5 ms·

There is some confusion in this particular case stemming from the difference between altitude above sea level and above ground level. The UAV in question was s

There is some confusion in this particular case stemming from the difference between altitude above sea level and above ground level. The UAV in question was spotted at ~10,000ft above sea level, but it was only about 800-900ft above the local ground level at the time.

fnordfnordfnord 11y ago
There is no confusion. I assumed 10,000 ft above sea level. Starting at 9,000 ft doesn't help much. The air is quite thin at 10,000 ft; many small general aviation aircraft have difficulty reaching that altitude. Here is a YT video showing a few minutes of someone's drone flight allegedly at that altitude. It doesn't appear to me that sustained flight for any reasonable duration is possible with that particular drone, nor do I expect any/many other drones to perform much better. https://www.youtube.com/watch?v=WnnlAS5Zzfg https://www.youtube.com/watch?v=WnnlAS5Zzfg
- votingprawn 11y ago
  Apologies, your phrasing "make it to that altitude" made me think otherwise! Multi-rotors using BLDC motors cannot really be compared to small general aviation aircraft with infernal combustion engines. "Reasonable duration" is a nebulous phrase. Some of the airframe configurations we use here couldn't manage more than about 10 minutes at 100ft, whereas others could quite happily carry a gopro at 10,000ft for 10 minutes (assuming it didn't have to climb from sea level!)
  - fnordfnordfnord 11y ago
    re: my phrasing, I did assume that the drone pilot in question started from a much lower (unknown to me) altitude. >Multi-rotors using BLDC motors cannot really be It's got nothing to do with the type of motor or fuel. >"Reasonably duration" is a nebulous phrase. So So is "quite happily carry". I'll admit that it appears that it's not impossible for such a flight, but I don't agree that any drone is "happily carrying" anything at that altitude.
    - votingprawn 11y ago
      Well I gave a payload (gopro, 350-400grams with gimbal) and a duration (10 minutes), I'm not sure what else to say! The aircraft was completely controllable with no memorable handling differences apart from a very slightly reduced response to high thrust demands, but as we fly our platforms for stability rather than speed it wasn't something that we really noticed.
      - ChuckMcM 11y ago
        The thing is, at 10,000' you have to move a lot more air in order to maintain the thrust necessary to stay at altitude. This additional RPM requires more current and that cuts into your battery life. It would be an interesting comparison to run a number of drones at 10K' to see how they held up.
        votingprawn 11y ago
        One of my colleagues has done some investigations into the effect of density on multirotor performance before. I don't have time to run the model for this specific case, but from a similar case with a lower air density of 0.71kg/m^2 (40c at 4km altitude) a simple quad uses ~20% more power in a stationary hover than at sea level. Interestingly at high flight speeds (15-20m/s) you could actually start to see power savings at this altitude, as the parasitic drag of the vehicle body is reduced.
        Vexs 11y ago
        There was a post on reddit/r/multicopter a while ago of prop thrust at different air pressures with the same current. It's pretty much exactly what you're asking for. https://i.imgur.com/Z5toIVm.jpg https://i.imgur.com/Z5toIVm.jpg Assuming air pressure at 10000 ft is ~69kpa, and sea level is ~100kpa, there really doesnt appear to be much of a difference, except for medium-sized props, which had very interesting thrust outputs- gaining efficiency at low rpm and loosing a lot at high rpm. Anyway, point being is a 250 class quad could pretty easily fly at 10000 feet. Not particularly well, but considering 1:4 (or even 1:8) thrust ratios arent uncommon with 250 class quads they could still pull it off. Now, idk about bigger quads like the phantom/iris, their props have quite different geometries which probably act quite differently.
- mikeash 11y ago
  Flight at that altitude is mostly problematic for GA aircraft because of the engines. Less air into the engine means less power produced means climbing is harder. Most small drones these days are electric, so that doesn't happen. It's a bit harder to generate lift from the props in thinner air like that, but you also experience less drag, which balances that out to a large extent.
foobarbecue 11y ago
I fly standard quadcopters above 10,000ft ASL regularly. Soon I will be on a trip where I will be taking off from aronud 20,000ft. Increase prop size or pitch to compensate and you don't lose any thrust, but you do lose some efficiency, esp. if you go for higher pitch. On Erebus (~ 0.65 atm) I had about half my sea level battery life fully loaded. https://www.youtube.com/watch?v=gMh8kM4o6tk https://www.youtube.com/watch?v=gMh8kM4o6tk

