Cause.

Fatigue or damage to the propeller.

Fix.

Diligent pre flight checking as indicated by the BMFA in their member's handbook should detect any fatigue before power is applied to the propeller. That said any unusual propeller noise at any time is good cause to to immediately discontinue it's use whether the exact cause is found or not.

Comment.

A recent BMFA News article in Issue No. 141 April 2017, raises an important safety warning.

Noticing intermittent noise and vibration on an electric powered aeroplane, which gradually became more prolonged with further flights, a thorough investigation in the workshop revealed some tiny fish scale like markings near the hub of the prop. On flexing the propeller lightly the blade snapped off at the markings. A replacement propeller of the same type would hardly flex at all.

The propeller concerned had zero rough landings or impacts, so almost certainly had degraded/fatigued over time. It had been in use in the aircraft since 2010.

The motor is 1.2kW, (1.6BHP), and the prop was an APC thin type glass filled nylon electric, correctly sized for the motor.

For the full details do read the article referred to above in BMFA News.

The following points were made about the risks involved:

1. Shedding a blade at high speed in flight will likely wreck the motor and airframe with the massive vibrations generated, before the model pilot can react and reduce throttle.

2. Shedding a blade on the ground in the pits, while in restraints, could have devastating consequences. Hence the practice of full throttle checks performed on the strip with fellow pilots out of the propeller arc.

Please Note. BMFA news is circulated in print to all members. It is can also be read online at http://edition.pagesuite-professional.co.uk//launch.aspx?pbid=4ca59cf2-c704-499a-94ac-dafea806e0a6 The most recent copy available (roughly 4 months behind the the original publication date) is automatically displayed, however clicking the "Archive" tab will open a list of magazines which at the time of this publication goes as far back as June 2006.  

Please Note. While the specific case discussed concerns an electric powered aeroplane, such fatigue possibilities could apply to IC powered aircraft and helicopter rotor blades. 

The last few lines of APC's Propeller Safety Section from their website, are really interesting and state:

"In summary, please abide by the safety practices recommended by propeller manufacturers. This is especially important for high performance propellers. Assume that propellers can fail at any time, especially during full power adjustments on the ground. Never stand in or expose others to the plane of the propeller arc."

The APC website has more information on all aspects of propeller use and safety, see www.apcprop.com  

Bibliography

BMFA News Issue No. 141 April 2017, Ian Pallister. Confessions. Electric Propeller Safety.

 APC's safety concerns for propeller use

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