[simpits-tech] Brainstorming a static & pitot system

[dabigboy at cox.net]

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