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Rotor blade balancing

This page about main rotor blade balancing goes hand in hand with the Walkera Dragonfly 4 page and is intended to help you improve the flight stability of your electric rc helicopter, by showing you how to balance the main rotor blades statically and dynamically if you don't have access to a blade balancing tool.

Such tools are the ideal way of balancing your rotor blades, and if you're going to get serious about flying rc helicopters then it would pay to buy one.
They're not expensive at around $40 for one such as the KSJ1470 Universal Blade Balancer shown below:

But... if you don't have access to a proper rotor blade balancing tool and buying your helicopter has cleaned you out of cash, then outlined below is a crude but effective (and free!) method of balancing your main rotor blades.

Why balance blades anyway?

Good question, and the answer is simple - to stop any unnecessary vibration in the helicopter.

If you've ever driven a car with a front wheel out of balance, then you'll know how annoying that horrible vibration through the steering wheel can be.
Unbalanced helicopter rotor blades have the same effect, but the vibration that they cause is more than just annoying - it impedes the flight performance of the helicopter and at worst can shake parts loose over time.

Having said that though, don't be too upset if you can't elminate all vibration. The very delicate nature of low-priced FP electric rc helicopters means that they are bound to show a certain degree of vibration; only heavier, more robust models will run completely smooth. But your job is to minimize the vibration as much as possible.

It's also important to note that while rotor blade balancing is the primary way to remove vibration, rotor blade tracking is also important, and the blades should be tracked properly as well.

The blade balancing method(s)**

This crude but effective method doesn't take long, but it is worth taking your time and doing it carefully and as accurately as possible. 30 minutes or so should see your blades balanced both statically and dynamically, providing you get all the necessary bits together first. These are:

• your main rotor blades
• a lever (an item with a single, narrow edge - see photo for examples)
• a sharp pencil
• duct tape (or proper balancing tape)
• some fine thread such as cotton
• bolt & nut (a wingnut is good) to fit the hole at the blade root

[**There is a variation on the static balancing method, this is talked about further down the page.]

Dynamic balance - this is the part you do first, and this ensures that both blades' centers of gravity (CG) are at the same point for each blade, or lie within a 1mm or less tolerance range.
By dynamically balancing the main rotor blades, the CGs are following the same line within the rotor disc.
Here's what you do...

Step 1:
Sit your selected lever (in this case a penknife blade) on a flat surface. Gently place one rotor blade on the top edge of the lever, holding each end between your two index fingers.
Slowly slide the blade left to right on the lever until you find its point of balance (CG), and then mark with a sharp pencil the leading and trailing edges exactly where they are sitting on the lever. Be careful not to move the blade as you're marking.
Do the same all over again with the second blade - try to sit the blade on the lever in the same position as you sat the first one ie whether the leading edge was at right angles to the lever etc.

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Step 2:
Lay one blade on top of the other so that the outlines match exactly.
Move the top blade back just enough for the leading edge CG mark of the lower blade to show, and compare the positions of the two marks on each leading edge.
If the marks are more than 1mm apart then you need to change the CG that's closest to the blade tip.
To do this, transfer the CG mark that's closest to the root of the blade onto the other blade. Repeat this process for the trailing edge marks.

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Step 3:
Separate the blades and add squares or strips of tape to the underside of the blade that has the new mark.
Keep the tape close to the root and recheck the blade's balance on the lever every time you add a piece, until it balances on the newly marked CG.
You shouldn't need more than a few small pieces of tape to make the correction.

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Static balance (method 1) - dynamic balance sorts out the blades' centers of gravity, but now you need to balance the weight of each blade to match the other one. The following steps outline the simpler method, the second method is talked about afterwards.
Here's what you do (method 1)....

Step 1:
Join both blades together with the nut and bolt and set the blades apart symmetrically ie how they would be in flight.
An accurate way of doing this is to use a straight edge (eg a spirit level) and hold it over the blades. Look down vertically on the edge and move the blades until your straight edge lines through both blade tips and the central nut.

