Radio Control Helicopters
Paul Tradelius, 4620 Barracuda Dr., Bradenton FL 34208
Last month I talked about pitch curves—what they are and how they are formed—and I gave a formula for an easy way to set up your computer radio to give a straight-line "curve" once you have determined the end points. Now I'll take another look at pitch curves: initial setup and adjustments, and how to tell what changes are needed after you have flown your helicopter.
One of the most important aspects of setting up a pitch curve is making sure that the basic mechanical setup of the helicopter is correct.
Mechanical setup
Although all helicopters are not the same, virtually all of them have things in common, and I would be surprised if the following does not apply to your helicopter. The basic setup of the pushrods, bellcranks, and control horns should be set up per the helicopter instruction manual. This should be rather easy to check just by looking at your helicopter.
With the radio installed and all servos in their neutral positions, make sure that the servo arms are perpendicular to the pushrods. Notice that I didn't say the servo arm should be 90° to the servo body, because on many radio installations the servos themselves are mounted in an off-center position and are not aligned with the helicopter frame.
Once the servo arm is correct, check to make sure the pushrod is attached at the correct distance from the center of the servo arm. Many instructions specify a servo arm distance as an initial setting to make the helicopter feel right on its initial flights. Remember that the farther out on the servo arm the pushrod is attached, the more sensitive the controls will be—which is why it's a good idea to follow the manufacturer's instructions for the first few flights. Then you can make changes to suit your flying style and personal feel.
Check the other end of the pushrod. What is it attached to, and is that arm in its perpendicular position or centered per the instructions? If this arm is not centered correctly, the controls will have more effect in one direction than the other, and the collective will not feel the same as it changes from positive to negative. Although I am referring to the collective pitch control, this procedure applies to all pushrods to ensure correct feel and movement at the stick extremes.
Disconnect the pushrod from the servos and move it back and forth as the servo would. Is there any binding in the movement that would cause the servo to work harder than it should? Can you get full throw in the positive and negative directions? Once you are convinced that the mechanical setup is correct and free, reattach the pushrod to the servo and turn on the radio. The travel should be set to 100% in both directions, and the servo arm should move quickly, freely, and equally in both directions.
Place your pitch gauge on a main rotor blade and see what you get for collective pitch at the maximum and minimum collective points. Many instruction manuals give a reference collective pitch range for that particular helicopter. If you don't have any numbers to go by, the following have proved to be good for me throughout the years for general flying, and will be in the ballpark for your initial flights:
- High = 9°
- Mid = 3°
- Low = −3°
"They are only initial settings to get me off of the ground, then I can make adjustments to suit my flying style and the performance of the helicopter."
This provides a total pitch "window" of 12° (−3° to +9°), with the center being 3°. That means you will have a linear match of the collective pitch to the movement of the collective control stick. However, if you are just learning the hovering process, I believe the above pitch range is too large, and it will make the collective pitch too sensitive until you get used to it. The 9° maximum pitch will allow the helicopter to climb quite rapidly, and the −3° will allow the helicopter to screw itself in the ground if you let it. Therefore, a more conservative collective pitch setting for the novice would be:
- High = 6°
- Mid = 3°
- Low = 0°
This 6° window makes the collective stick half as sensitive as in the first example.
This collective pitch setting will have you hovering above half-stick, and will limit the amount of climb that you can get from the helicopter (since excess altitude is not your friend while learning to hover), while also eliminating the negative collective pitch that could damage the helicopter during a panic landing.
Setting the pitch window mechanically
These collective pitch settings should be set mechanically—without the use of the computer functions of the radio. An easy way to do that is to check the collective pitch "window," or range, by noting the pitch at full and bottom collective stick positions. If the range is not what you want (12° in the first example and 6° in the second), reduce it by moving the pushrod closer to the center of the servo, or increase it by moving the pushrod further out on the arm. With a couple of tries, you should be able to find a position on the servo arm that will provide your desired window.
Once the window is set, check the maximum and minimum collective pitch readings to see if they are as desired. If they aren't, don't just adjust the pushrod at the servo, since you already made sure that it was set up properly to other arms or bellcranks. Make the adjustment at the rotor head, usually with the pushrod going from the swashplate to the mixing arm. It's easy to do by giving full collective at the radio and making an adjustment until the desired maximum collective is reached. Now that you have the maximum collective you desire, and the window has already been set, the minimum collective must also be as desired.
These initial settings are nothing more than "ballpark" numbers. No pitch gauge can accurately measure the collective pitch within a degree or so, because so much depends on the position of the swashplate and the flybar. If they are not level, the pitch gauge readings will be slightly off, but that has never really concerned me. They are only initial settings to get me off of the ground, then I can make adjustments to suit my flying style and the performance of the helicopter.
Fine tuning in flight
After making initial pitch adjustments, consider how the helicopter performs when making a max climb or a rapid descent. If the rotor speed starts to decay during a max climb, and the engine sounds as if it's lugging, there is too much pitch at the top end, no matter what the pitch gauge indicates. If the helicopter descends too rapidly during a quick descent, there is too much negative collective. It will take some experimenting during a few flights to get the settings as you like them, but try to make the changes mechanically, by adjusting the pushrods, rather than using the computer functions of the radio. Once the basic setup is as you like it, you can use the computer functions in the radio to fine tune the helicopter as you desire.
Let me know how this technique works for you, and share with me and other readers any other techniques that you have.
Transcribed from original scans by AI. Minor OCR errors may remain.
