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Flying the MPX Heron with 6Ch TX and 7Ch RX with all control functions available

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Flying the MPX Heron with 6Ch TX and 7Ch RX with all control functions available


Old 05-07-2015, 12:34 PM
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Default Flying the MPX Heron with 6Ch TX and 7Ch RX with all control functions available

The Heron from Multiplex looks like a nice plane. It is a 2.4m elapor motor-glider that has 4 wing servos, 2 tail servos and a motor.

The specs say that it has 5 control channels, which in my opinion is just plain wrong. There is no way to implement the control inputs: elevator, rudder, aileron, throttle, camber setting and spoiler (butterfly) with a 5 Channel Radio System.

The conventional way to implement all 6 control inputs stated above would be to use a 7 Channel Radio System, do all the mixing in the TX and use the 7 Channels for the servo signals (6 servos) and throttle signal.

Unfortunately I only have a 6 Channel TX (Graupner mx12). But since there are only 6 control inputs, I was sure it might work with my 6 Channel TX, when using a 7 Channel RX (I would actually have to use the Graupner gr16 with 8 Channels, since there is no Graupner 7 Channel RX).

My first idea to use the channels for the control inputs and do the mixing with the RX cannot work, since most receivers only support very few and basic mixing. After long hours working on the problem, I finally came up with a solution.

You set up your TX like you normally would, programming all the mixers, but you ignore one of the wing servos. Then you program 3 linear mixers in the RX from the 3 working wing servos to the ignored wing servo. Now the really hard part is setting up the mixers in such a way that the previously ignored servo behaves in such a way, as if it was programmed with a 7 Channel Radio. I did some math and was surprised to find out that you could indeed achieve just that with 3 simple linear mixers.

Lets just look at the wing servos, because for elevator, rudder and throttle you inevitably need 3 Channels. That leaves us with 3 free Channels and 4 wing servos that need to respond to the control inputs aileron, camber setting, spoiler. According to the Heron manual setup each control input has effect on every one of the 4 wing servos.

To explain the setup, I am going to use Variables for the control inputs:
a for aileron: a = -1 means stick full left, a = 0 is stick centered, a = 1 means stick full right.
f for camber flap setting: f = -1 means flaps down for thermaling, f = 0 is normal flap, f = 1 means flaps up for speed/acro
s for spoiler: s = 0 means no butterfly at all, s = 1 means full butterfly for landing

Instead of using variables for servo-signal, I am using variables for surface deflection in millimeters.
Looking from behind, i call the surfaces from left to right: A, B, C, D.
So A is left aileron, B is left Flap, C is right Flap, D is right aileron.

Now we can express the Heron manual setup with the variables:
when f = 0 and s = 0:
a = -1...0...1 => (A,B,C,D) = (16,10,0,-8)...(0,0,0,0)...(-8,0,10,16)
when a = 0 and s = 0:
f = -1...0...1 => (A,B,C,D) = (-2,-3.5,-3.5,-2)...(0,0,0,0)...(2,3,3,2)
when a = 0 and f = 0:
s = 0...1 => (A,B,C,D) = (0,0,0,0)...(22,-26,-26,22)

Now lets say you programmed your TX so this works for B, C and D. Then you can use 3 linear mixers (B->A, C->A, D->A) in your RX to implement the following function: A = 12/5*B - 12/5*C + 1*D
Or when you programmed so A, B and D work, you need to implement C = -5/12*A + 1*B + 5/12*D

I didn't calculate B as a function of (A,C,D) or D as a function of (A,B,C) yet, but that will work too, since the setup is symmetric.

Keep in mind that the values of (A,B,C,D) do not resemble the servo signal. So when implementing the three mixes, you cannot directly use the formulas above: You also need to use the relations between each servo signal and corresponding surface deflection.

Here comes the sad part: I am probably not going to try this on the real bird in the near future, because I simply do not have the money to buy the Heron right now. But I have simulated the Control surface response to the control inputs in an excel program, and it works beautifully.

All the stuff I have written is probably hard to understand from an outside view, but maybe it inspires some of you to try and get the most out of your equipment, before spending lots of money on fancy and expensive stuff.
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