Engineering: balancing propellers

You wouldn’t put up for long with an unbalanced wheel on your car – but when did you last have your aircraft propeller dynamically balanced, like they do at every tyre fitter’s depot?

In the aviation world we really put up with more noise and vibration (and often outdated technology) than we should. There’s no great expectation that old aero engines should run that smoothly and ? because propellers have traditionally only been balanced at rest on a bench (statically balanced) and are rarely checked once they are installed ? we even put up with new units that could actually be set up far better than they are.

It is relatively easy for someone with standard DIY skills to mount parallel knife-edges on a bench and balance the typical Permit aircraft two-blade propeller under static conditions. However, even this basic kind of prop becomes part of a far more complicated rotating system when it is mounted via a flange that might ? or might not ? be running true to an engine that might itself be in, or out, of balance (some of the older designs were made on dated machinery and subject to wider tolerances than we’d accept today). If a spinner is fitted, this too can contribute to vibration, either through not being mounted correctly or being out of balance ? or a combination of the two!

Even if you have a new propeller ? one that has quite possibly been dynamically balanced by its manufacturer ? fitted to a new-generation engine, you might find it would benefit from a ‘whole system’ check once the unit has bedded in. In particular, the blade bearings of variable pitch propeller are said to take five hours or so to settle, and balance may suffer slightly as they do so.

Dynamic balancing ? checking and rectifying the problem while the engine is running ? is the ideal solution and is something that has been made easier by the advent of portable electronic systems. You can buy the kit, but actually doing the job safely and correctly is something best left to the experts and there are now a number of companies and individuals around the country offering a service for everything from microlights to turboprops. For the typical GA single it is something that can be done in half a day for a total cost of a few hundred pounds.

To balance any rotating mass, one needs to know how much out-of-balance force there is, and establish which direction it is acting in. To measure the force, an accelerometer is attached to the crankcase of the engine, as close as possible to the propeller drive flange. To determine the direction or ‘phase angle’ a tachometer is fitted to the engine or cowling. Using the signals from these two devices, it is relatively easy for the clever bit ? the computer built into the readout ? to calculate the position and mass of the necessary balance weight(s).

Dynamic balancing is not a universal panacea: if the vibration is severe or would require excessive balance weight to correct, the propeller should be removed for static balancing. Nor can it fix any underlying problem with the engine or engine mounts. However, it will at the very least give you and your passengers a smoother ride and may prevent cowlings and other components cracking or becoming worn due to excessive vibration ? for example, the US Navy has discovered that regular balancing extends the TBO of reduction gearboxes by 100 per cent. Even in GA you may well avoid serious vibration-induced failures like a broken exhaust.

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Getting a propeller dynamically balanced may cost a bit more than having the same thing done on a car tyre, but it has the potential to save more than a few pounds in the long run.