How to balance a Prop or Fan
There are various methods used to balance a propeller. The simplest and easiest is static balancing - this method equalizes the weight of each blade. The best balancing method is dynamic balancing which balances the load on each blade while the prop is spinning - most homebuilders will not have access to dynamic balancing equipment (unless you happen to live next door to an airfield with maintenance facilities for light aircraft!).
I use two simple techniques to statically balance props.
The see-saw balance
There are various ways of making a simple see-saw balance. The one I use is made from a few bits of timber and a couple of modeling knife blades.
The knife blades are glued into two blocks of wood so that the blade edge is horizontal to the wood surface. These two blocks are then attached to another section of flat timber so that they are parallel to each other and vertical. You should take care when handling and storing the balancer to make sure that the blade edges do not get damaged or chipped.
To use the balancer you will need a shaft that is the correct size to fit through the prop centre. I try to use a shaft long enough to counterbalance the weight of the prop thus allowing it to hang past the end blades. If your prop is heavy, you can construct the balancer such that the prop hangs in between the two blades and attach the balancer to a bench with the blades and prop hanging over the edge.
If you have a removable blade prop (PowerFin, UltraProp, Ivo, etc) there are a couple of things you should check before using the balancer. First, weigh each blade and write the weight on the blade surface. Then mark the centre of gravity along each blade by balancing the blade on a shaft. If the blades are unequal weights or have differing CofG then you should try to adjust the weight until they are equal (CofG within 1mm and weight within 4g or so would be pretty good). To adjust the weight or move the CofG, either sand a small amount from the heavy end of a blade over as large a surface area as possible, (make sure that the blade manufacturer permits sanding first!). Or you can add small amounts of weight by paint spraying onto the blade surface (again, check with the manufacturer before doing this). When spraying paint, you will need to spray a bit more than you need as the solvent will evaporate and remove some mass - just spray a little at a time.
Firstly, you setup the balancer so that it is absolutely level both along each blade edge and between the two blades - very important! The other thing is that you need to do this in a draft free room - the slightest air movement can start the prop rotating - you should try to avoid moving around quickly while close to the prop (or even breathing on it!).
Place your prop and shaft onto the balancer. I find it useful to gently roll the shaft back and forward a bit to remove any particles that may be on the shaft - this makes the mechanism less sticky.
The object is now to get the prop to remain exactly horizontal without dropping one blade. You can either add weight to the lightest blade (the one that rises up) or remove it from the heaviest blade (see above for suggestions for removable prop blades - the manufacturer may also have a method for adding or removing weight). The best place to add or remove weight is at the centre of gravity of each blade.
The prop shown above needs a small amount added to the left blade to lower it. The amount of weight was equivalent to a three second burst from a spray can - about 0.2g! Once you have the blade balanced, rotate it 180 degrees and check it's still in balance. If not, then your blade edges aren't level or the shaft is out of true. You can spend as long as you wish on balancing - once you get to the point where the blades will only rotate a few degrees from level then that's probably more than good enough for a hovercraft which has much lower prop rpm than an aircraft.
If you have a solid blade prop (UH wood for example) and the prop is quite far out of balance then adding paint or sanding may not be enough, In this case what I do is to either add weight to the hub by fitting larger nuts or studs to the 'light' side of the blade or screw stainless steel wood screws into the hub section of the prop.
The hanging balance
This balancer is designed to hang the prop from it's centre and allow it to freely drop in any direction. You need to make a small dowel or shaft to fit into the centre hole of the prop. It should have a hole drilled through it longways exactly through the centre. This hole should be made to provide a neat fit for a length of thin wire (strong enough to take the weight of the prop). When the adapter is put into the prop hub, the wire should exit the adapter as near to the centre of the hub as possible (half way into the hub).
To use the balance, just hang it and the prop from a convenient hook (ceiling?). The prop will almost certainly drop or rotate. Keep touching and gently pushing the prop to try and get it to balance - I usually add weight to the hub at various points around the hub centre by taping washers/nuts/etc to it. It's a bit like trying to balance one ball on top of another - any minor unbalance will cause it to drop very quickly. The final objective is to get the prop to hang level (check it visually against a level line marked on an adjacent wall).
This type of balancer is useful for balancing a prop in all axis - particularly good for wide blade wood props that are likely to have side-to-side imbalance due to the variation in wood density. You may find it easier to balance the prop on a see-saw balancer first to get rid of any gross error before trying to use the hanging balance.