Balance

Balancing

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Setting up the balance screen, where the two sensors go, and which sensors work.

A balance result is a magnitude and an angle. Both come from two sensors: a vibration sensor and, optionally, an index sensor giving one pulse per revolution. Two placement errors produce a wrong result with no visible sign on screen: a vibration sensor on a compliant mount measures a filtered version of the bearing's motion, and an index mark on a blade produces one pulse per blade instead of one per revolution.

Screen setup

Set the blade count (2 to 6) and the rotor type. The screen converts the measured angle into a blade reference ("between blades two and three") in addition to the angle.

For a ducted fan the correction is given as an angle on the hub, because a blade tip inside a duct is not accessible:

Sensor placement on a test rig

• The index mark goes on the motor bell, not on a spinner. The bell rotates with the shaft, is rigid, is present with any propeller, and can be observed from below where nothing crosses the sensor's line of sight. A sensor aimed at a spinner from the front looks across the disc and counts blades.
• The mark is a pen line, not reflective tape. Anything attached to the bell is mass on the part being balanced.
• The vibration sensor goes on a rigid part as close to the bearing as possible. A compliant mount is a low-pass filter.

Sensor placement on a complete aircraft

• The accelerometer goes flat on the firewall, not on the cowl. The cowl is a fairing, often rubber-mounted, and moves relative to the motor mount.
• A three-axis sensor mounted flat has two axes in the firewall's plane, across the shaft. Use one of those and ignore the third.
• The index mark goes on the bell. A spinner is removed for transport and refitted at a different angle, which invalidates the phase reference of the previous balance.
• Tie the aircraft down. A free-standing airframe is a spring in series with the measurement.

Sensor selection

Without an index pulse there is no phase reference and balancing uses the four-run method: a baseline run, then a trial mass at 0°, 120° and 240°. The method does not measure phase, so a constant sensor delay does not affect it. It needs bandwidth: at 10,000 rpm (revolutions per minute) the fundamental is 167 Hz, which an IMU (inertial measurement unit) streaming fused orientation at 100 Hz cannot resolve. Use an analogue sensor (piezo or analogue accelerometer) into the coprocessor's ADC (analogue-to-digital converter).

With an index pulse the number of runs halves, and only the variation of the sensor delay matters; a constant delay cancels because the trial run measures the whole chain. An analogue sensor has a delay of about 15 µs, under 1° at 10,000 rpm. A fused IMU has about 5 ms of delay, neither constant nor specified, which is 300° at 10,000 rpm.

What the measurement needs

An accelerometer and, optionally, an index sensor on the coprocessor. Neither is fitted, so the screen runs from the model and carries the MODELLED mark. The setup, the blade arithmetic and both placement guides work without them.