Home » Simpit Discussion » simpits-tech » [simpits-tech] Brainstorming a static & pitot system
| [simpits-tech] Brainstorming a static & pitot system [message #2603] |
Tue, 24 January 2012 22:24  |
 dabigboy
Messages: 208
Registered: September 2010 |
Senior Member |
|
|
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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| Re: [simpits-tech] Brainstorming a static & pitot system [message #2604 is a reply to message #2603 ] |
Wed, 25 January 2012 02:41  |
Ben Jones
Messages: 39
Registered: March 2009 |
Member |
|
|
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@simpits.org
[mailto:simpits-tech-bounces@simpits.org] On Behalf Of dabigboy@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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Simpits-tech@simpits.org
http://www.simpits.org/mailman/listinfo/simpits-tech
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page. Thanks!
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| Re: [simpits-tech] Brainstorming a static & pitot system [message #2606 is a reply to message #2604 ] |
Wed, 25 January 2012 07:31 |
dabigboy
Messages: 208
Registered: September 2010 |
Senior Member |
|
|
Thanks for the idea. I actually had thought of something along those lines: making a much smaller reservoir, with a piston poking into it that is connected to a servo. I hadn't thought of a syringe though, that would work even better. My main concern is that the relatively short throw of the servo (and necessarily, the large effect that even small movements of the syringe) would make, would cause erratic or imprecise gauge movements. I'd also need a hefty servo (I think?) to put enough force on the syringe. It's worth a try though.......I may give this a shot for the pitot system, it would basically cut the complexity and parts count in half if it works out. Thanks!
Matt
---- bjones@pipecomp.com.au wrote:
> 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@simpits.org
> [mailto:simpits-tech-bounces@simpits.org] On Behalf Of dabigboy@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
> _______________________________________________
> Simpits-tech mailing list
> Simpits-tech@simpits.org
> http://www.simpits.org/mailman/listinfo/simpits-tech
> To unsubscribe, please see the instructions at the bottom of the above
> page. Thanks!
> _______________________________________________
> Simpits-tech mailing list
> Simpits-tech@simpits.org
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> To unsubscribe, please see the instructions at the bottom of the above page. Thanks!
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To unsubscribe, please see the instructions at the bottom of the above page. Thanks!
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| Re: [simpits-tech] Brainstorming a static & pitot system [message #2607 is a reply to message #2606 ] |
Wed, 25 January 2012 08:04 |
Ben Jones
Messages: 39
Registered: March 2009 |
Member |
|
|
Matt,
Ok if your using a servo (rc aircraft type) you will need to extend the
servo arm by a long distance and have it coupled to the syringe by a
extension also to give you the travel needed ie most of the syringe
travel, in saying that the resolution you will get will be very jumpy, due
to the indexing of the servo between movements, my suggestion would be a
stepper motor.
What i missed in my first reply is that you will have to hook the static
of the ASI to your "Synthetic static generator" otherwise you will get
erroneous readings from the ASI with altitude due to the loss of pressure
compensation.
Anyhow throw it all together on a test rig and let us know how she goes :)
Ben
-----Original Message-----
From: simpits-tech-bounces@simpits.org
[mailto:simpits-tech-bounces@simpits.org] On Behalf Of dabigboy@cox.net
Sent: Wednesday, 25 January 2012 11:32 PM
To: Simulator Cockpit Builder's List
Subject: Re: [simpits-tech] Brainstorming a static & pitot system
Thanks for the idea. I actually had thought of something along those
lines: making a much smaller reservoir, with a piston poking into it that
is connected to a servo. I hadn't thought of a syringe though, that would
work even better. My main concern is that the relatively short throw of
the servo (and necessarily, the large effect that even small movements of
the syringe) would make, would cause erratic or imprecise gauge movements.
I'd also need a hefty servo (I think?) to put enough force on the syringe.
It's worth a try though.......I may give this a shot for the pitot system,
it would basically cut the complexity and parts count in half if it works
out. Thanks!
Matt
---- bjones@pipecomp.com.au wrote:
> 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@simpits.org
> [mailto:simpits-tech-bounces@simpits.org] On Behalf Of dabigboy@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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| Re: [simpits-tech] Brainstorming a static & pitot system [message #2608 is a reply to message #2607 ] |
Wed, 25 January 2012 08:17 |
Gene Buckle
Messages: 1071
Registered: January 2009 |
Senior Member
Administrator |
|
|
On Thu, 26 Jan 2012, bjones@pipecomp.com.au wrote:
> Matt,
>
> Ok if your using a servo (rc aircraft type) you will need to extend the
> servo arm by a long distance and have it coupled to the syringe by a
> extension also to give you the travel needed ie most of the syringe
> travel, in saying that the resolution you will get will be very jumpy, due
> to the indexing of the servo between movements, my suggestion would be a
> stepper motor.
>
I'd suggest using a stepper motor like this:
http://www.ebay.com/itm/Miniature-ODV76-L92121-P2-Linear-Bi- Directional-Stepper-Motor-12VDC-/110810764668?pt=LH_DefaultD omain_0&hash=item19ccd6157c
You can firmly mount the motor and attach the threaded shaft to the
syringe plunger. You could add a microswitch that the plunger base would
trigger in order to set the "home" position and you're ready to go.
You might also want to check out Mike Powell's Building Simulated Aircraft
Instruments - he's got a great gear-driven ASI project that uses a stepper
motor.
g.
--
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| Re: [simpits-tech] Brainstorming a static & pitot system [message #2609 is a reply to message #2608 ] |
Wed, 25 January 2012 10:11 |
wledzian
Messages: 18
Registered: November 2010
Location: Auburn, Wa |
Junior Member |
|
|
In theory, what you propose does not sound technically difficult. In practice, though, it may be quite a bit more challenging. I'll address the issues one instrument at a time.
Airspeed indicator:
This is essentially a delta-P gauge calibrated to read in knots instead of pressure. You've got your static port taking ambient pressure and your pitot port taking total pressure. For this gauge, you can either apply vacuum to the static port, or you can apply pressure to the pitot port. Either will work, but simply moving a syringe to a set position will not be sufficient unless the system is completely leak-free.
Altimiter:
This is a simple pressure gauge calibrated in distance instead of pressure. For any positive altitude, a vacuum connected to the static port will provide an altitude reading.
In addition, while the motion may be 'smoother' than a servo-driven mechanism, it will still have a step size limited by the resolution of whatever device is driving it. To be accurate to +/- 10 feet over 40,000 feet, you'll need at least 12 bits of precision and very little slop in your hardware.
VSI:
This is a very sensitive rate-of-change meter. Fortunately, once the control of the vacuum plenum for the altimeter is worked out, simply connecting the VSI to the same plenum should suffice.
My suggestion:
Three plenums as follows:
-- One large plenum as a pressure sink, kept pumped down as low as your pump will keep it.
-- One smaller plenum for the airspeed indicator 'static' port.
- connect this plenum to the sink tank via a needle valve.
- Use a second needle valve (servo controlled) to adjust leakage into the airspeed indicator plenum. Pressure within this plenum is to be monitored and controlled via a PID controller to set desired delta-P.
-- One smaller plenum for the altimeter and VSI. Connected and controlled similar to the airspeed indicator plenum, pressure to be actively controlled to match the desired altitude.
With this setup, the high level of precision is required only of your electronic bits, specifically the pressure sensors and the ADC used to read them.
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