glass instruments (no room for a
monitor, so a couple of graphic LCDs and an Arduino should do the
trick...and lots of software, of course). Plus, the plane I will
ultimately simulate, the Learjet 35, usually has a glass HSI and AI
anyway.

Unfortunately my sim is still sitting on its side in a corner of my new
garage, have not had time to set things back up yet. Plus my main priority
at the moment is to remove my ghetto-ized homebuilt yoke system and adapt
the fancy-shmancy PFC yoke I just got my hands on. :)

I can't wait to get started. :)

Matt
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