Tuesday, 4 October 2011

Blade Tracking and Balancing for Remote Control Helicopters



WARNING: Tracking adjustment is a dangerous procedure. Keep at least 5m from the spinning rotorhead. It is also recommended to wear eye protection. 

Before following this section, if you are working on a new build, or have yet to mechanically set up the rotor head, please read this first: CCPM Mechanical Setup. It will make sure that you have a good setup on your machine before attempting to balance and track your blades. By the way, my blog will list the latest radio control helicopter guides and tutorials. Sometimes you will get lucky, and have perfectly balanced blades out of the packet. The same goes for tracking, you may not need to track your blades at all. Either way you should definitely follow this procedure to make sure they are operating correctly.

Blade Balancing

This is the easy bit. Essentially, when the rotor blades are manufactured, there might be very slight imperfections in the weight of each blade. Not noticeable to you or I, but once spinning at 1,800 rpm or more, this will cause vibrations in the helicopter. The greater the weight difference, the greater the vibrations.


Helicopter Blade Balancer

A blade balancer comes in useful at this point, you will need to bolt your blades on to it and see if they "balance". If they don't, its time to use the pretty stickers that came with the blades. On the lighter blade, place the metallic sticker at a point along the blade where it makes them balance about there pivot point. Just lay it on initially without removing the adhesive back. When you are sure of the position, fix it in place. You should now have a set of balanced blades. On to the tracking.


Blade Tracking


The first thing you need to do is see if they are actually out of track or not. Time to get out some blade tracking tape, or electrical insulating tape will do. Make sure any edges of the piece of tape are towards the trailing edge of the blade. That way the tape wont come off due to air getting under it. Ideally a different color tape on each blade. Again, if not, just make sure one is well marked.
You will now need to spool the RC helicopter up to a decent head speed, around that suitable for a hover. I personally strap the heli to a workbench, but you can also bring the helicopter into a hover around head height and check from there. What you are looking for is wether one blade bends more than the other. This will be evident as an obvious doubling of the imaged rotor blade edge. With one blade higher than the other. If this is the case you need to adjust some linkages, as your blades are not tracked.

The Reasons Behind Out of Track Blades

Why does this happen i hear you say?! Well, simply, like the balancing, there are very slight imperfections in some blades. So, when a large external force is applied to them they can behave differently. In this case it is one blade flexing more than the other. this is not the only cause, in fact, the more usual suspect, is that when you tried to get zero pitch in your blades during the CCPM setup. There may be some slight pitch in the blade that is impossible to get out during setup.

So, as you may well have guessed, if there is more pitch on one blade than the other, the it will generate more lift, causing it to flex more than the other, and fly higher than its counterpart. Luckily this is very easy to fix.

Effects on Helicopter Headspeed
One thing to think about is headspeed, if you want to get all technical. Say the blades are out by a random figure, lets say 10mm. You could adjust the links so the one flying low comes up to meet the top one, or you could do it the other way round. We have to consider how this will effect the headspeed. As we are changing the pitch of the blade. The safest bet is to raise, and lower each blade respectively by the same amount, so the tracking meets in the middle for the helicopter.

To sum up these effects: If you are happy for the headspeed to increase, the higher blade needs to be lowered. The same is true if you want a lower headspeed, raise the lower blade. As i said above, making them both meet by adjusting equally, will maintain your current hover headspeed.
To actually adjust your blades, you will need to change the lengths of the linkage rods that control the pitch to your blades. It is wise during this step to refer to the manual that came with your RC helicopter, as they can be different from heli to heli. I will provide below a quick guide on how to adjust these on a T-Rex 500 helicopter.
The Align T-Rex 500 Method

Helicopters sharing a traditional flybared rotor head will usually share the same linkages from the swashplate upwards. So the instructions for the T-Rex range will be similar, if not identical to that of other manufacturers. But again, always check in your particular helicopters manual, for vendor specific instructions.

WARNING: Tracking adjustment is a dangerous procedure. Keep at least 5m from the spinning rotorhead. It is also recommended to wear eye protection. 

So, after following the previous instructions with the heli running and inspecting the blades. Look at the path of the blades carefully. If they are rotating on the same path, no further action is required. If one is higher than the other, then apply tracking corrections immediately.
In the image to the right links C and D, connecting the top seasaw arms to the blade grips are used to adjust for regular/medium amounts of trim to blade tracking offset. E and F are to be adjusted if you need to trim out small amounts of deflection on the tracking.

The blade with the higher path has to much pitch. Lengthen links C or D, depending on which blade is out of track. As above, if it s slightly off track, shorten linkage rods E or F. Remembering if you want to keep the same headspeed as before adjustments, to bring one blade slightly down, and the other slightly up to meet in the middle.

If you have followed the CCPM setup, and this tracking guide, you should now have a good mechanical set up on your RC helicopter. Happy flying!


1 comment:

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