Written by Mark Christy Monday, 08 February 2010 15:47
First a bit of perhaps "state the obvious"..
You may (or may not) have noticed that the geometry of everything above the swashplate seems to happen 90 degrees out of phase to the expected control input.
For example.. if you give forward elevator you would expect the rotor blade to loose pitch above the canopy and gain pitch above the boom.. therefore increasing lift at the back, decreasing it at the front, and causing the nose of the model to pitch down. The same with the paddles. However the blades don't move at all here and in fact receive no input from the swashpate at all.. it is only when they are 90 degrees to the swashplate that they move. Therefore the advancing blade will loose pitch 90 degrees before the canopy. Why? How? Eh?.. the reason behind all of this is that a helicopter is one huge gyroscope. And for some deep beard scratching mathematical reason things happen 90 degrees out of phase with gyros (and anything else that spins round).
So now we have that covered all is well and good. If we want to go forwards we need to decrease the blade pitch 90 degrees before hand. As with all these things it is great in theory.. but.. never quite so in practice.
Owing to the rubber dampers in the head allowing the spindle to lead/lag, the slop in links, the flex in flybars, aerodynamics, and much more.. the chances of your helicopter reacting at perfect 90 degree intervals is quite slim.. far more likely to be 87 or 94 degrees.. So what?
Well.. a small angle at 2000rpm can make quite a large difference! Have you ever wondered why when you roll your model does a good interpretation of a barrel?.. or when you try to loop you end up coming out of it 45 degrees to when you went in? With 90% of the helicopters I've owned (MA, JR, Hirobo, Avant, Kalt, Kyosho) I've always found I need to mix on the transmitter a little down elevator with left aileron and some up elevator with right aileron to get a really straight roll (say 10% mix). Same with elevator to aileron to have a model that loops well.
What this is doing is changing the swashplate timing. On some machines the washout has a "Phase Ring" which you can rotate around the mainshaft to change the angle of the washout (and therefore the entire swashplate inner ring). This has the same effect as the mixing on the transmitter. What you are doing by moving this (or mixing on your transmitter) is sorting out that 2 or 3 degree timing issue in the gyroscopic procession. Simply put, by adjusting the mixing, or the washout, you are advancing or retarding the inner ring of the swashplate from the outer. Therefore the inner ring will communicate the control request earlier, or later, than the normal 90 degrees.
If you don't have a machine with an adjustable washout of sorts then you are going to be reliant on having a transmitter that does support swashplate timing.. or having enough free mixers to do it manually. Remember though it's only ever the equivalent of a few degrees that are needed so your mixers should never really go much above 10%. And finally.. if you change your flybar length, paddles, blades, etc.. yes.. you're probably going to have to adjust the swashplate timing slightly too if you want a perfectly accurate machine..
Recently I have had to help people with machines that despite being perfectly balanced, no bent shafts, tracking spot on… simply shook.
After lots of head scratching and double checking of everything, trying different blades, dampers, etc.. it dawned on me that perhaps the static tracking was out. Static what? Some of you be thinking..
I first came across this on the original XCell 60 series of helicopters in the early 90’s. They had a couple of grub screws in the base of the head block that tightened against the main shaft. The purpose of these was to allow you to adjust the static tracking of the blades.
Static tracking the blades is much like normal tracking.. however you do it without the head spinning. Simply measure the height of one blade tip and the rotate the blades through 180 degrees and see where the other blade tip is. I do this by extending the aerial of my transmitter so it just brushes the bottom of one blade tip. Ideally they should be under ½ cm in difference. However these machines I mentioned earlier were suffering from blade tips that were several cm’s out. Adjustment on these machines was pretty straight forward as they had 2 piece head blocks. A quick slackening and re-tightening of them allowed us to shift them enough to bring the static alignment into an acceptable region.
If your machine has a one piece head block and has recently been “pranged” then there is a good chance that this might be ever so slightly out. Maybe not enough to visibly wobble.. but that amplified at the end of the blade tip it can be quite visible.
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