It is important that propellers be well balanced. The propeller should also sit square to the engine prop driver. Check for an equal height under each tip with the prop sitting on a flat surface.

Many props (especially moulded nylon types) will have an uneven bottom (and / or top) surface due to uneven material shrinkage. Check this before checking for equal heights below each tip.

It can happen that the shaft hole isn’t square to the rear face of the hub, for this it is a good idea to step or taper ream the prop, leaving only a small amount of the hole at the required diameter.

Please note, unlike machine made products which should be perfectly balanced (but often aren’t), hand made props will require some balancing and finishing. Just to be safe, all props should be checked.

We use 4 types of prop balancers

1) Basic double cone type. This is the common easy to use type. Take care to hold this balancer square between the fingers. It is easy to create a false reading..

2) The Pin and Cone type. This type is good for checking balance in 2 directions, a) along the length, b) across the hub. The position of the cone adjusts the sensitivity. High = insensitive, Low = sensitive or 'overbalance'. Make a series of spit sleeves to fit larger shaft diameters. This type of balance is very good for multi blade propellers.

3) Tru-Spin type. The Tru-Spin style of balancer has 2 sets of rotating wheels onto which the prop is placed via a shaft which is fitted through the hub of the propeller.

These are the most expensive, but accurate and hardest to use. Caution, we have found the alloy wheel types to be far better than the plastic types.

These balancers are also very good for balancing spinners. Note - many spinners are out of balance.

4) Magnetic type. To a large degree these work on a similar principle to the basic double cone type, but without the friction of the fingers. The good versions are as accurate as the Tru-Spin type. They are at a disadvantage when balancing large, heavy propellers.

At Bolly we use all 4 types at different stages for different jobs when we factory balance a propeller.

Always balance by evenly removing material (sanding) from the top (curved) side of the blade. To remove material from the bottom may change the pitch and to remove material from the blade length or chord will create a dynamic imbalance.

When removing the material, the choice of abrasive paper will depend upon the type of prop material. When modifying nylon base props, use a very fine abrasive paper as leaving any scratches on the surface is very dangerous, 1 deep scratch is potentially fatal. Wood props are easy to work with any appropriate abrasive paper. GRE / CRE props are very tough and will often require a very course paper for initial work, finishing with finer grades.

It is common for the prop to be heavy one side (across the blade). It is necessary to evenly remove material from the LE side of one blade and the TE side from the other, i.e. it is best to use a pin and cone or Tru-spin type balancer.... See the next page...

It is surprising how many modellers have never used a prop balancer, and even more surprising as to how few know how to read what the balancer is telling them.

With any balancer, and any propeller, the prop should stay stationery at any point throughout it’s 360 degrees of rotation. Sounds simple, but it isn’t. If you have ever had a prop which refuses to stay put, or one that is different when rotated through 180 degrees, it is almost certain that the prop has a heavy side ... across the blade (chord) as opposed to along the blade (diameter).

The technique we recommend is this ...

If the propeller produces slightly too much load for you engine, don't be afraid to trim the diameter slightly. If you would rather not reduce the diameter, your other options are, a) reduce blade thickness, b) reduce pitch or c) reduce blade chord.

Glass or Carbon propellers have the advantage of being easy to cut, file and sand. Don't be afraid to 'play', it is a good learning process. Carbon is easier to work than glass.

If you thin down your propellers, be sensible about it. Care must be taken on the back face, as this angle determines the pitch. Do not modify nylon types of propellers.

Epoxy resin propellers can be modified by twisting the blade under extreme heat, ie - boiling water or a heat gun. It is important that an accurate pitch gauge is used when attempting this form of modification. Take care to not over-do it, and avoid damaging the structural integrity of the prop.

After spending time balancing the propeller, take care to keep it in balance on the field. Frequently clean off residue, ie - grass, insects, earth, etc from the blade.

If you insist on mowing terra-firma chances are you may split or feather the tip. These splits can often be repaired by a drop or two of thin cyno glue. If the split is too long of the cyno fails to repair the split, the propeller should not be used. A badly feathered propeller is often best trimmed to a lower diameter.

If using a wood or nylon type propeller, these should not be used if any damage or stressing is evident. Avoid storing propellers in a stressed position or hot environment, ie - model nose down, resting on the prop. The propeller may warp under these conditions. Nylon needs to maintain a moisture content for optimum properties. Most manufacturers moisture condition the props before sale.

Propellers are potentially dangerous. Please treat with care, respect and common sense. Modellers have died from injuries caused by propellers. Remember a static propeller is safe, it is how they are used that causes the problems. Your safety (and those around you) is your responsibility.

At Bolly we have a test bench where we frequently test our products, when testing a 20cc engine we had a prop kick loose (the prop nut wasn’t tightened sufficiently), the prop flew forward 3 metres and hit the roof which was 4 metres above. It then bounced back to the test stand. It was a very good example of the potential dangers, luckily no one was nearby when it happened.