Friday, August 12, 2011

Helicopters: How to get rid of those shakes

How to get rid of those shakes.

By Lood Birk

Ermelo Radio Flyers

RC Helicopters are probably THE most amazing piece of engineering available to the man in the street. The mechanics and aerodynamics of the heli is truly amazing and at the same time mind boggling. In this also lies the problem. The heli has so many moving parts that interact with one another that finding something as simple as a vibration or a shake can be a monstrous task. Well, my aim today is to give a rough guide at how to go about getting rid of the shakes.

Well, let us start by talking about why vibrations are present in your heli. There are three main reasons why a heli can give you the shakes. The first is quite simply that something is out of balance. The second is that something is loose and the third is that something is bent. The trick is to find the offending part or problem.

So, with all those intimidating moving parts in your heli, how do you go about finding the offending part. Well, I normally tackle the problem from big to small. Let me explain a bit better. Firstly you determine where the vibration is coming from. Once you know if it is coming from the tail rotor or the main rotor you are halfway there. Then it just becomes a matter of following a good couple of rules and you can sort out your vibration problem chop-chop.

Right-o, so let us start at the beginning and take it down in steps. Firstly, find out where the vibrations are coming from, the main rotor or the tail rotor. To do this, start up your chopper and have a look at the oscillations you find on the chopper. If it’s a slow shake with the tail mostly moving in an up/down motion, the problem most probably lies at the main rotor blade. Beware though, a badly installed motor can also give you the same symptoms as a unbalanced rotor head. Also, a too low rotor speed can also cause a slow up and down movement of the tail, so just be aware of it. If you get a high frequency vibration and is mostly from side to side, the problem most likely lies with your tail rotor assembly. So now you know in which area you are looking at to find your fault. I am now going to break this article up into two sections. One will give a breakdown of possible problems in the rotor head and the other in the tail rotor.

Main rotor

Finding vibrations on the main rotor is generally not that difficult, but it can become quite a bit of work, so be prepared to be at it for some time.

  1. Check the blade tracking. As I described in a previous article on blade tracking, a pitch difference can cause vibration.
  2. Make sure your rotor blades are balanced.
  3. Are the main rotor blades tightened equally?
  4. Also check if your rotor head (complete unit) is out of balance. To do this, you have to use a balancer like the one on the WiFly site, Extreme accuracy Prop/Rotor balancer. In essence what you do is place the entire rotor head in the balancer and see if it balances.
  5. See if anything is bent. If anything is bent, it will most probably be the feather shaft, the main shaft or fly bar. To check these two shafts properly, you will have to remove them and I normally roll them on a piece of flat glass or of you are fortunate enough to have a clock gauge, you can use that as well. Do not forget to check all the arms and levers to be straight.
  6. While you have the feather shaft out, check the damper rubbers to be in a good condition. The damper rubbers are situated inside the main rotor head housing and the feather shaft goes through it.
  7. Are the paddles from the fly bar level and at the same distance from the center of the main shaft? As a matter of interest, I have heard of professional flyers that balances their paddles, but normally the difference of the paddles are not of such a nature that it would influence a beginner or intermediate pilot. Though, might be worth the while just to check.
  8. Check if anything is loose. Check that all the nuts and bolts are fastened properly. A loose arm flopping about can also cause vibration. At the same time look for extensive slop on the ball links.
  9. Is the feathering shaft installed to tightly or to loosely? If the nuts on the ends are turned too tight, the rotor blade grips will not be able to move and vibration will occur. Too loose and the shaft will move from side to side causing vibration.
  10. Is anything binding or tight in the head? A too loose head causes slop and a too tightly assembled head will prevent the parts from moving freely as they should and that can also cause vibration.
  11. Does the main shaft move up and down?
  12. Are the bearing and bearing blocks OK and fastened well? This is something that is generally overlooked.
  13. Is the main and autorotation gear running true? It’s not wobbling or oval in shape. Yes, I have found that. Are all the teeth present on the main gear?
  14. Is your auto bearing OK? Properly lubed and working properly? It must be run freely in one direction without sticking or grinding to the main shaft.
  15. Has the motor been mounted too tightly to the main gear? If you do this, there is no room for any play between the main gear and the motor pinion and this can also be a source of vibration.

If by now, the vibration that originates from your rotor head is still there, I would suggest you disassemble the entire head and re-assemble it from scratch. When you do this, make sure you have the manual for the chopper at hand to check if all the bits and pieces are there and assembled in the correct order. Most of the decent choppers out there have got websites where you can download the manuals.

Tail rotor

The tail rotor moves about three times faster than the main rotor. For this reason diagnosing vibration problems on the tail rotor unit can be more difficult. I have had instances where there are no vibration on the tail rotor until a certain RPM and then it would shake. All I can say is keep to the list below and go over the setup again and again till you find the problem.

  1. Are the tail rotor blades balanced?
  2. Is anything bent? To check if your tail rotor shaft is bent, you have to remove it and roll it on a piece of glass.
  3. Is the tail casing loose? This and a bent tail rotor shaft are probably the most common reasons for vibration in the tail rotor assembly.
  4. Is the tail rotor shaft moving from side to side?
  5. If your heli has a belt drive, is the belt tensioned correctly? When you turn the main rotor head, is it skipping teeth on the belt?
  6. Check to make sure that the tail pitch slider and tail pitch arms are not loose or sticky.
  7. Is your tail pitch control rod properly supported? A vibrating tail pitch control rod can be the cause of vibration and normally wrongly diagnosed as a faulty part in the tail rotor assembly.
  8. Is your tail servo loose or worn?
  9. And then lastly, is your Gyro gain set too high. This is rarely the case as mostly vibrations progressive in nature.

Now for those of you that are flying glow helis out there, there is an added section that I briefly want to touch on. Having an engine turning your heli brings added problems and is sometimes very difficult to find due to the high precision needed. A clock gauge here is, I would say, a must. So if you have done all of the above and you are still struggling, look at the following that could possibly be wrong.

  1. Incorrectly installed clutch lining. This can cause shudders and vibrations that are mostly found during spool up. BUT, if your clutch is unbalanced due to incorrectly installed clutch lining, it will cause vibration even at high RPM.
  2. Worn or faulty clutch. A worn clutch normally causes imbalance due to the fact the one arm normally expands more than the other when worn, which unbalances the clutch
  3. Damaged clutch bell.
  4. Loose lock nuts. If your locking nuts are not tightened properly it will cause play in the assembly and in turn cause a vibration.
  5. Bent shafts. Not common but a very real possibility.
  6. Imbalance inside the motor. This is really not very common as the motors are normally balanced at the factory and I have not had a case like this myself, but I have read on forums that it is possible.

As you can see from the list, vibrations are in general caused by something that is imbalanced or loose. I hope that this gives you a basic guideline to sorting out your helis Parkinson’s disease.

Happy landings.

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