fnordfnordfnord 11y ago

There is no confusion. I assumed 10,000 ft above sea level. Starting at 9,000 ft doesn't help much. The air is quite thin at 10,000 ft; many small general aviation aircraft have difficulty reaching that altitude. Here is a YT video showing a few minutes of someone's drone flight allegedly at that altitude. It doesn't appear to me that sustained flight for any reasonable duration is possible with that particular drone, nor do I expect any/many other drones to perform much better. https://www.youtube.com/watch?v=WnnlAS5Zzfg https://www.youtube.com/watch?v=WnnlAS5Zzfg

votingprawn 11y ago
Apologies, your phrasing "make it to that altitude" made me think otherwise! Multi-rotors using BLDC motors cannot really be compared to small general aviation aircraft with infernal combustion engines. "Reasonable duration" is a nebulous phrase. Some of the airframe configurations we use here couldn't manage more than about 10 minutes at 100ft, whereas others could quite happily carry a gopro at 10,000ft for 10 minutes (assuming it didn't have to climb from sea level!)
- fnordfnordfnord 11y ago
  re: my phrasing, I did assume that the drone pilot in question started from a much lower (unknown to me) altitude. >Multi-rotors using BLDC motors cannot really be It's got nothing to do with the type of motor or fuel. >"Reasonably duration" is a nebulous phrase. So So is "quite happily carry". I'll admit that it appears that it's not impossible for such a flight, but I don't agree that any drone is "happily carrying" anything at that altitude.
  - votingprawn 11y ago
    Well I gave a payload (gopro, 350-400grams with gimbal) and a duration (10 minutes), I'm not sure what else to say! The aircraft was completely controllable with no memorable handling differences apart from a very slightly reduced response to high thrust demands, but as we fly our platforms for stability rather than speed it wasn't something that we really noticed.
    - ChuckMcM 11y ago
      The thing is, at 10,000' you have to move a lot more air in order to maintain the thrust necessary to stay at altitude. This additional RPM requires more current and that cuts into your battery life. It would be an interesting comparison to run a number of drones at 10K' to see how they held up.
      - votingprawn 11y ago
        One of my colleagues has done some investigations into the effect of density on multirotor performance before. I don't have time to run the model for this specific case, but from a similar case with a lower air density of 0.71kg/m^2 (40c at 4km altitude) a simple quad uses ~20% more power in a stationary hover than at sea level. Interestingly at high flight speeds (15-20m/s) you could actually start to see power savings at this altitude, as the parasitic drag of the vehicle body is reduced.
      - Vexs 11y ago
        There was a post on reddit/r/multicopter a while ago of prop thrust at different air pressures with the same current. It's pretty much exactly what you're asking for. https://i.imgur.com/Z5toIVm.jpg https://i.imgur.com/Z5toIVm.jpg Assuming air pressure at 10000 ft is ~69kpa, and sea level is ~100kpa, there really doesnt appear to be much of a difference, except for medium-sized props, which had very interesting thrust outputs- gaining efficiency at low rpm and loosing a lot at high rpm. Anyway, point being is a 250 class quad could pretty easily fly at 10000 feet. Not particularly well, but considering 1:4 (or even 1:8) thrust ratios arent uncommon with 250 class quads they could still pull it off. Now, idk about bigger quads like the phantom/iris, their props have quite different geometries which probably act quite differently.
mikeash 11y ago
Flight at that altitude is mostly problematic for GA aircraft because of the engines. Less air into the engine means less power produced means climbing is harder. Most small drones these days are electric, so that doesn't happen. It's a bit harder to generate lift from the props in thinner air like that, but you also experience less drag, which balances that out to a large extent.

foobarbecue 11y ago

I fly standard quadcopters above 10,000ft ASL regularly. Soon I will be on a trip where I will be taking off from aronud 20,000ft. Increase prop size or pitch to compensate and you don't lose any thrust, but you do lose some efficiency, esp. if you go for higher pitch. On Erebus (~ 0.65 atm) I had about half my sea level battery life fully loaded. https://www.youtube.com/watch?v=gMh8kM4o6tk https://www.youtube.com/watch?v=gMh8kM4o6tk