losing altitute with big x8

R

robyr1

Member
My coaxial x8 is equipped with U8 motors and 28"props..my main problems is the losing altitude!! Especially when i go forward of when i use the yaw
I tried all the gains but nothing happen! I can adjust all the gains with my knobs..but nothing happen.
We are talkimg of Devourer x8 from Foxtechfpv...

All the people with my same rig/setup but with Gemini fc works better with high wind also!!
 

ary

ary

Member
changing your gain will not help
Vibration effecting your IMU.
balance your props & motor. Also check your motor bearing
 

econfly

econfly

Member
Losing altitude when moving forward or yawing usually indicates the vertical gain setting is too low. It could also indicate a power problem (battery can't deliver needed amps, wires not able to carry current, etc.).
 

R

robyr1

Member
thanks guys! i'm not the first owner of these big X8 with A2 flight controller, and all ther other owners with my same setup has the same problem! With Gemini or Xaircraft flight controllers on my same setup the craft flight well with all wind conitions! i tried also all kind of vertical gains...nothing change. Wires and current are enough for the motors...
 

O

ovdt

Member
This problem exist on many users; on rcgroups you will find lots of X8 users are facing this problem. And this is also not related with A2, it also happens with MK FC.
 



Carapau

Carapau

Tek care, lambs ont road, MRF Moderator
I think it is something to do with the DJI algorithms for the X8 mixer rather than anything being wrong with the physical design of their kit. Its a bummer for sure but you quickly learn to compensate with the throttle easily enough. The Gemini is better in this respect but from my experience that's about it's only strength. My Gemini didn't fly half as nicely as my A2 or generally work as well.
 

R

robyr1

Member
do you fly with a X8 ? because the problem is only on this configuration..what i have to do now is to buy a Gemini, and try to sell the A2, because i can't wait "untill" they will make a new firmware update!
 

Bartman

Bartman

Welcome to MultiRotorForums.com!!
are you guys reporting this as a problem when in altitude hold or when just flying around without FC assistance?
 


Vojec

Vojec

Member
This is normal with low KW motors. When I switch from high KW to 400KW been so afraid, talk to some expert (Kopterworx) and they told me that is normal behavior with heavy birds. My lose around 2-5 m if I step fast forward. I've heard with red on it is normal up to 10 or 15m

cheers.
 


R

robyr1

Member
But with Gemini fc the story is different..and i see my same drone with same setup perfect fly with high wind too
 

Pumpkinguy

Pumpkinguy

Member
robyr1 said:
Happen always, especially when you use the yaw..or you go forward..no way to control the altitude
Click to expand...
If this happens with low kv motors why would you want to use them? U7's/490 with 17 or 18" props will give better performance at half the price. I've seen so many x8 rigs with the above motor prop combination that fly beautifully.
I also, think there is a gap left unfulfilled in the tmotor product line. The gap being between u7 and the next motors up. U8, u10 and u11.
 

JoeBob

JoeBob

Elevation via Flatulation
How does your FC measure height?

The Pixhawk uses a barometer to measure changes in air-pressure, which it then interprets as changes in height. Some configurations create a slipstream effect in fast forward motion that increases air pressure around the FC. The drop in air pressure when you stop forward flight makes the FC think that it is rising, so the FC drops the UAV to keep the air pressure stable, sometimes into the ground.
 

Bartman

Bartman

Welcome to MultiRotorForums.com!!
when the helicopter is level all of the lift is acting vertically to support the helicopter. if you don't change the throttle setting and lean the aircraft forward, that lift is now on an angle with vertical and horizontal components. the vertical component is now less than the weight of the helicopter so the helicopter will descend.
add a bit of throttle when leaning the helicopter forward and it won't descend. if there is an altitude hold function then that will compensate for the reduction of the vertical component of lift by adding throttle for you.
 

Carapau

Carapau

Tek care, lambs ont road, MRF Moderator
The reason for the drop is simple. As air increase flows over the pressure sensor, a reduction in pressure becomes apparent (Bernoulli's theory). Thus the sensor thinks that it has gained height and so it asks the machine to loose height to compensate. This is also why, when you stop, the copter regains the lost height. If the algorithms were written well, this could be compensated for. Of course the other option is to place the IMU out of the airflow. With regard to DJI's controllers, and this is where I am making a pure guess, it seems that the algorithms to deal with this are pretty bang on unless you are in the X8 configuration. DJI are not massive fans of coax copters hence why I suspect little work has been done on this configuration's code.
 


JoeBob

JoeBob

Elevation via Flatulation
robyr1 said:
ok thanks, for this reason i will try to us the IMU outside the center frame
Click to expand...

Hmmm. Not sure that's going to help.
Here's an in depth discussion of the problem :
http://diydrones.com/profiles/blog/show?id=705844:BlogPost:1802585&page=1#comments

3DR tried to mitigate the problem by adding open cell foam inside the Pixhawk Flight Control case over the Baro sensor.
All agree that it's best to mount the FC inside and out of prop wash.

Some people tape shut every unused opening in the FC to lessen the pressure shift inside.
Others have encapsulated the Baro sensor and run a tube out to either the front of the UAV (trying to get ahead of the pressure bubble of an oncoming UAV) or up the GPS mast in order to take Baro readings in air less disturbed by prop-wash.
 

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