Rudder/stick movement and servo travel - RC Groups
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Mar 05, 2012, 09:15 AM
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1Man1Mountain's Avatar

Rudder/stick movement and servo travel


Im setting up my helicopter with a new Gyro and i seem to come across one of the same problems that i had on my old gyro

I am currently useing a DX6i and a Align gp780 gyro and one of those blue small from hobby king servos (THX900?) i will upgrade the tail soon but anyway thats the current spec. i used a HK401 before wich only worked in ratemode.

Anyway to the question
When i move me left stick on the DX6 only about 1/4th of the possible travel for the stick the servo moves to full tail pitch. this is the same if i use rate mode or HH mode. this is with the heli on the ground i might add. not sure if its the same in flight.

Is this normal? in HHmode i understand it will probably calculate travel distance so it will stay on full servo movement since the heli itself dosent move to input, but in rate mode?

I have no expo or negative expo. travel limit is set on the GP780.

If anyone have an idea how i can test this in another matter please fel free to comment.

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Mar 05, 2012, 12:58 PM
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1Man1Mountain's Avatar
Do i perhappes have to test the spin speed in air or turntable?
Mar 05, 2012, 05:12 PM
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1Man1Mountain's Avatar
I will stop worrying

Found a post by Jasmin2501 and i think it applies to my question in case someone is interested how i "solved" it.
"With your gyro, remember it's a fly-by-wire system. Your stick inputs do not equal servo positions. When you put the stick in the center, you're saying "keep the tail steady" or more precisely "make the rotation rate equal zero" - and the gyro moves the servo until your command is executed. So, when the helicopter is not flying, and the tail can't push the helicopter around, the gyro doesn't understand what's happening - it moves the servo and nothing happens, so it moves the servo some more, still nothing happens... then you center the stick and you give the "keep the rotation rate equal to zero" command - but the helicopter is sitting still and the rotation rate IS zero - so it doesn't move the servo from wherever it happens to be. I made a video explaining this a while back...

i Guess flying will be the only option to make sure.

Thanks jas
Last edited by 1Man1Mountain; Mar 05, 2012 at 06:38 PM.
Mar 05, 2012, 10:26 PM
An Ordinary User
Not at all.

You need to use the limit pot on the gyro and set your rudder end points.

The limit pot on the gyro tells the gyro how much movement it's allowed to make. You do this by putting it in "rate mode" and giving full rudder in both directions. Adjust the limit until the tail slider just bottoms out on the right and left. Ideally, center stick should have the tail slider in the center and full left and right has it just hitting the sides.

After you have the limit set in "rate mode", you can proceed to adjust the end points so that it doesn't go full left or full right when you barely touch the stick.

Get things working right in rate mode on the ground, then HH will work right when it's in the air.
Mar 06, 2012, 04:22 PM
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824TSV's Avatar
The issue that concerns me is when you switch to rate mode your basically turning the gyro off. Your rudder servo should return to centre without giving any rudder inputs. If it's not doing that, there is an issue you havn't found. There is probably a great set up vid on Helifreak in Finless Bob's tech room that covers the Align GP780.
Mar 07, 2012, 06:45 PM
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1Man1Mountain's Avatar
Thanks for the suggestions guys.

I tried it on a turntable and the more stick input i give the faster spin so i think im allright.
For ratemode its mechanically setup and the GP780 gyro keeps it limit setting in rate.
Mar 08, 2012, 10:00 AM
An Ordinary User
As you found, it's much easier to do this in rate mode. On my DX6i, I use the gyro switch to switch between the two modes. I have "0" configured for rate mode (30%) and "1" configured for HH mode (70%).
Mar 09, 2012, 05:38 PM
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1Man1Mountain's Avatar
Most say its easier in rat mode but I don't see why. If you Knight heading hold will hold and compensate for change your good. Now if you have slight drift I can see that would mess things up but so would drift in rate mode
Mar 09, 2012, 08:07 PM
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norcalheli's Avatar
Originally Posted by 824TSV
The issue that concerns me is when you switch to rate mode your basically turning the gyro off. Your rudder servo should return to centre without giving any rudder inputs. If it's not doing that, there is an issue you havn't found.
Not quite right. First you have to understand what they mean by "heading hold" and "rate" modes. Heading hold says if something blows the tail, measure how much it moved, and move it back to where it was as best you can. (Nothing's perfect.) Rate mode says if something blows the tail, detect the rate of change and adjust to make the rate of change zero. A gyro in heading hold will try to do just that, hold the heading you set until you change it. A gyro in rate mode merely tries to catch the tail and stop it from moving more. It has no concept of where it started, so it doesn't try to move you back to your original heading. That's why moving a stick on an initialized gyro in HH, or moving the heli, will cause the tail servo to slowly drift all the way to one side or the other. Jasmine's explanation is spot on, so no need to repeat it here. Since rate mode only seeks to stop rotation, moving an initialized gyro, or moving a stick, only causes a momentary movement of the servo, after which it returns to center. After all, sitting there, the heli's rotation rate IS zero.

As for which is easier, most who fly scale like to fly in rate mode, as the tail will "weathervane" to some extent behind the heli in a turn, without requiring much rudder to keep the turn coordinated. In Heading Hold mode, turns require rudder to keep the turn coordinated, and it takes more effort to make the turn look smooth.

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