|Thanks goes to Mark Kiner for this easy way to set the cyclic to throttle mixing.|
Step through setup.
Section 2: Limitation of the Futaba 8U and 9C and what to do about it
Section 3: What is this for? Why not just use a govenor? How do you eliminate servo binding?
|Section 1: Step through setup|
|This technique does not require you to
readjust your throttle end points (EPA/ATV) or throttle
curve. It's very easy to do.
Radios that require the sub-trim trick to prevent the mix from overdriving the servo:
Airtronics RD6000/8000, Futaba 6X, 6E, 7C, 8U, 9C (not the Super version), Hitec Eclipse 7, JR 622/642/652/662/6102/8103, and PCM 10's before SX-II.
Radios that do not need the sub-trim trick:
Futaba 9C-Super (use 'swash->thro mix'), 9Z (use the 'limit' mode), JR 10 SX-II/X (use 'throttle mix') and 9303.
|Step 1||Pop the link loose from the throttle servo arm|
|Step 2||Turn on the radio/heli and move the throttle stick all the way up. Note: the top of your throttle curve should be 100%|
|Step 3||In your radio adjust the center position of the throttle up until the servo no longer moves counter-clockwise. As you increment this value, the servo arm will move a click each time until you have reached the limit. Stop right there and don't go beyond that. The center position is named different for different radios. My Airtronics RD6000 labels it CNT (center). JR radios call it sub-trim.|
|Step 4||The servo arm is now extending to far to put the link back on, so take the servo arm off the servo and reposition it so that it aligns back up with the link. In some cases the teeth on the servo spline will not let you install the arm at the correct angle, in other words one way is too far and the other is not enough. In this case try a different arm on the servo horn. In most cases if you rotate it 180 degrees you will find that in-between spot. If you do have to use a different arm then measure the distance the ball is from center and place it in the new position with an equal distance from center. Confirm that the link matches up and snap the link back on the servo arm.|
|You have done the magic steps now that will keep the servo from binding the linkage due to the cyclic mixing. For some strange reason the radio manufactures allow the mixes to overdrive the servo. You would think that when you set the end point for a servo that it would consider that the stopping point, but no, the mixes will ignore this stop point and go right past it.|
|The way this technique works is it sets the top end point to the maximum that a servo can go. So if your throttle is all the way up and the mix try's to go further, it can't because the servo is already as far as can physically and electronically go so the extra amount is just ignored. Your radio gives a position command by using a pulse width that varies from 1 milli second wide to 2ms wide. For example 1ms would be full clockwise and 2ms would be full counter-clockwise while 1.5ms would be center. You radio uses end point values from -150 (1ms) to +150 (2ms) note: -150=2ms & +150=1ms if servo is reversed. So if your end point is say 88 then by changing the center (CNT on RD6000, sub-trim on JR) from 0 to 62, that will make the top of the throttle 88+62=150 which is equal to 2ms. Servo control standards only except 1-2ms so the mix can not cause the radio to go beyond the 1-2ms range. Therefore the mix will only add throttle up to but not past your top end point.|
|Step 5||Set the elevator to throttle mixing. On the Airtronics RD6000 go to the "etc" menu and down to the "MAS 1" and put "EL", then go down one to "SLV 1" and put "TH", then go down one to "E->T 1". Move the elevator stick up and set the up mix to 20%. Move the elevator stick down and set the down mix to -20%|
|Step 6||Set the aileron to throttle mixing. On the Airtronics RD6000 go to the "etc" menu and down to the "MAS 2" and put "AI", then go down one to "SLV 2" and put "TH", then go down one to "A->T 2". Move the aileron stick left and set the left mix to 20%. Move the aileron stick right and set the right mix to -20%|
|Section 2: limitation of the Futaba 8U/9C|
|I had the Futaba 8UHFS and unfortunately whoever
wrote the software for this radio limited the sub-trim
range and it will not reach a high enough value to push
the top end ATV to the 1 or 2ms pulse limit. I had to max
out the sub-trim at -120, then increase the top ATV until
the servo no longer moved. That gave me a value of 111.
While helping someone else with a 9C I found it also has
this limited sub-trim range.
Step 1: First step
is to make sure the top of the throttle curve is 100%.
For the 8U go to 'TH-CRV NORM' position 5 and set the
value to 100%. For the 9C go to 'THR-CURVE NORM(NORM)'
and set POINT 5 to 100%.
|Section 3: What is this for? Why not just use a govenor? How do you eliminate servo binding?|
|When you move the aileron and/or elevator you are adding pitch to the blades. So in addition to the collective pitch position you are adding up to 6 degrees or more to the blades. This increased load would slow the rotor speed down without any other corrections. That is where the cyclic to throttle mix comes in. You use two of the programmable mixes in the radio. One is aileron to throttle and the other is elevator to throttle. So as you move the cyclic stick away from center it will add throttle. Some people do not want to bother with this and say it is easier to simply install a governor. A governor is a device you install on the helicopter that monitors the head speed. You set it for a certain speed and when it senses a change it can tell the throttle servo to increase or decrease to make the speed the same. One thing to remember here is that it waits until the head speed does change before it compensates. With cyclic to throttle mixing you are adding the extra throttle at the same time you add the extra load.|
|What about servo binding?
When you use a programmable mix in the radio it completely ignores the end points you setup on the throttle servo. In other words if you have the throttle stick all the way at the top (at 100% in the th. curve) this would have the carburetor opened all the way. Now with the mix setup if you move the aileron or elevator the servo will try to move further then the limit. Since the carb is already opened all the way one of two things will happen. Either the link could pop off in flight if it is a little worn, or the servo will draw a lot of current from the battery and pull the voltage level down. In the 2nd case if you already have a few flights on the battery charge then you could see the helicopter jump just like it got some radio interference.
So how do we get
Each channel is give 2 milliseconds (2ms). For
the first 1ms the signal remains high. This indicates the
beginning of the signal. The next millisecond is the
position the servo needs to go to. In the picture you see
a signal that drops at the 1ms mark and the servo is at
the full clockwise position. This would represent a
throttle servo that had the bottom of the throttle curve
set at 0%, the throttle trim was all the way down, and
the end point (EPA/ATV) was maxed out.
So in the picture below you can see the entire
range 100% to 100% end points sits in the middle of the 1ms
range. This is with the center (CNT/sub-trim) set to 0.
Zero means make the center at 1.5ms.