Written by Ashley Davis Sunday, 06 March 2005 00:00
I'll illustrate throughout with pictures using main blades and tail blades.
The procedure :
1. Find the lightest blade
2. Find the center of gravity of each blade
3. If the center of gravity of each blade matches go to step 5
4. Move the center of gravity of the lightest blade to match the heaviest blade
5. Add weight to the center of gravity of the lightest blade until both blades balance
Find the lightest blade
Bolt the two blades together at the blade root using a long bolt and two nuts. The blade bolts are normally long enough for this purpose.
Once they are bolted together make up two straight edges that you can place the bolted blades onto to create a make shift balance. For the pictures below I just used two kitchen knives held parallel to one another. They must of course be horizontal or the bolt will try to roll and the balance will not work properly.
Pictures below using tail blades, just copy this for main blades. I didn't have a couple of nuts of the right size to show this for main blades so the illustration uses tail blades, however the procedure is exactly the same.
The blades must be bolted tightly together to be able to act as a balance. Which ever blade hangs lowest is the heaviest. We want the lighter (higher) blade, mark it with a felt tip or something similar.
Find the center of gravity of each blade
This is very straight forward. Using a large sharp knife place it sharp side up and secure it in a horizontal position.
Place your rotor blade to be balanced onto the knife at an angle of 45 degrees. Find the point where it balances and then just push down on the rotor blade to make a mark on the rotor blade with the knife sharp edge.
Now place the blade at 45 degrees again but at the opposite angle. Find the balance point and push down on the rotor blade again to make a mark with the knife sharp edge. When you look at your blade you should now have an 'x' on the bottom, the center of the 'x' is the lengthwise and chord wise center of gravity of the blade.
Repeat the procedure for your other rotor blade.
(this procedure is very hard to do with tail rotor blades as it is very difficult to make them balance on the sharp edge of the knife, get it as close as possible, probably just teetering one way and then the other,and then push down to make the marks)
Illustrative pictures below using main blades :
Make each blades center of gravity match
This only needs to be done if the two marks (above right) are not at the same point along the length of the blade.Note:
Trim tape is used to weight the blade. Cut a small piece that is long enough to wrap around the blade and about 1/4 inch wide. Pick the lightest blade to have it's center of gravity moved.
If the center of gravity needs to move towards the blade root then put the tape at the root of the blade (at the widest part of the blade before it narrows).
If the center of gravity needs to move towards the blade tip then the tape should be placed at the blade tip.
Add tape in a trial and error way until you get the center of gravity matched to the heavier blade. You should have only added trim tape to one end of the blade to achieve this.
Balance the blades
Now repeat the procedure for finding the lightest blade at the top of the page.
Having found the lightest blade we now add trim tape (1/4 inch wide and long enough to wrap once around the blade) a bit at a time until the blades balance. The tape should be added at the center of gravity (the 'x') until the blades balance (sit perfectly horizontal on your balance).
Pictures below of the main blades pictured above with trim tape added at the center of gravity to balance. No work was required to move the center of gravity on these blades as they already matched.
That's all there is too it. Generally balancing main blades is a necessity but on a micro helicopter balancing tail blades is very difficult at best and in most cases not necessary.
As previously stated the above procedure is correct whether you are using a proper blade balance or not. I recommend the use of a proper blade balance as they are far more accurate in balancing the blades than using the method of bolting blades together at the root as shown.
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