Model Helicopter Control Setup Overview
Hopefully this article will give people starting out the upper hand by explaining the how and why about adjusting linkages. Manuals usually do not mention why but only give dimensions which can be inaccurate, being somewhat of a ball park figure in nature. A few manuals give exacting and accurate dimensions for the control system based on various flying styles and if followed are best. Understanding where problems can occur will help with all model helicopters.
There are basically two mechanical setup extremes, the beginner and the 3-D type. What will be different between the two is the amount of control system throw or the maximum blade angles available for both the cyclic and collective mechanisms. This will apply to all model helicopters.
Common to both setups will be a mechanically centered control lever geometry. On the beginner and advanced machine we adjust the half travel position of the collective to make all rotating levers horizontal. These would be specifically the washout and mixer levers. Keep in mind the flybar must be 90 degrees to the mast or level when checking this specification. Once we have met the above criteria the blade pitch will be set with respect to the flying style. Zero at mid collective travel for 3-D and 5 degrees for the other types of flying are normally used. The mid collective blade pitch angle will be set with the link connecting the blade grip to the level flybar mixer using a level cyclic swashplate position. If your machine does not have adjustable pitch links you can manufacture them. If you don’t want to use adjustable pitch links then a compromise will be made with the control geometry at some point in time. As with any rule of thumb there are exceptions. In the case where the mixer is mounted to the blade grip pitch horn both the long and short rods will need to be adjusted together to raise and lower the blade pitch, thus maintaining a level mixer lever.
Just what is mid travel on the collective mechanism must be asked. Mid travel will allow equal and maximum safe collective travel in both directions to occur and without the cyclic binding at extremes. As an example, some machines can allow the swashplate to be lowered so low that when the cyclic is deflected it can contact the top mast bearing collar. Other machines could have the washout disengage the drive pins under the same conditions or be dangerously low in contact depth. Conversely at the other end, the same pins could contact the center of the swashplate when cyclic is inputted. Another way of thinking about it is that the washout mid travel with respect to its sliding on the drive pins limitations, should be located with the washout levers level. The rotating control rods to the paddle pitch control arms will be adjusted to meet this end causing the washout to rise or fall at mid travel location. This is provided that the rotating paddle control rods attaching to the swashplate outputs are adjustable, as many are. Some machines have a movable washout driver which can be slid up and down the mast to help with matters. When you think of a mid collective travel position think safe engagement, ponder any possible linkage restrictions and consider the number of adjustment locations. It can be generally stated that the wider the collective range the more critical the centered mechanical setup will have to be; or the beginner can get away with a much larger adjustment tolerance.
Now with all the above considered let me say there are very well designed machines out there which have collective limitations met by the travel stops of the mechanism between the side frames. This is done is such a way so as to avoid end travel pitfalls when the system is mechanically centered correctly during setup. Fixed or non-adjustable links ensure the swashplate setup height location is fixed. This is a very wise feature and will eventually become more common place as manufacturers of the mechanical collective/cyclic mixing systems learn more and more through evolution. The only down side I can think of is that a change in mast length and/or swashplate thickness will be difficult in some cases.
Once we have the collective control centered with level mixers and washout arms, the collective blade angles set to the flying style angle, these positions are then matched with the collective servo by connection at mid travel (90 degrees to the control rod). At this mid-point the control rod attached to the collective servo may be adjusted. As mentioned previously 3-D will have a zero collective pitch value while the beginner will use about five degrees. We can now set the travel extremes based on full servo ATV or servo wheel rotation. The mid point on the transmitter pitch curve (50%) if not already completed, will center the collective servo electronically at half stick during this process, then the servo arm is reinstalled at 90 degrees to the control rod to match. The length of the servo arm will be dictated by a 100% ATV and the mechanical stops of the control system. If the control system has no physical stops care is needed with respect to previously mentioned engagement and interference issues. Find the hole in the servo arm that most closely meets these requirements, then trim back or forward the pitch curve end points as needed.
