Aerodynamics behavior question

[kns1313]

Flying today in winds 0 -10 knots. Over the course of an hour, programmed flight plan altitude stayed constant, but the aircraft would not hold altitude. Highly variable, at one point enroute altitude set for 13 feet yet the aircraft touched down. Other times altitude would deviate -3 to -6 feet from that programmed.

Questions: Does a quad respond the same as a fixed wing aircraft to increasing tailwind, i.e. it can drop out of the sky (flies at 10 knots with sudden tailwind of 10 knots = 0 forward airspeed and converts from aircraft to rock)? I can see where it would, but is this reality? Would there be something else that causes erratic altitude control?

 

[kns1313]

FYI, using Litchi under a Waypoint mission on a Nexus 2013.

 

[Meta4]

Flying today in winds 0 -10 knots. Over the course of an hour, programmed flight plan altitude stayed constant, but the aircraft would not hold altitude. Highly variable, at one point enroute altitude set for 13 feet yet the aircraft touched down. Other times altitude would deviate -3 to -6 feet from that programmed.

Questions: Does a quad respond the same as a fixed wing aircraft to increasing tailwind, i.e. it can drop out of the sky (flies at 10 knots with sudden tailwind of 10 knots = 0 forward airspeed and converts from aircraft to rock)? I can see where it would, but is this reality? Would there be something else that causes erratic altitude control?

The Phantom shouldn't be affected by a tailwind as a fixed wing plane is because the Phantom doesn't have a back and front - every direction can be the front.
It's not a good idea to program a waypoint mission at low altitudes.
You could try a new IMU calibration and see if that does anything to stabilse the Phantom.

 

[Mark The Droner]

...at one point enroute altitude set for 13 feet yet the aircraft touched down.

You know altitude is not really altitude, it's height from launch point. So if elevation is plus 13 "enroute" it will touch down at 13 feet "altitude"

 

[MacCool]

Questions: Does a quad respond the same as a fixed wing aircraft to increasing tailwind, i.e. it can drop out of the sky (flies at 10 knots with sudden tailwind of 10 knots = 0 forward airspeed and converts from aircraft to rock)? I can see where it would, but is this reality? Would there be something else that causes erratic altitude control?

No it's not reality. The formula that you propose for "0 forward airspeed" and "dropping like a rock" has no basis in reality and violates the most rudimentary principles of aerodynamics, fixed-wing aircraft or rotorcraft.

 

[flyNfrank]

The P3's propulsion system is actually outstanding in doing a very nice job at maintaining altitude while in flight.

However, there is something that comes to mind in your situation. First I have no experience in this particular issue. But I do think it might be possible that your Aircraft is getting strong enough tail winds that is causing pressure changes with the barometer and introducing a mixed signal. Now that I think about it, I do have some experience with what I mentioned. I make Top Speed Flights which consist of staging the Aircraft in a hover while waiting for the biggest invisible gust of wind to come along. At times the wind will push the Aircraft from it's position, but never can I remember it descending due to a pressure change being staged.

 

[kns1313]

"The formula that you propose for "0 forward airspeed" and "dropping like a rock" has no basis in reality".

Please read https://www.faasafety.gov/files/gsl...7/FAA P-8740-40 WindShear[hi-res] branded.pdf, p. 5, para 1 under Situation 1. Aircraft see gusty winds the same as wind shear. The differences between us probably arise from my statement "0 forward airspeed", as it is relative to the air mass, not the ground. Fly an RC aircraft at 10 knots in a 10 knot crosswind. See what happens when you turn into the wind and when you turn downwind. The result is what prompts my poorly articulated situation.

My altitude was set via reference to the launch point. I did a practice run to see what indicated altitude was necessary to avoid obstacles, then added 6' for a bit of a pad. This value was programmed into Litchi, using the same T/O point, so AGL heights were the same between the practice run and the actual flight. However, this was not maintained. The only reason I could see would be wind effects. The IMU was recently calibrated, but it is a good suggestion to recalibrate.

Any other possibilities out there for the aircraft descending uncommanded? I won't fault Litchi, but there is certainly a natural gremlin afoot.

 

[NimpoCub]

Altitude is "held" by barometric pressure. If your bird experienced a lower pressure (turbulence) it would think it was too high, & descend.
That's my 2c worth. I accept checks.

 

[kns1313]

Hmmm, both pennies, not sure I can fit that into the envelope . . .

I know the different altitude references on the bird, I guess the question really is "How does it process the information?" I've had it descend from 10' on a waypoint mission, bounce once in tall grass, then climb along flight path quickly back up to cruise altitude (10' AGL). Winds where I was standing were calm, but among the hills and tall trees the microclimate could change and the aircraft experiences a gust and reacts accordingly in a negative way. Understanding that could help alleviate some of the problems others have reported concerning controlled flight into terrain.

Just want to understand what is really going on internally. Anyone can pick up a controller and fly, but until they understand all the ins and outs to safe flight, they aren't really a good pilot. It's the difference between the cook following a recipe, and the chef who knows how the tastes, smells, and visual presentation interact to make a good meal.