Blades Suddenly out of Track

This problem is also known as 'woof and poof'. When I had my raptor about half a year, ago the following happened. I had moved the servo rod for the collective from the middle to the outer hole (middle hole is 12mm, outer hole is 17 mm from the center of the servo). I did this to reach the pitch requirements for 3D flying: -9 to 10.5 degrees. However, my blades went heavily out of track (I estimate some 10cm!!) suddenly during the first test flight. I was hovering at a height of maybe 1 meter, and it looked perfectly stable until this problem occured after some 4 minutes of hovering, while slowly drifting to the right. Someone else checked the blades, the play, the connections etc, but did not find anything. But at a second test flight the problem occured again after some minutes of hovering, so it was no coincidence.
I put the servo rod back to the middle hole, and that resolved the problem (no problems in some 10 flights), so this blade tracking problem was certainly caused by the servo rod length. However, this did not give me my required pitch range!
The local shop owner suggested that the flybar rod or the feathering shaft might be bent. But we decided that vibrations would be there all the time, not suddenly after a few minutes flying.
In another shop I heard that the NiMH batteries may be the cause. When the servos are fully loaded or overloaded, they say that the current they draw increases dramatically, up to 600mA. If multiple servos would draw this amount at the same time, this might be enough to lower the supply voltage of the battery (due to the high internal resistance of the NiMH batteries), causing the servo to give way. As the blades rotate further the load suddenly ceases and the servo has a small time to recover, but after a full turn of the head the process repeats.
I replaced the batteries with NiCd (1400mAh, I could not get the 1700mAh I liked). I also made a custom servo rod with a hole exactly between the standard 12mm and 17mm hole (thus, at 14.5mm). This allows exactly enough range for 3D flying, if I set the servo throw to 140%. This way, I minimised the required servo torque. I have made hundreds of flights without any tracking problems with this configuration. I also never heard anything sounding as a anything coming close to the tracking problem. Concluding, I suspect the NiMH batteries unable to deliver enough current in slightly more extreme than standard setups.

Some time ago Jim Chapman mailed me that he and 5 people he knows have similar problems. He suggests that there is some eigenfrequency that you may hit and things start oscillating then. He suggested that changing batteries changes the weight of the system and thus the eigenfrequency, resolving the problem. I asked for more info on weights, used servos, servo arm lengths and rpms, to find out the relations, but I never received that info. Also, my NiCd batteries are not much heavier than the NiMH, and it makes hardly any difference on the total weight of the helicopter, and therefore I would expect the problem to re-appear at a slightly different rpm, which was not the case.

Maybe this problem is related to the low-frequency wobble problem: if your servo's are not strong enough (eg, a battery problem or servo problem), the low-frequency wobble may evolve rapidly into the blade tracking problem.

There are a few newsgroups having threads on this, for instance at rotory (local copy here).

© W.Pasman, 23/8/02