Balance of Align blades

[Bronek]

What scales did you buy?

[helimadness]

I got these ones. Not the most expensive but as long as they are consistent I guess. Might weight the same blade 10 times and see what sort of range I get. Only 8 quid including delivery. There were cheaper ones but steered away from them. These come with a rating of 4.7 out of 5 after 550 votes so I am assuming not too bad. Says calibrated before shipping and they come with a 100g calibration weight.

http://detail.tmall.com/item.htm?spm..._u=qpoj6se8292

Have both sets balanced as per finless bob so will give them the scale calibration and see how they go. Just have to find some heavier tape. I have had to put 5 laps of the skinny light sellotape on and its kinda bulky.

Was thinking for the COG balance if it might be more accurate to get 2 scales and put one under each end of the blade. So weighing each end separate. Not sure if that would be better or worse than rolling it on a pin. Fairly hard to tell the exact spot.

[nobbycopter]

Will be interesting to see the difference in weight between the blades. I've never had to put much tape on, one maybe 2 bits at most.

The "teeter totter" method will be the best I'd have thought, cant see how you could balance them easier the other way.

[helimadness]

So got my new scales and weighed the blades. They were about .25 g difference and the one I had the tape on heavier. So took all the tape off and they were perfect to within .01g So the only way the seesaw could have been out was COG being wrong. So started again. Sitting the blade on the shaft and rolling it to find the COG maybe the best way but I think fairly hard to get accurate and that is what was throwing me off. So stuck some weight on the end of the blades to get the seesaw balance correct then put the same amount of tape on the root of the other one to get the overall weight of the blade the same.

As the weight was fine and only the COG out I figure no point adding the balance weight at the cog as that was what I was trying to move. End result both blades perfect weight and on the seesaw dead level.

Check the Vbar log and a lot less vibe warnings so must be better.
Then I had a string of red power and signal failure extreme warnings on the log... But I think that was as the heli hit the ground nose first with speed and splintered my perfectly balanced blades into a million bits.

Oh we have it sorted for next time.

cheers

[nobbycopter]

So got my new scales and weighed the blades. They were about .25 g difference and the one I had the tape on heavier. So took all the tape off and they were perfect to within .01g So the only way the seesaw could have been out was COG being wrong. So started again. Sitting the blade on the shaft and rolling it to find the COG maybe the best way but I think fairly hard to get accurate and that is what was throwing me off. So stuck some weight on the end of the blades to get the seesaw balance correct then put the same amount of tape on the root of the other one to get the overall weight of the blade the same.

That will have put the COG out again lol...you have to put the tape on the COG of the other blade to balance the weight...

As the weight was fine and only the COG out I figure no point adding the balance weight at the cog as that was what I was trying to move. End result both blades perfect weight and on the seesaw dead level.

Check the Vbar log and a lot less vibe warnings so must be better.
Then I had a string of red power and signal failure extreme warnings on the log... But I think that was as the heli hit the ground nose first with speed and splintered my perfectly balanced blades into a million bits.

Oh we have it sorted for next time.

cheers

All that time balancing and then you go and stick it in....that's my sort of trick!! Unlucky on the crash by the way

[Vikki]

Can I just say simply, that by the time the blades are connected together via the feathering shaft, the CoG should be at the centre-line of the main shaft.

Let's say you put the the blades on a balancer. The left one is heavier so you need to add weight to the lighter one. If you need to add 0.5gm to it at the end you would find that if you had decided to correct the weight in the middle of the blade you would have needed MORE weight. If you tried to do it at the root it would requaire a large amount of weight. It's all about the fulcrum. Might need to prove that since it would make scales useless. Right, proved that with a ruler and some weights.

Think of a car wheel being balanced, they don't cut it in half and get the CoG right for each half before welding it back together for balancing. They spin the puppy up and a microswitch or some description monitors where the out of balance is and flashes a srobe light in time to it. Therefore you see the point requiring the weight. OK, I don't balance wheels so that was simplistic. But, that point of balance is good for the whole range of speeds that the vehicle travels at - not just the speed that the wheel was balanced at.

I tried the CoG thing and found that I was 3mm out. To bring its CoG back to the other blades CoG I had to resort to adding modelling clay at the root in small lumps.

So, the blades are connected to the feathing shaft by the blade grips. Now run that over your rod and see where the CoG really is. Get the CoG dead in the middle and that's the CL of the mainshaft. Of course, just use blade balancer, it's easier.

Putting tape close to the tip uses far less of it.

[helimadness]

My process was
Weigh blades and found exactly the same.
Check COG and add weight on the tip to get right.
Then add weight to the other to get the actual weight back to square.
Then check COG and had to make some small adjustments to get COG and weight balanced.
If I added the weight at the COG on the light blade I would have had to add a lot more weight on the tip of the other.
I had to play around with a few bits of tape but the COG and the actual weight are now both right.

I have a brand new set of blades brought today as spares so will give them a test.

Got 7 lipos through her today and all is good. Vbar log file is very good now. About what it was just before I crashed. Here is a sample before and after I balanced the blades all before the crash. Blade balance was the only thing I changed. Shows a big improvement so must be better. The odd "raised vibration warning" is not so bad. There are 2 more levels above that. The blades were actually not that bad. I got them to within .02g so very close. Might have a look at the tail next :-)

vbar log before balance.JPG log after balance.JPG

[Vikki]

Think of the one bladed helicopter where one blade is taken by a hefty hunk of metal to balance the other blade.

[helimadness]

Exactly the important thing is the cog. doesn't matter where the weight is as long as the COG is right.

[cjcj1949]

I think the cog is a red herring. The cog of both blades (together) is important and should be in the middle at the centre of rotation. A common method in maths and physics is to think of small parts like strips across the blade. Each small strip exerts a force during rotation and that force is proportional to the distance from the centre of rotation. (mass X radius X omega squared) To get rid of vibration the forces on one side must be the same as the forces on the other. We work out the total force by adding up or integrating the contribution of each strip. That is complex, but luckily the moment or torque of each strip due to gravity is exactly the same function. ie Delta mXrXg. So balancing is exactly the same as equalising the rotational force. There is no need to get the cog of each blade in the middle, but it will do no harm, it's just not relevant.

There are many rotational systems where a mass at one end balances a distributed mass at the other. The masses do not need to be equal, just balanced. It is easy and it works.