Altitude not correct, causing issues


I’m running a pixhawk on a little 450 frame. I’ve recently rebuilt it from an APM2.something to the PixHwak. The stabilize works fine. I’m not a great pilot but the thing stays in position and so on. I’d rather fly in loiter mode, but i’m having trouble getting this thing to behave properly with regards to it’s altitude.

I’ve attached a logfile with the issue. It’s a super short flight, because quite quickly after take off i can hear mission planner going down in altitude, while the thing goes up. If i switched to loiter or pos-hold then i’m quite sure that it would go haywire. Because the PID’s are not setup correctly, going down quickly could easily cause a crash.

I’ve looked at the barometer. As far as i can see, that registers correctly that the thing goes up. So does the GPS. But if i look at the POS->Alt telemetry, then it goes down?

I’m not an expert, but i’m really puzzled by this one… Could somebody please help and review the logfile?

Logfile can be found at

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This is often a result of high vibration which is the case here. You have very hight vibes on the Z axis with accel clipping indicated. How is the FC mounted and are the props balanced? is it one of those cheap F450 frames with the plastic arms?


Thanks for the response!

Yes, it is one of those cheap frames. I’ve drilled holes and put the blue rubber mounts into the chassis. Then a normal plate above and the pixhawk on top of that. The rubber mounts are stretched. So the holes at the bottom are wider than the top. Not sure if that makes sense.

The props are not balanced, they are just cheap plastic things. I might rotate them to see if that’s any better.

What would I need to change to make this better?

Would you mind also explained why vibration is causing this? The baro and gps altitude logs seem fine…

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The first thing you should do is to balance your props



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Vibration doesn’t effect the barometer or GPS it effects the acclerometers. The Z-axis accel is used for vertical stability. Your’s is a classic example of the result of Z-axis clipping.

The props must be balanced and I don’t understand what you are descibing with the FC mount. Post a photo.


I realize my description of the mounting wasn’t so clear :slight_smile:
So, here’s a few photo’s:

In the past, i’ve not balanced the props. I need to figure out if i am able to do it as i don’t have the special tooling for that at the moment. I guess i can make a little rig to try and do it.
Normally i’d just order a balancing rig, but i’m currently on holiday and not in a location that easily allows this to be ordered.

That mount should be OK if the case of the Pixhawk is not touching the frame. I can’t tell from the photos if that’s true. Where do you mount the battery?

You will need a prop balancer for sure. The one I use is ~$8 and it works fine.

Thanks again!

I’ve made sure the pixhawk is free to move, even checked the cables. I can’t really do anything about the smoothness of the cables, but i have made sure it’s free moving.

I’ll try to see if i can build a little rig to balance the props. I’ll also try a flight without the front camera mount, as i suspect that might also cause some issues.

Thanks for the help so far!

You can remove the isolation and replace it with 3m tape and balance the props if you like then re calibrate.

Hi Again!

So, i did a poor-man’s job on balancing the props. Just hold a 2mm round tube in it horizontal and then see if they balance. Two props were very out of balance (the green two on the photo). I’ve just used a bit of tape to balance them as much as i could.

The result is a much better vibration on the z-axis (it stays below 30). I’m sure it can be better, but for now it’s ok.

The copter is an experiment for me. I know that with a better frame/hardware/props, it will fly a lot better!

Many thanks for all the help. Love learning about this stuff!