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Old 04-16-2011, 01:20 AM   #1 (permalink)
Justin Pucci
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Join Date: Oct 2009
Default V-Bar Tail Tuning and Troubleshooting Guide with Mikado Logo Example

Hey all,

This thread is in response to a few posts and requests for a sticky regarding Logo/V-Bar tail issues, but I'd like to emphasize that this is applicable to pretty much any heli or FBL system. For troubleshooting purposes and since this was originally motivated by questions in the Mikado Logo Helicopter forum, I'll stick with the stock Logo plastic tail assembly as the detailed example below, but if you have an aftermarket tail on your Logo or if you're reading this and have another heli entirely, you can apply it without a problem and should just look for the same issues, but on the specific component details for that heli design. Please feel free to post your feedback, experience with this or other methods, or suggestions for modifications or improvements. Thanks!

It's of critical importance on any heli to make sure that your tail is behaving itself, as few things can more readily rob a pilot of confidence than a bad tail. The most common tail issue is a wag, which can manifest itself in may different forms.

NOTE: Before performing any tail tuning on a new or troublesome helicopter, turn off your headspeed governor!
The governor control loop can cause tail wags very similar to the issues discussed below if it is not tuned correctly. Once you get your tail tuned on a flat throttle curve, you can enable your governor and tune it knowing that you're not fighting your tail mechanics or the FBL system.

The first thing that I tell everyone is to check the smoothness of your tail slider assembly. To be clear, this means your entire tail control assembly from the servo all the way to the blade grips. I've seen tons of people say it's smooth, but then when I feel it, it's chunky, rough, and just bad. This is not to say ANYTHING about you or your building skills, but a lot of Helifreaks suffer from not having perspective that is only gained from building a lot and flying around others. Trust me, I've built a TON of reallllyyyyy "smooth" tails...

1.Checking the Tail Slider Mechanics:
First, pop the control rod link off the belcrank and move it over it's travel range. Is it less smooth at certain points along the shaft? Right rudder (slider towards the tail case) seems to be the most common portion of the travel to be rough. This is usually due to a combination of the smoothness of the plastic balls or other linking mechanism on the blade grips and the sizing of the pitch slider links.

Right Rudder
The plastic Logo grips come out of a mold and the mold lines run right through the center of the balls. Clean them up with some sand paper and give it another go. This is usually an iterative process and you should be mindful of not over-sanding the balls. Once they appear to lack any sign of mold lines, you should check the smoothness again, but you may still need to go over the actual plastic link arms with a ball link sizer. Anytime you're using a sizer, less is cannot put plastic back if you screw up, so be patient and careful! For other helis or tail slider linkage designs, check to make sure that your linkages are not binding in the right rudder region of travel. This could happen due to too tight links on metal balls or some roughness between the link and bushing in the bushing-type designs. Again this applies to all helis, but you'll just have to think about your particular design to figure out what specifically to address.

Left Rudder
If the binding occurs in the "left rudder" portion of the travel range, it's more often then not due to the plastic cup on the bellcrank that the pitch slider ball sits in. The height of the cup can sometimes be excessive, which causes it to rub on the underside of the pitch slider housing. Sand the cup from the top side slowly so as to not take too much material off and recheck.

Full Travel Range
If the binding/roughness occurs over the entire range of travel, then it's most likely your shaft/bushing interface. I like to use Mothers Aluminum/Magnesium polish with a micro-fiber cloth and polish the shaft a bit so that it's silky smooth. To do this, remove your main blades and get a nice microfiber cloth with some Mothers on it. Spin the head while holding pressure on the tail output shaft with the cloth. Give it a good 30 seconds of spinning and then find a clean portion of the cloth and repeat. The end result should be a super-polished output shaft. Clean it off with a bit of alcohol and then re-grease it with Tri-Flow and re-assemble.

W.O.D. Check
WARNING: Only attempt this if you are fully confident in your ability to control your helicopter in any situation!
If you get spooked easily, are not confident in all orientations, or are not comfortable with controlling the helicopter during full-speed pirouettes, then DO NOT attempt this!

A good check for whether binding is the issue is to get your heli in a hover at an altitude that you're comfortable with and do a few full-throw piros with hard stops ("dropping the stick") in each direction. If the heli enters the dreaded Wag of Death (W.O.D. is characterized by a mild-to-wild 15 to 90-degree tail wag that comes on upon stopping the piro and makes a loud, angry buzzing), then remain calm and simply give rudder input in the opposite direction and it should stop. Now you know you've got a binding issue...

