|
|
 |
|
|
| Issue 36 (June 2009) |
Words: Dan Goldstein |
Today is the day you set out to make your first flight on a new model. With starting equipment in hand and the power switched on, you get the model spooled up and start to lift off. As soon as the skids leave the ground, the heli heads straight for the nearest tree! You get it back under control, but it still wants to drift towards that tree.
There is no wind, so why does the model keep heading in the same direction? The trim of the model is almost certainly out of whack. All R/C models can benefit from some form of trimming to help neutralize the flight characteristics. This guide will outline how to refine your model's tendencies. Proper model flight trimming is not difficult and only takes a small amount of time, but it yields worthwhile results. Despite an assembly manual's suggested linkage lengths and radio setup (or an RTF model being pre-assembled by factory workers), any model will still need tweaking to fly as best as it's truly capable of.
|
|
Find the bind!
Nearly all model helicopters have control linkage binding somewhere. This is very easy to check by popping off each linkage at the servo and operating it by hand. In doing so, you should find that the control moves freely and with little to no resistance. If you feel any resistance, it is very easy to troubleshoot where it is coming from by process of elimination.
For instance, while I was checking the tail pushrod at the tail servo on a model, I found that the pushrod would develop increased resistance towards its extremes in both directions. This particular model had a pushrod running from the servo arm to a bellcrank, which connected to another pushrod, which then connected to the tail pitch slider. First, I made sure that the pushrod ball links were moving freely on their balls by twisting them. They checked out.
As a side note, if you find that your linkages are sticky, then you'll need to use a ball link reamer tool to remove some material from each link until you achieve near frictionless movement. If you pop off one end of the linkage and it doesn't move freely, then that link is in need of reaming. You don't want to ream out too much material, otherwise you'll induce slop and the link will wear out prematurely. Use the reamer sparingly so as not to remove too much material, otherwise this could cause the link to pop off in flight.
Anyway, after disconnecting the tail pushrods and finding no resistance, I went on to moving the tail pitch slider. Again, I felt a bind at its extremes. I began to look more closely at the tail mechanism to see what could be causing the binding. I removed the screws from the bushed tail grip links and found that they were somewhat tight and would stick in position. Some slight loosening of their respective screws resolved this. I also made sure not to tighten the tail grips' link screws down too much when I reattached the links. I checked the pitch slider movement again and the binding at the extremes was gone.
You can apply this same troubleshooting process to every linkage on a model to find the cause of binding or slop. Either way, this method ensures that whatever inputs you give the model; it will react smoothly and without causing unnecessary wear and tear on your servos. Now your trimming efforts won't be inhibited by control system binding or slop and will go much more smoothly.
Tilt the swash
Next, you'll want to trim the swashplate to counter translating tendency. "Translating tendency" is what occurs when you lift a helicopter off the ground and it starts to slide. In a clockwise main rotor system, the tail pushes air to the right; this causes the model to slide to the left upon liftoff. To counter this phenomenon on a clockwise rotation rotor head, ensure that all of your cyclic digital trims are centered or set to zero, then you'll need to decrease the pushrod length of the right roll servo and increase the length of the left roll servo each by one full turn. This adds a smidge of right cyclic to counter the translating tendency.
In order to obtain the best trimming results, you'll want to go out on a day with the least amount of wind. Of course, not all of us live in an area devoid of wind, so just try to pick as calm a day as your area allows. For electric helicopters, flying indoors provides the optimal trimming environment. Make sure you fly in a larger space like a gymnasium or arena, otherwise the model will be susceptible to ground effect or other vortices created by the rotor wash.
Digitally speaking...
Next, you'll want to ensure that all of the digital trims in all flight modes are centered. I recommend making your trim adjustments by mechanically adjusting the linkages on the model. I prefer this method for a couple of reasons. Some radios allow you to change the digital trims for each flight mode. Checking your radio is simple: Turn on the radio, and as you flip through the flight modes, adjust one of the digital trims. If the servo moves, the digital trim works for that flight mode. By leaving the digital trims at zero and adjusting the trim mechanically on the model, it ensures that the trim of the model stays the same for each flight mode. I prefer to use the digital trims for making small trim adjustments for day-to-day changes in weather conditions like wind and temperature.
Tails First
Now we'll get the tail rotor trimmed out. There is some debate these days as to whether it's worth the time to trim the tail rotor since nearly everyone flies their gyro in heading hold mode. In practice, a quality heading hold gyro will hold the tail regardless of the tail pushrod's length. However, not adjusting the tail in most cases can adversely affect the amount of pitch throw on either side of your hover tail pitch.
If you're going to fly in heading hold all of the time, you can set up the tail on the bench so that the pitch slider is at the center of its travel, the tail servo arm is 90° to the incoming pushrod and the rudder stick is centered. The suggested setup by the majority of gyro manufacturers is to adjust the pushrod to trim the tail rotor for no drift with the gyro in rate mode. To get you nearly spot on you can use a micro-heli pitch gauge to set 8°-10° of pitch on the tail blades with the gyro in rate mode with the rudder stick centered and the pushrod coming into the servo arm at 90°.
In keeping with most gyro manufacturers' guidelines, hover the model in rate mode and make pushrod adjustments to get the tail to hold without drifting. For beginners, you can accomplish this easily by getting the heli light on the skids and noting which direction the nose drifts. Assuming you set up your servo direction and gyro compensation direction properly, note the direction the tail pitch slider moves on the output shaft when you give left and right rudder stick input. If the nose drifts right, adjust the pushrod to give a slight left input. If giving right rudder moves the tail pitch slider in on the tail output shaft, you'll want to lengthen the pushrod to decrease tail rotor pitch and move the pitch slider out on the output shaft to compensate and vice-versa. This method works on all models regardless of which side the tail assembly is oriented. Once the tail is trimmed, you can continue to fly in rate mode if you prefer, or switch to heading hold.
To finish trimming the tail, dial in the gyro gain. I recommend starting low on the gain and then working your way up until the tail starts to wag and then backing off a few points. Starting out at a high gain puts unnecessary stress on the tail servo. If you're a beginner, you're done for now. If you're capable of forward and backwards flight, make sure to check for wag in both directions and adjust the gain as needed to eliminate any perceivable tail wag.
Cyclic, getting it right and left...
Next, you'll dial in the right/left cyclic trim. Since you adjusted your swashplate on the bench to compensate for translating tendency, the right/left cyclic trim should be pretty close. Lift the model into a hover and note whether the model tends drift consistently in a particular direction. Adjust either the right or left roll servo's linkage until you don't have to constantly give corrective input for the same direction. You'll know the trim is nailed when the model randomly drifts left or right.
To finish, you'll dial in the fore/aft cyclic trim. If your model hangs level when you hold it from the flybar with a full tank of fuel and the main blades parallel to the boom, your trim should be close. If the model is not level, then you can adjust this by moving the receiver pack or adding weight to where it allows the model to level out. Now, lift the model into a hover, note whether it continually drifts fore or aft, make a linkage adjustment on the model to compensate. Once the model randomly drifts fore or aft, your trimming is spot on.
Conclusion
With a perfectly trimmed helicopter, you may want to note your linkage lengths so in case of a crash you'll be able to reproduce near perfect trim. Now that your model is trimmed, your control inputs will be nice and responsive. The model should sit in a hover with minimal fuss instead of you trying to wrangle it from certain doom. The extra time you spend tweaking the model will pay off by making it more precise and enjoyable to fly.
|
|
|
Subscribing to RC Heli is easy! Just fill out the form below, and you will be invoiced for $24.99 for your first year.
Offer is valid for US residents only. |
