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I am building an RC aircraft, 10kg weight range. It is supposed to perform long flights (20km+), so reliability is my main concern, mainly on servos controlling Flight control surfaces.

How reliability is maintained on RC planes?

I am afraid if any flight control surface servo would be jammed while manouvering, crash would be quite horrible :-)

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Get the most reliable servos (and other gear) you can in the first place and over spec the servos and their attachments greatly so any mechanical stickyness can be forced out of the way.

If you are still not convinced that you have enough reliability, then look into how commercial airliners get higher reliability:

  • Have multiple batteries, all of them feeding the bus though diodes, so that if one battery craps out it will not bring down the others.
  • Split control surfaces and use independent servos to control each, mixed so each control surface is held below 25% authority in normal operations, that way you can count on being able to counter a stuck surface with the one left operational, just by switching off the excursion limit and accepting a lot of drag.
  • Fuse each servo at the central controller, so it cannot bring down the power bus if it shorts out.
  • Use servos which can report back their position and other data, like motor current like: http://www.openservo.com/ so the flight controller can detect stuck servos and try to fix the problem or at least let you know.
  • Run all servos from a controller that is able to power down individual servos, so if one servo goes nuts, then it can be turned off and allowed to weather vane.
  • Stick a complete autopilot with GPS and IMU in the plane (see: ArduPilot), to take it back to the launch site, if the radio craps out.

In the end I doubt any of those enhancements would do a lot of good in practice as all the redundancy will also add complexity and thus new sources of errors, just look at all the problems Airbus had getting their flight systems debugged.

... but I bet a whole lot of fun could be had building all of that redundancy.

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It really depends on the servos you are looking at, I personally would use the ones with brass or metal gears (usually higher end servos or bought as an upgrade kit). You will have to do some math on what kind of torque/power you will need to control those surfaces. Reliability will depend on lots of factors, but most RC plane servos last for quite a bit of time as long as you don't crash lol.

With My micro servos in my blade CP helicopter I had very few issues with the small servos until i crashed horribly, the crash was my fault..... after that it had chipped the gearing in the servo. In total i had about 25 flights with those servos over a year time span.

As for maintenance on the servos themselves, very little to none, but it never hurt to give them a little grease here and there.

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