Friday, January 21, 2022
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Flight Trimming 101

For 700/800-Class Electric Helicoptor Series

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 makes a bee-line for the nearest tree! What the heck? You reign it back in, but it still wants to be one with 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 RC 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 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 at its best.

Find The Bind!

Flight Trimming 101
The linkage should operate smoothly with little to no resistance and requiring minimal force.
Nearly all new 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, you’ll need to troubleshoot where it is coming from by the 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.

Flight Trimming 101
Ream the ball links only if needed. Do so sparingly to minimize slop.

This particular model had a pushrod running from the servo arm to a bell-crank that connected to another pushrod that then connected to the tail pitch slider. First, I made sure the pushrod ball links were moving freely on their balls by twisting them. They checked out. However, if you find that your linkages are sticky, then you’ll need to use a ball link reamer tool to remove some material from inside of each link until you achieve near frictionless movement. If you pop off one end of the linkage and it doesn’t move freely, the 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 or worse yet, it could pop off in flight and cause an untimely demise to your new toy. Use the reamer a little at a time, test and repeat if necessary.

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, 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.

Flight Trimming 101
Binding can cause undue stress on your servos. Try to reduce binding wherever possible.

You can apply this same technique 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 creating unnecessary wear and tear on your servos. Now, your trimming efforts won’t be negated by control system binding or slop and will go much more smoothly, literally.

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 left for a clockwise rotating main rotor system. 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, insure that all of your cyclic digital trims are centered or set to neutral, then you’ll need to decrease the 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.

Flight Trimming 101
You’ll only need to tilt the swash about 1 or 2-degrees to correct for translating tendency.

In order to obtain the best trimming results you’ll want to go out on a day where there is little to no 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 and get up to eye level, otherwise the model will be susceptible to ground effect or other vortices created by the rotor wash.

Digitally Speaking…
Next, we’ll want to ensure that all of the digital trims in all flight modes are centered or set to zero. I recommend making your trim adjustments by mechanically adjusting the linkages on the model. I prefer this method for a couple of reasons. Firstly, 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, you ensure 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-today changes in weather conditions like wind and temperature.

Flight Trimming 101
Set the radio trims to zero (centered) and make any trim adjustments via the mechanical linkages.

Flight Trimming 101

Tails First
Now we’ll get the tail rotor trimmed out. There is some debate these days as to whether or not it’s worth the time to trim the tail rotor since nearly everyone flies their gyro in heading hold mode. In practice, a good 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 the pitch slider is at the center of its travel, the tail servo arm is 90-degrees 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-degrees 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-degrees.

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. This is 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.

Flight Trimming 101
Left: If the model’s nose turns right on lift-off, you’ll want to lengthen the
tail pushrod to decrease tail rotor pitch. Right: If the model’s nose turns left
on lift-off, you’ll want to shorten the tail pushrod to increase tail rotor pitch.

To finish trimming the tail you’ll 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 up into a hover and note whether the model tends to 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.

Flight Trimming 101
Checking the fore-aft balance of your model is very easy. Just fill the tank with fuel and lift it with your fingers by the flybar.

Finally, 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 until it allows the model to level out. Now, lift the model up 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.

Flybarless Less Trimming?
Just because you have a trio of gyros to tell your servos how to compensate for drift doesn’t mean you’re exempt from properly trimming your model. There are a few common setup requirements among the current crop of flybarless systems. Most of them require that the servos are all set to neutral, your swash is level when the cyclic servos are at neutral and that at full cyclic deflection you get exactly 6-degrees of cyclic pitch. Make sure your linkages are all set to length as per the model manual’s specs. You’ll also want to make sure the balls are all set at the appropriate distances on their arms as per the manual too. Once this is all in place and you’ve made sure the links all move smoothly on their balls, you’ll want to jump into your flybarless system’s manual and follow the steps exactly.

Unlike a flybar model, I don’t recommend adjusting the links on the model to fix drifting problems with a flybarless system. Instead, you’ll set everything up by the book on the bench. Similar to a pesky drifting tail, most of the trim problems with flybarless models are more related to vibration adversely affecting the gyros than an out of whack linkage. Preventing and/or fixing sources of vibration is a whole other story for another article. However, presuming you built your model really well and everything is tight, moving smoothly and your electronics are setup properly, then you should not have any issues.

Flight Trimming 101
With some minor tweaking you can make your
model practically hover itself!

The other phase of trimming a flybarless model involves tests similar to what you would perform to insure your tail gain is set properly. You’ll want to lift up into a hover and switch through your idleup curves and watch for any oscillation on any one or combination of control surfaces. If your model doesn’t pass this test, then you’ll need to go back to your flybarless setup and decrease those respective control surface gains until the oscillation is eliminated. Once that’s sorted, you’ll take the model into forward flight, backwards and inverted in all of your flight modes. If you’ve made it this far without any oscillations, then if you’re a capable enough pilot you can try some 3D to really stress the system. Maneuvers like funnels, pie dishes, tail slides, snakes and tic-tocks can force a gyro to show its hand if the gain isn’t set properly. If your model makes it through these tests without exhibiting any twitching or weirdness, you should be good to go.

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 probable doom. Since your model is now so sweet to fly, you may want to note your linkage lengths so in case of a crash you’ll be able to reproduce near perfect trim. The extra time you spend properly trimming your model will pay off by making your controls more predictable and flights more fun.

By Dan Goldstein