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Dunk into the ground during low-altitude flight

Flying fast means it's pitched forward meaning the highest pressure is at the rear and even behind the bird (rear props provide more trust). The barometer is thus located slightly forward of the pressure bubble, resulting in a gradual drop af at least a foot until it settles at that speed. Easing the pitch will make it gain a foot again. You can't fool a barometer. It's reading the pressure as it is. Flying high it doesn't make much of a difference and you wouldn't notice it. But seeing a -0.2m/s VS is scary when flying very low and fast.

Low = slow if you want to keep the pressure constant.
 
Thanks once again to everyone who chimed in and sorry for the delay in my reply, I've been travelling the past few days.

Roll on the day they install forward and downward laser rangefinders and a full terrain following mode
Ahhhh, then we will get to reminisce about the old days when the REAL pilots flew full manual with no obstacle avoidance and imperfect sensors :rolleyes::D

If the barometer is giving conflicting signals and you're below 10 feet, I'm going to guess the controller is going to more rely on the VPS, and WHAM.
Thank you for your input! Yeah, it makes sense that the VPS overrides the barometer, otherwise I guess the I1 would not follow terrain slopes like it does. So it follows that if the VPS is giving off incorrect readings like it would when operating at low altitude, high speed flights, it can easily cause a crash, rather than prevent it. Takeaway for me personally: avoid flying outside VPS operating specs, but if you ever do, keep VPS off!

One pretty simple workaround I thought of for achieving the same effect I was going for without taking any crazy risks is to simply fly slow while in low altitude and then speed up in post-production.

If there were any trees and slight wind in the area, you could have been the victim of turbulence. Remember there is an 8 X 1 ratio from the direction the wind is blowing AND in cold conditions the air mass is heavier adding the the accelerated downdraft
I was also thinking of the possibility of a downdraft contributing to the crash (I classify it under theory 3 - "something else that I do not know about caused the AC to lose altitude"). I was flying about 50m into a lake that had frozen over, so the terrain was close to level and there were no trees or significant topography around. I am not sure I understand what you are saying about the 8:1 ratio from the direction of the wind, but there was slight wind (I would guess around 2-3 m/s).

VPS was only intended to assist during low hovering and landing at places where GPS reception (like indoors) is too low, and to help when hovering low in Atti. Nothing else. It's not meant to be an extra aid for low flying.
To be honest, considering this, I think DJI should be blocking VPS commands to the controller when the AC is flying outside VPS operating limits.

Same as the auto gear. I had it coming down a couple of times during a very low flight and that's not good. Never switched that back on again after switching off 2 years ago.
Doesn't sound like too bad of an idea, I always keep the switch to the correct position anyway and lower the gear manually when landing, before the auto-lowering is activated. One question though - does the AC still lower its gear when executing RTH with the auto-gear switched off?

Flying fast means it's pitched forward meaning the highest pressure is at the rear and even behind the bird (rear props provide more trust). The barometer is thus located slightly forward of the pressure bubble, resulting in a gradual drop af at least a foot until it settles at that speed. Easing the pitch will make it gain a foot again. You can't fool a barometer. It's reading the pressure as it is. Flying high it doesn't make much of a difference and you wouldn't notice it. But seeing a -0.2m/s VS is scary when flying very low and fast.
Sorry for my incompetence in aerodynamics, but I'd be great if you can clear up some of this above:
- Why is it that the highest pressure is in the rear/behind the AC? My intuition says that the highest pressure should be at the front of the bubble - I am picturing the AC pitched forward, forming something similar to an upside-down aircraft wing (sloped toward the direction of movement rather than away), with the highest pressure forming at above the hull towards its back (where the battery status LEDs are) and lowest pressure under the hull at the very front (where the gimbal mount is).
- What causes the rear rotors to provide more thrust (given they are the exact same motors)?

