My Phantom grounded - did not pass the test. Need help

[djphantom2015]

Hi everyone. I need some feedback, maybe one of you experienced the same.

I grounded my phantom since 10 days because in my last 3 flights the front LEFT motor was playing strange. It all started while in idle, minimun speed after start. The front motor after 1 minutes beginns with making a very short buzz, the light goes off for a split second and the propeller slows down. This happened every 5/10 seconds. Nonetheless I have flown it with caution and the phantom behaved without any problem. No sudden movement, no strange noise while flying. I carefully looked at the footage to see if I could get some hints. Nothing. Now I tested the propeller speeds with the laser. The back motors are running at the same speed (around 2400 rpm), the front right motor is runnig at 3300 rpm while the LEFT at 2300 rpm.

So, I thought there is truly an issue here. I changed the esc first and noticed that the buzz disappeared nonetheless the speed is the same!

So I ordered a new motor. Mounted the motor and nothing has changed. Still running the same speed. I boosted the speed to around 4500 rpm on the right front propeller and noticed that the left was turning at a slower speed but the difference appeared to be much less (maybe 400 rpm).

So now I’m really confused. I don’t feel comfortable to fly until I understand what is going on.

All 4 motors after testing were warm the same.

I’m wondering if the problem could be on some misbehaviour of the Naza module. As long as it’s in idle things seem not ok. I also turned 180° the phantom to see if maybe there would be a change in speed. Moreorless it’s the same. I’m really curios if any one of you measured the rpm of the motors while in idle. If not maybe you should.

Any suggestions??

 

[mcphipps]

I wouldn't think that all the motors would run exactly the same except when they are at idle. You would think that they would be very close in speed. I am curious to see if mine does the same? I got to see if I can get a strobe to check rpm on mine.

 

[N017RW]

Testing motor rpm with out props or while the quad is held to the ground can be inconclusive.

The FC and ESC form a closed-loop control system.

When you give a flight command or stick input to the FC it evaluates the command and then instructs the 4 motors to operate in a way to carryout the command. At the same time the FC (via the IMU) is looking to see some change (feedback) in the quad's attitude indicating the command is being executed.

When you 'interrupt' this feedback by removing the props or securing the quad to the ground, bench, etc. it may not respond in the way you expect.

 

[djphantom2015]

I forgot to add that I tested the motors with the propellers on. I put on each tip the reflective strip and then used the laser. I tested and retested many times and always get similar results. I'm sure that once I give trust the motors behave correctly but I'm wondering if this is a sign off a potential problem.

 

[mcphipps]

Testing motor rpm with out props or while the quad is held to the ground can be inconclusive.

The FC and ESC form a closed-loop control system.

When you give a flight command or stick input to the FC it evaluates the command and then instructs the 4 motors to operate in a way to carryout the command. At the same time the FC (via the IMU) is looking to see some change (feedback) in the quad's attitude indicating the command is being executed.

When you 'interrupt' this feedback by removing the props or securing the quad to the ground, bench, etc. it may not respond in the way you expect.

So in reality, then you can really only test this with a strobe when the bird is at idle speed. Sure can't strobe it while it's flying...LOL

 

[N017RW]

I don't see 'idle' being a valid test point or condition either.
This does not mean it can't be or isn't, I just consider any situation other than flight to be open-loop.

Keep in mind if left idling, the P2 motors will slowly increase RPM to the point of a tip-over. This would seem to indicate drift in the IMU and the stationary nature does not provide inertial or movement feedback.


While flying (hovering) it's obvious if there is a problem as it will drift, tilt, oscillate, spin, etc.,anything but hover.

 

[mcphipps]

Yes
I agree.
But how else could you or even would test like this.
Is there any other way than let's say other than just observing the flight to see if your motors are not in sync?

I would think that the OP said that his flew just fine. Just the noise at idle speed. Is that something we should be worried about? I can see his concern on that problem. Wouldn't want to be 300 ft in the air and it decide to really screw up...I don't know how comfortable I would be in his shoes.

 

[djphantom2015]

Yes
I agree.
But how else could you or even would test like this.
Is there any other way than let's say other than just observing the flight to see if your motors are not in sync?

I would think that the OP said that his flew just fine. Just the noise at idle speed. Is that something we should be worried about? I can see his concern on that problem. Wouldn't want to be 300 ft in the air and it decide to really screw up...I don't know how comfortable I would be in his shoes.

Thank you for your understanding. I'm just looking to prevent a disaster. I never flew a rc and started with this phantom 2 13 months ago. Now I have some 130 flights and no crash. Maybe I was lucky but I learned to follow my guts. So if I notice something different than usual I want to understand what is going on. Since 2 months I'm flying in FPV and I brought my "aircraft" up to 1 mile distance from the top of a mountain at 1000 feet height. Imagine if the motor fails. I still think this "toy" is a work of art, but there are many scenarios we need to investigate.