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This thread is privately moderated by Andrey_Rakhmanov, who may elect to delete unwanted replies.
Jul 22, 2018, 05:14 PM
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Arlequin FrSky S8R based flight stabilizer, 3D, vertical takeoff and landing videos

Previous stabilization scheme for the 2-engine Arlequin used Eagle Tree Guardian flight stabilizer. This stabilizer is quite old (more than 10 years) , so I decided to replace it with a more modern and popular one. It is not easy to do because very few stabilizer can match qualities of Guardian. Finally, the choice fell on a popular receiver with built-in stabilizer - FrSky S8R.

I tested FrSky S8R with classical scheme model, and what I found:
- Relative small price - $ 40 - $ 50 - 8 channels for a telemetry receiver with stabilizer isn't bad!
- It has telemetry channel - it is possible to measure the voltage of the battery.
- Can output data in SBUS format - allows to avoid PWM wires mess. Now receiver can be connected with one cable.
- An ability to do full configuration directly from the transmitter (Taranis + Lua scripts).
- A large number of flight modes (3D, horizontal stabilization, hover, knife).
- Active support from FrSky - new firmware releases, utilities, etc.
- Receiver configuration menu features minimum possible number of settings.
- Complex and unobvious processes of installation, configuration and calibration - for the understanding it's better to watch the video on YouTube.
- Support - only 3 basic types of plane models (classic, delta wing, v-tail) 'hover' and 'knife' flying mode aren't enabled for delta wing and V-tail type models.
- Not very obvious stabilization scheme. Impossibility to set the "general" stabilization gain in all modes, except 3D aerobatics.
- Relatively large dimensions and weight (21g).

Signal Mixer.

Nevertheless, I set myself the goal to feature Arlequin with this FrSky S8R. But 2-engine Delta wing model isn't very popular, so I also have to build a signals mixer on the base Arduino Micro Pro:

The mixer is similar to the existing mixer for Arlequin with Eagle Tree Guardian but it have some differences:
- Input signal to the mixer is fed by one wire via the SBUS. It is necessary to use SBUS -> TTL NPN transistor as a signal inverter.
The mixer has 4 PWM outputs (THR1, THR2, ELV1, ELV2) - for 3D aerobatics better to set S8R stabilizer to 'classic flying model.
- To feed servos with 6V, LM7806 (KREN6B) is used. It allows to increase the speed of the servos and also use the popular quadcopter ESCs without BEC.
- Receiver FrSky S8R is powered directly from the battery voltage (which makes it possible to transfer this voltage through telemetry back to the transmitter - the flight voltmete -buzzer is not needed.).
- The mixer also outputs "failsafe" signal, which can be connected to buzzer, but since the model is aerobatic type, the feature is not activated, but for FPV models such function is very useful).

Mixer building and programming.

I will not describe in detail the manufacture and programming of the mixer, because it is quite similar to the old version mixer. You can download the sketch from here. Now, program is very simple - having whole input data in digital SBUS, no need to use timers and interrupts for PWM signals decoding.

Start-up diagnostics.

If the mixer is correctly soldered and programmed after powering it should start working immediately. When the power is turned on, the mixer LED should blinks:
- 1 flashing - the voltage is applied to the mixer.
- 3 flashes - The SBUS signal is normally processed.

Mixer calibration

Since the mixer performs non-linear signal mixing operations, it needs to know the minimums and maximums of the control signals. The data have to be collected by calibration operation. It is quite simply to do:
- Set the full sticks ranges on the transmitter, switch off FrSky S8R stabilizer.
- Press and hold the mixer button. The mixer LED should go off. Move the THR-RUD, AIL-ELEV sticks to the minimum and maximum positions (making "squares").
- Release the mixer button. If the calibration is performed correctly, the mixer LED should blink three times. In the case of the error - the LED will not blink and new calibration data will not be recorded.

Configuration and setting stabilization parameters of FrSky S8R

I will not describe the procedure here - large number of publications on the topic exist. If you have a popular Taranis X9D plus transmitter, then the configuration can be performed by Lua scripts directly from the transmitter menu - here for Arlequin it is necessary to choose not "delta wing", but the "classical" scheme because the mixer itself read "classical" signals data and performs mix of the signals featuring "delta wing" scheme.

Results of the tests

- 3D aerobatic mode - the dynamics is very similar to the use of the Orange stabilizer, but the stabilization is more reliable and precise. Ability to change the general stabilization factor by knob on the transmitter.
- 'horizontal stabilization' mode is also quite precise, but without the possibility to change general stabilization factor.
- "Hovering" mode. The aircraft actually turns into a bi-copter, which makes it possible to do vertical take-off and landing.
- "Knife" mode - somehow works, but it isn't very stable because for Arlequin stable knife angle is about 10 - 20 degrees from vertical, but stabilizer tries to feature knife mode about 45 degrees from vertical.


In general, FrSky S8R appeared to be a good replacement for the aging Guardian. Its principal new feature - stabilized hovering flight mode is quite useful for take-offs and landings. But here it is necessary to understand that in the room vertical take off/landing will not a big problem, but outdoor landing it is quite difficult - the model is light enough and any gust of wind and unevenness of the ground can overturn it, so you will need to have some practice here.

In-room flights

Home Arlequin fying (0 min 43 sec)

Outdoor flights

Arlequin 3D pilotage (3 min 51 sec)
Last edited by Andrey_Rakhmanov; Aug 20, 2018 at 03:30 PM.
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Jul 27, 2018, 05:01 AM
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RCvertt's Avatar
Great job! Looks very stable.
Jul 27, 2018, 01:33 PM
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Thread OP
The major problem is - the pilot skills - I am not very good one, I built the model as 'pilotage trainer' - it is safe for people and robust in collisions with ground - propellers placed inside the wings.
Jul 28, 2018, 06:56 PM
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RCvertt's Avatar
I agree. It's a very good design for a tail sitter. I don't have good flying skills either. The more automation I get, the better I fly.

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