Using DynaVibe to Balance Rotorcraft

DynaVibe can balance rotors as well as propellers, and key to accurate rotor balancing is proper placement of the lateral and vertical accelerometers as well as the phototach sensor. 

Lateral Accelerometer Placement

The purpose of the lateral accelerometer is to measure the mass imbalance that occurs once per rotation (1-per) of the rotor.  The best way to measure the mass imbalance is to maximize the sensitivity to the mass imbalance while minimizing sensitivity to other vibrations. 

To do so, mount the lateral accelerometer as high as possible on the rotor mast to maximize sensitivity.  It’s also preferable to mount the accelerometer on a rigid structure.  If the accelerometer is mounted on a bracket or other secondary structure, that structure may absorb vibration or modify the signal due to structural resonance.  Furthermore, to maximize sensitivity, orient the accelerometer so that it is sensitive to the axis of minimum rotational inertia.  For rotorcraft, this is usually the roll-axis, hence the reason for laterally mounting this accelerometer.

To minimize non-mass vibrations, point or align the accelerometer through the rotor shaft.  If the accelerometer is mounted ahead of or behind the main rotor mast, then the accelerometer will receive vibration from the main rotor imbalance plus vibrations from any torsional vibration.  This occurs frequently in analyzing engines: the vibration signature will change if the accelerometer is not pointed through the rotating element.

When measuring mass imbalance, a key area of focus is measuring the vibration when there is minimal forward airspeed.  If the two blades are not identical in lift and drag, then blade asymmetry will cause a 1-per vibration. Measuring the mass imbalance at high forward speeds will cause the sensor to register both the mass imbalance and blade asymmetry.  For this reason, it is best to collect mass imbalance data while in a step decent, power off.

When the lateral accelerometer and phototach are setup correctly, and the forward speed component of the 1-per vibration is eliminated, then balancing the rotor is just like balancing a prop.

Phototach Sensor Placement

Position the phototach so that it has a clear view of the rotor system, making sure that it is at least six inches away from the reflective tape target to get a good signal.  A piece of reflective tape is applied to rotor system so the DynaVibe computer can determine the relative position of the blades to the vibration signal received by the accelerometer.  The phototach position is determined by the polar charts that you will be using, the manufacturer's recommendation, or by talking with a DynaVibe team member.

Vertical Accelerometer Placement

The purpose of the vertical accelerometer is to measure vertical hop and pitch axis vibration.  This requires placing the vertical accelerometer in the forward cabin as pictured.  If vertical hop is present at the 1-per, then its cause is blade asymmetry.  If it is present twice per rotor rotation (at the 2-per), its cause is the advancing blade lift and drag.  It’s usually possible to correct a 1-per vibration through tracking: making tracking better or worse depending on the blades.  The 2-per vibration, however, is generally difficult to eliminate.  For helicopters, DynaVibe engineers recommend using trim tabs to adjust the forward flight 1-per vibration.

For rotorcraft balancing, the full spectrum analysis capability the DynaVibe GX2 provides is excellent for troubleshooting.  However, even the basic balancing capabilities of the DynaVibe Classic provide all that is necessary for rotor smoothing.  The only frequencies requiring analysis are the prop, 1-per rotor, and 2-per rotor, which are within the scope of the DynaVibe Classic feature set.  What also facilitates accurate rotorcraft balancing is expertise and knowledgeable customer service of the DynaVibe team.

Learn more by visiting the DynaVibe Rotorcraft FAQ.

DynaVibe is easy and economical to use for prop and rotorcraft balancing.  For mechanics, shops and service centers, it adds profit to the bottom line.  To learn about using DynaVibe to dynamically balance propellers and rotors, enter your email address below, visit the RPX Technologies website, or call:  405.896.0026.