“Aircraft” instruments in a Jeep?

Miscellaneous and Personal Stuff I Must Share
1

  Since getting my Jeep, I have found myself, in some circumstances, wanting to have an objective sense regarding its attitude and movement, that I associate with aircraft-type instruments.

  Most specifically, but not the whole of it, is that I often find myself wanting to know, with objective certainty, whether I am going uphill or downhill, gaining or losing elevation, and at what rate.

  To a lesser degree, I want an objective sense of what my pitch and roll angles are, relative to what they would be on flat ground.

  My direct, seat-of-the-pants sense, as well as that which I get from the few gravity-based instruments that I have obtained and installed are subject, of course, to being seriously distorted by movement, by acceleration, deceleration, and turning.

  At some point, I was thinking that what I really wanted was a genuine artificial horizon instrument, as commonly present in aircraft, along with a rate-of-climb indicator. On Googling artificial horizons, I have found that in general, they are prohibitively expensive for my application.  One thing I had sort of dared to hope is that somewhere, there is a market for old aircraft instruments from crashed or scrapped aircraft, not deemed reliable enough for aviation use, but still reliable enough for use in my Jeep.

  I have since come to understand that the range of pitch angles that I experience in my Jeep is really not great enough to register clearly on an artificial-horizon type display, that to get a useful sense of my pitch, I need something that either displays the angle numerically, and/or in a more exaggerated sense than an artificial horizon would.

  I did buy one multipurpose instrument, that among its features, implied a claim that it was a gyroscope-based inclinometer, but that representation appears to have been false; the inclinometer function behaves exactly as I would expect from a gravity-based inclinometer; showing the exact same skew from turning and acceleration/deceleration.  One thing that I do still believe that I want is an inclinometer that is truly gyroscope-based.

  Another parameter that I want to be able to accurately measure is elevation above sea level.  A few of the trips that @Seanette and I have taken have been about “getting high”, and I’d like to be able to directly and accurately measure just how high we get.

  The first instrument that I got for my Jeep was an altimeter, that turned out to be of disturbingly poor quality for its price.  Build quality issues aside, we quickly took notice of a rather significant discontinuity.  It we calibrated it for sea level, and then went along a freeway leading uphill, that had elevation signs along the way, we noticed that this altimeter would remain consistent with those signs, up to about 2,000 feet, but would quickly deviate above that point, so that by the time we got to 3,000 feet, it was reading several hundred feet lower than what the signs said.  On one trip, at Sonora Pass, elevation 9,624 feet, I calibrated it there to match that elevation, and on the way back down, noticed the same issue in reverse; it was consistent with the roadside elevation signs, down to about 3,000 feet, but then deviated badly so that by the time we were down to 2,000 feet, it was reading several hundred feet high.

  A different, portable altimeter that we later obtained, seems to show the same discontinuity, so perhaps this is a genuine discontinuity in the air pressure in that altitude range.

  Barometric altimeters for use in aircraft must be more sophisticated, and must include some way to account for this discontinuity, that cheaper altimeters do not.

  The one multipurpose instrument that I described earlier, does include an “altimeter” function, but it is GPS-based, and not barometric.  It seems to be the best elevation indicator that I yet have, usually being within a hundred feet of whatever elevation signs we pass.  But it must be possible to do better.  There must be some dark art to getting a truly accurate measurement of one’s elevation above sea level.

  I guess that’s about as well as my thoughts and experiences on the subject are formed, at this time.  Anyone have any advice for me, as to what instruments I might obtain at reasonable cost, to help me to better measure…

  • Elevation above sea level
  • Rate of climb or descent
  • Pitch and roll angles of my vehicle, not distorted by acceleration, deceleration, or turning

2

A few random comments:

A friend of mine had a lean-and-tilt indicator in his Land Rover. I never looked inside, but I assume it was gravity-based, as you described. According to him, all the “built Ram tough” and other such commercials that showed trucks charging over bumps and bouncing off the ground were bullshit. The best technique for off-roading was to use the low-range gearbox and go slowly up hills and over obstacles. If that’s the case, then there won’t be any effects of acceleration or sudden movement, and a gravity-based gauge would be fine. That said, if you do manage to rig up one with a gyroscope, that would be pretty cool.

