Aviation propellers need to be balanced.
The best balancing can be achieved by dynamic balancing (that is when the propeller turns) However, this type of balancing needs expensive equipment and not very many people can afford it.
Another way to balance a propeller is the static balancing. This means the propeller is placed on a device that will act as a balance.
Most propeller balancers I have seen, work on the longitudinal axis of the propeller only. This way of balancing is not very accurate.
For the best possible static balancing, the propeller needs to be balanced on the longitudinal and transversal axis as well.
Here come into play the so called "string type" balancers.
The principle is simple: instead of being inserted into a cylindrical axis at the hub that rests on two supports, the propeller is hung thus the device and the propeller are suspended by a string, giving us the possibility to judge the balance on both axis.
The very few "string" balancers that I have seen, fit a certain size of hub hole.
Some time ago, I learned that Mark Damon of Illinois built a new string balancer.Following his advice, I built mine.
The balancer I will present is universal and can adapted to fit most of our popular propellers used on different Power Paraglider Motors.
The main part of the balancer is an expansion plug found at almost any hardware store.
The 1" diameter rubber fits the most common hub holes.Tightening the nut will cause the rubber to expend and this way, the plug will fit different propellers.
After purchasing this plug, I took it completely apart (removing the rubber as well) and I drilled a 3/32 hole into the top of the bolt. Then, using a fast curing epoxy, I inserted a cage string into the drilled hole and applied the epoxy.
One can even adjust the sensitivity by turning the nut on the bottom plate.
The way the balancer works is as follows:
1) Insert the plug into the hub hole.
2) Tighten the nut to have a good fit.
3) Holding the string by hand or better, suspending it from a support, we can see which side of the propeller needs more weight.
4) Instead of spraying varnish on the light side, I suggest applying some fast epoxy on the propeller tip and then using a file, remove the excess of the glue until a perfect balance is achieved.
Using a lacquer spray instead, will force us to be very careful and will prolong the operation since as it dries, the lacquer (varnish) becomes lighter and the propeller will be out of balance again. It would take many layers of varnish.
5) The transversal balance is a little more difficult. Sometimes, we may need a lot of weight. Alex Varv told me he once used two 22 caliber Lead bullets to balance a prop. Lead is a good material to use. In order to apply it, we need to drill a small "bed" for the weight.
We must remember that the epoxy will also add some weight.
It is better to use small amounts of Lead because we can easily add or remove weight. When the final balance is achieved, we plug the hole with epoxy, which will also increase the weight. As we see, the transversal balancing is a little more time consuming.
One last thought: a propeller that is out of balance will cause detrimental vibration and even if not felt by the pilot, will cause the reduction bearings to wear sooner. It may even cause (in time) a fracture of the reduction plate or other parts in flight or on the ground and endanger the pilot and bystanders.
A wooden propeller should always be stored horizontally so it will absorb moisture more evenly thus maintaining the balance much better.
It is not true that once balanced, a wooden propeller can be run forever. A conscientious pilot will recheck the balance from time to time.