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Gyro problems. read this. Great information


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jathanjr
Extreme 3D
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Joined: 21 Jan 2013
Posts: 75
Location: League City, Texas

PostPosted: Thu Apr 11, 2013 3:15 am
PostPost subject: Gyro problems. read this. Great information
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Most contemporary gyros support both a Rate Mode and a Heading Hold or Tail Lock mode. Rate Mode allows the tail to "weather vane" in the direction of flight, smoothing out and coordinating turns, but will resist smaller changes, like a gust of side wind in a hover or torque from increasing or decreasing throttle and pitch, allowing the helicopter to be controllable by mere mortals (I learned to hover on a GMP Cricket without a gyro... damn... that was hard!). Heading Hold mode attempts to keep the nose of the helicopter pointed in a constant direction, regardless of how much wind is pushing the tail, allowing for flight in all orientations including sideways and backwards. Most pilots today fly exclusively in Heading Hold mode.

Gyros typically have a "gain" setting, which controls sensivity: how much movement it will allow before trying to oppose it. Too little gain, and the tail will "blow out": either allow the helicopter to weathervane into the wind, or allow the helicopter to spin counter to motor torque. Too much, and the gyro ends up fighting itself, causing the tail to wag back-and-forth as the gyro overcorrects. Gyros with a controllable gain setting will have two receiver plugs: one to receive the rudder signal, and the other to receive the "gain" signal. The gain plug usually has only a single wire and is generally red or orange rather than black in color. Gyros without a controllable gain usually have a pot for adjusting this setting. Heading Hold gyros consider the mid-point of the gain signal to be 0% actual gain, the low end-point to be 100% Rate Mode Gain and the high-point 100% Heading Hold Gain - though some, like Nine Eagles, are the reverse of this, so again check your manual.

Consult your transmitter manual for how to set gyro gain. Some transmitters, such as most Spektrum models, have a "Gyro Gain" setting that runs from 0% to 100%, which controls the signal sent to the "gear" channel of the receiver, where 50% is the mid-point. Adjusting up from the mid-point adds Heading Hold gain, while adjusting down adds Rate Mode gain. So, 30% transmitter gain means 40% Rate Mode gain (50-30 = 20*2 = 40) and 60% Tx gain means 20% Heading Hold gain. There are generally different settings for both up and down positions of the gear/gyro switch, allowing one to change between two different gain settings (two Rate settigns, two Heading Hold settings, or one of each; most people set one of each). Some transmitters may use a different range, such as -128 to 128, or may not have a "gyro gain" setting at all, in which case you can adjust gyro gain via the "end-point adjustment" of the channel the gyro's gain plug is connected to (you can even do this with Spektrum Tx's and avoid the 50% messyness - thanks for the tip pmackenzie!). Other transmitters may just have a knob that can be twisted to change that channel's value.

The "limits" setting available on some gyros sets how far the gyro can move the tail servo when making corrections and in response to rudder input. Therefore, the rudder channel does not directly control the position of the servo, it instead informs the gyro how quickly to yaw or pirouette the helicopter; setting the "end-points" on the transmitter controls piro speed, not servo throw. The limits should be set to provide the maximum throw without stalling the tail servo. On the Trex 450 v2 and clones, setting the limit too far can cause the tail linkages to "lock" on one side, making the heli uncontrollable, so be sure to check for this. If your gyro does not have a limits setting, the same effect can be achieved mechanically by moving the ball on the servo horn in for less throw and out for more throw. To get more throw in one direction than the other, simply offset the horn by more or less than 90 degrees. Moving the ball in also provides more torque and resolution, allowing for a higher gain setting before the tail starts to wag.

Gyro delay, available on some gyro models, helps to compensate for the speed of the tail servo. The most noticable effect of too low a delay setting is that the tail will "bounce back" several degrees when quickly stopping a piroutte, or "hunt" back and forth a few times. Too much delay makes the tail feel "mushy". Counter to "common wisdom", delay is not just for Analog servos: the performance of digital servos can sometimes be enhanced by adding a delay setting, depending on the servo, helicopter, and other mechanical factors.

Here's my gyro set-up check list to get both heading-hold gain and delay set properly for any class helicopter with any kind of tail servo.

INITIAL SETUP
Power up the transmitter (Tx).
If gain can be controlled by a gear/gyro switch, program the Tx to give 20% Heading Hold in one position, and 20% Rate Mode in the other. On a Spektrum Tx, this means 60% and 40% Gyro Gain.
If there's only a single setting available, use 20% Rate Mode for now.
Change the "Analog" or "Digital" switch on the gyro to match the type of tail servo you have. The Analog setting is safe for digital servos, but may impact performance. The Digital setting will most likely cause permanent damage to an Analog servo in a matter of minutes.