2. V-Bar Tail Tuning:
If you believe that you've exonerated all of your potential mechanical causes, then move on to the V-Bar. Are you running stock settings? In other words, check that your P-Gain = 80, I-Gain = 60, Common Gain = ~90 (Common Gain will vary with heli size and type, but for Logos, I usually start at 90 to 100). Note that P and I gains are found in the advanced menu while Common Gain is the slider on the front panel.

How does the tail behave with stock settings? Does it overgain (hard wag with buzzing...i.e. the tail sounds pissed) in hard maneuvers like backwards inverted tail slides, funnels, or hurricanes? If so, start off by dropping your common gain a few points at a time while repeating the maneuvers. If you're not able to do any of the above moves yet, then try flying in fast forward or fast backward flight. If it doesn't get aggravated with that, then throw some hard turns into your routine. My rule of thumb is to find the gain value where you can JUST BARELY hear the tail get angry in a hard funnel and then back it off by 2 or 3 points. Find the right common gain value and once you're confident that the tail will hold and not overgain, then move on to the next step...

Fast Wags
Too high of a P-Gain can cause what most would consider to be a "fast" (>2-3Hz), but small amplitude (a few degrees) tail wag. I like to decrease it by 5 points from 80 until I get down to around 60. If it doesn't change the behavior at all, then I would automatically assume that I've missed something mechanically...Go back to #1 and try again. If a lower P-Gain does fix it, then re-run the common gain optimization as described in the above paragraph and you should be set.

Slow/Medium Wags
Too high of an I-Gain can cause what most would consider to be a "slow" (1Hz or less), but small-to-moderate amplitude tail wag because, as you may recall, I-gain represents the heading-hold portion of the tail control loop and when it is too high, it results in an overshoot-reaction to sudden changes. As in the case of P-Gain, decrease the value from 60 by 5 points each time until you get to 40.

If you do change your I-gain, be aware that you may need to adjust your precomps as well. I-gain and precomps are inversely proportional in effect. In other words, if you lower your I-gain (reduce the amount of overshoot you allow), then you need more precomp to get the same effect. Conversely, if you increase you I-gain, you may need to lower your precomp. If you've gotten rid of the wag and checked the precomp, then simply re-run the common gain optimization and fly! If it still exists, then check your mechanical setup again, but this time if the tail linkage mechanics pass the test from #1 above, then move to on to the next step.

3. Advanced Mechanical Checkouts:
On the tail, check the tail output shaft and tail rotor hub...are they bent? It only takes a little bit to aggravate things when the tail is spinning at several thousand RPM. Is there any excessive play in the tail grips either in the radial (along the rotor hub shaft) or axial (along the tail output shaft) directions? Disassemble the tail rotor and make sure that you've installed the radial and thrust bearings and their associated spacers correctly.

If you have radial play, then you may need to resize one of both of the rotor hub shafts.See if you can match the grips to the correct shaft by finding the combination that gives you the least radial play. If you still have some play, you can VERY LIGHTLY and PATIENTLY sand the end of the rotor hub shaft that is longer than necessary until, when matched with the appropriate grip, you have a nice snug feel. Do not overtighten the screws, as they're too small to handle you kung-fu death grip. Degrease and apply a threadlocking agent and then tighten them hand-tight.

Next move on to the main rotor head and drivetrain. Worn dampers, bent shafts, bad bearings, an out-of-round maingear, over-tight gear mesh and other similar issues can manifest themselves as tail wags on FBL systems, so be thorough. Of all the FBL helis I've seen with these issues, I think I've only ever seen one of them with a head issue that was causing a tail wag and it was due to a slightly bent mainshaft. By "slightly" I mean that the shaft had approximately .001" doesn't take much.

Once you're satisfied that your advanced mechanics are in order, go back to #2 and tune your tail as described above. At this point, you should have a heli with a rock-solid tail and minimal if any tail wags. Keep in mind that nothing is perfect, so don't kill yourself trying to get that last millimeter of wag out of your machine. I've flown helis before with extremely slight tail wags in a hover that hold like a rock and fly wonderfully! Depending on the FBL system, it's actually not as uncommon as you might think to see a very mild wag in a hover. It's when the wag starts affecting your tail control and confidence in flight that you need to take notice and fix it.

Good luck!


Last edited by Justin Pucci; 04-29-2011 at 03:14 AM.. Reason: Updated some descriptions to include more information.
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