Helicopter Maintenance: Balance that Fan!

An ounce of prevention is worth a pound of cure. This adage certainly applies to aircraft preventative maintenance. Take, for example, significant main rotor imbalance: it’s relatively easy to detect, because the pilot can often feel the vibrations from the imbalance in their back or the seat of their pants. This sensation usually prompts some preventative maintenance, before there is significant damage to components and impact to the bottom line. 

With something like a tail rotor or fan wheel imbalance, however, the vibrations are rarely identifiable in the cockpit. Fatigue cracks and an unplanned out-of-service ship may be the first, obvious sign that something is wrong. What follows is a repair that is often time-consuming, expensive, and rarely conforms to your schedule. It’s more cost-effective and operationally efficient to invest in preventative maintenance than to repair damage. 

Balancing fans are an aspect of preventative maintenance that can save expensive repair bills. For instance, consider the Robinson R22, where Robinson documentation states that “Fan balance must be checked upon installation; fanwheel imbalance can cause damage.” As the accompanying photographs show, this damage often reveals itself in the form of fatigue cracks in the engine cowling, requiring expensive repairs.

With the proper diagnostic equipment, such as the DynaVibe GX3 dynamic balancing system, checking for fan imbalance becomes a relatively simple procedure. You can use DynaVibe to objectively measure whether the fan wheel has a significant imbalance that may slowly but steadily be creating a fatigue-crack time bomb. A fan wheel vibration check is done with the same accelerometer and optical tach used to balance the main rotor, by simply moving the optical tach and left accelerometer to the fan, with the accelerometer typically pointed toward the centerline of the fan rotation as shown (the DynaTrack optical blade tracking accessory is not required for this procedure). Because in-flight aerodynamic factors are not involved, the entire procedure is typically done using ground runs exclusively, without hover or flight runs.​

This image shows the optical tach (upper right) and accelerometer (near center) affixed to the fan housing for a fan balance procedure. The positioning will vary depending on the helicopter model and helicopter manufacturer's instructions.

If the analysis shows that the fan is outside of the acceptable balance parameters, the DynaVibe system will step you through the balancing procedure. Follow the helicopter manufacturer's procedures. It may be helpful to refer to the RPX supplemental DynaVibe user manual and helicopter-specific application notes to assist with DynaVibe setup and operation.
 
Select the "Contact Us" link near the bottom of this page to discuss fan balancing with an RPX representative or to learn more ​about RPX helicopter track-and-balance solutions.