Long servo arms
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Having large arms helps by preventing some of the interactions between the controls. With small arms and high rates on your radio, you will notice that at full collective the elevator servo does have enough travel to affect the correct input and you will see the swashplate move vertically (interaction between cyclic and collective). This is due to the fact that the end of the servo arm travels in an arc rather than a straight line, and there is not enough travel left for both inputs, so they get mixed into each other. With larger arms this situation is greatly alleviated, because the radius of the arc is larger, so there is more vertical movement for a given servo rotation. It’s too bad there are not linear servos available at this size. WES-Technic makes some 3.0g servos that work well in very small helicopters. Oh well. If using the stock arms, use the outermost hole. My large arms have a radius of 18 millimeters from the center of the screw, to the hole, while the stock ones are 12 mm. Having this large radius allows rates to be turned down on the transmitter, but that’s for another section. Next, get the servos centered. Install the servo arm without the screw on each servo. Move the servo by hand to see where its endpoints are. Don’t force them if they do not move freely. Turn on your transmitter and move it a bit to get past the tight spot. Re-install each servo arm so it is perpendicular to the servo case when neutral and that it travels equally in both directions. It will not be perfect, and you will need to use the sub-trim function on your transmitter to get it perfectly centered. Use the transmitter trims for fine-tuning. If it requires more than 5 clicks of trim (subjective) you may as well change the rod lengths from the servos to the swashplate. Here are some photos of the servos set up for 120 degree CCPM without mounts. The pictures show the servos collective pitch travel range.    |
James and I use Hitec HS-85BB servos for swashplate control, and in my opinion, the stock arms are just not long enough. While using longer arms is not essential, it is very helpful because it allows more linear controls at extreme control inputs. Large arms can be found at