I'd love to gain deeper understanding of these in effects. I also think DJI should be analyzing and building systems to compensate for these effects into their controllers (they should be, after all, relatively consistent).
 
To be honest, considering this, I think DJI should be blocking VPS commands to the controller when the AC is flying outside VPS operating limits.
Agreed.

Doesn't sound like too bad of an idea, I always keep the switch to the correct position anyway and lower the gear manually when landing, before the auto-lowering is activated. One question though - does the AC still lower its gear when executing RTH with the auto-gear switched off?
The correct position is meaningless anyway when you have auto-gear ON.
Yes the landing gear comes down with RTH, in fact it's the first thing it does as soon as it's over the homepoint. Stupid actually because a VRS is much easier to get with gear down. Comning down from 50 meters with landing gear down all the way is asking for trouble.
- Why is it that the highest pressure is in the rear/behind the AC?
Because the rear props are turning faster and are pushing the quad forward. The front props are turning slower hence the pitch forward.

My intuition says that the highest pressure should be at the front of the bubble - I am picturing the AC pitched forward, forming something similar to an upside-down aircraft wing (sloped toward the direction of movement rather than away), with the highest pressure forming at above the hull towards its back (where the battery status LEDs are) and lowest pressure under the hull at the very front (where the gimbal mount is).
You're talking about the draft the airframe produces. The propulsion always needs to overcome the draft. So the pressure under/behind the propulsion (whatever it is, in this case propellers), are producing a higher pressure than the pressure the airframe produces (draft), otherwise it wouldn't go forward. Draft at the front will be positive (pushing), while at the back negative (pulling).
What causes the rear rotors to provide more thrust (given they are the exact same motors)?
The IMU manages the electronic speed controllers (ESC's). Every motor has it's own ESC and the IMU controls them separately and at the same time.
 
Righty, so I completely missed taking propulsion pressure effects into consideration :confused: :D

I appreciate you clarifications, I am much clearer on the aerodynamics and what you are referring to in your previous posts now. I am still wondering whether these aerodynamic effects are not consistent enough for DJI to be able to program corrections into the controllers (given speed = calculated barometric measurement deviation -> respective correction to avoid unwanted altitude effect)?
 
Righty, so I completely missed taking propulsion pressure effects into consideration :confused: :D

I appreciate you clarifications, I am much clearer on the aerodynamics and what you are referring to in your previous posts now. I am still wondering whether these aerodynamic effects are not consistent enough for DJI to be able to program corrections into the controllers (given speed = calculated barometric measurement deviation -> respective correction to avoid unwanted altitude effect)?

That would be very nice for people who don't have a clue about what they're doing. But now you know, you take the aerodynamics in account with every type of flight and situation. Much better than a programming job that might suck in the end (when it's to late).

It's very easy to fly an Inspire but as soon as the limitations of the system kick in many people are lost.
 
That would be very nice for people who don't have a clue about what they're doing.
Can't say I agree with you:

1. The Inspire series is a prosumer/ entry-level professional line built mainly for film-making applications. There is basically no way to learn about the effects we are discussing (from courses, manuals, etc) short of getting an aeronautics degree. Even then, I don't think it is at all obvious, you would have to put in some thought to figure out the specifics. And you can't reasonably expect that every Inspire owner/pilot out there will be an aeronautic engineer. The other way of learning is the way I am learning right now - trial and error, investigating crashes and digging through forums, but you always run the risk of missing some vital piece of knowledge when its critical (just like I did).
2. Even when armed with the knowledge, it's not always possible to manually correct for deviations such is this one as well as a computer controller can. You are right about the limits and knowing about them, but DJI should be striving to be continuously pushing those limits and making sure their customers know about them in as much detail as possible.

I am not at all trying to put blame on anyone, I still think DJI did an amazing job with the Inspire. I am actually glad about what happened (especially considering the limited repercussions) and the learnings I got from it. Just sharing my thoughts and suggestions.
 
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