Dynamic Prop Balancing!
How does propeller imbalance arise?
In an ideal world your propeller should have been statically / bench balanced prior to fitting to your aircraft so that it was hopefully reasonably well balanced within itself before it ever got anywhere near your aircraft. It then gets fitted to your aircraft and all of a sudden the vibration that is transmitted through your engine mounts to the airframe is now a combination of the rotating propeller mass combined with the mass of the other moving parts it is attached to which can include spinner, spinner bulkhead, starter ring and components within the engine / gearbox (if applicable) e.g. crank / prop shaft etc. If you then factor in the unbalancing effects of a few "repaired" stone chips or re-lacquering of a wooden prop and it is possible for your prop / engine installation to have found it's way out of balance.
Why should I bother with Dynamic Prop Balancing?
When you change a tyre on your car it is pretty much universally standard practice these days for the tyre fitter to dynamically balance your tyre / wheel combination by spinning it up to speed and then fitting a lead weight to the rim of the wheel to balance the combination up. This procedure is done as it is recognised that an out of balance wheel can cause premature / uneven tyre wear.
In an aircraft installation any unbalanced rotating mass is acting as an undesirable force on your prop shaft bearings at the very least and in more severe cases excessive vibration is responsible for cracked exhaust systems / attachment brackets, premature failure of exhaust springs, engine mounts etc to say nothing of the avoidable fatigue that you as the pilot and your passenger experience.
When is prop balancing not applicable?
There are some basics that you should ensure are right before you even consider a dynamic prop balance. It is a pointless exercise to balance your prop and drive train if you have a dual carb engine (most Rotax engines) and excessive vibration is being experienced simply because your carbs are out of balance. Carb balancing is usually a fairly quick operation assuming that your carbs have been serviced correctly and your throttle cables aren't sticking. It also makes sense to ensure that your engine mounts are in good shape. A propeller balance is not a cure all for vibration arising out of poor, defective or worn parts on your engine / airframe. Dynamic prop balancing / spin trim balancing will only improve the vibration caused by mass unbalance of the externally rotating components of the propulsion system. If your carbs are out of balance or your engine mounts are in poor shape it will be very hard, if not impossible, to isolate and correct any vibration that your prop / drive train is responsible for.
How is dynamic propeller balance achieved in practice?The same fundamentally principles of operation apply to all dynamic prop balancing equipment and they are that a prop balancer must provide ...
- A means of measuring the vibration level.
- A means of measuring the angular reference of the vibration.
- A means of determining the relative trim weight adjustments necessary to reduce the vibration of the propulsion system.
We use PB3 prop balancer from Smart Avionics. This measures the level and phase angle of the propeller / drive train's vibration and display this information graphically. It indicates where to position a trim weight and provides an relative estimate as to what that weight should be. The weight is fitted and another measurement is taken and further adjustment is made until the IPS value is below 0.1 IPS (Inches Per Second).
Polar Chart view.
Numeric Display view.
Graphical Display view.
Spectrum Display view.
Please see the table below for an idea of how IPS of prop imbalance relates to the vibration experienced.
Guide to vibration levels and their effect.
|Vibration||(IPS) Peak Amplitude||Recomendation|
|Danger||1.25 IPS||The propeller should be removed and a static balance performed.|
|Very Rough||1.00 IPS||Propeller can be dynamically balanced; however a large amount of weight will be required. A propeller static balance is recommended. Operation at this vibration level could cause damage.|
|Rough||0.50 IPS||Propeller definitely requires dynamic balance. Long term operation at this vibration level could cause excessive wear.|
|Slightly Rough||0.25 IPS||Dynamic balance will improve passenger comfort.|
|Fair||0.15 IPS||This is the maximum acceptable vibration level after dynamic balancing.|
|Good||0.07 IPS||Vibration levels below 0.07 will not be detectable by pilots or passengers.|
Dynamic Propeller Balancing is charged at an hourly rate. See pricing page for prices. Please phone to discuss your requirements..