H-Quad flies like a dream and them...

HI Guys

I have built 2 almost identical H-quad's - Galactica and Pegasus. I find the layout and configuration spacious enough to house my camera, battery, flight controller ect. without exposing them. I do not use my quads for acrobatic endeavours, but still need them to be responsive enough to follow a flight path closely. The basic part of the configuration is :

1. Carbon fuselage

2. Aluminium motor arms through the fuselage held by silicon foam.

3. Turnigy 4206 620kV motors with 30A Afro ESC's(Simon K)

4. APM2.5 flight controllers with 3.1.2 firmware

5. frSky telemetry receiver and radio unit in 9X Transmitter.

6. BEC powering the APM, GPS and Receiver.

I experienced strange behaviour on Galactica in that it will fly perfectly for around 1 or 2 minutes and then start a wobble and drops. I've replaced the ESC's and did run tests on the motors but the symptoms remain. I had a look at the logs and beside some vibration (disappointing because of the anti-vibration construction) I can not find a cause for this. The only coarse that remains would be to replace the flight controller and receiver, but before I do this I would like to ask a second opinion. I attach my logs, if someone can have a look and volunteer an opinion it would be greatly appreciated.

Thanks,

Antonie

2014-02-18 08-16.kmz

2014-02-18 08-16.log

2014-02-18 08-16.log.gpx

Replies

MR60

Hugues February 20, 2014 at 12:00am

Hi,

Looking at your log you will notice that your input controls are absolutely not followed by adequate response from APM. For example the graph below shows your roll (in) and pitch (in) commands, and shows the respective outputs by APM (Roll, pitch) that are way way off your inputs. You have I think very bad PIDs. What are they ?
- Antonie Kruger > Hugues February 20, 2014 at 7:51am
  
  Hi Hugues, dang, I thought you were onto something. I looked at your plot. If you compare the roll with the rollIn you need to plot them on the same side of the graph to share a common scale. You've compared the rollIn with pitchIn left and Roll with pitch right, so its not comparing apples with apples. The R next to the item means right on the graph(I think).
  
  Thanks for having a look anyway. Attached the comparison plot from my log.
Antonie Kruger February 18, 2014 at 9:12am

HI Forrest, thanks for the quick response. The motor substructure is rigid. The idea was distributing any remaining vibration in the structure around the fuselage. The other quad with the same design flies very well.

Looking at how the failure happens, it seems like there is exactly that - something fails.

My APM is also mounted on silicon foam - I do not think it is the vibrations knocking her down.

Hi Michael - the first prototype had no brackets, the second had them very close to the fuselage. This is the third and fourth. I tend to agree that the foam pads and carbon supports this far out can cause excessive friction and vibration.
- Todd Hill > Antonie Kruger February 18, 2014 at 9:19am
  
  After looking at your logs you do have an excessive amount of vibration.
- Antonie Kruger > Todd Hill February 18, 2014 at 9:50am
  
  Thanks Todd, I have done the balancing. I have issues balancing the bells though. I've tried but seem to find vibration happens at different speeds once adjusted. I saw am IMU sensing solution to this somewhere - I'll try and track it down and see if I get better results. I've replaced some shafts and prop mounts after the upside down landing.
  
  I have spare motors. I'll swap them out and see the effect on vibration.
  
  Yeah, the supports brackets - Turning them vertical is a good idea, thanks. Losing the foam landing pads is essential. I'll laminate some carbon "feet" to sit under the fuselage over the weekend and re-orientate the support to vertical whist I'm at it.
  
  Thanks for the good suggestions and comments.
Todd Hill February 18, 2014 at 9:02am

Antonie, nice builds! Have you balanced your motor and props? Have you tried reflashing? Have you tried flashing an earlier firmware? It sounds as if you are having a build up of errors, thus causing the undesired flight attitude after a couple of minutes. I just downloaded your logs and will have a looksy.
- Todd Hill > Todd Hill February 18, 2014 at 9:07am
  
  Didn't see Forrests reply, but he is right. Using foam and other "anti-vibration" materials often times exacerbates problems with stabilization. In my experience I have found nothing that works better than a rigid frame that has well balanced motors and props. I don't even use the anti-vibration pads that come with most controllers. Thin Double sided Dubro Twin Stick is what I use.
Michael February 18, 2014 at 8:53am

Have you tried moving your carbon reinforcement brackets closer to the fuselage and away from the downdraft of the propellers?
- criro1999 > Michael February 18, 2014 at 9:20am
  
  i agree. I have also a H-shape quad and using additional bracket is not helping, since affect the airflow. maybe you can try to put them vertically eventually. try first to remove completely.
MR60

Forrest Frantz February 18, 2014 at 8:52am

My first suspicion would be that the anti-vibration is causing vibration. This is real easy to find out. Remove the foam and hot glue the motor masts to the carbon fuselage. Try it again.

Do two things. Look at your logs and see if vibration became less (it probably will). The stiffer the ship the better.

Then if it still wobbles, your PIDs are not optimized for your throttle setting. But before you go there, let's get a stable frame.

of 2

This reply was deleted.

ArduCopter User Group

Replies