I wonder if the inaccurate alitmeters you tried are assuming a linear relationship of pressure to altitude. I think it would be opposite of what you described; that as you gain altitude the gauge would read lower than the correct altitude, not higher. It’s also possible that the weather is changing between when you calibrate the altimter and when you drive to some high place. If a low- or high-pressure system moves in, that will affect your reading. Pilots get weather updates so they can recalibrate their altimters en route.

I don’t know how much stock I’d put in the elevation signs by the side of a road. They have to be placed where conditions allow, and that may not be exactly at 3,000, 4,000, or whatever elevation they say. I’d be more inclined to trust the GPS, within its displayed margin of error.

3

  No disagreement, here.  Nevertheless, I have found myself in positions where, between tree cover, unlevel ground, and other factors, I find myself wishing for a much better, more objective sense of my actual attitude; free from any distortion caused by any motion.

  More prominently, even on smooth road, I’ve wanted a similarly reliable, objective sense of whether I am going uphill or down, whether I am gaining or losing elevation, and at what rate. So far, my best sense has come from judging how hard I feel the engine working to maintain speed, but that’s subject to distortion not only due to acceleration or deceleration, but also to what my absolute speed is, and even, to some degree, to the engine having to work harder at higher altitudes, in thinner air, to maintain the same about of power.

  The first time or two that I observed the discontinuity in reading from my first altimeter, I attributed it to possibly-changing weather conditions.  I am well aware that barometric pressure can vary, even at a fixed elevation, and that is the entire point of a barometer as a weather tool.  But if that was the case, I would not expect the error to be so consistent, on multiple trips, on multiple days.  The discontinuity appears to be real, and consistent, at least in the mountains east of Sacramento, between 2,000 and 3,000 feet. Calibrated for sea level, each barometer continued to read accurately up to 2,000 feet, but was off by several hundred feet by the time it got up to 3,000 feet.  Calibrated at Sonora Pass, for 9,624, the first barometer read accurately down to 3,000 feet, then was off by several hundred feet by the time it got down to 2,000 feet.  I only just now got around to looking up how to calibrate the newer electronic barometer, so I have not yet seen an accurate reading from it above 3,000 feet, but I have observed that above 3,000 feet, the absolute amount of the error remains fairly constant.

  I guess I’ve taken it on faith that those signs are placed by people who have access to better technology than I have, to measure the elevation accurately.  They do seem to be fairly consistent with the readings form my GPS-based altimeter, to the degree that GPS is able to provide an accurate elevation.  But it is well-known that GPS is not nearly as accurate about vertical position, as it is about horizontal position, ad @Seanette and I have observed significant deviations in the elevation readings therefrom, even when stationary.  It does seem that for now, GPS-based technology is what gives us the best elevation readings.

4

Graph of pressure / altitude.
wikipedia

Correction tables

5

Heck, I had an aftermarket 2 axis tilt instrument on my motor home in the 1970s. They must still be available.

6

I would think the main reason to worry about the attitude in your jeep is to know how close you are to rolling over. A gravity based gauge would be more accurate for that than a gyro one. (you can roll over on flat ground with too fast a turn).

You could get a real aircraft gauge to put in your jeep, something like uAvionix AV-20-E Multi-Function Display. Cheap by aviation standards ($895).

For altitude the cell phone apps for a phone that has a barometer input are probably as good as anything.

7

For altitude I usually use the GPS.

I’m coming from a Jeep that I owned for 10 years. Now I have a Subaru Outback and it has a handy incline meter built right in. It is very handy to have this data available to the driver.

8

Have you checked eBay? All sorts of odd stuff there.

9

Look at Garmin devices. They do have a line of aircraft GPSs but I’m thinking one of the handheld / eTrex models might have all you’re looking for.

10

The inclinometer in my Outback is nice to have, but it is affected by accel and decel. And probably by lateral g also although I have not looked for this. So, not optimal.

11

You might want to look into instruments for hang gliders etc cheaper than the ones that go in 2 planes and many of those are portable like a GPS receiver of yore.