The radio programming will determine that full cyclic servo throw is available through the ATV , EPA or servo travel volume. These names or definitions mean the same thing and are used by various radio manufacturers simply to confuse all of us! The maximum high and low points on the pitch curves do the same for the collective servo as ATV does for the cyclic. As mentioned once we know the collective servo will move full travel with full transmitter stick deflection we can then decide upon the length of the servo horn control link attachment point. For the specific process, one will raise the collective stick to maximum on the radio and match the collective mechanism on the helicopter using a servo arm length. Try for 12 degrees using a pitch gage. At this point the control rod and its ball location on the servo horn will be fitted but the control rod length will not be altered. This maintains the half stick settings we worked so had to achieve. Finally we will fully lower the collective stick on the radio and make sure no binding occurs and that safe washout pin engagement is evident. We will again be checking full cyclic deflection at both collective extremes. We use the same methodology of using 100% cyclic ATV to adjust the servo arm to control rod attachment point based on the mechanical limitations. By selecting a shorter servo arm for a beginner setup one can still reduce the collective or cyclic range and also advantage full servo resolution. When you learn to fly well very little disruption to the collective linkage will be needed to access more pitch range and move towards a 3-D setup. The shift to the advanced collective setup will mean a longer servo arm and an adjustment to the pitch links between the mixer arms and the blade pitch horns. For the cyclic longer servo arms will be needed.
The fine tuning adjustments to the collective control parameters will now be completed at the radio through the pitch curves as the correct mechanical setup has been completed. In other words mechanically you have made the best compromise of what you have! The final radio settings for top (and possibly bottom for 3-D) collective will be based on the available engine power. Once everything is finalised one last ground check must be completed in the name of safety.
The collective will be positioned to full high and maximum cyclic held. Maximum cyclic by definition means both lateral and fore/aft cyclic, in other words the cyclic stick is held boxed. The airborne system will be turned off at this point. Slowly rotate the main rotor and see if the servos move back towards neutral. This same check will be carried out at full low collective stick position and again with maximum cyclic. If the servos move you will need to reduce the cyclic throw.
Generally speaking I set the throttle hold pitch curve for the maximum collective I can get at top stick, and about minus three or four degrees at the bottom, with the same five degree hover point I use at on the normal pitch curve. For the 3-D configuration generally the upright hover point is at 3/4 stick throw and the inverted hover is at 1/4. You will get more available top pitch the further you move away from the 3-D collective setup.
I limit powered pitch curves to about +/- 9 degrees. For the rank beginner I look for plus 9 and minus 1 degrees. For the cyclic I try to get the maximum (+/-6 to 7 degrees) for aggressive sport flying and 3-D. For the beginner I feel 4-5 degrees is good each side of a level swashplate (+/-4 to 5 degrees). It is easiest to check this specification at zero collective.
The above applies to all models but it does not mean that other parameters like flybar weight, blade and paddle weight inclusive of design should not be fully optimised once proficiency is reached. A system with non adjustable rods connecting to the swashplate inputs will need the mid travel setup procedure based on this fixed limitation…in other words you cannot easily or methodically work exclusively from the rotor head back. Because of this it can be stated that adjusting from the servos forward will work better in some situations. A Shuttle or Raptor might be a good example. Alternately the mechanical mixed X-Cell system offers full adjustment and flexibility, but with additional complexity. Some helicopters have a fixed or non-rising/falling swashplate. Examples here would be some Kalt, Kyosho and TSK helicopters. Setup is easier because there is no mechanical cyclic/collective mixing but the flying style at the blade grip will still be adjusted with the pitch links. This does not mean anyone control system is much better than the others but only that different setup methods may be employed.
One setup method cannot be applied to all model helicopters due to many different designs, but if you apply common sense, look and apply for a best centered control system you will be better off in the long run. The poor individual helping everyone setup their machines at the field will appreciate the fact that people have taken the time to understand their helicopter and appropriate terminology.