4 servo support for tradionnal helicopter

Hello everyone

I need to fly a Vario Benzin autonomously. however, the present code does not support the 4 servo swash plate of the Vario.

I am aware of a custom building on the version 3.3.3 which could support 4 servos, but the internal gouvernal option is not available on that version and the Vario Benzene is a gas Heli.

Is there a newer build which could support a 4 servo swash plate?Thank you

Mmmm, the sweet smell of unnecessary point of failure. :slight_smile:

can you elaborate a bit more ?

Every additional servo, means another point of failure, be it it’s electronics, potentiometer, motor, gears… for a traditional heli, I’d go with a few critical servos as possible (three on swash + one on tail) then you have the engine throttle, but it’s working less hard, and not necessarily that critical if failed.

I understand. However, I’m already in possession of the helicopter and must work with what I have. So based on that, can you help me?

Actually four-servo swashplate is a redundant system. One servo can fail and the helicopter can still be flown.

@Francois_Chauvin we presently do not support four-servo swash but changes are being made to the heli outputs to switch them to the servo library. And further changes in the pipeline to get rid of the three-servo adjustable position convention and go to pre-defined swashplate types like commercial FBL units use. This won’t happen until Copter 3.7, although I have some of the code done in my dev branch. H4-90 will be one of the supported swashplate types when I get this all done

In the mean time four-servo swashplates are being flown with ArduPilot by using a FBL unit that supports four-servo downstream of the ArduPilot flight controller.

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I should add that the mixer for H4-90 is trivial. It is a H3-90 or HR3-90 with an extra servo. One of the problems in the old code was the trims weren’t accounted for, which would cause servo binding with a H4-90. @bnsgeyer fixed that.

Once the servo outputs are done (Bill and tridge are working on that) then we can get rid of the three-servo limitation that is in the current code. We will be able to assign any servo an output, even duplicate outputs, with the servo library.

When I added support for the H3-140 swashplate and it became obvious the current system of using assignable servo positions around the swashplate does not work for the H3-140, I started looking at using the pre-defined swashplate types. There is only really three in common use, and the rest are somewhat rare these days. But H4 is very popular in bigger helicopters like Vario and Velos Rotors and it has been a requested feature for a very long time. Fabi even submitted a PR for it once and it was shot down due to being “too complex” because it didn’t fit in with the musical chairs servo system current in use. Scrap that and then we can move forward.

So we’re working on it :grinning:

Thank you for your very complete response! However, I would have some further questions.

  1. Is the code you spoke of in your dev branch is Flyable?

  2. If not, can you tell me a bit more about the commercial FBL unit that I could use and how to use them with autopilot. I currently have a Pixhawk 2.1 if a commercial code can be added to it.

Yes, it is flyable but only supports 6 different swashplate types at present - all three-servo.

Just about any FBL unit that has SBus input will work downstream of a Pixhawk. I suppose you could use one with PWM inputs too, but it will be a bit messy. The way it works is that your Pixhawk will supply attitude and navigation, the FBL unit handles rate control and servo outputs. The Pixhawk will be set up with H1 swash output, flybar mode turned on, VFF (fastforward) turned up, and RATE PID’s turned off. There is no tuning done in ArduPilot - it just provides the same outputs to the FBL unit as your RC receiver would, with the addition that it supplies autopilot control.

Do you have some names in mind ?

One of more popular ones for four-servo seems to be the Spartan Vortex.

Thank you! This conversation has been very helpful for our project!

I would have another question. We had a Futaba GY701 that came with our Vario and I was wondering if it would work with it. I know that it’s not a FBL unit, but the Vario already have a Flybar and the GY701 could handle the gyro and the governor. But there is still the 4 servo swash plate …

I saw on a forum that someone was using a GY701 with a Pixhawk but I don’t know how he could handle the 4 servo.

If you could help me with that it would be great because I’m starting to get confused!