Total noob to traditional heli!

I saw Robert at Drone Con #1 talking about the traditional heli and now I want one! I'd like to get into this a cheap as possible and still get pretty good quality gear. I do FPV with everything else I've got so I'll want to do that too. So what kind of trouble can I get into with this new branch of APM?

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  • Well I finally solved the twitching problem. It's the servo cable picking up the 433Mhz telemetry signal. If I touch my finger on the servo motor, it stops twitching. If I shield the servo cable, twitching is gone as well. I'm gonna say these 306g are good servos, they are so quick that they can react to such fast signals.

    However I think I'm not going to dissemble my heli for shielding the cables. The shielding is not perfect, sometimes the wave can still penetrate it. Also the shielding makes the outside of the cable conductive, and I have to isolate them, maybe using heat shrink, it's too bulky and the cables become so hard to bend, not good for the PX4.

    Too bad 900Mhz is illegal in the Netherlands, maybe an Xbee 2.4Ghz can be an option? I guess 5.8Ghz wireless module is not that common.

    • I would have a long look at the grounding of the servos.  When I had that problem, that was the key.  The servos aren't actually moving at 433Mhz.  It's the noise impacting the PWM waveform.  It makes it dirty, and the servo is getting the wrong reading.  That's what makes it move.

      I had that problem with Xbees 900MHz.  Grounding the servos better to the APM helped a lot.

      • Yes I meant the noice in the PWM signal, it changes the pulse, that's why I wanted to use oscilloscope. I have measured the ground, all the Gnd connection are connected together, also connected to any Gnd connector of PX4 and the battery. Is there anything I should check specifically?

  • Brian and Rob,

    It does make some sense that something may happen inside the wire and circuit loop. The magnetic field disturbance generated by the servo is very weak, and the magnetic flux density decreases with a quadratic relation. Since the GPS module is very small, the magnetic flux goes through wouldn't be strong enough to influence the performance.

    Brian can you try wrapping the cable of your GPS with some copper foil? If the cable was picking up the magnetic noise this trick will eliminate or reduce the effect. If it doesn't improve the GPS performance, there may be something more complex we need to think about.

    • I don't think it's inductive coupling between the wires at all.  The servos must be generating noise near the 1.5GHz frequency which is the same as the GPS signals.  The problem is not that the GPS can't communicate with the APM, the problem is the GPS puck is not receiving a good signal from the satellites. It's a very very weak signal, and easy to interfere with.  

      • Robert and Xin,

        Having the servos running in very close proximity to the APM, but not connected to the APM (i.e. running on a separate power and signal source) produces no interference. It only seems to create a problem when the servo is directly connected to the APM.

        I was wondering if putting a low-pass filter between the servo and the APM might make a difference. Would I have to put a filter on all three wires, or just on the signal wire?

        • Hi Brain, I'm not sure whether a low-pass filter may work, most PWM signals for servo has a frequency of 50Hz - 499Hz as far as I can remember. A LPF with a bandwidth of about 1Khz seems to match, but it's very close to 1.5Ghz of the GPS signal, so I'm not sure about the effect. I'm sure a LPF will help if it's on Vcc, we can see a ferrite ring on most of the BECs. Maybe Rob can help you further with filters.

          Having discussed with my father (he's an electrical engineer) today about the interference problem we have been talking about in this thread, and I feel there is still a lot to do if we want to really nail down the interference and fix it.

          His point is, as the interference comes from high frequency and probably some pulse /short steam of signal from different frequencies, there can be too many cause of what we have seen. It can be ground loop, it can be wire behaving as antenna, it can be disturbance in Vcc, it can be cross-talk between the components, there are simply too many of them. By running a component connected and disconnected can't always reveal the source of interference because such change in the system is actually too much, we have eliminated too many source of interference when unplug the component.

          He told me some tricks and simple checking, like changing the input impedance of the component, insert a capacitor parallel, shield the suspicious source or cable, check the ground loop, filter, etc. These are some simple checking method we can do without an oscilloscope. But I guess I'm going to buy an entry level oscilloscope, it's good to learn about electronics and also I don't want my flying machine to hurt anyone :)

  • I was using the Corona... I think 939MG HV servos for a while.  They were pretty decent other than the fact the mounting ears snapped off every time I crashed.  I had to keep replacing them because of that.  The plastic was very fragile.

    • I went with the 929MG servos, they seemed to be a little more beefy than the 939, and they're around the same price as the 939. I'm attaching the full excel sheet here :)

      HK-450 heli build list.xlsx
  • I just saw that this thread has been revived, and I'm happy the dead has risen again. I have a question about replacing an old 450 kit. I have an old MX-400 heli that I could never get tuned out that used mechanical mixing. It was a nightmare to tune, and I spent a year just trying to get blades on the same level, which never happened. It has a Heli-Max Brushless 6 Pole Motor at 32A and 2580kV, a Heli-Max Brushless 25A ESC, 30C 3S lipos, and a bunch of Hitec HS-55 plastic gear cheap servos. Assuming I upgrade the ESC, because I was young and naive when I bought it and should never have, could I just transfer successfully over to an HK-450 frame kit using my current setup? I was specifically looking at either this kit or this kit, but I assume I need a flybarless controller for the latter one. I'm rebuilding an absolutely massive 750mm flybarred camera ship heli, and I need a trainer at low cost. From reading this thread, I was thinking about the belt driven flybarred kit, so my experience with the head would transfer over. Any advice? Thanks!

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