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Step 2:
You now need to hang the blades from your thread, and here's where it can get a bit tricky.
I modified a wingnut to take the thread (I used fishing line), as the photo shows. If you don't want to go to this trouble, place the thread between the nut and top blade before tightening (don't over tighten!) or tie the thread round the blades at the central point.
However you do it, it's imperative that the thread is in line with the center of the blades, otherwise this whole exercise becomes pointless!

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Step 3:
Lift the blades by the thread, or tie a loop at the top of the thread and hang them from a thin, round object - a small gauge drill bit or a thin screwdriver shaft are ideal examples. Clamp it to a solid edge with an overhang.
Let them settle and hang on their natural position. Eye them front-on and see if they are hanging dead level, or if one is hanging lower than the other.
If one is low and one is high, then they are not balanced statically and you need to do something about that....

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Step 4:
Cut strips of tape (say, 1/2" wide x the blade chord [width]) and add them, one by one, to the higher (lighter) blade.
It's very important that you place the tape at the center of gravity of the blade ie symmetrically each side of the pencil marks. If you don't do this, you're going to change the dynamic balance of the blade and undo everything that you did before!
Add tape until the blades hang exactly level - that's it! Your rotor blades are balanced, but there's something else you need to do before you fit them to the helicopter...

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Static balance (method 2) - this method involves removing the rotor blade holder from the head, and balancing both blades and holder as one.

Step 1:
To balance the blades this way, you need to have two 90 degree metal brackets, readily available from any hardware store. They don't have to be too big, the ones that I use are 2 inch brackets.

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Step 2:
With both rotor blades still in place, remove the blade holder from the rotor head assembly. First you'll need to unclip the two fybar connecting rods from each side of the head, then carefully prise the head off of both bearings.
This can be a bit fiddly, a small flat-bladed screwdriver might help release the holder. Take care not to force anything, and be careful with the other components of the rotor head assembly!

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Step 3:
As outlined in Step 1 of the first method further back up the page, straighten the blades so that they are how they would be in flight.
Sight down a straight edge (such as a spirit level) held directly over the blades, and adjust the blades so that the straight line runs centrally through the blade holder middle, both screws and out to each blade tip.
You'll probably need to adjust one or both blades a few times to get the line right.

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Step 4:
Place both brackets on a flat surface, standing upright and opposite to each other.
Now gently rest the blade holder and blades down onto them, adjusting the position of the brackets so that the point of contact is with each ball on either side of the holder. This is the point of least resistance with the brackets.

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Step 5:
Finally, stand back and let the blades settle naturally. They may take a while to stop rocking, but eventually they will stop and you'll be able to see if one blades hangs lower (is heavier) than the other.
If the whole unit hangs level, then they are balanced. If not, add small bits of tape over the blade CG as outlined in Step 4 of method 1, further back up the page, until they do balance correctly.

Next... Rotor blade tracking and flybar balance and paddle pitch.

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When you've sorted out the tracking and flybar, return to this page and make sure you're OK with...

Fitting the main rotor blades correctly

Once you've balanced the flybar, you can correctly fix the main rotor blades back into the blade holder of the rotor head assembly.
Place them into the holder and screw in the screws. How tight you do them up is very critical to the stability of the helicopter.

Don't tighten the screws so that the blades can't move in the holder - if you do then they won't be able to 'self-straighten' on initial start-up and it's crucial that they can do this.

So as a rough guide to setting the correct torque of the screws, do them up until the blades do not move freely but you can move them with just a very small push at the leading edge. Try and get both blades the same, as close as you can.

Now, when the rotors start spinning the centrifugal force will take care of equalizing the line of the blades through the rotor head.
Another advantage is, of course, that the blades will fold back on impact - rigid blades will just snap!

It's important to keep checking the torque of the screws - they can work loose during flight, and if one blade is being held tighter than the other, vibration will start.

Related pages

Rotor blade tracking - an often overlooked task.

Flybar balance and paddle pitch - more ways to stop vibration.

The Walkera Dragonfly 4 - what you get and what to do with it all.

Your first hops - tips & advice for to get airborne.

Main frame replacement - guidelines for the worst job of them all!

Index - to all Walkera Dragonfly 4 related pages.