endotherm
Second Officer
Flight distance : 503241 ft
Australia
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Nigel_ Posted at 2017-4-29 07:56
I'm sure it was assisted by the wind, however when it first went to 400ft it could apparently cope easily with the wind, I wouldn't expect huge and sudden changes in that area yesterday, a 40mph constant wind would be unusual at any time.
How do you explain that when it turned around to fly home under RTH, it pitched forward to fly fast back home, but it started moving away faster? It seems like it turned to the wrong direction as though it had a compass calibration problem. You can't trust the yaw, pitch and roll data from the logs if the compass was badly calibrated, it is not accurate data, and so you can't tell if it was flying against the wind or with the wind which means you can't calculate the wind speed.
Of course it can be dead calm at the surface, yet be quite windy a few dozen meters higher. You have no way of knowing what is going on at altitude as it is generally quite invisible. Sure, when it took off and went to 400ft it was in control, I previously mentioned this. The wind may have been 30mph gusting to 34mph. Once again, the p3 can handle that speed. It has enough power to correct any lateral forces and stay in control, holding position. Yet if the wind picks up by 5-10 mph you are going to have problems. That's nothing drastic/abnormal/unusual. It's not a huge change, but it's enough. There is no proof that it was coping with ease in these conditions, it was either stable, or it was out of control. When we notice it moved off, it lost GPS mode and went to ATTI mode. There were no pilot inputs holding it in position any more, and GPS mode wasn't contributing anything, so the only forces left is wind. This is what the data shows. The logical explanation is that it is wind. The most obvious solution is probably the right solution. The counter argument is to ignore all the data as unreliable on the basis of a few error messages, and assume the aircraft developed its own intelligence and decided to fly off on its own, or was so confused it had no idea what it was doing. Somehow that means it can fly faster than physically possible while maintaining a constant altitude, instead of just corkscrewing into the ground.
When they fly back in RTH the "nose" will turn to point towards home, the front motors slow in relation to the rear, This pitches the aircraft into negative pitch. The combined motor output spins up to 10m/s, leaving some capacity in reserve to maintain GPS mode stabilisation. Despite trying, it is unable to overcome the headwind, and it will pitch forward in the correct orientation into the wind, somewhere around -10° to -30° depending on how strong a wind it encounters. However it will travel backwards as it is pushed away. RTH will not come home "fast", it's more like medium speed. If you want full power you need to come home with full forward stick input which did not happen. I analysed the stick movements. They weren't present significantly when the aircraft flew off. They were activated in bursts to rotate the aircraft port and starboard. The compass headings changed in unison and consistently. The stick movement data is not dependent on compass calibrations,there is no reason to distrust them. Therefore there is no evidence that the compass readings are wrong.
For hundreds of entries there were compass readings recorded. There were no "compass error" warnings associated with them. Why do we need to discard all of them as untrustworthy?
There is no evidence of a faulty compass calibration, merely a speculation.
All a compass error is going to do is cause confusion regarding its heading and which direction it is facing. It won't affect the GPS coordinates being logged from a different subsystem. Even in ATTI, it is recording GPS location, just not relying on GPS for stabilisation.
I'm not privy to DJI's system firmware code, but it is quite logical to assume that the "compass error" message is a sanity check code. A value that is calculated or expected isn't making sense... "it does not compute". I haven't looked at them all, nor am I saying that this is the reason, but I saw a couple of examples where there was a stream of data, then there was data loss (broken connection with the RC). The data resumed for 100-200ms, then there was another gap, then the data resumed. During the one or two stranded lines of data, it showed a large change in orientation or speed from what was there previously. It looks like inconsistent data, but it could just as easily be correct in that the aircraft legitimately changed orientation and speed during the blackout. Curiously, there was a corresponding compass error message reported, but it disappeared once the data stream resumed. There were numerous dropouts and absent data periods during the flight.
With regards to the speed, it is calculated by the firmware, not me, It's a product of the change in GPS coordinates. The heading has no bearing on its speed. It can be easily verified by examining GPS coordinate pairs over time and calculating the difference in distance. Similarly the roll/yaw/pitch data is derived from an accelerometer which is calibrated independently of the compass. If there were this many errors in place, there would be a slew of IMU errors reported and the aircraft would barely fly. |
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