Hackaday Prize Entry: Density Altitude Gauge

Despite what extraordinarily overpowered quadcopters suggest, the air pressure of whatever a flying machine flys at is extremely important. Pressure is dependent on altitude and temperature, and there are hundreds of NTSB investigations that have concluded density altitude – pressure altitude corrected for nonstandard temperature variations – was the reason for a crash. Normally density altitude is computed through a slide rule or a flight computer, with the pilot entering in altitude and temperature, but somehow accidents still happen. For his entry to The Hackaday Prize, [Neil McNeight] is building an automated density altitude calculator to automate the process entirely.

Instead of having a pilot enter the altitude and temperature into a flight computer manually, [Neil]’s device grabs the current altitude from a GPS unit, and reads the temperature with a tiny sensor acquired from SparkFun. With just a little bit of math, this device will spit out the altitude an airplane or ‘copter thinks it’s at.

While the FAA won’t allow instruments that are cobbled together on a breadboard, this does have a few applications in the RC world. There are extremely high performance racing quadcopters out there now, and knowing how the craft will perform before flying it will save a few props.

The 2015 Hackaday Prize is sponsored by:

16 thoughts on “Hackaday Prize Entry: Density Altitude Gauge

  1. Why grab the altitude from a GPS receiver? The density altitude is based on pressure altitude, not altitude above mean sea level. It seems that every inexpensive MEMS barometer is temperature compensated, so they include the temperature sensor. An LPS25HB, from ST Microelectronics, would work perfectly.

  2. For several years I flew with ag (aerial spray) pilots (guys about three shades crazier than bush pilots). Density altitude was *crucially* important to them: they carried every ounce they could in payload because, well, it’s PAYload. Performance was cash at the end of the day. The good ones had an uncanny sense of exactly how much they could load on, which decreased as the day warmed up. They told me *humidity* was as important as temperature. On hot, humid afternoons the payload might decrease by a factor of two compared to the morning. As density decreases wings lose lift, props lose efficiency, and engines lose power. Triple whammy. A drone wouldn’t have to worry about engine power loss or wing lift loss, though the helicopter pilots I flew with (a far saner bunch), worry at least as much about it because their power margins are much slimmer.

    It’s fun to look at the performance charts for an underpowered craft like a Cessna 150 (or even a 172), and look at their required takeoff distance vs. density altitude.

    What’s really needed in the calculation is pressure, temperature, and *humidity*. Inferring the local pressure from the altitude (and depending on a satellite constellation to do it!) and assuming a standard atmosphere sounds convoluted and bass-ackward. Just measure it, dammit!

    1. Not an ag pilot – if they make helo pilots look sane, you have to wonder – but I fly in the soup of Midwestern summers in 172s and PA28s…late afternoon means long runways.

      FAA claims that humidity’s importance is a function of decreased engine output rather than aerodynamic effect:
      “Humidity is not generally considered a major factor in density altitude computations because the
      effect of humidity is related to engine power rather than aerodynamic efficiency. ”
      http://1.usa.gov/1S6xW9j [pdf]

      If they are to be believed, it likely wouldn’t affect non-internal-combustion powered vehicles.

    2. I met an ag pilot that was “out there”, he had a collection of props on the wall in his hangar, all with dings, dents and chunks missing.
      When I asked about them, he said “that one was a fence, that one was a power line” eyc

      Flying really low seems to do strange things to a persons mind…

    3. I am generally of the opinion Ag pilots have both a screw loose, are missing a few and don’t have them in order either. On top of that they must be afraid of heights. I’ve seen some Czech pilots that never went above 30 meters altitude. Not even on the way to or from the job. Just skimming the fields all the way.

      (There is no substitute to being woken up at 5 in the morning by a grumpy 9 cilinder radial reluctantly hacking and sputtering to life about 12 feet from your head though)

      1. Yes, and there’s little like a big supercharged radial running rich, taking off in pre-dawn light. Snarling flames coming out the exhausts, prop tips hitting supersonic and generating vortexes of fog. From the other end of the runway it looks and sounds like a portal to Hell itself coming at you. Good times, but maybe I was more easily amused in the ’80s.

  3. I have a printed circuit board that has all the components (and a PIC processor) to do this. Bare board is free if anybody wants one. I’ll add a page to my ranarchy.org pages about it. I think I have a public version of the PCB at OSHpark

  4. Just make sure the altitude is from a few measurements. GPS are notoriously bad a finding altitude even at the best of times. Giving +/- 50 feet in the best of times, presuming you’re not using differential GPS.
    You’re also not really getting the altitude above sea level. GPS give you the altitude above the 1984 datum, which may or may not be close to 1984 sea level depending on your location on the globe and the error inherent in the ellipsoid model.

    If you’re in the US, and have an internet connection, there are a few APIs you can use to find elevation based on lat/long and some government data sets from things like the shuttle missions and the USGS.

  5. Fun stuff!

    Original project shows Humidity & Temp (Adafruit HTU21); Pressure sensor (Bosch BMP180); and other goodies. So all set for density altitude. AWOS at GTU (Georgetown Texas would give the density altitude on hot day… basically in a Cessna 172 I was good with 1200 ft, make it a 104 low humidity afternoon…. and I better have 1800 ft…

    Cade making a living crop dusting in a Turbine AgCat…. getting that close to power wires, no thank you.

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