String Method, by forum members Gill with a little help from Klaus and Don.

"I have found this method most satisfactory. It gives a high degree of accuracy for the static balancing of a prop.

Here is an action shot with no movement evident.

With this method the prop is a beam balance and the string entry to the plug is the fulcrum.

A little balance theory now:

There are 3 basic actions (movements) of the beam about the fulcrum. These are influenced by the centre of gravity in relation to the fulcrum.

The first action is Accelerating. This is bought on by the centre of gravity being above the fulcrum. When balanced, if the beam is deflected from balance, it will continue to move at an accelerating rate to the limit of it's travel.

The second action is Neutral. This is bought on by the centre of gravity being on the fulcrum. If deflected from balance, the beam will remain in that position.

The third and the one we use is Vibrating. This is where the centre of gravity is below the fulcrum. If deflected from balance the beam will return to the balance state.

As the centre of gravity is moved from just under the fulcrum to a much lower point, the beam goes from being Sensitive to Dead. This means that more weight is required to bring about the same change in balance.

Here is the stub off an F&P shaft I made to plug the hole and provide a fulcrum with a centre of gravity adjustment.

The main thing to note if you make your own, is that for consistency, the thread must be firm and the string must be tight through the string hole.

Method:

• Pick a dead calm day or go indoors where no draughts.
• Fit the plug to the prop.
• Lower the C of G (centre of gravity) for a dead action.
• Add weight for a horizontal balance.
• Raise the C of G and adjust with weight as required.
• Repeat this step until a very sensitive action is achieved.

Never fear, most will go to far to a neutral or accelerating action and get odd and confusing readings. Just go back a little.

Well that's how I do mine. Hope you have fun.

Gill"

"Hi Gill,
Let me see if I got that

1. what you call the 'fulcrum' is the top end of the threaded rod, where the string exits?
2. the balancing 'action' positions are adjusted by screwing the threaded rod in and out (up or down actually) on the splined stub?
3. your propellor hangs backside up and the washers (or lack of them) are your balancing weights?

Did I get it? Very clever, your method that is.

Klaus"

"Your spot on working out what I didn't say. It's a YES to each question though the backside facing upward is because the CofG is closer to that face on this prop.

If the blades protruded forward and the C of G was to be near or even further forward than the front of the hub, then that face would need to be uppermost. In such cases the threaded rod can protrude past the splined stub to position the fulcrum in correct relation to the C of G even if the C of G appears to be in no-mans-land.

Gill"

Don Brown has made the following notes...

"The sensitivity of the balancing arrangement will be affected by the length and weight of the bolt or threaded section used for balancing.

Unnecessary length (and hence weight) lowers the centre of gravity of the combination of the blades and the bolt. This makes it more difficult to approach the toppling point, where the centre of gravity of the blades plus bolt is above the point at which the suspension line enters the bolt.

Just before, or at, the toppling point is where the balance will be most sensitive.

This consideration is of particular importance for small and/or extremely light blade sets, especially when the weight of the bolt becomes a significant percentage of the weight of the blades.

The basic message is that, for maximum sensitivity, keep the bolt short and light."

Good point, thanks Don.