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Old Nov 09, 2013, 09:57 PM
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Coordinated Turns

G'day,

Question: How does a UAV do a coordinated turn?

This is something I've been mulling over for a while, because my UAV so far just slips through turns with no rudder control at all. In a manned aircraft there is generally a turn and bank indicator or a tube in a ball somewhere. When the ball rolls away from the center point the aircraft is in a slip or a skid.

So from that I've worked out that lateral (side to side) acceleration is a symptom of an uncoordinated turn and that applying rudder deflection in the opposite direction of the acceleration will balance and coordinate the turn. Is this correct?

In straight a level flight, if there was a gust of wind for example, this would cause a lateral acceleration and consequently a rudder deflection. The rudder deflection would cause an adverse roll and change in heading, which the autopilot will try to correct for by turning . . . it seems like there might be potential for the aircraft to become unstable. I'd like to hear any thoughts about this.

Cheers, Camel
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Old Nov 10, 2013, 09:32 AM
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On the simplistic level, mixing rudder and aileron differential to an amount appropriate to the dynamics of the subject airframe is all that is necessary to coordinate a turn and/or remove adverse yaw.

jeffparisse
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Old Nov 10, 2013, 10:28 PM
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Quote:
Originally Posted by workshop View Post
On the simplistic level, mixing rudder and aileron differential to an amount appropriate to the dynamics of the subject airframe is all that is necessary to coordinate a turn and/or remove adverse yaw.

jeffparisse
Ahh, right. This would probably take a bit of tuning I suppose but it would certainly do the job. Thanks!
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Old Nov 10, 2013, 11:13 PM
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That was a trick I used with the PicoPilot that they later adapted into to their SOP... A couple of manual flights with an on-board mixer and programmable servos means one can trim out the plane for coordinated turns without any TX mixing. That make it easier for the AP to command.
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Old Nov 11, 2013, 08:59 AM
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#define ADVERSE_ROLL 0.2

servo_out[CH_RUDDER] = servo_out[CH_ROLL] * ADVERSE_ROLL;

That's basically how the original Ardupilot does it.
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