Overcoming Propeller Imbalance

Lord's in-flight propeller balancing system (IPBS) is military tested


Propeller imbalance is a key problem on all turboprop aircraft — whether military, commercial, or general aviation. Overcoming propeller imbalance is crucial to the efficient operating performance of turboprop aircraft. Propeller imbalance leads to excessive vibration which causes fatigue in aircraft components and subsequent structural failures. The negative effects of vibration are also responsible for passenger discomfort and excess fuel consumption.

Keeping vibration under control and monitoring its effects to predict potential failures requires significant expenditures. The need to take aircraft out of service to periodically balance the propellers results in increased labor and maintenance costs. There are safety considerations to take into account as well. Engine failure due to vibration or fatigue could cause an accident, or the necessity to abort a flight.

 

In-flight balancing

To counteract the negative effects of propeller imbalance, a new technology — an in-flight propeller balancing system (IPBS) — has been developed. Active balancing technology for aerospace and military applications has been in development for almost a decade in conjunction with the U.S. Air Force (USAF) and the U.S. Navy. It has logged millions of service hours in applications from turbo-compressors to large extraction fans to high-speed machining spindles.

The IPBS continuously and actively balances propellers during flight operation. The result is propeller balance maintained at the highest possible level during the entire flight. By reducing propeller imbalance at the source — the propeller — vibration and its negative effects are alleviated. With an IPBS, structural fatigue is reduced, engines run more efficiently, spare parts will run longer, and safety concerns are mitigated.

Presently, to overcome propeller imbalance, an aircraft must be taken out of service every few months. For example, on the USAF’s C-130H cargo aircraft it takes approximately two to three days and a crew of three to five workers to manually balance the propellers. The traditional method of dynamically balancing propellers — adding counterweights — allows for proper balance at one specific operating condition, and for only a limited amount of time since the propeller blade angle and aerodynamic loads will change during flight.

A manually balanced propeller is only effective for the conditions on the ground during that particular point-in-time. As soon as the aircraft resumes operation, the propeller will be out of balance again. Manually balancing the propellers is not only ineffective, it is a labor-intensive and costly process that keeps an aircraft out of operation for several days.

 

Balancing benefits

The IPBS consists of one balancing device per propeller and a single controller per aircraft. This computer-controlled system uses input from the rotating propeller to automatically adjust the balance as needed to minimize total system imbalance for each operating condition. It features a series of imbalance weights configured in a ring. The ring is then connected to a centralized controller.

With IPBS, propeller imbalance is continuously monitored. When an imbalance condition occurs, a series of magnets within the balancer are actuated. These magnets shift the imbalance weights and bring the propeller back to a balanced condition. Not only does the IPBS eliminate the need to rebalance propellers on the ground, the system itself requires very little maintenance.

Controlling vibration with an online, automated balancing system offers the benefits of increased structural durability, less stress on engine components, reduced maintenance workload, and lowered operations costs. Safety concerns due to improper propeller imbalance are eliminated, and a smoother ride is provided for the air crew.

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