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Old May 31, 2011, 09:42 AM
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Reducing RC Heli Vibrations - Beginner's Guide

So one of the many battles the RC'er fights when building/assembling a RC helicopter is eliminating vibrations. After building nearly a dozen RC heli's over the past few years, I figured I'd write a quick beginner's how-to in order to simplify the process, as there is a lot of conflicting information out there on the web and some of the info is rather daunting.

What sort of harm do pronounced vibes cause on a RC Heli? Well, first of all, too much vibration can literally shake your heli apart during flight making it dangerous to fly, a badly vibrating RC heli is a crash hazard and should not be flown. Secondly, pronounced vibration can interfere with Gyro operation, making it difficult to control your heli during flight.

Here are some tips that I hope will help beginners...
(1) Don't be so quick to install all your blades and paddles at once before you spin up that motor for the first time! Spin up with no blades or paddles in the head or tail to get an idea of how bad your heli vibrates (if at all) with NO blades installed. Ideally you should not have noticeable or bad vibration here, but if you do, double check that you have tightened your main blade grips to your feathering shaft and that there is NO play in the blade grips (they should not "jiggle" back and forth). Also make sure your tail grips are tightened in the same manner. Double-check your flybar measurements with a digital caliper, you need to be as accurate as possible to ensure flybar length is equal on both sides! Bent main and tail shafts can and will cause noticeable vibration, so if you have a lot of vibration and don't have your blades installed yet, double-check those main and tail shafts, make sure they are true. Sometimes just rolling a main shaft on a table will allow you to see whether or not it's bent.

(2) Balance, balance, and re-balance those main rotor blades and tail blades. If you lack a heli blade balancer, you can use a bolt, a nut, and a vise (or 2 blocks of wood) to balance heli blades. Here's a simple guide I like to use that outlines proper blade balancing techniques:

http://www.rchelisite.com/rc_helicop..._balancing.php

Make sure you do the same for the tail blades and the flybar paddles. Out-of-balance flybar paddles will cause vibes! I use blenderm tape on my blades to balance them on the CG point. Blenderm tape is nice because it's very light and VERY sticky, ensure you wrap the tape in a manner that WON'T interfere with airflow over the blades! Tape should overlap properly so that the air can flow smoothly past the leading edge of the blade and over the tape without disruption.

If you find blades are badly out of balance, don't be afraid to return them, especially if they are Align or Curtis Youngblood branded blades that cost you a small fortune. There is no excuse for expensive heli blades to be way off balance, that should be considered a defect. If you find yourself wrapping tape more than one rotation around the blade, I'd get them exchanged. With cheap off-brand blades, well, you get what you pay for, and may need to use extra tape to balance them. Btw, I haven't bought ONE set of Align blades that didn't need at least SOME tape to balance them out. I've also had generic blades that were balanced perfectly out-of-the-box and required no tape at all.

(3) Spin up with your heli strapped or bolted to a heavy work bench before you try test hovering it. If your heli vibrates itself apart, better to do so strapped down to a bench than in the air. Some rocking during spin-up is normal and not necessarily indicative of balance issues, the blades are just finding their place in the grips and things should smooth out as the head comes up to full RPM. Bad vibes are usually visible - you'll see your boom supports vibrating like crazy, the vertical tail fin will be "blurred" from vibes, or the boom/canopy will be blurred from bad vibes. If you can't visually see vibration on your heli, you're probably in good shape and do not need to take further action. Resonance/sound from the motor will make your heli frame and boom "buzz" but it shouldn't shake the frame or cause any visible "blur" on the frame or boom when you are up to full RPMs. If you see anything blurred on your frame or boom, you probably need to rebalance the main or tail blades, or possibly the flybar paddles.
Some RC'ers will spend hours trying to eliminate every single vibe, some just can't be eliminated. Again, resonance alone from the motor and high RPM spinning head/tail will cause some very high-frequency vibration that is NOT visually detectable. These vibrations are not a concern and cannot be eliminated entirely. This is why you must THREAD LOCK all screws going into metal.

If you want to see how vibrations affect your heli in the main or tail blades, simply wrap some tape around one of the blades and spin up with the heli STRAPPED tightly to a work bench, preferably outdoors. Note the behavior -- out-of-balance tail blades produce a different vibration pattern than out-of-balance main blades, it's good to know the difference so that you can more easily figure out where your problems reside.

So, your heli STILL vibrates badly even after spending a lot of time balancing EVERYTHING?? Time to check your main, feathering and tail shafts. Bent main shafts cause noticeable wobble at full RPMs, a bent tail shaft will cause your boom to vibrate badly. A bent tail shaft is generally easy to recognize as your boom or boom supports will be blurred/vibrating badly while your canopy will have little or no vibration. A bent main shaft wreaks havoc on the main frame of the heli, often vibrating the canopy, the frame, as well as the boom. A bent feathering shaft will cause your blades to be out-of-track which will result in additional vibration. It's good practice in general to replace the main, feathering, and tail shafts after a nasty crash, MOST heli crashes are nasty, unless you remembered to kill your throttle before your heli hit dirt.

Overall, make sure feathering shafts and grips are tight, and all blades/paddles are balanced and 90% of your sources of vibration are eliminated up-front.
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Last edited by ChrisWNY; May 31, 2011 at 09:58 AM.
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