Throttle Mixing

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Thanks goes to Mark Kiner for this easy way to set the cyclic to throttle mixing.
Section 1: 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%.

Step 2: Next move the throttle stick all the way to the top.

Step 3: Now max out the sub trim to 120 *note confirm you are going the right way so that the barrel of the carb would be rotated counter-clockwise. This could be either -120 or +120 depending on the servo you have. For the 8U go to 'SUBTRM' and select channel 3 'THR' then max the value out. For the 9C go to 'SUB-TRIM' then select channel 3 'THR' then max the value out.

Step 4: Next move the throttle servo end point up until the servo stops moving. This will most likely be around 115%. Whatever you find the top value ends up being, set the bottom end point to the same. So if you found the top value needed to be 114 then set the bottom also to 114. For the 8U go to 'ATV-TH' and again with the throttle stick all the way up increase this value past 100 until the servo stops moving. Whatever value you get then move the throttle stick down then set that side to the same value. For the 9C go to 'E.POINT' then select channel 3 'THR' now with the throttle stick all the way up increase the value past 100 until the servo stops moving. Whatever value you get move the stick down and set the bottom value to the same as the top.

Step 5: Next take the servo arm off and reposition it so that with the throttle stick in the center the arm is 90 degrees to the imaginary line drawn from the center of the servo spline to the center of the throttle barrel. Now adjust the rod length so that it is an equal distance from the ball on the servo arm. In other words with the throttle all the way up and the throttle link pushed all the way toward the back then check if the ball goes past the link, is right on, or is not far enough. Do the same with the throttle down and the link pulled all the way forward (BTW bottom throttle curve point should be zero). If it is right on both directions then you are good to go. However if the ball goes past the link both directions then move the ball in to the next hole closer to the center of the servo arm. If the ball does not go far enough to the link on both directions then move the ball out.
Tip: If you find you need to move the ball on the servo arm in, then first try moving the ball on the throttle arm of the engine to the outside location. Some throttle arms allow you to do this, others only have one location for the ball.

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?
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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 around this?
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We take advantage of the way the radio transmits the signals. The radio sends out a signal for each servo sequencially. As you can see in the picture below the duration of the pulse varies depending on the position of each servo.

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.

The picture below would be the same as above (throttle stick all the way down, throttle curve at 0%, trim all the way down) however in this case the end point (EPA/ATV) is set to 100% instead of the 150%.

The picture below shows that if a signal is sent at 1.5ms then the servo goes to center position. 1.5ms is half way between the 1.0 starting mark and the 2.0 end.

The picture below shows the servo with the throttle stick all the way up and the throttle curve set at 100% at the top. Notice that the signal still has some more space before reaching the 2ms limit.

The picture below represents the throttle stick all the way up, throttle curve at 100%, and the end point (EPA/ATV) maxed out. Some radios have a max value of 150% while others are 140%, and some at 120%. Whatever the max value is, that will represent the 2ms pulse as seen below and the servo rotate all the way.

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.

In the picture below you see the same 100% to 100% end points but the center has been adjusted so that the full counter-clockwise end point is on the 2ms limit. In most cases the center position will be (max end point - the top end point value you have). So in this example it is 150-100=50. This is just a quick tip but you should verify this be confirming the servo does stop moving at the 50 point.

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what about setting atv to 150%? I hear a lot of people say just set the end points to 150%. That could be done but you would have to move the ball very close to the center of the servo arm to keep it from binding. This causes the speed to greatly decrease so this is not a good technique.