Expert: Paul Soderman - 7/23/2008

Question
Hi...I'm involved a heated debate about balancing  model helicopter blades. The discussion is centering around static & dynamic balancing, predominantly the importance of the blade spanwise Centre of Gravity. I'm hoping an "expert" in the Aeronautics can provide a definitive answer. My argument is that the CoG of each blade needs to be the same, as does the weight of each blade, in order to have good dynamic balance - the opposition states that the CoG does not have to be the same, nor does the weight of each blade have to be equally. His argument is that good dynamic balance is gained by both blades having equal moment arms. As such, good dynamic balance can be acheived by simply placing the blades in opposition (180deg) on a simple see-saw and adding weight to the tip of the lightest blade until the blades balance horizontally. My counter argument is that by placing weight on the tip of the light blade, you have displaced that blades CoG from it's original position and have altered the dynamic balance of the rotor. In effect, all that has been achieved is good static balance with no regard for the dynamic balance of the rotor.
Can you help ?
Thanks
Will

Answer
Hi Will
You are on the right track.  Proper helicopter rotor balancing requires that the spanwise and chordwise mass distribution and centers of gravity (COG) of each blade be as close as possible. If the mass distribution or spanwise COG of the blades differ, the angular momentum of the blades will differ - this causes improper tracking and vibration.  Imagine two blades - one with an even mass distribution and one with the same COG but more mass at the ends than the center.  The blades would have different vibration characteristics. Likewise, if the chordwise COG are different, the blade pitching moments will differ and tracking will be affected. Small balancing weights are allowed.  The blade shapes should be very similar also.  The following website shows the complete static and dynamic balancing process that is done on real rotor blades:

http://www.rwas.com.au/blade-balancing.html

The same is true for propeller blades.

Good luck.
Paul