MECHANICAL SETUP
Power up the model, but make sure the motor won't start up accidently by either disconnecting the motor plugs, or hitting throttle hold (the former is HIGHLY recommended).
Zero the rudder trim & make sure the servo horn is at 90-degrees to the pushrod. Adjust with subtrim.
If supported by your radio gear, rebind the Rx to lock in fail-safe value for the now-centered rudder.
Adjust the tail linkage so the tail-control slider ball is roughly in the middle. In a hover, there needs to be about 6-8 degrees of pitch pushing the nose to the left in order to counter torque from the main motor.
Switch Tx to Rate Mode
Adjust "Limits" on the gyro to give the maximum tail slider movement without binding. Check both sides! This setting must be done on the gyro, not on the transmitter. Some gyros don't have the ability to set limits, or can only set overall limit rather than separate limites for each side, in which case you can try offsetting the servo horn away from 90-degrees to mechanically give more throw one side than the other.
Turn "Delay" up to maximum. Don't try to fly this way - the helicopter tail response will be very mushy if you do.
Power off, plug in the motor, then power back up (outside!)
Switch to Rate Mode
Bring the helicopter light on the skids and watch for rotation.
Mechanically adjust the tail linkage to remove helicopter rotation, until the tail is stable in a hover.
If the model spins uncontrollably in one direction, double check your gyro's servo reverse switch. When you rotate the heli by hand, the gyro should provide a counter command to the tail servo.
If a rudder input spins the heli the wrong way, correct the rudder channel reverse setting on your transmitter: left rudder stick should move the nose left.
If you made any adjustments, recheck the limits for binding and locking.

GAIN ADJUSTMENT
Increase Rate Mode gain on the Tx until the tail starts to wag in a hover, then lower it a couple of points. On a Spektrum Tx, this means LOWERING the Gain Setting to increase Rate Mode gain. Go figure.
If you are capable, do a few 360-piros clockwise and CCW with a hard stop, to make sure there is no wag. Adjust gain if necessary. The stops will be mushy, because you still have max delay.
I like to lower Rate Mode gain an extra point or two here (increase the Gain Setting on a Spektrum Tx), just in case I start getting a Wag of Death in HH mode... in which case I can try switching to Rate Mode and Idle-Off to avoid a crash.
Land and switch to HH mode.
Repeat the above, but this time INCREASING Tx gain until the tail starts to wag, then DECREASE a point or two. Again, do a few left and right piros with hard stops to make sure the wag is gone.
Switch to Idle Up and make sure there is no wag.

DIALING IN DELAY
Now let's find your servo's ideal delay setting.
Use divide-and-conquer to find the best setting that gives a hard stop after a full piro, but no kickback, in both directions.
Lower delay to 50% and check for kickback.
If there is kickback, try 75% delay. Or, if no kickback, then try 25%.
Keep going zeroing like this to get the best response.

DIALING IN THE PIRO RATE
Finally, adjust your Tx rudder end-points to give a consistent piro rate in both directions, at a speed that you are comfortable with: lower end-points will give a slower piro rate, higher a faster rate up to the mechanical limit of your tail.
Make sure to check and adjust both left and right setting.
To tame the tail down at mid-stick, but still have a fast piro rate, add some Expo to the rudder channel (see your Tx manual to determine if the Expo should be + or -). I tend to use between 15% to 25% on most of my models.
Take her up and have some Fun!
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tombo242
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Joined: 04 Nov 2008
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Location: Santo Estêvão, East Algarve, Portugal. Now 82, but still feels 22.

PostPosted: Thu Apr 11, 2013 6:44 pm
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My only comment on this excellent artical is that one should NEVER use 'Sub Trim' on the tail. This can upset the newer gyros to the point of instability.
Your servo horn should be as close to 90º as you can get with no trim at all and leave it there. That is the center as far as the gyro is concerned any sub trim is considered to be an input from the Tx (which it is of course) and therefore can cause problems.

Tom.
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jathanjr
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Joined: 21 Jan 2013
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Location: League City, Texas

PostPosted: Fri Apr 12, 2013 11:23 am
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You are absolutely correct tom! thank you
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Andy456
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Joined: 11 Mar 2013
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PostPosted: Fri Apr 12, 2013 12:03 pm
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Really informative and knowldgeable article shared here by the Jathanjr .It would be prove very helpfull for the users of the heli's and they can get a lot of information from this post.I appriciate this type of work..........

Regards.
Andy.
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jathanjr
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Joined: 21 Jan 2013
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Location: League City, Texas

PostPosted: Sat Apr 13, 2013 12:36 am
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thank you Andy, i put this out because i had allot of problems with setting my gyro up, but with allot of research i finally was able to set it correctly and now my heli flys like a dream. so i figured that other people may have problems as i did, which is the reason for this article.

happy flying!!!
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sbobby
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Joined: 07 Nov 2009
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Location: Melbourne, Australia

PostPosted: Sun Apr 14, 2013 6:17 am
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good article.
Head holding works great for me and from reading other articles from internet most people always using it. So I don't understand why they still make gyro with rate mode. Is there any particular sitiuation you need to use it ?
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tombo242
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Location: Santo Estêvão, East Algarve, Portugal. Now 82, but still feels 22.

PostPosted: Sun Apr 14, 2013 5:40 pm
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Sure thing Sbobby, you need rate mode to set up the tail as HH will always cause a degree of drift unless the heli is is powered up close to hover. Bench mechanical setup on the tail must be done in rate mode. Rate mode is also used by many to tell some gyros where center is when powering up. Usual method is to toggle the gyro switch from HH to Rate and back again 3 times, then in HH advance the throttle.

Rate mode is also handy to find the actual tail incidence required to overcome torque at hover. This is the real tail center that you require, setting zero incidence for center is for planes. The heli rotors are in balance when the tail holds against the main rotor torque, and this setting can only be accurately found in rate mode. If you have zero incidence at servo center then to hold straight the gyro has to give the tail a constant incidence to overcome the rotor torque - so your 'center' is not really center anyway and you may get a little drifting.

However, having said all that, you normally only ever fly in HH mode, but I would not set up a heli without using Rate mode.

Tom.
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