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P3 altimeter gain over flight duration

Discussion in 'Pro/Adv Discussion' started by BillP3P, Jun 16, 2015.

  1. BillP3P

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    I'm a new P3 pilot with only about a dozen flights on it, but have flown electric helis for last 6 years. I've started reviewing my flight data playbacks just to check for any possible error messages and noticed something peculiar with my altimeter (Height on playback). I start the flight at 0 feet but when I land in the same location after a 15 min flight or so, the altimeter shows anywhere from 10-30 feet. It appears to be biasing the altitude higher during the flight. Two questions: 1) Has anyone observed this and have a suggestion?; and 2) Does the RTH use this "height" for RTH fly back altitude and landing? Haven't used RTH yet but would like to know it will work (with this apparent increasing altitude bias) if I ever need it.
     
  2. Sammynels

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    I have observed this. Yes, the RTH does use the changed altitude. However, when using RTH the sonar senses the ground approaching and always lands "pretty" gently
     
  3. BillP3P

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    Thx Sammynels. The RTH auto landing was my major concern (slamming into ground thinking it was still 30 feet away). I wondered whether the sensors would compensate. If RTH uses it, I may need to bump my return altitude up to compensate for the bias. Better than RTH coming in low with obstacles... :)
     
  4. Rick Stef

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    RTH is awesome, Phantom3 was appx 1/2 mile down my street flying via my i6+, i attempted to land in friends back yard, i dropped below tree line and lost communication, Phantom3 increased altitude attempting to return home, i regained communication and continued the flight, hovered over their house filing, hit my RTH button, and within 2 minutes it landed within 3 feet of takeoff point.
     
  5. bbfpv

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    Anyone who has flown something w/ an altimeter has experienced this. Air temp (and thus barometric pressure) is lower than on the surface, which leads to a reading that says you're higher than you actually are. On a windy day you can actually observe the FC compensating for the colder air hitting the barometer. Hover like 30 feet off the ground and you'll notice the quad will yo-yo a few feet as it tries to figure out it's altitude as the wind gusts and then stops.
     
  6. caffeinated

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    So... given this does vps need to be on to cover for this scenario or is sonar engaged regardless? Given how many people turn it off either temporarily or permanently....
     
  7. BillP3P

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    I must say I'm a little surprised the IMU doesn't take precedent over the barometer. It should know pretty precisely how much the aircraft has moved in the X,Y,Z axis after startup. I can work around it since I'm aware of it. Will have to watch flight data files to see if bias is always (or at least mostly) in same direction... Thx for the input.
     
  8. bbfpv

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    Sonar is always engaged, but even if it weren't, it wouldn't be a problem. Regardless of how high the altimeter thinks the bird is, the motors disengage once there is no difference in barometric pressure from t0 to t3,, ie, it has stopped descending (whether it's auto-landing or you're doing it manually) and is sitting on top of something. The V+ also had different altitude readings depending on the climate and flight, and it was able to land w/o Sonar just fine. Of course, it would be a completely different story if the bird came in hot and slowed to land based on how high it thought it was. We all know that isn't the case.

    In short, VPS or not, false altitude readings or not, it will still disengage the motors once it has stopped descending. HTH
     
  9. BillP3P

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    I have a question relative to your response bbfpv. The temperature change affects the barometer? This made me think. I almost always take my aircraft from an air conditioned environment (car or house) and fly it almost immediately (after initializing but almost always within 5 minutes of the aircraft being at a 15-20 degree cooler environment for an extended time). I may try and let aircraft come closer to the outside ambient temperature before flying and see if that changes the symptoms...
     
  10. bbfpv

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    Yep. The molecules in warm air move faster than in cold air, which means pressure in warm air is higher than in cold air. Think of it in terms of a basketball in winter (or car tires)... cold air = slow moving molecules in the ball = a flatter ball. In the quad, the barometer is cheap and doesn't compensate well for the changes in pressure, thus the discrepancy in altitude. It's a problem in real planes too, but the computers are smart enough to compensate.